Current Relays Commercial Refrigeration

On start-up, the amp draw of the compressor is near LRA before the rotor starts to turn. The high current in the power leg going from M, through R, to the run winding, energizes the relay coil between terminals L and M. The coil pulls in the normally open (NO) contacts L and S. Line voltage is sent through the closed contacts to the start winding (S), which starts the rotor turning. The compressor begins to run.

When the motor is up to speed, the amperage in the power leg to R starts to fall. The decrease in current in the relay coil between L and M reopens the circuit to the start winding (L to S). The motor continues operating on only its run winding at its RLA.

Learn more about Commercial Refrigeration by enrolling in our Commercial Refrigeration for HVAC Techs Course.

About The Author

Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.