Basic Electrical Contactors

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.

Parts of a contactor Contacts Control Voltage Terminals Coil Line Voltage Terminals

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.

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

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

Contactor with a sliding armature

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

Contactor with a swinging armature

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

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.

The magnetic field that closes a contactor is created by a coil wound around a laminated iron core.

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.

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!

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)

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.

Two pole contactor is required for single phase systems. A three pole contactor is required for three phase systems.

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.

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.

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.

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!

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.

Contact conditions Pitted and burnt contacts Good Contacts

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.

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

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.