AC Contactor A Comprehensive Guide With Tips And Tricks

The AC contactor is a switch that consists of a metal plunger, some sets of electrical contacts and a control coil. The switch receives low voltage from the furnace to turn the air conditioning unit on and off as required in order to maintain the desired temperature.

The contactor is a standard magnetic relay and low voltage usually flows through the coil to create a magnetic field which pulls a piece of metal in order to connect both sides of the high voltage circuit (usually 120V or 240V).

Therefore, once the circuit is closed, high voltage powers the fan and other compressor motors. Contactors with a single magnetic coil connecting one circuit are known as single pole contactors and contactors that have two magnetic coils connecting two circuits are known as dual-pole contactors. 

Operations And Failure

ac contactor

Normal AC contactor operation

In most residential units, a voting rating is clearly indicated on the control coil. Although the control rating is usually 24 vac, it can also be 240 vac or 120 vac. This voltage is supplied to the control coil by the air conditioner thermostat. 

When the voltage is supplied to the contactor, the current will flow through the control coil in order to create a magnetic field. The main purpose of the magnetic field is to attract the plunger and pull it to the center of the control coil. This causes the air conditioner contacts to close. 

Each contact is usually composed of one movable and one fixed contact. The contacts are coated with silver so as to enhance conduction. The plunger is connected to the movable contact and when the plunger moves to the center of the control coil, the fixed contact comes into contact with the movable contact. The circuit is completed and this provides power to the ac components.

When the thermostat senses that your home does not require any further cooling, the control voltage to the air conditioner contactor shuts off. The magnetic field collapses when current stops flowing through the control coil. The plunger is released and it springs back to the normal position and it also opens the contacts. However, it is vital to keep in mind that the ac contactor can fail either mechanically or electrically. 

ac contactor

Mechanical failure

One of the normal mechanical failures of the contactor is when it sticks closed. The signs of this failure are that the outer part of the condensing unit will not shut off. Since the thermostat is off, the air handler or the blower on the furnace is of but the system (condensing unit) is still running. Normally, you will ice covering the evaporator coil and the refrigerant lines. This failure happens when the silver coating of the contacts wears off. 

As the contacts close without the silver coating, an electric arc is created and the contacts can easily be welded shut. You should replace the contactor in case it is sticking. On the other hand, the contactor can fail to close if something gets into the mechanism and physically prevents it from closing. This can be caused by mice or insects. 

Electrical failure

Electrical failure can occur in three ways and you can use a simple multimeter to check for electrical failure. The control coil can become grounded, open or shortened.

The coil becomes shortened when insulation between wires breaks down. This failure can be found by simply shutting off power and carefully taking a resistance reading particularly between the terminals. The resistance reading should be about 20 ohms. You should replace the contactor if the reading is significantly less. 

The coil becomes grounded isolation on the coil’s outer wires breaks down creating a path to other metal components. You can find this error by taking resistance readings from all the coil terminals and the metal case of your air conditioner. A low resistance reading means that the coil is grounded and you should replace the contactor. 

In case the coil is open, the ac contactor does not close when control voltage is applied. The failure can be found by simply shutting off power to the air conditioner and the air handler/furnace and taking a reading across all terminals. The coil is open if the resistance reading is higher than 20 ohms and you should replace the contactor. 

ac contactor

Selecting a replacement 

There are three main things that you should match when looking for a replacement ac contactor. These include; the control voltage, the contactor’s resistive amps rating, and eth units contact rating. In most residential air conditioning units, the resistive amps rating is usually 20, 30, or even 40 amps.

You can replace a unit with a contactor with a higher amps rating but you should not use one with a lower amps rating. On the other hand, you should match the contactor’s number of poles. Contactors usually come in single pole units, two pole units, or three pole units. 

How should you replace a defective contactor?

· Begin by shutting off power to the unit and the air handler/furnace. Use a multimeter to verify that power is actually off. 

· Next, mark the wires while noting their location on the ac contactor. Remove the wires and then proceed to remove the mounting screws. 

· Mount your replacement contactor and connect the wires. Tighten all the wire connections and then turn on the power to the air conditioner and the air handler/furnace. 

Complete the ac contactor replacement process by observing one complete cooling cycle. 

ac contactor

Packard C140A 1 Pole 40 Amp Contactor 24 Volt Coil Contactor

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