Contactors in HVAC Systems
The contactor coil must be in good condition to create enough magnetic force to pull the contacts closed. Rarely does the coil of a contactor become so weak that it cannot pull the contact closed? If a contractor is not closing and there is the voltage applied, it is usually due to excessive friction on the mechanical linkage.
Coils
A coil can either be good, open or shorted. Both opened and shorted contractors can be checked with an ohmmeter. An open contractor has infinite resistance. A shorted contactor has a resistance reading of zero. A small measurable resistance usually indicates a good coil. (If you have a good contactor measure it for your reference).
Contacts
The contacts must be in good enough condition to pass the voltage on to the load. In most cases, a visual inspection is all that is needed. If the contacts are pitted and burned they should be replaced. A voltage drop of more than one volt across the contacts indicates a bad contact.
Our interactive voltage measurement exercises let you measure, test, and diagnose contactors.
Do you want to attend HVAC classes and become an HVAC Technician? Sign-up for our online HVAC training.
by Ron Walker
- A contactor is used in both residential and commercial HVAC units. It is used to energize or de-energize a load (usually a fan motor or compressor).
- A contactor is an electrically controlled switch that allow the control of a high voltage, high power load with a low voltage low power control.
- A contactor consists of a coil that when energized creates a magnetic field around a laminated core. This magnetic field has enough strength to pull the contacts closed, applying power to the load.
- There are springs that open the contacts once the magnetic field is de-energized.
- In residential HVAC systems, the largest load controlled by a contractor is the compressor. Unlike a larger commercial system which has a contactor for each load, a residential system's compressor and outdoor fan are usually controlled by the same contractor.
- The coils of a contactor can be energized by 24, 120, 208/230, and 460 volts (the control voltage). It is important to match the replacement contactor's control voltage with the original as they are not interchangeable.
- The contacts are made of silver and cadmium, an alloy that resists sticking.
The contactor coil must be in good condition to create enough magnetic force to pull the contacts closed. Rarely does the coil of a contactor become so weak that it cannot pull the contact closed? If a contractor is not closing and there is the voltage applied, it is usually due to excessive friction on the mechanical linkage.
Coils
A coil can either be good, open or shorted. Both opened and shorted contractors can be checked with an ohmmeter. An open contractor has infinite resistance. A shorted contactor has a resistance reading of zero. A small measurable resistance usually indicates a good coil. (If you have a good contactor measure it for your reference).
Contacts
The contacts must be in good enough condition to pass the voltage on to the load. In most cases, a visual inspection is all that is needed. If the contacts are pitted and burned they should be replaced. A voltage drop of more than one volt across the contacts indicates a bad contact.
Our interactive voltage measurement exercises let you measure, test, and diagnose contactors.
Do you want to attend HVAC classes and become an HVAC Technician? Sign-up for our online HVAC training.
by Ron Walker