HVAC Contactors and Relays Explained

By | December 24, 2016


HVAC Contractors and Relays Explained
Contactors and relays are used on almost all heating, cooling, and refrigeration equipment for the operation of loads in the system.
 
Contactors relays are similar in their operation, because both contain sets of cant and a coil that is used to open or close the contacts.
 




 
The contactor is larger and capable of carrying more amperage than the relay. The same procedure can be used to check both contactors and relays. There are three areas of problems encountered with contactors and relays the contacts, the coil, and the mechanical linkage. Any one of the the three areas can cause a contactor or relay to malfunction.
 
Contacts
The contacts of a relay or contactor must make good direct contact when energized for the device to function properly.
 
One of the problems often countered with contactors and relays is the contacts’ inability to make good contact.
 
The contacts can be burned, pitted, or stuck together. A set of contacts that are burned or pitted can cause a voltage drop across the contacts.
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There are several methods of checking a set of contacts to determine they are burned or pitted enough to warrant changing the device. The easiest method is to make a visual inspection.
 
Most contactors have movable cove which allow easy visual inspection. Most relays are sealed and visual inspection is impossible. A resistance check can also determine the condition of a set of contacts. The device must be energized to check normally open contacts. Normal* closed contacts must be checked with the device deenergized.
 
If the resistance is greater than 2 or 3 ohms, the contacts should be considered faulty.
 
Contactors and Relays
A voltage check can also determine the condition of a set of contacts. When a voltage check is made, the contactor or relay should be energized.
 
To make a voltage check on a set of contacts, take a voltage reading from one side of the contacts to the other. The reading will show how much voltage is being lost.
 
The load must be energized when a voltage test is being performed. If the voltage loss across the contacts exceeds 5% of the line voltage, the contacts are faulty and the contactor or relay should be replaced. The contact alignment can also cause a contactor or relay to malfunction.
 




 
Contacts should close directly in line with each other and seat directly in line with good firm contact. The major cause of contact misalignment is a faulty mechanical linkage. If the contacts are out of alignment, the contactor or relay must be rebuilt and any faulty components replaced.
Coil
 
The coil of a relay or contactor is used to close the contacts by creating a magnetic field that will pull the plunger into the magnetic field. If the coil of a relay or contactor is faulty, the device will not close the contacts.
 
A coil of a contactor or relay should be checked for opens, shorts, or a measur-able resistance. If a coil is shorted, the resistance will be 0 ohms and the coil should be replaced. An open coil will give a resistance reading of infinity and this coil should also be replaced.
 
A measurable resistance indicates that the coil is good. Almost any measurable resistance indicates a good coil be-cause of the variance in coil voltages. A shorted contactor coil will cause a transformer to burn out, and the service mechanic should take caution not to allow this to happen.
 
Mechanical Linkage
The mechanical linkage of a contactor or relay can cause malfunctions in many different forms, such as sticking contacts, contacts that will not close due to excess friction, contacts that do not make good direct contact, and misalignment of contacts. The best method for detecting a faulty mechanical linkage is by visual inspection.
 
On contactors and some relays this can be done by removing the device and merely looking it over. However, most re-lays are sealed and inspection is impossible. A sealed relay must be checked by determining if the contacts open and close when the relay coil is energized or de-energized.
 
A mechanical linkage problem can cause a contactor or relay to stick open or closed or cause misalignment of the contact. If a contactor or serviceable relay has a mechanical linkage problem, it should be replaced.
 


 




 

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