three phase / single phase motors &circuits nre 8...

NRE 8 Three Phase/Signal Phase Motors and CircuitsCompiled by Bruce Davison, Greg Riach & Robert Baker

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Three Phase / Single Phase Motors &CircuitsNRE 8

Student Resource Package No: NRE8.

Delivery: Competence in this training program can be achieved

through either a formal education setting or in the

workplace environment.

Recognition of Prior Learning: The student/candidate may be granted recognition of

prior learning if the evidence presented is authentic

and valid which covers the content as laid out in this

package.

Package Purpose: This package provides the student with the

underpinning knowledge and skills to identify and test

three / single phase motors and their associated

power / control circuits. In addition the operation of

three phase motors, contactors, thermal overloads,

single phase motors and their starting devices.

Suggested Resources: Australian Refrigeration and Air Conditioning Vol 1&2.

Electrical Principles for the Electrical Trades, J.R.

Jenneson

Assessment Strategy: The assessment of this package is holistic in nature

and requires the demonstration of the knowledge and

skills identified in the student package content

summary. To be successful in this package the

student must show evidence of achievement in

accordance with the package.


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Assessment:Grade Code: 72

GRADE CLASS MARK (%)

DISTINCTION >=83CREDIT >=70PASS >=50

Assessment Events:

1. Practical Test 20/40 50%2. Theory Test 40/40 50%

Practical Test: Simulated practical exercises in regards to three phase motors, single phase motors, DOL contactors and associated circuits..

Theory Test 1: Short answer Questions, multiple choice questions.This assessment covers the contents from sections 1 to 9 in the student resource package.

Summary of Contents: Page No

Delivery & Assessment Details: 1

Section No: 1 Three Phase Electrical Systems: 4

Section No: 2 Common Motor Information: 9

Section No: 3 Three Phase Induction Motors: 17

Section No: 4 Three Phase D.O. L. Motor Starter: 26

Section No: 5 Three Phase Motor Protection: 35

Section No: 6 Split Phase, motors Starters and Overloads: 41

Section No: 7 Capacitor, Shaded Pole & Synchronous Motors: 50

Section No: 8 Series Universal Motors: 57

Section No: 9 Servicing Motors and auxiliary equipment: 60

Answers to Review Questions: 64

End of Package Assessment: 70

This package is designed as a self paced package with all relevant information and references contained. At the end of each section there are a number of review questions which require responses. (Answers to Review Questions page 64).


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Delivery: Competence in this training program can be achieved through either a formal

education setting by completing the attached student resource package or in

the workplace environment.

Prerequisite: The following resource packages or equivalent is a prerequisite:

Occupational Health & Safety (7793T) Refrigeration Electrical 1 & 2.(NRE 1 & 2) Refrigeration Electrical Wiring (NREW)

Recognition of Prior Learning:

The student/ candidate may be granted recognition of prior learning if the

evidence presented is authentic, sufficient and valid which covers the content

as laid out in this package.

Competency:

Upon successful completion of the Occupational Health & Safety (7793T),

Refrigeration the: Electrical 1& 2 (NRE 1&2), Refrigeration Electrical Wiring

(NREW) and Three Phase / Single Phase Motors & Circuits training programs

you will be assessed against the following competencies:

UTE NES208EA: Disconnect & Reconnect fixed wired electrical equipment (up to 1000Volts).

UTE NES 209EA: Attach flexible cords and plugs to electrical equipment connected to 240 volt supply

UTE NES 210EA: Attach flexible cords and plugs to electrical equipment connected to a supply up 1000 volts AC ( Three phase 415)

UTE NES106EA: Install electrical/electronic apparatus and associated circuits, refrigeration and air conditioning.

UTE NES 206EA Maintain and repair electrical/electronic apparatus and associated circuits, refrigeration and air conditioning.

UTE NES505EA Locate and rectify faults in electrical equipment intended to operate off fixed wired supply up to 415 volts A.C.

Suggested Resources:

Jenneson, JR., 1995, Electrical Principles for Electrical Trades, McGraw Hill, Sydney.

Australian Refrigeration and Air Conditioning Volume 1 & 2.


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Section No: 1

Three Phase Electrical Systems:

Purpose: The purpose of this section is to provide you with the underpinning knowledge and skills to identify and test three phase voltages, currents and three phase electrical connections.

Reason for Study. In the HVAC&R industry three phase is mainly used to power electric motors because: Three phase motors have greater outputs for their physical size. Three phase motors have more uniformed starting torque. Three phase motors are self-starting. (See later notes). Three Phase systems require less copper in conductors to transmit the power, i.e. P=VI and

if the voltage is 415 less current is needed to make the same power as 240V. Less current means smaller conductors.

At this stage you should be familiar with: Production alternating current. Ohm’s Law. Impedance. Resistive, Inductive and Capacitive Circuits. Power in Single & Three Phase Alternating Circuits.

Note: Reference to Refrigeration Electrical 1 & 2 may be necessary to revise single and three phase alternating circuits and their characteristics, before the commencement of this student resource package.

Three Phase Electrical Production:

Revision

Three phase voltage is produced with a device called an alternator. The alternator has three sets of windings, mechanically fixed 120 electrical degrees to each other which when passed (rotated) through a fixed magnetic field will produce an Electromotive Force that is proportional to the strength of the magnetic field and the speed that the windings pass through the field. The current available from the alternator is limited by the cross sectional area (size) of the conductor in the alternator’s windings. Because there are separate voltages, each one can be used as a single phase power source provided a neutral connection is available.


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Types of three phase connections:

References: * ARAC page 14.7. * J.R. Jenneson chapter: 11, Page 218 – 221. Electrical Principles For The Electrical

Trades

There are two ways three phase can be connected to form a working system;

Star (Y) Delta ( )

Star connection (Y) Sometimes referred to as the WYE system for electric motors.The star connection is formed, by connecting three similar ends of the windings together as shown in Fig.1. Either the start or the finish ends of the windings can be used but not dissimilar ends.

Fig 1

Star Systems; This is the way three phase is delivered to the consumer. The delivery system is referred to as a ‘four wire system’ because there are the three phase cables and the neutral.

The three actives (lines) usually referred to as phase A. B. or C. they have a colour code of Red, White and Blue and the neutral is Black.

The voltage between these three actives is called the Line Voltage and the current flowing through the lines is referred to as the Line Current.

The Line Voltage is not equal to the Phase Voltage because two windings are connected in series with two Line Voltages.

Line Voltage (V line) = 415volts Formula: VVline 415

Phase Voltage (Vp) = 240volts3

VlineVp

Note: for calculation purposes 732.1

415Vp

voltsVp 240

The line current equals the phase current. Formula: IpIline Where: I line = line current

I p = phase current

Note: the common connecting point is called the star point.


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Delta connection ()The windings of a three phase alternator can also be connected in delta as shown below. The delta connection forms a closed loop with dissimilar ends joined together with the line cables joined at these junctions. (fig 2)

Fig 2

In a Delta connection, each phase winding is connected across two line voltages and therefore the line voltage equals the phase voltage. VpVline The line current in a delta connection is a combination of the phase currents. If the phase currents are all equal, then the line current is equal to;

√ 3 x phase current or simply put: I line = √3 x I phase

Consumer Electrical Supply in Australia.

In Australia the three-phase power is delivered to the consumer via a four wire Star connected system. The Delta does not have a neutral and is not used much as a commercial system, but is used to transmit high voltage around Australia to consumer’s transformers. The transformer’s high voltage side is internally connected in Delta and the low voltage side is internally connected in star, so that the consumer can have the four wire system.


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Electrical Power:The amount of power consumed by a three phase load is determine by using the following formula: P = √3 x V x I x pf

Where: P = power in watts or kilowatts (w or kW) √3 = 1.732

V = voltage in voltsI = current in amperespf = power factor (0 to 1)

Example: A 415 volt three phase motor draws 6.5 amperes with a power factor of 0.84. Determine the total power consumed.P = √3 x V x I x pf

84.05.6415732.1 PwattsP 5.3924 or kW925.3

Note: the power consumed is measured and charged as a kilowatt hour kWHr

Example: If the above motor operates for 24 hours and the cost per kWHr is 12 cents determine the total cost of operation over 24 hours.

timekWHrtkWtTotal /coscos 2412925.3

100

4.1130 cents

30.11$

Therefore the total cost over a period of 24 hours equals: $11.30

Service Points.Remember to check the voltage on all three phases, 415 volts between phases, and 240 volts to earth. When testing the supply always check at a non-load point. i.e. at the input terminals of

the contactor. The supply voltage should not be below 5% of the rated voltage. Check for continuity

of supply at the terminals of the device it is powering. i.e. the motor terminal block. It must be the same at no load and full load, and within 5% of the rate voltage on the nameplate of the device it is supplying.

Symptoms of low voltage supply.Motor overheating.Extra noisy motor.Motor fails to start.Overload tripping when motor not loaded.

Causes of low voltageDirty contacts, or loose terminals.Supply from the electrical authority.Overload building wiring.

Always tong amp test at least two of the three phases although it is best to tong all three. They should be the same value and no greater than the Full Load Amps on the nameplate of the device it is supplying.

Set the meg-ohmmeter to 1000V when testing three phase motors.1. A reading less than 2Mohms indicates a burnt out motor.2. A reading less than 25Mohms indicates moisture in the motor.3. A reading greater than 25Mohms generally indicates no short to earth, but

remember a motor starts with infinity resistance so anything less means it is wearing out. W

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Review Questions Section No: 1

Q.1 List the two methods of connecting a three phase systems?

Q.2 Can a single phase motor be connected into a three phase electrical supply system

and are there any additional requirements:

Q.3 What is the line and phase voltage of a three phase delta system?

Q.4 A three phase delta supply draws a phase current of 5 amperes from a 415, volt 50 HZ power supply. Determine following in the space provided:

line voltage phase voltage line current phase current:

Q.5 A star connected 415 volt three phase motor draws a phase current of 3.6 amperes. Determine the following: in the space provided:

line voltage phase voltage line current phase current:


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Section No: 2Electric Motors, Common Information

Purpose: The purpose of this section is to provide you with the underpinning knowledge and skills to identify, disconnect, diagnose and service various types of single and three phase motors which are commonly used in the refrigeration / air conditioning industry.

Reason for Study: The A.C. electric induction motor is rugged and has a high degree of reliability, and because of these quantities they are the most common type of motive force used in refrigeration.

Introduction

The induction motor gets its name from the fact that the current that causes the magnetism in the rotor is induced it is not drawn directly from the supply

Construction.

Stator is for an Hermetic compressor.

Stator

The stator is the magnetic field producing stationary part

of the motor and is made up of laminated steel

punchings with slots cut to accept the windings. The

slots are insulated with thick paper type insulation

(leatheroid), which acts to prevent the winding’s

conductors from rubbing against the steel laminations as

the magnetic flux cause everything to vibrate. Each

lamination has a build up of oxidized metal on one side

so that when they are placed together the oxidized

surface will create an area of high reluctance for the

magnetism produced by the stator windings, this then

gives each lamination its own magnetic field, as opposed

to a solid steel block having one magnetic field.

Windings; The windings are made from varnish

insulated copper wire, which is wound as coil sets,

the number of coils in each set is determined by

the number of magnetic poles in the motor. Single

and three phase motors’ have different numbers of

coils for the same number of poles.


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Types of Rotors

Squirrel Cage; gets its name from the shape that the rotor conductors form without the laminations. The rotor conductor’s are mainly made from solid aluminium which is cast into the laminations when it is manufactured although there are some made from solid copper.The only insulation between the rotor conductors and the laminations is the oxide on the aluminiumthat comes from the casting.Insulation is not necessary because the induced voltage on the rotor is very low, but the current is very high to produce the required magnetism.The rotor bars are short circuited together so that the induced EMF has a circuit in which the current can flow. The rotor has low resistance that allows high starting current, to create the starting torque. They are available in a number of designs that provide different starting torque characteristicsby having different widths for diameter and closeness of the rotor to the stator.

Advantages of a squirrel cage motor include:

simplicity and rugged construction

No sparking contacts in the motor so they can be used in explosive atmosphere

(provided; the appropriate enclosure is used).

Wide range of shaft speeds (when used with electronic frequency controllers).

Disadvantages of squirrel cage motor include:

Relatively poor starting torque

Fixed characteristics.

Rotor / ShaftThe rotor is also made from laminated steel punchings for the same reasons as that of thestator. The rotor is designed to fit neatly in the stator so that the air gap between it and the stator is kept to a minimum. The closer they are together, the greater is the magnetic attraction, because air gap has a higher magnetic reluctance.

The holes that accommodate the conductors are often skewed across the curved surface of the rotor to help with the inductive process during starting. It also helps reduce the magnetic humming noise.

The shaft and rotor are supported by ballrace bearings mounted in the endplates.

Squirrel Cage Rotor

It will be a rare occurrence to find any other type of motor being used in refrigeration other than the Squirrel Cage Induction Motor.


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Wire Wound (slip rings); (normally three phase), has a rotor winding the same

as the stator, except that one end of the coil windings are connected in star,

and the other ends are brought out through slip rings and brushes to an

external circuit, where equal but variable resistance is connected to the rotor

windings. The resistance is decreases as the speed increases. This limits the

starting current, increasing the starting torque.

They are not used much in refrigeration industry because of their cost and

their complex control.

Synchronous Motor; is not an induction motor and it is not the power tool

motor. It has a stator the same as the induction motor but the rotor is wire

wound with each individual coil leads attached to a segment of a commutator.

Now the rotor is now referred to as an armature. Both the stator and the rotor

are supplied with A.C., and because of the way the windings are connected to

the commutator on the rotor the A.C. is converted to D.C. for the rotor

windings which will create a constant position magnetic field on the rotor. The

constant position magnetic field reacts directly against the rotating magnetic

field. This combination causes the shaft to rotate at synchronous speed, and

has its strongest torque at synchronous speed.

Sometimes used to correct power factor as it causes the current to lead the

voltage, other than that not used in HVAC&R industry.

Small Synchronous Motors, however small synchronous are used

extensively in the industry for timer motors because they keep accurate time,

and small modulating motor systems. They use permanent magnets in the

rotor instead of the DC supplied magnetic field.

Synchronous motor used on a domestic defrost timer


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Frames

The frame (motor housing), carries the stator and is also the mounting for the end shields (end

plates), that house the bearings.

It is also the means of sercuring the motor to what it

is driving.

There are several different types of frames;

Open, which means you can see the rotor and stator

windings from outside.

Drip proof that has a closed upper half, while

allowing ventilation through the lower half.

Weather proof is designed to be out in the rain.

Hose proof types they are designed to prevent the entry of forced

water.

Flame proof, meaning any burnout will be contained within the frame enclosure.

Weatherproof motor frame

Drip proof

Whenever you are servicing an electric motor, check that all the mounting bolts are secure and that there are no cracks in the frame or its mountings. Electric motors must be held down firmly, for they develop large amounts of torque which strains the mounting points, and will twist the frame if it is not secure.


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Basic Operation of an Induction motor.

At the instant of start up an induction motor is

a transformer, because as the electricity is

applied to the stator winding its magnetic field

will spread out cutting through the rotor

winding inducing a large current but low

voltage (step down transformer). The

magnetic field produced in the rotor is

opposite to the stator field, so the rotor will not

turn (unlike poles attracted fig 1).

To make the rotor turn, a force is needed to

make the rotors magnetic poles to be out of

alignment with the stator. (fig 2).

The following text describes the different types

of induction motors and how they cause the

stator and rotor magnetic poles to misalign

and start rotation.

Types of Motors and equipment covered in the

following text.

1. Basic Motor Information.

2. Three Phase Motors.

Contactors.

Overloads

Motor Starters

3. Single Phase Motors

Shaded Pole.

Split Phase.

Capacitor Start Induction Run.

Capacitor Start Capacitor Run.

Permanently Split Capacitor.

Synchronous.

4. Single Phase Motor Starting Relays

5. Single Phase Motor Overloads

6. Servicing of all the equipment.


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Nameplate Information

Power

This is the rated shaft power, not the electrical input. The electrical input should be more and

the difference between the two is regarded as losses for electric motors. The losses can be

either electrical or mechanical and cause heat.

Supply Voltage

Single phase motors have a supply of 240V.

Three phase motors will be supplied with 415V, make sure that

the motor has its internal connection arranged to accept the

voltage (star or delta), it will be on the nameplate.

Supply Frequency

Should be 50 hertz (cycles/sec). (60Hz USA motors)

Motor Current

Full Load Amps (FLA), is the maximum current that the motor is designed to draw at

full load while maintaining its rated speed.

Starting Current.(not on nameplate), this can be from 3 to 9 times the FLA, and is

something that should be kept to a minimum. eg. If a motor has a FLA rating of 10

amps, and is started fully loaded it can draw 90 amps to start. If this motor was to start

at your house the lights would dim until the motor reached running status.

Locked Rotor Current (LRC on hermetic compressor’s nameplates) is the current

that the motor draws if the rotor is locked by a too heavy load. eg. a compressor seize.

One horse power = 0.746 kW

Always check motor voltage

before energizing it.


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Motor Speed

Rev/min. (Shaft rotation on nameplate) is the speed that the shaft should turn at, even when

fully loaded.

Rotational speed of the electric motor is controlled by the number of poles built into the stator

and the percentage of slip needed to maintain rotoation.

Terms Related to Motor Speed

Synchronous Speed (not on nameplate) of an electric motor is the speed at which the

magnetic field ‘rotates’ around the stator windings.

The synchronous speed is not the speed of the rotor shaft, it is the speed that the magnetic

flux appears to rotate around the stator. It travels faster than the rotor so its magnetic field will

continuously sweep through the rotor causing the reaction within the rotor windings maintaining

rotation and torque.

This can be calculated by knowing the frequency of the supply, the number of magnetic poles

(norths and souths always an even number), wound into the stator, and a constant of 120.

120 x frequency

nsyn = ———————— number of Poles

Where nsyn = synchronous speed

Frequency = Hertz (cycles/sec)

Poles = magnetic poles

120 = a constant.

The synchronous speed remains constant regardless of the

load placed on the motor.

120 x 50nsyn = —————— = 1500 rev/min. 4

Shaft Speed

The speed the shaft travels at is the rated speed on the nameplate of the motor.

The difference in speed between synchronous and shaft (actual) speed is referred to as slip

and is expressed as a percentage in calculations, usually around 4%.

( syn - actual speed) 100slip = ———————— x — Syn 1

Both three phase and single phase 4

pole motors have the synchronous

speed of 1500 rev/min,

The voltage intensity has

nothing to do with the rotational

speed of the motor, but it does

have a lot to do with the ability

of the motor to turn the load.


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e.g. A 4 pole motor (syn 1500 revs/min), has a rated shaft speed of 1440 revs/min, calculate the percentage of slip?

( 1500 - 1440) 100 60 100slip = ———————— x — = x = 4% 1500 1 1500 1

Another calculation that can be used when referring to motors is how to calculate the actual

motor speed when the % of slip is given.

If the motor was running 4% slow it means it is running at 96% of synchronous the 96% can be

converted to a decimal (0.96) to make the calculation easier.

Actual speed is synchronous multipled by the slip

Service information.

An electric motor must turn at its rated speed. If the speed isn’t maintained the motor will have

less inductive reactance meaning, less impedance which will allow more current to flow

causing the motor to overheat, tripping the overload or burning out the motor’s stator windings.

Terminal Block

All electrical connections made to a motor must be completed in the motor’s terminal block.

Flexible conduit and multi-strand conductor must be used (preferably flexible conductor).

The conduit must be secured by an approved fitting or in the case of flexible cable to a small

fan motor an approved gripping type grommet.

The earthing connection must only be made at the terminal marked as the earth.

120 x Hertzn = —————— X % of Slip Number of poles

All electric motors must beearthed at the earth terminal in the terminal block.


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Section 3Three Phase Induction Motor: Reference: J.R. Jenneson pages, 253 – 260.

Three phase induction motors are said to be self starting

because as the electricity is applied to the motor it will

develop a “rotating magnetic field”, It doesn’t require any

external equipment to cause the rotor to start to move.

Three Phase motors are used in the HVAC industry to drive

large loads like commercial compressors and large fans.

ConstructionThey have three separate sets of coil windings placed

120oE apart in the stator. Each winding is connected to two sequential phases, (see diagram

below). They have squirrel cage rotors.

Operation

Because of the winding configuration, and

the nature of three phase supply, as power is

supplied to the motor having two magnetic

poles (3000 rev/min synchronous), only one

set of coils will have maximum magnetic

strength, another set of coils will have a

rising magnetic field, and the other a falling.

As this is happening the rotor is being

affected by all three magnetic fluxes which

are moving at different angles through the

rotor conductors inducing three currents and

three magnetic fields which react with the

stator fields producing a very strong starting

torque.

The magnetic fields of the stator appear to

rotate around the stator, but actually they

only grow stronger and weaker in sequential

order around the stator windings.


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Three phase motors made here in Australia are designed to

operate from our 415 volts 50 Hz four wire system.

The neutral is needed for single phase motors and control

circuits.

Connection

There are two ways to connect the coil windings of three phase

motor so that 415 volts can be applied to them.

They are; Star or Delta.

Some three phase motors can be connected in either Star or

Delta. These motors will have six terminals in their terminal

block and their nameplate will indicate that they can be wired to

415V. They will have terminal ‘bridging links’ to connect the

ends of the coil windings together so that they can be configured to either star or delta. When

the motor is connected in Delta will draw more current from the supply then when it is

connected in Star.

The ‘lines’ are then connected to each set of windings at the motor terminals.

Some motors are imported from

overseas and may not be suitable for

our supply, so check the nameplate

information before you make the

connections. They may need a

transformer to be supplied

separately, or have a transformer as

part of package. Some imported

motor have lower voltage and 60Hz

frequencies and must be connected

in star.


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If the motor has only three terminals it is internally connected in either Star or Delta and should

just have the ‘lines’ connected to the terminals. (Check the nameplate for voltage rating).

Most motors used in refrigeration and A/C will be connected internally in Star and there will be

three terminals for the connections.

Reversal of three phase induction motors:

Since the rotor follows the rotational magnetic force (RMF), to reverse the direction of a three

phase induction motor you must reverse the direction of the RMF. The directional change can

be achieved by changing the phase sequence (swapping two supply lead connections).

Reguardless if the motor has six terminals or three the reversal procedure is the same.


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Hermetic Three Phase Motors.

An hermetic motor is basically an open framed motor directly coupled to the compressor it is

driving. They are both suspended on springs inside a fully sealed metal housing.

The suction vapour of the refrigerant used in the system is used to cool the motor windings.

Electrical testing of three phase induction motors:

The stator windings represent a balanced load that is each phase winding should have a

similar resistance

An ohmmeter is used to measure the resistance of each winding against the

manufacturers, requirements. Winding resistance readings can be between 10 to 100

ohms depending on the motor output capacity in kW, (more kW less resistance).

The windings that are in good condition will have similar resistance of several ohms or

more.

Damaged windings will have open circuit resulting in an infinite reading of resistance

and a short circuit will result a much lower resistance.

The motor windings are normally internally connected, therefore you can not do a

between winding insulation test with the megger.

The motor windings to earth insulation test should be well above 25Mohms. (see

earlier notes for resistance values).

Do not megger an hermetic motor when under a vacuum.

When connected to the supply the frame of the motor must be solidly earthed at the

marked earth terminal.

Check the motor line currents with a clamp on ammeter for correct operation.

There should be three similar line currents that do not exceed rated current on the

nameplate.

Cable protection such as flexible conduit must be secured at the motor.


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Practical Exercise

Three Phase Motor Connections

Aim: To test the supply of a three phase system and to draw a sketch of the component layout needed to supply power to a three motor.

Equipment: Electrical training boards in workshop. 415 volt power supply Isolation switch Fuses or circuit breaker D.O.L. motor contactor 240 volt control circuit ON/OFF control switch Overload Three phase motor Note: drawing to be done on attached page.

Procedure: Complete a risk assessment of the task. On completion of circuit diagram check with you teacher/mentor and proceed

test a power circuit and a motor wired in Star and there Delta:

Electrical meters: Megohm-meter Multi-meter Tong ammeter

Record Results:

Delta: Resistances across each winding: 1 2 3

Star: Resistances across each winding: 1 2 3

Supply Voltage

Line voltage:

Phase voltage:

Current:

Star. L1

L2

L3

Delta. L1

L2

L3


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Three Phase Electrical Wiring Diagram


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Q.1 When testing a three phase motor the tong tester is placed:

(a) across the active or neutral cables(b) between any phase and earth cables(c) around any two phase cables(d) around any one phase cables.

Q.2 The stator core is laminated to:

(a) improve starting torque(b) provide silent running(c) reduce eddy current loss(d) reduce hysteresis loss.

Q.3 Rotor bars are usually made from:

(a) steel or copper(b) copper or aluminium(c) carbon or copper.(d) steel or aluminium.

Q.4 Slip speed is the difference between:

(a) standstill and rotor speed(b) rotor and stalling speed(c) Synchronous speed and rotor speed(d) Synchronous speed and standstill speed.

Q.5 A typical three phase stator winding would be expected to have a resistance of:(a) 10 to 100 ohms(b) 100 to 1000 ohms(c) greater than 1 megohm(d) almost zero ohms.

Q.6 On completion of an insulation test between each of the phase windings and earth a three phase induction motor would pass if all test results were:

(a) less than 2 ohms(b) between 10 kilohms and 1 megohm(c) The greater the better.(d) between 2 ohms and 100 ohms.

Q.7 The rotor windings of a wound rotor induction motor is usually connected in:

(a) delta(b) series(c) parallel(d) star.

Q.8 To reverse the direction of rotation of a three phase squirrel cage motor you would:

(a) interchange any two supply lead connections(b) replace the squirrel cage rotor with a wound rotor(c) change the delta connected stator winding to star(d) remove the rotor and reverse end for end.

Q.9 At what speed is the RMF produced by the stator winding rotate at?


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Q.10 How would you change the direction of a three phase motor?

Q.11 Briefly explain why the rotor speed of an induction motor is always less that

the speed of the stator RMF:

Q.12 A three phase 415 volt, 50 HZ, 4 pole induction motor operates with a 4% slip.

Determine the synchronous speed and the actual rotor speed:

Q.13 From the details below determine the voltages for a three phase power supply

between:

Any two lines: =

Any line and earth: =

Any line and neutral: =

Earth and neutral: =


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Q.14 The diagrams below (fig 1 & 2) represent a schematic layout of the motor windings for a six terminal three phase motor. Complete the diagram from the three phase supply (L1, L2, and L3) to include the phase windings for both a star and delta connections.

Q.15 Indicate on the diagram below the main components of the squirrel cage induction motor:

A1 B1 C1

A2C2B2

L1 L2 L2 L3Delta

Fig. 2

A1 B1 C1

A2C2B2

L1 L2 L3Star Y

Fig. 1

L2


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Section No: 3Three Phase Motors Starters

Purpose: The purpose of this section is to provide you with the underpinning knowledge and skills required to identify a number of three phase motor starters and in particular the construction and function of Direct On Line (D.O.L.) motor starters utilising the latch and interlocking control methods for three phase motor circuits.

Reason for Study: Three phase motors are used extensively in the HVAC industry you needto have a thorough understanding of how they are energized and de-energized.

A.C. Motor Starters (Contactors)

Motor starters (contactors) are devices that are connected between the supply lines and motor in a functional system, for example:

The operating characteristics of an induction motor are that current and torque conditions are considerably different during the starting sequence and then running conditions. In the refrigeration industry, the compressor motor is the largest single load that has to be switched by the control circuit. The control circuit can not directly handle the large starting currents or switch the 3 phase for the motors, so it operates a contactor or intiates a startersequence that is in the power circuit.

Contactors

A Contactor is multi-poled device for repeated establishing and interrupting an electrical circuit with a large current draw, and a Starter is an electric controller for accelerating a motor from rest to normal speed. (Starters control is covered in greater detail in Systems Control).

The contactor has an electromagnetic coil that moves an armature which closes the contact points.The magnetic coil can have a different voltage ratings to the motors, some typical voltages are 24V, 240V, and 415V. (24V is common on Air conditioning systems because untrained operators tend to fiddle with the controller adjustment). The physical size of the coil will not relate to the voltage rating. The control circuit is connected to terminals A1 and A2 on the contactor


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Contactor Rating

The contact points are rated for different types of load. ie. The same contactor will be rated for resistive load (heater bank), and inductive load (motor). The rating is done this way because the heater will have a constant current draw whereas the motor has a very high starting current.Contactors normally have three main sets of poles so that the three phase supply can be switched on/off to a three phase motor. The pole connections are marked by numbers, 1 through to 6. Odd numbers on top evens on the bottom.There can be other contact points on the contactor these are referred to as the auxiliaries, which can be used as part of an interlock system, or used to energized some other auxiliary piece of equipment, (eg. A contactor for fan motor or water pumps).

Auxillary contacts have very low current ratings and should not be used to switch inductive loads.They are numbered 13 - 14 for normally open contacts and 21 - 22 for normally closed contacts.

Contactor Selection Guide.

In refrigeration contactors are used for two different purposes;

1. Switching on Heaters.

2. Motor Starting.

1. To switch on Heaters. a contactor from the AC1 category is sufficient, as the loads have a

power factor of greater than 0.95.

2. Motor starting, you need contactors of the AC3 category, as the contactor’s contact points

are designed to handle the high starting current of the motors (8 times F.L.A. for three phase

motors).

If you have a motor that has to be reversed while it is still rotating in the other direction you

need to use a contactor of the AC4 or AC2 category.

Rated Voltage.

The voltage rated on the contactor is stated as the voltage between phases. (Line voltage)

Rated Current.

Current rating take into account the operational voltage, rated frequency, the category of

operation and the environmental conditions.

e.g. 40amps resistive

15 amps inductive


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Motor Starters

There are a number of three phase motor starters used within the refrigeration, air conditioning and electrical industries and they are as follows:

Note: further reference: ARAC, page 14.11 – 14.19 D.O.L. (Direct on Line)

Reduce Voltage- star- delta starter- primary resistance starter- secondary resistance starter - auto-transformer starter- part winding starters.

Electronic (soft- start) starter.

Direct on Line Starter (D.O.L.)

The basic D.O.L. motor starter, consist of a contactor with a thermal overload relay that will trip in the event of an overload condition. The D.O.L. starter has the added feature in that it can be operated by manual or automatic control devices.

A simple ON/OFF control switch is all that is needed to energize the contactor coil via theoverload. This will create the electromagnetism that causes the contactor’s armature to pull in and close the three Line contacts and switch any auxiliaries on the contactor.D.O.L. motor starter uses its three line contacts to simultaneously energize the motor winding. The windings receive full line voltage so they will draw up to eight times their running current.

Control Circuit Cycling Devices:

A number of control devices can be used to stop and start a three phase motor control circuit utilising a D.O.L. starter. These devices can be stand alone or used as part of the other circuits:

Switches like, Thermostats, Pressure Controls. Relays with Latching circuit Other contactors with Interlocking circuit.

Advantages of D.O.L. motor starters: Fail safe operation of the control circuit Simple operation Low initial cost Remote stop and start features of control.

Disadvantages of D.O.L. motor starters: No current limiting ability, therefore maximum current occurs at start.

Note: reduced voltage and electronic soft starters will be covered in more detail in another student resource package.

All reduced voltage starters require the motor to be unloaded to start.


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Components in Starter Circuits

Latching circuits:

Latching uses the auxiliary contacts on contactors to hold (latch) circuits for as long as the contactor is energized. A typical example of this is a three phase pedestal drill with a stop start switching arrangement.

Interlocking circuits:Interlocking uses the auxiliary contacts on contactors to automatically bring in other components, or stop them if a fault appears in another circuit.

The interlocking circuit represents the control circuit for a refrigerated coolroom operating with a three phase evaporator, condenser fan motor and compressor drive motor.

Note: The diagram “interlocking” compressor drive motor in a refrigerated coolroom. The compressor drive motor contactor will be energised when the evaporator fan motor contactor is energised via EF/1. The normally closed auxiliary (EF/2), will indicate fan failure.


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Practical Exercise

Three Phase D.O.L. Motor Starter

Aim: To construct an electrical wiring circuit diagram for a three phase exhaust fan motor in accordance with the following details:

415 volt power circuit Isolation switch / fuses etc D.O.L. contactor Star connection. Control circuit consists of: STOP/START switching and overload protection for

the fan motor.

Note: Complete circuit diagram on the page provided below.

Procedure: Read the procedure completely.

Complete a risk assessment for the task.

On completion of your circuit diagram proceed to electrical workshop and hard

wire on three phase training boards, using the correct meters tools and safety

procedures.

Test run on completion of connection and check current and voltages under

supervision of your teacher or mentor.


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Three Phase D.O.L. Motor Starter Circuit Diagram


32

Practical Exercise

Three Phase Motor Connections

Aim: To draw a complete wiring diagram for a three phase motor for star and then delta in accordance with the following details:

Electrical training boards in workshop. 415 volt power supply Isolation switch Fuses or circuit breaker D.O.L. motor contactor 240 volt control circuit ON/OFF control switch Overload Three phase motor. Condenser fan to be energized via contactor’s N/O auxiliary. Note: drawing to be done on attached page.

Procedure: On completion of circuit diagram check with you teacher/mentor and proceed to:

Isolate the power supply and proceed to connect the motor circuit (STAR).

Test the motor windings resistances and record.

Switch the power supply on and measure the operating voltages, current and record results.

Reverse the direction of the motor.

Electrical meters: Megohm-meter Multi-meter Tong-ammeter

Record Results:

Star: Resistances across each winding: 1 2 3

Line voltage:

Phase voltage:

Current:

Delta: Resistances across each winding: 1 2 3

Line voltage:

Phase voltage:

Current:


33

Three Phase Electrical Wiring Diagram


34


Q.1 Describe the operation of a DOL motor starter:

Q.2 What are the three main components of a three phase contactor?

Q.3 What is the major advantage of a D.O.L. contactor over an isolation control switch?

Q.4 List three types of motor starters used to operate three phase induction motors:

Q.5 Draw an electrical control circuit for D.O.L. contactor controlled by a start / stop switch. (space provided on the next page).

Q.6 With the aid of the above circuit diagram, explain how the control circuit operates:


35

Section No: 5Three Phase Motor Protection

Purpose: The purpose of this section is to provide you with the underpinning knowledge and skills to identify various motor protection devices and how they operate within three phase motor circuits.

Reason for Study: In the refrigeration industry motor overloads must be selected and applied with considerable care to reduce any nuisance tripping but protect the motor from faults that cause having to replace the motor.

Motor Protection:Motor protection is required to prevent motor windings being damaged or burnt out. Three phase motor breakdown is typically due to electrical or mechanical damage.

Excessive rise in motor phase current is due to one or more of the following: Mechanical load is greater than the electrical rating (kW). Rotor shaft becomes locked. Partial loss of supply voltage. Reduction in supply phase voltages. Winding insulation breakdown.

Irrespective of the cause of motor breakdown the effect is often a rise in motor temperature due to an increase in motor current.

Motor Protection Devices:

Inverse time characteristics of Protective Devices:Protection devices installed in the electrical circuit must be capable of discriminating between normal operating conditions and conditions which may cause damage. To meet this needprotective have a design feature called inverse time characteristic which reduces their operating time when there is an increase in overload conditions and prevents nuisance tripping under normal starting currents.

Protection devices are designed to either monitor short duration high current overload conditions, short circuits or long duration medium to low current overload conditions.

Types of Protection Devices:.1. Fuses and circuit breakers:

C/Bs and fuses are there to protect the wiring to the motor from damage due to short circuit, and to degree they provide protection for motors against excessive current overloads.

The most common type of fuse used on three phase motors is the (HRC) this of fuse operates with a high rupturing capacity, A combination of the HRC fuse and circuit breaker are used in most cases. On some older installations you may come across re-wire-able fuses (outlawed under AS3000). C/Bs should have at least a 3kV rating when used to protect motors.

2. Overloads.Overloads are designed to protect a specific piece of equipment that is connected to the electrical supply. They are not designed to protect the circuit wiring mainly because the circuit wiring should be capable of carrying a far greater current than the O/L setting. Overloads are designed to absorb “start up current” without tripping; but will heat up during an “overload” condition.

Always have the current rating of the overload matching the

FLA of the motor it is protecting.


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Overloads for three phase motors will either be;

1. Current sensing2. Heat detecting.

Current Sensing

Thermal Overload Relay.These are current sensing devices that have all three phases flowing through individual branches so that any phase can cause the device to trip. They are fitted directly to a contactor so that the current flowing to the motor flows through it first.If a current greater than its rating flows through the overloads heaters, the heat produced will affect a bi-metal strip that moves a lever to trip open a normally closed switch (relay). The normally closed switch is connected to the control circuit via terminals numbered 95 – 96 on the O/L. The overload relay is connected in series with the control circuit for the contactor coil. (A1 A2).The Thermal Overload Relay has a limited adjustable current range, that allows the tradesperson to fine tune the setting for the motor’s protection. They can be manually or auto reset.

Magnetic Overload

The magnetic overload is current sensing, and will only operate if the current flow through it exceeds its rating. They are normally only wired in series to two of the phases and operate a set of contacts in series with the control circuit.They have fixed ratings and are found on imported package air conditioning systems.

Heat sensingThese devices are usually embeded in the motor windings when it is manufactured.They are connected in series with the control circuit supplying the contactor coil and open circuit if the motor windings temperature exceeds their rating.The new types are made from semi-conductor material,(thermistor), while the older types used bi-metal strips that were manufactured as a set ofcontacts.Both embeded types are non-adjustable, and the motor has to cool down before they will reset automatically.


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Setting and re-setting overloads: Only thermal overload relays can be adjusted and the correct setting must be

made to the motor specifications. Set at motor full load amperes.

If the setting is to low nuisance tripping may occur.

A motor may burn out due to a high setting. This is particularly relevant for modern motors because their design efficiency has increased along with operating temperatures.

Under Voltage and Over Voltage Relays:

The under voltage relay will respond to low voltage on the control and motor circuit and the effects of under voltage are as follows:

Relays and contactors will not operate correctly.

Sensing devices do not respond as required.

Increase in temperature of motor windings dur to the lack of inductance with will

allow more current to flow.

The over voltage relay responds to over voltage on the control and motor circuit and the effects of over voltage are as follows:

Increase power consumption in the control circuit.

Sensing and controlling devices operating incorrectly.

Increase in motor temperature because more current will be push by the

higher voltage..

Electronic motor protection relays:The electronic motor protection relays offer the ultimate protection functions by incorporating thermal, magnetic and voltage protection.


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Isolation Switch:

Motor Timer Protection: (lock out timers)Timers can be used to protect three phase motors against the problems associated from short cycling. These problems include:

Increased heating of protective devices.

Increase in temperature of the motor

Extra stress placed on the motor.

Control contacts, both power and control, have increased operation which will result in lowering the life span

Coupling between motor and load is subjected to increased stress.

Fault finding in three phase motor installations:

An electric motor may not operate due to a fault within the motor or in the circuit supplying the

motor. A fault in the load the motor is intended to drive, can also affect the motors

performance. Remember a motor is a component that converts electrical energy to mechanical

energy and therefore a mechanical fault will be reflected in the electrical circuit.

The general areas where faults in motors and their associated circuits occur are:

The electrical supply and controls to the motor

Within the motor itself

In the load being driven by the motor.

Note: To become competent in motor fault finding it is essential that you have on the job training guided by a licensed mentor in the workplace.

The AS/NZS 3000 requires that every circuit and apparatus has a point of isolation. In the event of

electrical or mechanical hazard, the circuit between the motor supply and its associated control circuit

must be capable of being isolated through the installation of an approved isolation switch. A licensed

refrigeration mechanic is allowed to repair and replace up to the isolation switch, but not the switch itself.


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Review Questions Section No: 4Q.1 List four problems that are associated with excessive current flow in a three phase

induction motor:

Q.2 List three types of circuit protection devices:

Q.3 List and describe two types of thermal overloads:

Q.4 List two main considerations that need to be taken into account when setting

overloads:

Q.5 Describe the operation and advantage of a thermal overload:

Q.6 List two problems associated with under voltage in a three phase motor circuit:

Q.7 List two problems associated with over voltage in a three phase motor circuit:


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Q.8 Electronic motor protection relays offer three protective functions. What are they?

Q.9 What is the function of an isolation switch?

Q.10 Timers can be used to protect three phase motors against the problems associated

from short cycling. List three of these problems:


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Section No: 6Single Phase Motors, Starting Relays & Overloads

Purpose: The purpose of this section is to provide the underpinning knowledge and skills required in the construction, operation, directional change applications and to identify the motor terminals for various types of single phase motors and starting relays.

Split Phase Motors: (ARAC, 14.1)

Development of starting torque in split phase induction motors:Split phase motors like all A.C. induction motors requires a rotating magnetic field to be produced in the windings so that it can act on the rotor and cause it to rotate. A split phase motor is designed to produce a directional, non-rotating magnetic field. The field is developed by placing two separate windings on the stator core at 90 electrical degrees to each other.At start up both windings are energized. The start windings magnetic flux is slightly ahead of the run winding (see lagging and inductance), the runs stronger magnetic flux pushes the start's flux to the side which gives the moving magnetic field through the rotor windings allowing it to react with the run’s field, resulting in the rotor turning.After the motor reaches approximately 75% of its rated speed, the start winding should be isolated from the supply by the start relay, to prevent the winding from burning out.

Run & Start Windings:The run and start windings are electrically dissimilar in that they are wound using different gauge wire, with a different number of turns and are physically positioned at different depths in the stator slots which affects their inductance

Applications: anything that doesn't require the motor to start loaded; Domestic compressor (equalize or the off cycle)

Fans, Load comes with speed increase.

Pumps, the same as above

Characteristics.Run and start winding. Run winding thicker wire, lower resistance high inductance deeper in the stator laminationsStart winding thinner wire, higher resistance low inductance positioned on top of the run winding in the statorStart relay needed either a, current coil, solid state or centrifugal switch.Medium starting torque.

Symbol

Picture of an open frame washing machine motor.

If a split phase motor is designed for two speeds it will have three windings;One will be the start winding so it will have the highest resistance.The second highest resistance will be the slower of the two speeds, because the there needs to be more poles for the slower the motor, therefore more copper windings, more resistance.The third winding should have the lowest resistance. This means less copper windings for fewer poles.


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Reverse Rotation

Reverse connection of either (but not both) starting or running windings connections are swapped around.

Even though both terminal links are moved it only changes the point that the A.C. is first applied to one winding.

Characteristics of a split phase motor: The run winding by itself does not develop starting torque. The start winding, working with the run winding, is needed to develop starting torque. Starting currents are around five to seven times the rated motor current. The starting current is not damaging because it decreases as soon as the motor

accelerates. Because of the high starting current the starting torque developed is around 1.5 to 2

times the rated full load torque.

Winding connections behind terminals


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Split Phase Motors in Sealed compressor:The signal phase hermetic motor/compressor unit is used in both refrigeration and air conditioning systems consists of a compressor with a drive shaft connected to the rotor of the motor which is hung on springs that are attached to an internal frame. The whole unit is fitted into a two piece pressed metal housing, called the dome and hermetically sealed by welding the two parts together

Correct electrical terminal identification:

There are three terminal connections on a hermetically sealed motor compressor and are as follows: Common (C), Start (S) and Run (R). To identify the correct terminal connection the following procedure applies.

The highest resistance reading is between the start and run terminals The middle resistance reading is between the start and common terminals. The lowest resistance reading is between the run and common terminals.

Testing and fault finding:The insulation resistance between the motor frame and all windings and active conductors must be greater than 30 meg-ohm. Remember when it was new its resistance started out so high it could not be measured (infinite).

Damage to start windings could be due to one of the following:

Starting device not opening because of a sticking centrifugal switch or a malfunction of the current sensing device (current coil / solid state relay).

Frequent stop / start Excessive load on the motor Incorrect winding connections Low supply voltage.

Possible causes of non-starting are as follows:

Loss of supply voltage Overload reset button needs to be reset Open circuit in either winding Locked rotor Faulty relay Excessive load on compressor motor (Trips out on overload)

Advantage: operates on a single phase supply.Disadvantage: low starting torque and high slip on heavy load.Rating: output ratings are: 250 watts – 1.5kW.

Note: the electrical motor terminal connections are as follows: start winding connects between S and C run winding between R and C C is common to both windings.

Hermetic Compressor


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Practical Exercise

Split Phase Motor

Aim: With the aid of the ohms scale of a multi-meter and meg-ohm meter determine the motor winding resistances and check for earth leakage on a split phase sealed motor compressor.

Procedure: After obtaining the results place the appropriate resistance readings across each winding connection and indicate the Common (C), Start (S) and Run (R) terminals on the diagram below.

Recorded results:

Hermetically sealed motor compressor motor windings


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Single Phase Motor Starting Relays

Centrifugal switch. (Fitted to open type motors only)

ConstructionThe centrifugal switch is a mechanical device that relies on the centrifugal action of the motor’s spinning shaft for its operation. It is made up of two parts. One, the Centrifugal Flyweight androtating element which is attached to the shaft of the motor, and the other part is a normally closed switch plate attached to the motor’s end shield. The switch plate also acts as the motor terminal block.

Operation When the motor is at rest the springs hold the flyweight in, which keeps the rotating element pushing against the switch closing the contacts. As the motor accelerates from rest centrifugal force starts to be exerted by the flyweights against the springs, when the rotor speed reaches about 75% of its rated speed the centrifugal action will overcome the spring tension allowing the rotating element to release the N/C switch opening its contacts disconnecting the start winding from the power supply.

Different speed motors have different tension spring. e.g. A 2880 rev / min motor will have springs with a greater tension than a motor which operate at a speed of 960 rev/min.

AdvantagesIf the speed of the motor decreases (because of increased load etc.), the centrifugal action will also decrease allowing the switch to reset energizing the start winding. This gives the motor extra torque to try to get back up to speed again.

DisadvantagesCannot be used on hermetic compressors because the flash from the arcing contacts will burn the refrigerant. (The refrigerant vapour is used to cool the windings of the sealed unit compressors).

ApplicationsMotors that drive open compressors, pumps, fans or any application where the motor is air cooled.


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Sealed Unit Starting Relays.The purpose of the starting relay is exactly the same as the centrifugal switch i.e. it disconnects the start winding circuit from the supply as the motor speed reaches approximately 75% of its rated speed. There are several different types, and each use a different force for their operation.

Current Coil Relay. (CCR)Construction The current coil type relay is a low resistance (a few turns of thick wire) magnetic solenoid coil that is wired in series with the run (main), winding. The solenoid’s armature operates a set of normally open contacts which are wired in series with the start winding circuit.

Operation. When the motor is energized it initially draws Locked Rotor Current, which is sufficient to energize the relay coil lifting the armature closing the N/O contacts connecting the start winding circuit to the supply, which starts the motor’s rotation. As the motor builds up to its rated speed the increasing inductive reactance within the motor reduces the current flow through the run windings and the relay coil. At about 75% of the motors rated speed the current flow should be insufficient to hold the relay closed, opening the start winding circuit.

DisadvantagesCurrent coil relays are designed to suit individual motors because the LRC and running currents are critical to the operation of the relay. Current coil relays need to be mounted the correct way up, as they rely on gravity to complete the disconnecting process.

AdvantageAs with the centrifugal switch, if the motor slows for load reasons, the current draw will increase and some current coil relays will pull their start winding back into the circuit.


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Potential Coil Relay.

A potential coil (voltage coil) is used for the magnetic force needed to move the armature of the relay. The contacts in this relay are normally closed (N/C), and open only when sufficient “back EMF” is generated in the auxiliary winding.

Operation.At the instant of start up both the main and auxiliary windings are connected to the supply, as the rotational speed builds up so does the induced EMF in the auxiliary winding. The potential relay’s coil is connected in parallel across the auxiliary winding, and responds to the voltage flowing in that winding. At about 75% of the motors rated speed the induced EMF in the auxiliary winding is around 350 to 400 volts, which is the voltage needed to energizes the coil opening the N/C contacts that are in series with the auxiliarywinding.The motor continues to induce the high voltage in the auxiliary winding holding the coil energized. Current flow in the potential coil is restricted by the high resistance of the small CSA conductor of the potential coil. Potential coil relays are used with high torque, capacitor start motors, because the contacts are closed at start up and only opening as the motor reaches its running speed therefore the current will be low which will reduce arcing at the contact points.

If Potential Coil Relays needs to be replaced, one of the same coil characteristics should be selected so that the ‘pick up voltage’ (the voltage that is needed to make the relay work), will be available from the start winding, as the motor reaches 75% of its rated speed.

Wiring Diagram with a potential coil relay.

The high voltage produced in

the auxiliary winding is a result

of the transformer action

between adjacent coils.


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Solid State Relay. (SSR) The solid state relay has no moving parts. It is a positive temperature co-efficient (PTC) thermistor that increases its resistance when heated by the start winding current passing through it.

Operation At start up the SSR is cold allowing sufficient current to flow to the start winding to energize it. Within milliseconds the current flowing through the SSR causes it to heat up increasing the resistance, reducing the current flowing to the start winding to milliamps. Although there is still a 240 volts potential on the start winding, the current flow through it is so small it produces very little magnetic effect in the windings, but produces enough heating effect in the SSR to maintain the resistance.

DisadvantagesThe Solid State relay can be affected by other sources of heat, such as radiated heat from a discharge pipe and should be mounted away from any heat source. Be careful when handling Solid State relays, as they can get hot.This type of relay requires a cool down time between starts, and is not designed for operation on compressors that have a high number of starts per hour, such as commercial systems.

AdvantageUnlike the Current Coil and Potential Coil Relay, the SSR does not have to be exactly “sized” to suit the electrical characteristics of the motor. One relay can be used to control the start winding circuit for motors ranging in size from 0.25 kW to 0.37 kW.

Wiring Diagram for a domestic refrigerator.


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Single Phase Motor Overloads

Signal phase motor overloads are either heat or heat/current sensing.

Heat Sensing

External Bi-metal thermostat overload.

This type of overload is often referred to as a ‘Klixon’ (brand name), because it clicks off when it reaches a pre-determined temperature, and clicks on when the reset is pushed.This type of overload is fitted on the outside case of the motor, and it normally is accessible for resetting manually.# not used on hermetic motors.

Current and Temperature Sensing.

These are referred to as the ‘inherent protector’, as they have a bi-metal strip (deforming disk), that senses the motor housing temperature, as well as a heater that has the motor running current flowing through it, that when heated will also affect the bi-metal. The bi-metal is part of a single pole single throw switch that is in series with the motor supply, and will trip it if heated to a set point.The active conductor should be connected to the screw terminal on the overload.# Used on hermetic compressors.

These overloads automatic reset after they have cooled.They are fitted to the outside of Hermetic compressors in the terminal box next to the motor terminals. They must have good thermal contact with the body of the compressor. The adjustment screw should not be fiddled with.


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Q.1 Which of the windings is open circuited by the current sensing switch?

Q.2 Explain how you would reverse the direction of rotation of a split phase motor:

Q.3 Explain the operation of the following switching mechanisms on a split phase motor:

Current coil relay:

Solid state relay:

Q.4 When energised, a split phase induction motor fails to start. List in point form the

steps you would take to locate the fault:

Q.5 Describe the operation of a centrifugal switch when used on a split phase motor:

Q.6 When energised a split phase induction motor continually trips out on its thermal

overload. List a least four possible faults that would cause this condition:


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Q.7 A 50 HZ two pole split phase induction motor operates on full speed with a 4% slip.

Calculate the full load rotor speed:

Q.8 In the space below draw an electrical wiring diagram for a split phase compressor

motor controlled by a thermostat, current coil relay and overload protection. Include on

your drawing the motor windings and terminal connections.

Q.9 You are to replace the current coil relay with a solid state relay within the circuit above.

Draw an appropriate electrical circuit diagram to meet the requirements for a solid

state relay.

Q10. After checking a hermetically sealed motor compressor the following resistances were

measured between the terminals. Indicate the common (C), start (S) and run (R)

terminals on the diagram below:

22 ohms 25 ohms

47 ohms



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Section 7

Capacitor, Shaded Pole and Synchronous Motors

Capacitor Start Induction Run Motor.

Symbol/Schematic

Operation.

Much the same as split phase motors, except the capacitor is in series with the auxiliary (starting) winding which will bring the current in the auxiliary to be nearly in phase with the supply voltage. (See A.C. impedance Z). In doing this the "phase angle" difference between the main winding’s current and the auxiliary’s is increased. Having the main winding’s magnetic field at a greater angle to the auxiliary’s will result in a stronger starting winding torque.

Capacitors must be matched to the motor to obtain optimum starting torque. Manufacturers will specify the capacitor value required for a particular motor. Start capacitors can range from 40µf to 180µf. Start capacitors remain in the circuit for approximately three seconds and are limited

on the number of starts per hour. This depends on the motor design. Increasing the starting torque with this type of motor is accompanied by an increase in

starting current.

Application

Small commercial compressors that will cycle loaded, i.e., high discharge pressure.

Reverse rotation same as split phase motor.

Characteristics:

Requires a starting relay.

Centrifugal switch or current coil.

Run winding resistance less than start

winding.

High starting frequency. (starts per

hour)

Winding connection are now referred

to as;

Run = main

Start = auxiliary.

This is because the auxiliary will have a

current flow through it as the capacitor

charges and discharges from the A.C.


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Capacitor Start Capacitor Run Motor

Symbol/Schematic

Operation.

Similar to C.S.I.R. motor with the exception that two capacitors are connected in parallel and then in series to the auxiliary terminal (winding). Once the motor reaches 75% of its rated speed the starting capacitor is switched out of the circuit leaving the running capacitor connected in series with the auxiliary winding. This arrangement gives a high starting torque (reduced phase angle) and good running torque.

Application.

Large single-phase compressors that starts loaded. e.g. low temps.Reverse rotation same as Split Phase motor.

Characteristics:

Starting relay required;

Centrifugal switch or potential

coil are normally used.

Main and auxiliary windings.

Running capacitor

Starting capacitor.

High starting torque.

Capacitors must be

matched to motor


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Permanently Split Capacitor Motor

Symbol/Schematic

Picture of a Fan Deck motor assembly

Operation

At start up both windings are energized but having the

capacitor in series with the auxiliary will cause its

magnetic field to be less "out of phase" with the supply

voltage. This will result in the rotor's magnetic field to

misalign with the main windings magnetic field given the

rotor its rotation.

The capacitor and auxiliary winding remain energized

during operation, giving the motor its good running

torque. They are used on fans and pumps because with

these devices the load increases with speed.

They can all be used with compressor’s that unload on

the off cycle. i.e. capillary systems

Applications.

Evaporator fans, condense pumps and compressors in

residential. A/C.

Reverse rotation - Connect active to other side of capacitor.

Compressor motors should not be reversed.

Characteristics: No starting relay required.

On small fan motors both main and auxiliary

windings can have the same

resistance, but with compressors’ there

is a difference in resistance.

Capacitor 3 to 80MF

Quiet electrical characteristics, i.e. won't

upset T. V. at start up.

Torque

Low starting so, motor starts unloaded.

Good running torque.


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Capacitors Used for Single Phase Induction Motors:

Start capacitor features: AC electrolytic Connected during starting only Limited number of starts per hour (20/hour) Capacitance ratings between 30µf and 250µf. Voltage rating of 240 volts.

Run Capacitors features: Oil bath Permanently connected Capacitance ratings between 4µ and 35µf. Voltage rating over 400 volts.

Capacitors are used on single phase motors to correct the lagging current in the windings, which will improved starting torque in Capacitor Start Induction Run motors over Split Phase motors, and even better starting torque and running efficiency of Capacitor Start Capacitor run Motors. They are also used to create the phase angle in the Permanently Split Capacitor motor. Capacitors are very common in refrigeration because there are many motors that are started loaded, and even more fans being driven by PSC motors.

How to test a capacitor is covered in section 9 of this package.

Service Points.

Capacitor’s have a limited life so always test the capacitor on a single phase motor before you replace the motor.


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Shaded pole

Symbol

Picture

Synchronous Motors.

Symbol:

Operation.

A.C. is applied to the stator winding creating a magnetic field across the rotor. Rotation happens because the rotor's permanent magnets will react to the stator’s electromagnetism.Synchronous motors are so named because they spin at synchronous speed, (e.g. 2 pole motor will have a shaft rotation of 3000 revs per minute).

Characteristics:

Single winding of high resistance.

No starting winding or relay.

Permanent magnets in rotor.

No overload.

Synchronous motorused as a defrost timer

Characteristics

Low torque Can be stalled without

damage therefore they do not have an overload.

Has a single winding. No starting relay required.

Operation: The shading ring causes

the defection in the magnetic flux,

starting the rotor to turn.

Uses/ Applications:

-Small evaporator fans.

-Non critical timers

-Two position value/damper motors with spring return.

Reverse rotation:

Must be dismantled and rotor and shaft must be reversed in

stator.

Application

Timers (because they rotate at an exact speed).Small modulating motors.

Reversing Rotation, is the same as the Shaded Pole motor in as much as the motor must be dismantled and the rotor reversed in the stator.


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Section 8Series Universal Motors (commutator motors)

Purpose: The purpose of this section is to provide the underpinning knowledge and skills to identify the construction and applications for the series universal motor.

Reason for Study; The Series motor is not used much in the refrigeration industry as it is expensive to manufacture and requires a lot of maintenance compared to induction motors. They where used to drive modulating valves but they have been replaced with permanently split capacitor motors.

Construction: Components parts of a series universal motor are as follows:

1. Stator consists of: Frame, rolled steel or cast iron shell Field core, laminations pressed tightly together and contains the field windings. Field coils, few turns of wire and are connected in series with the armature.

2. Armature consists of: Laminated core having straight slots Windings housed in slots with the coil sides brought out to the commutator segments Both armature core and commutator are pressed onto the armature shaft

3. End plates and brush holders End plates house the bearings in which the armature revolves. One end plate has brush holders and can be removed. Brush holders and brushes provide the sliding electrical contact between the field

and the armature’s commutator.

Operation. The Series Universal Motor (Commutator Motor), is not an Induction motor, the current flowing in the armature (rotor) is not induced it is fed directly from the supply via a set of brushes and the commutator. The main magnetic field is set up by two field poles mounted on the stator. It remains in a constant position. The armature has overlapping coils wound on to it similar to an induction motor’s stator but each coil’s leads are connected to individual segments on the commutator. The commutator converts the AC to DC and because of the way the brushes are positioned, supplies the voltage only to the coil that is being affected by the field coil’s magnetic flux. This will position the armature magnetic field to give an alternating magnetic field reacting with a permanent magnetic field resulting in rotational. The Commutator motor can be supplied with either AC or DC voltage of the same intensity.e.g. 240V AC or 240V DC.

Applications: Portable hand held tools

- Drills (240 and Battery)- sanders- jig saws- routers- grinders

Domestic appliances:- vacuum cleaners- sewing machines- hair dryer.


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Review Questions Section No: 6, 7 and 8

Q.1 Name four methods used to switch the start/auxiliary winding of a split phase motor.

Q.2 Describe how would you change the direction of a shaded pole motor:

Q.3 Explain why a capacitor start motor can develop a greater starting torque than a split phase

motor.

Q.4 Explain how you would change the direction of a permanently split capacitor motor:

Q.5 Explain the operation of a potential coil relay:

Q.6 Which of the following motors is best suited for a single phase evaporator fan motor?

(a) Series universal motor

(b) Permanently split capacitor motor

(c) Split phase motor.

(d) Capacitor start motor

Q.7 The single phase motor which is most suitable for a domestic refrigerator evaporator fan

motor is the:

(a) Capacitor start motor

(b) Series universal motor

(c) Shaded pole motor

(d) Permanently split capacitor motor.


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Q.8 Which of the following motors would be best suited for a domestic refrigerator compressor?

(a) Shade pole motor

(b) Permanently split capacitor

(c) Capacitor start / capacitor run motor

(d) Split phase motor

Q.9 List a least three desirable characteristics of a capacitor start / capacitor run motor :

Q.10 Why would you use a capacitor start capacitor run motor in preference to a split phase motor

compressor on a refrigeration system that has a TX valve as a refrigerant metering device?


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Section 9

Servicing Motors and Auxiliary Equipment

Safety. Never attempt to change electrical components while the power is still turned on.TURN OFF, SAFETY TAG and LOCK THE ISOLATOR

Reason For StudyA refrigeration mechanic will be required to test an electric motor for electrical and mechanical faults. To this end the mechanic will need to have a good working knowledge of the basic electrical test instruments and have a feel for the small tolerances associated with the mechanical side of the motor.

It is hoped that you as a student would have successfully passed Electrical Testing Equipment, before attempting to use the meters.

The minimum testing equipment required is;

Megohm meter, (1000 volt), to check for the insulation quality for leakage to earth.

Resistance meter, (ohmmeter), to check the resistance of the windings, and continuity of the circuits, and also to do a field test for a capacitor.

Tong Tester (clamp on ammeter), to check the current without interrupting the circuit.

Voltmeter, to check the supply voltage at the motor terminals.

Develop a procedure that follows a sequence so that you check the motor in a logical manner.

Here is an example testing procedure, with explanation.

Power Supply Check the voltage at the switchboard, if a fuse is blown look at the way it has melted. Splatter fuse element to the terminals, usually means a short circuit, (do not replace fuse, megger circuit to find earth or short between windings), and if the fuse element has broken with small droplets of melted wire at the tips, usually indicates an overload, replace fuse and tong test for excessive current flow.Check the voltage at the terminals of the motor as it is starting where it is possible, or at a junction box. The supply voltage should not fall below 10% of the rated voltage on the nameplate. Remember to check the voltage on all three phases, 415 volts between phases, and 240 volts to earth.

Symptoms of Low voltage supply. Motor overheating. Extra noisy motor. Motor fails to start. Overload tripping when motor not loaded.

Causes of low voltage Dirty contacts, or loose terminals. Supply from the electrical authority. Overload building wiring.

If the voltage supply is OK, proceed to check the motor.

Disconnect the motor from the supply wiring, and remove any bridging connection so that the windings can be tested without getting errors via parallel circuits.

Check the windings’ continuity and resistance of each winding. Relate the results to the type of motor being tested, (single phase or three phase). Remember the motor resistance will be very low as the opposition to current flow is inductance of the windings, and inductance only happens while the motor is operating.


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Megger the windings to earth, at least 1Mohm for open type motors and 30 Meg-ohms for hermetic.Remember when the motor was new it had a resistance that was so great it could not measured,(infinite).

Do not megger a hermetic motor while it is under a vacuum, the risk of insulation damage is very high under vacuum conditions.*

Overloads.Overloads are generally not serviceable, and if the fail they are replaced.

Testing is limited to continuity check and checking of the mechanical action of the three phase overload relays, although you can purchase replacement bi-metal heaters for the larger three phase thermal overload relays.

If the overload is tripping, and the motor current draw is within the FLA rating do not attempt to adjust three phase overload relay current setting, as the overload tripping mechanism has failed through age or other means, replace the overload completely.

Always replace an overload with one of the same rating and calibration.

Some overloads are buried in the windings, so if they fail it will be necessary to fit a replacement externally. Fitting an overload externally requires careful selection as the heat generated by the windings will take a lot longer to conduct to the frame of the motor where the replacement overload will have to be fitted. As a general rule if is better to replace the buried temperature sensing type with a current sensing device that is set to the FLA of the motor

Testing CapacitorsTests can be made for continuity which will indicate short circuits or open circuits.

* On run capacitors disconnect the Bleed resistor from one terminal.

Test 1. Set the ohmmeter to the Rx1 scale, and do a continuity check between the terminals, if there is a circuit the capacitor has a short circuit between the plates

Test 2. Set the ohmmeter to the Rx10000 scale, do a continuity check. There should be an immediate indication of a short circuit, leave the meter connected and wait for the capacitor to ‘charge up’ as the electrons (from the battery in the meter), flow onto the negative plate. The number of electrons flowing will reduce as the plate fills with electrons, which will be indicated on the ohmmeter as if the resistance of the capacitor is increasing. This is an indication that the capacitor is serviceable, and could be put back into operation.

Test 3. Some capacitors have a metal case which can allow the outer wound plate to track to earth. A megohm meter is used to test for shorts to earth. Use the lowest voltage output of the megger to do this test. Connect one lead of the megger to one terminal, and the other lead to the case, operate megger, there should be a very high resistance to earth. Do the same of the other plate’s terminal. Do not megger between terminals as the dielectric between the plates is not designed to withstand the high DC voltage from a megger.

To accurately test a capacitor use a capacitor analyser.

Remember to reconnect the bleed resistor on run type capacitors.

Symptoms of FaultsRun Capacitors. An open circuit on the run capacitor connected to a PSC motor will effectively disconnected the auxiliary winding from the supply therefore the motor will not turn, it will just hum. An open circuit in the capacitor on a CSCR motor will reduce the running performance to about the value of a split phase motor. The starting relay may chatter, and the motor overload should begin to heat because of the excessive current flowing in the windings


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A short circuited capacitor on either a PSC or CSCR motor should cause excessive current flow.

Start Capacitor. An open circuited capacitor on a CSIR motor will disconnect the start winding from the supply leaving the motor to hum. A short or open circuited capacitor on a CSCR motor will reduce the starting torque only. A short circuited capacitor on a CSIR motor normally reduces the starting torque to a point where it will not start to turn against a head pressure.

Mechanical Testing

Electric motors are used because they are reliable and rugged, they rarely suffer from mechanical failure other than bearing replacement.

During routine servicing it is a good practise to check the mounting of a motor to ensure that it is held down firmly, and that the coupling to the compressor or fan is secure as well.

Another check that can be made on site is to feel for bearing movement. there should be no vertical play on the motor shaft, there can be horizontal slide of the shaft in and out of the end plates, but if you push it in it should spring back out, as there is a spring fitted to the back end bearing of some cheaper motors to ensure that the rotor lines up with the stator.

Be-careful of moving parts while you are doing this test. Place a long screw driver against the end plate of the motor and then cover the handle of the screwdriver with your ear you can hear if the bearings are making a grinding sound. If the bearing is making this sound it means that they will need replacement soon. This check generally can not be made on motors directly coupled to compressors, as the piston movement sound is transferred to the shaft and bearings.


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Starting RelaysCentrifugal Switch. Centrifugal switches suffer from mechanical wear because the switching arm actually rubs against the rotating element. Over time the switching arm will break off, this requires switch replacement. This problem is indicated by the motor not starting to rotate combined with excessive current draw. Another mechanical problem that can occur is that the springs that hold the flyweight and determine the speed at which the switch will open circuit break. Symptom of this is that the motor will not start each time, and when it does it appears to be slow at getting away. ie. The starting torque is reduced because the start winding is removed from the circuit too early. What normally happens is that the motor will kick off but slow down immediately causing the centrifugal switch to reset and give the motor another kick along. This will continue to be repeated until the motor overload heats up and trips open. There is no electrical test for this you need to dismantle the motor and look.Remember replace the spring or the centrifugal switch with one of the same rating.

Current Coil Relay. Because the conductor in the coil of the currents coil relay is so large in diameter not much can go wrong with it, other than mechanical breaks at the connection points. There are a few things that go wrong with the contacts. Burning is one which makes the contacts a high resistance point. This is indicated by the motor starting some times but not others. To check for this remove the relay from the motor, connect an ohmmeter between the pin connection points on the relay turn the relay upside down and check the ohm reading it should be a perfect circuit. Repeat this a few times, each time check the ohmmeter reading it should be the same. If there is any reading that is not a perfect circuit keep the relay upside down but shake it side ways to see if the reading will improve. Replace the relay if the readings are higher than 5 ohms. Another problem can be the contact points welding together. This is indicated by the motor starting but continuing to draw a high current and not getting up to full speed. This is checked the same way as before, except that when the relay is returned to the upright position the ohmmeter will continue to show a circuit. Replace the relay. It is critical to make sure the ohmmeter is working correctly for both of these tests.

Potential Coil Relay.The potential coil relay has one set of normal close contacts wired in series with the start winding. The contacts are moved by the potential coil that is wired in parallel across the start winding. The potential coil needs about 380 volts to work. This is obtained via the step up transformer action between the run winding (primary) and the start winding (secondary). Faults. Start winding stays energized. Either the potential coil has burnt out or the switch contacts have welded together, use an ohmmeter to test the coil, and a screw driver to feel for the welded contacts. The coil should have a high resistance, you may have to adjust to a high range setting on the ohmmeter. Motor hums but will not start. Normally dirty contact points resulting in an open circuit to the start winding. The contacts can be cleaned on a lot of potential coil relays so open it up and clean them with a points file or fine wet and dry paper.

Solid State Relay. The solid state relay is made from a Positive Temperature Co-efficient semi-conducting material which needs time to cool between starts. This is the biggest problem. If the relay hasn’t cooled it will not allow a current to flow to the start winding therefore the motor will not rotate but it will hum and drop out on its overload. If short cycling of the motor is indicated turn off the machine for 15 minutes then re-start it checking the operation of the relay with an ammeter to the start winding. Other than short cycling of the device the motor is connect to not a lot can go wrong, as the are no moving parts. Some early relays developed an oxide between the semi-conductor and its holder causing an open circuit.


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Answers to Review Questions:

Section No: 1

Q.1 Star and Delta

Q.2 Yes a single phase motor can be connected to a three phase supply as long as a neutral is present.

Q.3 Phase voltage = 415 volts Line voltage = 415 volts.

Q.4 Line Voltage: VpVline voltsVl 415

Phase Voltage: voltsVp 415

Line Current: IpIline 3 5732.1 amperes66.8

Phase Current: 3

IlineIp

732.1

66.8 amperes5

Q.5 Line voltage volts415

Phase voltage 3

Vline

732.1

415 volts240

Line current IpIl amperesIl 6.3

Phase current amperes6.3

Section No: 2

Q.1 (d)Q.2 (c)Q.3 (b)Q.4 (c)Q.5 (a)Q.6 (c)Q.7 (d)Q.8 (a)Q.9 Synchronous.Q.10 Swap any two phase connections.Q.11 because of the slip speed.Q.12 Actual speed = 120 x 50 X 0.96 = 1440 rpm.

4

Q.13 any two lines = 415 volts.any line and earth = 240 volts.any line and neutral = 240 volts.earth and neutral = zero volts.

Q.14


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Q.15 Electrical terminals

Bearing

Rotor Frame

Stator

A1 B1 C1

A2C2B2

L1 L2 L2 L3Delta

Fig. 2

A1 B1 C1

A2C2B2

L1 L2 L3Star Y

L2


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Section No: 3

Q.1 The three contacts when closed supply the electrical power to three phase motor windings. An ON/OFF control switch when closed energises the contactor coil KI through the overload and thus creating an electromagnet causing the three contacts close.

Q.2 Coil, armature, electrical contacts.

Q.3 Can switch the three phase appliance on and off automatically and provides overload protection.

Q.4 D.O.L. Star Delta Auto- transformer Primary Resistance Part Winding. Electronic Soft Start. Secondary resistance.

Q.5

Q.6 Pushing the start button energises K1 and closes K1 contacts and thus when start button is released K1 is latch in through K1 4.

Section No: 4

Q.1 Mechanical load is greater than the electrical rating (kW)Rotor shaft lockedPartial loss of supply voltageReduction in supply phase voltagesWinding insulation breakdown.

Q.2 Fuses, circuit breakers, thermal overload, magnetic overload.

Q.3 Thermal overload (bi-metal strip)Thermistors.

Q.4 Not to be set to low to prevent nuisance tripping Not to high causing motor burn out.

Q.5 A heater senses a rise in temperature on either one of the phases, this rise in temperature causes the bi-metal strips to warp and through a lever mechanism the overload switch is opened.

Q.6 Relays and contactors will not operate correctly.Sensing devices do not respond as required.Increase in temperature of motor windings.

K1

O/L

Stop Switch Start switch

Auxiliary Latching Contact (K1-4)

A N


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Q.7 Increase power consumption in the control circuit.Sensing and controlling devices operating incorrectly.Increase in motor temperature.

Q.8 Thermal MagneticVoltage

Q.9 To isolate electrical circuit between the motor supply and or its associated control circuit in the event of electrical or mechanical hazards.

Q.10 Increased heating of protective devicesIncrease in temperature of the motorExtra stress placed on the motorControl contacts, both power and control, have increased operation which will result in lower life span.Coupling between motor and load is subjected to increased stress.

Section No: 5

Q.1 Start

Q.2 Change either the start or run winding connections but not both.

Q.3a Circuit energised causes a high rush or current through the relay coil. Relay contacts close and energises both start and run. As motor reaches full speed current falls and relay contacts to open. The start winding drops out and leaves the run winding in circuit.

Q.3b Voltage is applied to the motor. As the current flows through the SSR it causes the semi conductor to heat up. The semi conductor will reach a temperature where its resistance will change for very low to

very high. This will reduce the current flow to a trickle maintaining the heat in the semi conductor.

Q.4 Ensure correct voltage is present. Isolate power supply to motor. Check relay and overload operation. With the motor terminals disconnected check the winding resistance against the

manufacturer’s specifications using an ohm meter or megger.

Q.5 When the motor is at standstill the centrifugal switch is closed. Both start and run are energised. Centrifugal switch will open at approximately 75% full motor speed dropping the start winding

out and leaving the run in.

Q.6 Locked rotorOpen circuit in either windingFaulty relayExcessive load on motor trips out on overload.

Q.7 N = 120 x f x slip N = 120 x 50 X 0.96 N = 2880 rpm P 2


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Q.8

Q10

R C S

O/L

PTC

A N

t°

Q.9

Split phase motor with a solid state relay

R R C

SO/L

A N

tº

Split phase motor with a current coil relay


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Section No: 6

Q.1 Speed sensing, current sensing or voltage sensing devices. Which include: centrifugal switch, current coil relay, solid state relay and potential coil relay.

Q.2 Dismantling and turning the stator end for end, relative to the rotor.

Q.3 The capacitor start motor has a start capacitor connected in series with the start winding.

Q.4 By switching the supply connection from one side of the capacitor to the other.

Q.5 The contacts are normally closed energising both windings (aux & main). The contacts will open at approximately 75% full speed due the back EMF generated in the

auxiliary winding. The auxiliary drops out and leaves the main in circuit.

Q.6 (b)

Q.7 (c)

Q.8 (d)

Q.9 Reduced noise and vibration.Improved motor efficiency and power factor.Current coil relay can be used on this type of motor.

Q.10 Because the refrigeration system would require a motor with high starting and running torques to compensate for varying loads. Split phase motor has both low starting and running torques.

22 ohms 25 ohms

47 ohms


R C S

three phase / single phase motors &circuits nre 8...

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