Normal AC Contactor Operation
In order to determine whether a part is operating properly or not you must
understand how it is designed to work.
The control coil has a voltage rating on it. In most residential systems, the
control voltage will be 24 vac but it can also be 120 vac or 240 vac. The rating will normally be written
on the coil itself. This voltage is usually supplied to the coil by the thermostat.
When the control voltage is supplied to the air conditioner contactor, current
flows through the coil and produces a magnetic field. The magnetic field attracts the plunger and pulls it
into the center of the coil. This action cause the contacts to close.
Each set of contacts is composed of one fixed and one movable contact and is
referred to as a pole. These contacts are made of steel and coated with silver for enhanced conduction of
electricity. The movable contact is connected to the plunger and when the plunger is pulled into the
center of the coil, the movable contact is brought into contact with the fixed contact. This completes the
electrical circuit and provides power to the air conditioner components such as the compressor and the
When the home's thermostat senses that no further cooling is required, the
control voltage to the contactor is shut off. When the current stops flowing through the coil, the
magnetic field collapses and the plunger is released. The plunger springs back into its normal position
and opens the contacts.
The air conditioner contactor can fail in one of two ways either electrically or