Presentation 12 contactors-handouts

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HVACR114 – 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 contacts that controls the voltage applied to some load in HVAC systems.

•  A contactor consists of a coil that opens and closes a set of contacts due to the magnetic attraction created by the coil when it is energized.

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Parts of a contactor

Coil  

Contacts  

Control  Voltage  Terminals  

Line  Voltage  Terminals  

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Applications

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

•  The contactor used in a small residential air conditioning unit probably controls the compressor and condenser fan motor.

•  Larger Air conditioning systems will have multiple contactors.

5  Simple schematic diagram of a contactor controlling a compressor 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 of contacts.

•  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

Armature  

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•  The sliding armature is mounted between two slots in the frame of the contactor and moves up and down in these slots.

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Contactor with a swinging armature

Armature  

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•  The swinging armature is mounted on a pivot or hinge and moves up and down in a swinging motion.

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•  The armature of a contactor is connected by a mechanical linkage to a set of contacts that causes a complete circuit when the armature is 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 magnetic field that closes a contactor is created by a coil wound around a laminated iron core.

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

•  The core then becomes a electromagnet of sufficient strength to attract the armature closing the contacts.

•  Some contactors have a spring mounted between the armature and the stationary contacts to ensure the contactor opens when the coil is de-energized.

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Coils

•  Coil characteristics depend on the type of wire and the manner in which it is wound.

•  Coils are designed to be operated on 24, 120, 208/230 and occasionally 480 volts.

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

Coil  voltage  is  marked  here!  

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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 can control:

»  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% of their 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 a strong enough electromagnetic force to pull in the contacts.

•  It’s uncommon for the coil to become so weak that it does not close the contacts, unless there is excessive friction to the mechanical 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 some measurable resistance.

•  A coil can also be checked by applying voltage to it and observing the contactor to see if it closes.

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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…. So BE CAREFUL!

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Contacts

•  The contacts must be in good condition to ensure that the proper voltage reaches the load.

•  A visual inspection is sufficient to diagnose bad contacts.

•  A voltage reading taken across the contacts of the same pole will show the voltage drop across the contacts.

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Contact conditions

Pi:ed  and  burnt  contacts  

Good  Contacts  

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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 can be 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 will completely replace the contact portion of the contactor.

•  Parts can be difficult to locate, it might be better to just replace the contactor.

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