What Is Contactor In Hvac System?

Contractor in hvac unit is a switch which is located in the outdoor part of the air conditioning unit. Contractor is similar to a relay only it has high electrical ratings. Contractor is controlled by a circuit having lower power level.

Contractors are used to control the voltage applied to loads. These systems work on electricity and are used to power motors and compressor of your hvac system. In home system control voltage is 24 vacs but it can also be 120 vac or 240 vac. Contractor starts or stop the compressor after receiving the signal from the thermostat.

Each set of contactor has one fixed and one movable contact. One of the contact of the contactor is fixed while the other is movable. The contacts are made from steel and then coated with silver for increasing the conduction of the contractors. In your hvac system thermostat applies 24 volts current is applied to both these contacts.


A contactor is made up of following components
• Control coil
• Metal plunger
• Sets of electrical contacts

Working of contactor

• As the current passes through the contactor, electromagnet starts building up inside the contactor and produces the magnetic field.
• Magnetic field makes the core of the contactor to wind up, this helps in energising the moving contact and both the contactors make a closed circuit.
• Because of the closed circuit current reaches the other circuit. Large amount of current is brought up to the initial position by the armature coil.
• When the home is completely cool the thermostat turns off the control voltage of the contractor.
• When the current stops the magnetic field is lost and the plunger is released.
• Plunger moves back to its place and the contacts are open.

Contractors control your air conditioning system by turning them on and off and after the continuous usage these contractors might get some damage. A faulty contactor can cause damage to your air conditioning system. So contractors are to be replaced from time to time.




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