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SECTION 4

ELECTRIC MOTORS

UNIT 19

MOTOR CONTROLS

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UNIT OBJECTIVESAfter studying this unit, the reader should be able to

Describe the differences between relays, contactors and starters

Explain why the locked rotor amperage (LRA) affects the choice of a

motor starter

List the component parts of contactors and starters

Name two types of external motor overload protection

Explain the differences between external overload protection devices Explain the conditions under which motor safeties can be reset

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INTRODUCTION TO MOTOR

CONTROL DEVICES

Relays, contactors and starters pass power to the

motor by closing sets of contacts

Contacts controlled by coils in the control circuit

Starting relays are only in the active circuit for a

short period of time

The type of motor control used is determined by

the size and application of the motor used

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L1 L2

CONTROLCIRCUIT

RELAY OR

CONTACTOR

MOTORRUN

START

START

RELAY

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L1 L2

CONTROLCIRCUIT

RELAY OR

CONTACTOR

MOTORRUN

START

START

RELAY

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MOTOR AMPERAGES Running load amperage (RLA)

Similar to full load amperage (FLA)

Amperage drawn by the motor while operating

Locked rotor amperage (LRA)

Amperage drawn by motor on startup

Five to seven times greater than RLA or FLA

Both LRA and RLA must be considered when

choosing a control device

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THE RELAY

Uses a magnetic coil to open or close one or more sets of

electric contacts The most common control voltage for both relays and

contactors is 24 volts.

Relays are not repaired. Replace on failure.

Used for light duty applications

Can be used as a pilot-duty relay

The relay contacts must be able to handle the amperage drawof the load being controlled

Pilot relays are designed to switch on and off larger contactors

or starters. They are very light duty and are not designed tostart motors directly.

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COIL

NORMALLY OPEN CONTACTS

NORMALLY CLOSEDCONTACTS

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THE CONTACTOR Larger version of the relay

Has movable and stationary contacts Often times only one set of contacts opens which opens only one

side of the power to condensing units. This provides power for off-cycle heat to the compressor.

Holding coils are rated at different voltages Can have one or more sets of contacts

Some are equipped with auxiliary contacts, which are usually ratedat a lower amperage than the primary contacts.

Contacts and coils can be replaced, the material most used on thecontacts is silver.

Use the exact replacement whenever possible

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HOLDING COIL

STATIONARY

CONTACTS

STATIONARY

CONTACTS

MOVABLE

CONTACTS AND

ARMATURE

COIL

CONNECTIONS

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WHEN THE COIL

IS ENERGIZED,

THE CONTACTS

ARE PULLEDCLOSED

The most common coil voltage is 24 volts.

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L1 T1 L2 T2 L3 T3

STATIONARY

CONTACTS

MOVABLE

CONTACTS

SPRINGSCOIL

STATIONARY

ELECTROMAGNET

ARMATURE

When contactor is energized what resistance would you expect to read

between L1 to T1, L2 to T2 and L3 to T3? -

If placing one lead of your voltage meter on T1 and the other on L1, what

voltage would you expect to read if the coil is energized?

Springs maintain pressure on the

contacts for current carrying capacityand they help obtain equal contacts

against the stationary contacts.

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MOTOR STARTERS

Contactor equipped with overload protection, whichactually are designed to protect the motor where fusesor circuit breakers protect the entire circuit.

High amp draw will cause the starter to open.

Coils, contacts and heaters can be replaced

Contacts become pitted over time Pitting increases the resistance across the contacts, if the

pitting is bad enough it can cause motor burn out.

The voltage across the contacts will increase

The voltage across a good set of contacts should be about zerovolts

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CLEAN CONTACT

DIRTY, PITTED CONTACT

Dirty contacts dont necessarily mean

they are pitted, the sure sign of pittedcontacts is increased voltage across the

contacts.

Sanding contacts will only speed

up the deterioration of the

contact surfaces.

The most common material used for the

contacts is silver.

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MOTOR PROTECTION Motors are expensive and must be protected

Fuses and circuit breakers protect the entire circuit, notthe individual circuit components

Motors can operate under an overcurrent condition for ashort period of time

Most small motors have no overload protection

The larger the motor, the more elaborate the method ofmotor protection should be

Motor protection can be inherent (internal) or external

Motor service factor is determined by the motors

reserve capacity

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L1 L2

3A25A

FUSES PROTECT THE

ENTIRE CIRCUIT, NOT

THE INDIVIDUAL

COMPONENTS

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INHERENT (INTERNAL) MOTOR

PROTECTION

Thermally activated devices in the motor winding. Internal thermal overloads

Usually embedded in the motor windings

Open on a rise in temperature

Thermally activated bimetal snap discs

Positioned so that contact is made between the bimetalcontrol and the motor

Snap action opens contacts if the motor temperature risesabove the desired level

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EXTERNAL MOTOR PROTECTION

Devices that pass power to the holding coil of the

starter or contactor

Devices open when an overcurrent condition exists

The trip point and type of overload protector aredetermined by the manufacturer

The overload device takes the service factor of the

motor into consideration

NATIONAL ELECTRIC CODE (NEC)

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NATIONAL ELECTRIC CODE (NEC)

STANDARDS

Sets standards for electrical installationsConductor sizes and ampacities

Cable materials and applications

Electrical devices

Sets standards for motor overload protection

The published code book should be consulted if

questions or concerns are encountered on the job

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TEMPERATURE-SENSING DEVICES

Bimetal elements

Devices called heaters wired in series with the load The heater is exposed to the current draw of the load

The bimetal warps and open when it gets too warm

The open bimetal de-energizes the starter holding coil

Solder pot

Uses solder with a low melting point

The solder melts when excessive temperature is sensed

Excessive heat results from the overcurrent condition Look @ Figure 19-21 on page 366

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MAGNETIC OVERLOAD DEVICES

The most accurate means of electrical overloadprotection

Device is not attached to the starter

Device is not affected by increased ambienttemperatures

The contacts within the device will open to de-

energize the motor at the desired amperage level

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RESTARTING THE MOTOR Motors should not be restarted immediately

Cause for the overload condition must first belocated and repaired

Motor must be given ample time to cool

Many control devices are manually reset, this is to

ensure that technicians are aware of a possibleproblem which caused the starter to trip in the firstplace.

Some controls reset automatically after a

predetermined time delay Time delay feature prevents short cycling

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UNIT SUMMARY Relays, contactors and starters pass power to motors

Contacts controlled by control circuit holding coils Relays are used for light duty, contactors for heavier

duty applications, starters have built-in overloads

Contacts on contactors and starters can be replaced

Fuses and circuit breakers protect the entire circuit,

not the individual circuit components

Motor protect can be inherent or external