I recently discussed the working components of the potential relay and the importance of having a thorough understanding of its function in single phase Refrigeration compressors. Now I would like to discuss the other popular relay which is the current or amperage relay used on fractional horse power compressors. On all electric motors the starting current is greater than the running current because the rotor is not turning on startup. The startup amperage of an electric motor is usually stated as locked rotor amperage. This is the maximum amp draw of the motor. The current relay uses this electrical characteristic of the motor to remove the starting circuit once the motor has achieved about 75% running speed.
The current relay is built much like a solenoid, with copper wire wrapped round a steel hollow core holding a steel plunger.
The contacts of the current relay are normally open but close at high amperage startup. As the motor comes up to speed the amperage decreases and when the amperage is low enough the contacts open to remove the starting circuit. When looking at the electrical schematic you will notice how the current relay coil is in series with the run winding so when L1 and L2 close, full start current will flow through the relay coil and pull the contacts closed. At the same time the start windings are energized in series with the start capacitor to increase the torque on the rotor and bring the motor up to speed until the amperage diminishes to release and drop the contacts. Current relays are easy to troubleshoot with an Ohmmeter.
The current relay has N.O. contacts that are easily checked by turning the relay upside down and checking the contacts with an ohmmeter at less than 1 ohm. The relay contacts will read open (infinity) when the relay is right side up. The coil of the relay is made of very large wire and has very low resistance, around 0 -1 ohm. If the coil reads more than that, more than likely the coil is bad and should be replaced.
copyright(c)Roger J. Desrosiers
About the Author: Roger is a contributing Faculty Member of HVACReducation.net
He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter, United Association and is 608 Universal Certified.