Presentation 12- Contactors-Handouts.pdf

8/10/2019 Presentation 12- Contactors-Handouts.pdf

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10/13/11

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HVACR 4 – Electrical for Gas

Heat

Contactors

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• A contactor is used to control an electric load

in a control system.

• Contactors make or break a set of contactsthat controls the voltage applied to some loadin HVAC systems.

• A contactor consists of a coil that opens and

closes a set of contacts due to the magneticattraction created by the coil when it is

energized.

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Parts of a contactor

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Applications

• The largest electric load in any cooling systemthat requires control is the compressor.

• The contactor used in a small residential air

conditioning unit probably controls thecompressor and condenser fan motor.

• Larger Air conditioning systems will have

multiple contactors.

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Simple schematic diagram of a contactor controlling acompressor and Condenser fan motor

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Operation

• Different manufacturers design contactors in

different ways. But they serve the same

purpose: Opening and closing a set ofcontacts.

• The armature of a contactor is the portion that

moves.•

This can be accomplished by two ways:» A sliding armature

» A swinging armature



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Contactor with a sliding armature

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• The sliding armature is mounted between two

slots in the frame of the contactor and movesup and down in these slots.

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Contactor with a swinging armature

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• The swinging armature is mounted on a pivot

or hinge and moves up and down in aswinging motion.

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• The armature of a contactor is connected by a

mechanical linkage to a set of contacts thatcauses a complete circuit when the armatureis pulled into the magnetic field produced by

the coil.

• This operation applies for both the swinging

armature and sliding armature.

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• The magneticfield that closesa contactor iscreated by a

coil woundaround alaminated ironcore.



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• When the coil is energized, a magnetic field iscreated around the laminated core.

• The core then becomes a electromagnet of

sufficient strength to attract the armatureclosing the contacts.

• Some contactors have a spring mounted

between the armature and the stationarycontacts to ensure the contactor opens whenthe coil is de-energized.

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Coils

• Coil characteristicsdepend on thetype of wire andthe manner in

which it is wound.• Coils are designed

to be operated on24, 120, 208/230and occasionally480 volts.

• To identify the coil voltage, the voltageis marked on it.

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Contacts

• The contacts of a contactor make a complete

circuit when the contactor is energized,

allowing voltage to flow to the controlled load.

• Contactor are rated by the ampere draw they

can carry.

•

There are two types of loads a contactor cancontrol:

» Inductive loads ( Motors)

» Resistive loads ( Heaters)



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• Contacts are made of silver and cadmium, which resists sticking.

• The chemical composition of contacts can

operate at cool temperatures up to 125% oftheir current –carrying capacity.

• Contactors are usually manufactured with two

or three poles.

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• Two pole contactor is required for single

phase systems.

• A three pole contactor is required for three

phase systems.

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Troubleshooting

• To diagnose a faulty contactor requires

looking at three sections of the contactor: – The coil

– The contacts

– And the mechanical linkage

• A defect in any part of these parts can cause

the contactor to fail.



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The coil

• The coil must be in good condition to create astrong enough electromagnetic force to pull inthe contacts.

• It’s uncommon for the coil to become so

weak that it does not close the contacts,unless there is excessive friction to themechanical linkage.

• A coil is diagnosed as either good, open or

shorted.

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• The coil can be checked with an ohmmeter.

•

If the coil is shorted, resistance will read 0

• If the coil is opened, resistance will read OL

• If the coil is good, the resistance will read somemeasurable resistance.

• A coil can also be checked by applying voltage

to it and observing the contactor to see if itcloses.

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• A voltage reading of the coil should be taken

before checking the coil to see if the contacts

should be closed.

• If voltage is applied to it, the coil will cause a

direct short and other damage could result….

SoBE CAREFUL



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Contacts

• The contacts must be in good condition toensure that the proper voltage reaches theload.

• A visual inspection is sufficient to diagnose

bad contacts.

• A voltage reading taken across the contacts of

the same pole will show the voltage dropacross the contacts.

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Contact conditions

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• Any voltage drop above 5% of the rated voltage is considered to be excessive.

• If you come across this, replace the contactor.

• Voltage must be applied to make this check.



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Mechanical linkage

• The easiest fault to diagnose.

•

Any problem with the mechanical linkage canbe detected by visual inspection.

• The mechanical linkage will usually fail do to wear, corrosion, or moisture

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Repairing contactors

• Contactors can be repaired by using

replacement parts.

• Some manufacturers do sell a kit that willcompletely replace the contact portion of thecontactor.

• Parts can be difficult to locate, it might be

better to just replace the contactor.

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Documents

Presentation 12- Contactors-Handouts.pdf