unit 2 industrial electronics

8/8/2019 Unit 2 Industrial Electronics

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Industrial ElectronicsIndustrial Electronics


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,KINAL Technical Center

SummarySummary

IndustrialIndustrialElectronicsElectronics

The Electric Motor. . .

The DC MotorCompounds of a DC

Motor

The Series DC Motor. ..

The Shunt DC Motor. ..

Control Diagrams . . .

Independent DC Motor. . .

Compound DC Motor. . .

Industrial Electronics 2


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THE ELECTRIC MOTOR (Generator,Transformer and Motor)

An electric motor converts

electrical energy into mechanical

energy. The reverse task, that of

converting mechanical energy into

electrical energy, is accomplished by a

generator or dynamo. Traction motors

used on locomotives often perform

both tasks if the locomotive is

equipped with dynamic brakes.

Electric motors are found in household

appliances such as fans, exhaust fans, fridges, washing machines, pool pumps and

fan-forced ovens. Most electric motors work by electromagnetism, but motors

based on other electromechanical phenomena, such as electrostatic forces and the

piezoelectric effect, also exist. The fundamental principle upon which

electromagnetic motors are based is that there is a mechanical force on any

current-carrying wire contained within a magnetic field. The force is described by

the Lorentz force law and is perpendicular to both the wire and the magnetic field.

Most magnetic motors are rotary,

but linear motors also exist.

Generator

An electrical generator is a device

that converts mechanical energy to

electrical energy, generally using



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electromagnetic induction. The source of mechanical energy may be a reciprocating

or turbine steam engine, water falling through a turbine or waterwheel, an internal

combustion engine, a wind turbine, a hand crank, or any other source of mechanical

energy.

The Dynamo was the first electrical generator capable of delivering power for

industry. The dynamo uses electromagnetic principles to convert mechanical

rotation into an alternating electric current. A dynamo machine consists of a

stationary structure which generates a strong magnetic field, and a set of rotating

windings which turn within that field. On small machines the magnetic field may be

provided by a permanent magnet; larger machines have the magnetic field created

by electromagnets.

TransformerA transformer is an electrical

device that transfers energy from

one circuit to another by magnetic

coupling with no moving parts. A

transformer comprises two or

more coupled windings, or a single

tapped winding and, in most

cases, a magnetic core to

concentrate magnetic flux. A

changing current in one winding

creates a time-varying magnetic

flux in the core, which induces a

voltage in the other windings. The

transformer is one of the simplest

of electrical devices, yettransformer designs and materials

continue to be improved. Transformers come in a range of sizes from a thumbnail-

sized coupling transformer hidden inside a stage microphone to huge gigawatt units

used to interconnect large portions of national power grids. All operate with the

same basic principles and with many similarities in their parts. A simple



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transformer consists of two electrical conductors called the primary winding and the

secondary winding. Whenever the amount of current in a coil changes (including

when the current is switched on or off), a voltage is induced in the neighboring coil.

The effect, called mutual inductance, is an example of electromagnetic induction.

The DC Motor

Generally, the rotational speed of a DC motor is proportional to the voltage

applied to it, and the torque is proportional to the current. Speed control can be

achieved by variable battery tappings, variable supply voltage, resistors or

electronic controls. The direction of a wound field DC motor can be changed by

reversing either the field or armature connections but not both. This is commonly

done with a special set of contactors (direction contactors). The effective voltage

can be varied by inserting a series resistor or by an electronically controlled

switching device made of thyristors, transistors, or, formerly, mercury arc rectifiers.

In a circuit known as a

chopper, the average

voltage applied to the

motor is varied by

switching the supply

voltage very rapidly.

Since the series-wound

DC motor develops its

highest torque at low

speed, it is often used in

traction applications such

as electric locomotives,

and trams. Another

application is starter

motors for petrol and smalldiesel engines. Series motors must never be used in applications where the drive

can fail (such as belt drives). As the motor accelerates, the armature (and hence

field) current reduces. The reduction in field causes the motor to speed up (see

'weak field' in the last section) until it destroys itself.



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This can also be a problem with railway motors in the event of a loss of adhesion

since, unless quickly brought under control, the motors can reach speeds far higher

than they would do under normal circumstances. This can not only cause problems

for the motors themselves and the gears, but due to the differential speed between

the rails and the wheels it can also cause serious damage to the rails and wheel

treads as they heat and cool rapidly. Field weakening is used in some electronic

controls to increase the top speed of an electric vehicle.

The simplest form uses a contactor and field weakening resistor, the electronic

control monitors the motor current and switches the field weakening resistor into

circuit when the motor current reduces below a preset value (this will be when the

motor is at its full design speed). Once the resistor is in circuit, the motor will

increase speed above its normal speed at its rated voltage. When motor current

increases, the control will disconnect the resistor and low speed torque is made

available.

One interesting method of speed

control of a DC motor is the Ward-Leonard

control. It is a method of controlling a DC

motor (usually a shunt or compound

wound) and was developed as a method of

providing a speed-controlled motor from

an AC supply, though it is not without its

advantages in DC schemes. The DC

output from the armature is directly

connected to the armature of the DC

motor (usually of identical construction).

The shunt field windings of both DC

machines are excited through a variable resistor from the generator's armature.

This variable resistor provides extremely good speed control from standstill to

full speed, and consistent torque. This method of control was the de facto method

from its development until it was superseded by solid state thyristor systems.

It found service in almost any environment where good speed control was

required, from passenger lifts through to large mine pit head winding gear and even

industrial process machinery and electric cranes.



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Compounds of a DC motor

In a rotary motor, the rotating part (usually on the inside) is called the rotor,

and the stationary part is called the stator. The rotor rotates because the wires and

magnetic field are arranged so that a torque is developed about the rotor's axis.The motor contains electromagnets that are wound on a frame. Though this

frame is often called the armature, that term is often erroneously applied.

Correctly, the armature is that part of the motor across which the input voltage is

supplied. Depending upon the design of the machine, either the rotor or the stator

can serve as the armature. This Java applet shows a direct current electrical motor

which is reduced to the most important parts for clarity. Instead of an armature

with many windings and

iron core, there is only

a single rectangular

conductor loop; the axis

the loop rotates on is

omitted. The blue

arrows indicate the

conventional current

direction (from + to -).

You can recognize the

magnetic field lines

(directed from the red

painted north pole to

the green painted south

pole) by the red color. The black arrows represent the Lorentz force which is

exerted to a current-carrying conductor in the magnetic field. The mentioned

Lorentz force is orthogonal to the direction of current and to the magnetic fieldlines. The orientation of this force results from the well-known third hand rule.

Industrial Electronics 7Exercise No. 1


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Now, read the text again if necessary and answer the following questions.

1. What is the main function of an electric motor?

2. What is the armature?

3. Number the compounds of a DC motor:

4. What is an electrical generator?

5. What does a dynamo machine consists of?

6. What is a transformer?

7. What range do transformers come in?

8. According to the picture in the previous page, what do the black arrows

represent?

9. Once the resistor is in circuit, the motor will:



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10. Why the dynamo uses electromagnetic principles?

THE SERIES DC MOTORS

Condition assessment of DC motors requires a basic understanding of the design

and operating characteristics of the various types available: the series motor, the

shunt motor, and the compound motor. Each type has unique operating

characteristics and applications. These characteristics enable the operator to

perform a wide variety of tasks.

DC motor fault zone analysis is a vital part of any DC motor maintenance

program. Visual inspection and

electrical testing of the armature

and fields give the maintenance

personnel an understanding of

the condition of the motor.

Implementing a predictive

maintenance program takes a PM

program to the next level. We

will review some case studies

that will illustrate the utilization

of an effective predictive

program.

Components of a series

motor include the armature,

labeled A1 and A2, and the field,

S1 and S2. The same current is

impressed upon the armature and the series field.

The coils in the series field are made of a few turns of large gauge wire, to

facilitate large current flow. This provides high starting torque, approximately 2

times the rated load torque. Series motor armatures are usually lap wound.



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Lap windings are good for high current, low voltage applications because they

have additional parallel paths for current flow. Series motors have very poor speed

control, running slowly with heavy loads and quickly with light loads. A series motor

should never drive machines with a belt. If the belt breaks, the load would be

removed and cause the motor to overspeed and destroy itself in a matter of

seconds.

In SERIES MOTORS, the field windings are connected in series with the armature

coil. The field strength varies with changes in armature current. When its speed is

reduced by a load, the series motor develops greater torque. Its starting torque is

greater than other types of dc motors. Its speed varies widely between full-load

and no-load. Unloaded operation of large machines is dangerous.

Common uses of the series motor include crane hoists, where large heavy loads

will be raised and lowered and bridge and trolley drives on large overhead cranes.

The series motor provides the starting torque required for moving large loads.

Traction motors used to drive trains are series motors that provide the required

torque and horsepower to get massive amounts of weight moving. On the coldest

days of winter the series motor that starts your car overcomes the extreme cold

temperatures and thick lubricant to get your car going.

THE SHUNT DC MOTORS

The shunt motor is probably the most common dc motor used in industry today.

Components of the shunt motor are the

armature, labeled A1 and A2, and the

field, labeled F1 and F2. The coils in the

shunt field are composed of many turns

of small wire, resulting in low shunt

field current and moderate armature

current. This motor provides starting



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torque that varies with the load applied and good speed regulation by controlling

the shunt field voltage. If the shunt motor loses its field it will accelerate slightly

until CEMF rises to a value sufficient to shut off the torque producing current.

In other words, the shunt motor will not destroy itself if it loses its field, but it

wont have the torque required to do the job it was designed for. Some of the

common uses of the shunt motor are machine shop lathes, and industry process

lines where speed and tension control are critical.

1. Its a vital part of any DC motor maintenance program:

2. It takes a PM program to the next level:

3. The coils in the series field are made of:

4. When the speed of the series motor is reduced by a load, what does it

develop?

5. What do common uses of the series motor include?


Exercise No. 2


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6. Which is probably the most common dc motor used in industry today?

7. If the shunt motor loses its field it will:

8. Which are some of the common uses of the shunt motor?

9. The coils in the shunt field are composed of:

10. A series motor should never drive machines with a belt, what might happen

if the belt breakes?

CONTROL DIAGRAMS

Process control diagrams depict that part of the plant or process that the

operator has selected. Color coded diagrams facilitate operator understanding. A

typical plant will have several such diagrams and corresponding Operator Control

Panels.



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Each Operator Control Panel is associated with a process control diagram.

Together, they represent a portion of the plant that can be reached by selecting

from a list of panels that is always available. The operator can view the operation

of the plant, set controls to automatic or manual, open and close valves, start and

stop pumps, set control points, etc. Typically, the panel is laid out in a way that

corresponds to the layout of the corresponding process diagram. Effective use of

color is made so that the operator's attention is immediately drawn to those items

that require intervention. Items that are out of tolerance or which may be critical

are displayed on a special Alarm Panel.

The Alarm control panel is available to the operator on demand. It pulls from all

other panels any meter that is out of tolerance. This gives the operator a single

screen to examine all points in the plant that may need attention.

If a critical point reaches an emergency level, this panel is automatically

displayed (critical points are outlined in red). The system can be set up so that

after a delay, emergency shutdown will occur.



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If a monitored value drifts out of bounds, an alarm is triggered. Alarms can be

either warning (yellow) or emergency (red) and can be viewed at any time on the

Alarm Panel. In the case of an emergency, the system can be set up to shut itself

down in an orderly fashion if an operator does not intervene. You can set the

intervals for how long the system waits before beginning shutdown as well as how

long the operator has to abort. In addition, an external siren can be triggered to

alert plant personnel when an emergency occurs.

Wiper Speed Control

A continuously working wiper in a car may prove to be a nuisance, especially

when it is not raining heavily. By using the circuit described here one can vary

sweeping rate of the wiper from once a second to once in ten seconds. The circuit

comprises two timer NE555 ICs, one CD4017 decade counter, one TIP32 driver

transistor, a 2N3055 power transistor (or TIP3055) and a few other discrete

components. Timer IC1 is configured as a monostable multivibrator which

produces a pulse when one presses switch S1 momentarily. This pulse acts as a

clock pulse for the decade counter (IC2) which advances by one count on each

successive clock pulse or the push of switch S1. Ten presets (VR1 through VR10),

set for different values by trial and error, are used at the ten outputs of IC2. Butsince only one output of IC2 is high at a time, only one preset (at selected output)

effectively comes in series with timing resistors R4 and R5 connected in the circuit

of timer IC3 which functions in astable mode. As presets VR1 through VR10 are set

for different values, different time periods (or frequencies) for astable multivibrator

IC3 can be selected.



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The output of IC3

is applied to pnp

driver transistor T1

(TIP32) for driving

the final power

transistor T2

(2N3055) which in

turn drives the wiper

motor at the selected

sweep speed. The

power supply for the

wiper motor as well as the circuit is tapped from the vehicles battery itself. The

duration of monostable multivibrator IC1 is set for a nearly one second period

Answer the following questions.

1. Items that are out of tolerance or which may be critical are:

2. What panel must be displayed if a critical point reaches an emergency level?

3. How can you vary sweeping rate of the wiper?

4. What can the operator do when there is an available list of panels?

5. What is the function of color coded diagrams?


Exercise No. 3


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INDEPENDENT DC MOTORS

The drive system for

Brannigan consists of two

tank-treads powered by two

independently controlled

Pittman motors. Selection of

wheels and tread material was

the first and most

fundamental process in

designing the drive system.

Since slipping between the

wheel and the tread would be fatal, toothed tread systems were investigated. Pre-

manufactured systems did not fit the specifications because they were either too

light-duty or too expensive. Eventually we came upon timing belts as the treads

and timing belt pulleys as the wheels. Timing belts and pulleys are generally used

in industrial power transmission applications, but we decided to modify the belts

and pulleys to drive our robot. The pulleys are generally manufactured in plastic or

steel. The ribs are important to prevent the belt from slipping off the side. Some

machining was required on each pulley, namely removing the hub protrusion. The

tread system has three wheels (each composed of two timing belt pulleys). The

center wheel is the

driving wheel, since it is

connected to the motor.

The two slave wheels are

smaller in diameter and

run freely. As shown in

the picture above, there

are two tensioners that

provide a large area of

contact between the belt

and the driving wheel. If

these tensioners were not

present, slipping between the belt and the driving wheel would likely occur.



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Timing belt specifications, like those found in Machinery's Handbook, suggest at

least six teeth in contact. The geometry of the belt path was governed by this

criteria and the overall dimensions of the robot.

To maintain the dimensions of the placement of the wheels and tensioners,

the housing of the tread system needed to be strong. It also needed to be

protective; using an upside-down U-channel provided two rigid bearing surfaces via

a through-hole in legs of the channels. We decided on aluminum 1 1/2" x 1/8" Wall

Square U-channels for several reasons: 1. the excellent strength-to-weight ratio of

aluminum coupled with the shape of channels provided complete coverage and

dimensional stability with a reasonable weight. 2. Aluminum channel is extruded,

unlike steel channel which is bent after being rolled. This provides: a. Sharp

corners and b. Better parallelism between the legs of the channels. 3. Machining

the aluminum channel is easier. Most holes in the channel were through-holes,

which ensures that the axes line up. Machining two plates would require the holes

to be matched up perfectly. Also, the channel could be refashioned by rechecking it

in the mill and touching off the original datums. 4. The two channels were the

structural grounding of our robot. Because of the large moment of inertia of the

shape of the channel, he forces required to deflect the channel any reasonable

amount over its entire length were very high. A plate between the two channels

was all that was needed to create a rigid and reliable robot.

Motor selection is one of the major decisions of any moving robot. We decided

on DC motors, rather than stepper motors because we believed that they could

provide the accuracy

we needed without

the complexity of

controlling stepper

motors. We also

wanted to purchase a

motor with a gear

head that would

provide the necessary

speed reduction and

torque increase.



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Answer the following questions.

1. What does the drive system for Brannigan consist of?

2. When were toothed tread systems investigated?

3. Where are timing belts and pulleys generally used?

4. Why did the tread system need to be strong?

5. Which is one of the major decisions of any moving robot?

THE COMPOUND DC MOTOR

When comparing the advantages of the series and shunt motors, the series

motor has greater torque capabilities while the shunt motor has more constant and

controllable speed over various loads. These two desirable characteristics can be

found in the same motor by placing both a series field and shunt field winding on

the same pole. Thus, we have the compound motor.

The compound motor responds better to heavy load changes than a shunt motor

because of the increased current through the series field coils. This boosts the field

strength, providing added torque and speed. If a shunt coil is added to a series

motor at light loads (when a series motor tends to overspeed) the added shunt field

flux limits the top speed, eliminating self-destruction.


Exercise No. 4


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Common uses of the compound motor include elevators, air compressors,

conveyors, presses and shears. Compound motors can be operated as shunt

motors by disconnecting the series field. Many manufacturing process lines are

designed this way. The reason being that, most off the shelf motors are compound

motors, and the series field can always be connected later to provide additional

torque, if needed.

rCom pound

motors can be connected two ways, cumulatively and differentially,

when connected cumulatively, the series field is connected to aid the shunt field,

providing faster response than a straight shunt motor. When connected

differentially, the series field opposes the shunt field. Differentially connected

compound motors are sometimes referred to as suicide motors, because of their

penchant for self-destruction. If perhaps, the shunt field circuit were to suddenly

open during loading, the series field would then assume control and the polarity of

all fields would reverse. This results in the motor stopping, and then restarting in

the opposite direction. It then operates as an unloaded series motor and will destroy

itself. Differentially connected motors can also start in the opposite direction if the

load is too heavy. Therefore, it is seldom used in industry.

Fault Zone Preventative Maintenance

Fault zone preventative maintenance on dc motors includes electrical testing and

visual inspection of the armature, commutator, brushes and fields. Over the years,

people have been performing insulation to ground tests on DC equipment to

evaluate the condition of insulation, particularly with regard to moisture and dirt.

These parameters are valuable readings when taken under similar conditions at

various times. High insulation resistance values do not necessarily indicate high

dielectric strength. Insulation that is mechanically damaged may show high



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resistance values but fail at relatively low dielectric test voltages. Insulation

resistance varies inversely to the temperature of the motor. As the temperature

increases, resistance will decrease. Approximately 8 to 15C temperature rise will

half the resistance.

Armature

Visual inspection of the armature should include the search for cracked or brittle

insulation, loose or broken banding, and any dirt or oil contamination. Leakage to

ground testing of the armature indicates the relative condition of the insulation.

Performing a bar-to-bar resistance check will indicate any shorted windings or

defective solder joints at the risers. Infrared inspection of the armature can reveal

overheating of the brushes, commutator, as well as loose or hot connections on the

risers. The ideal temperature for proper commutation is between 120-140 F.

1. What do common uses of the compound motor include?

2. What does Fault zone preventative maintenance on dc motors include?

3. What is the ideal temperature for proper commutation?

4. As the temperature increases:

5. when do we have a compound motor?


Exercise No. 5

unit 2 industrial electronics

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