90% of HVAC techs do not understand current relays or how they work. This is one area of knowledge that will take you one step closer to becoming a master technician.

You will find current relays used on single-phase, less than one horsepower motors that do not require high starting torque. You see them use very often in light commercial refrigeration in prep tables and small reach-in coolers and freezers as well as household refrigerators, and window ACs.

Here’s a great HVAC Training video on Current Relays

The current relay’s main function is to assist in starting the motor most the compressor. As mentioned about small refrigeration devices use capillary tubes as metering devices. Devices with capillary tubes or fixed-orifice metering devices equalize pressure during their off cycles. 

Current starting relays have a low-resistance coil (< 1 Ω) and a set of normally open contacts. The coil in current relay is different than found in a general purpose relay, as it is short in length and large in diameter.  The best way to identify a current relay by the large size of the wire in the relay coil. Larger wire must be used as it carries the full-load current of the motor. 

The coil is wired between the L (line) and M (main winding) terminals,  the contacts usually are wired between terminals L (line) and S (start).

A wiring diagram of a current magnetic relay. The diagram shows two lines from a supply connected to two compressor contactor contacts (open), L1 and L2. L1 is connected to the common terminal of a run winding which is further connected to one end of the start winding. The other end of the start winding is connected to an optional start capacitor. L2 is connected to the terminal L of the current relay circuit. The run terminal is connected to terminal M of the current relay. The other end of the start capacitor is connected to terminal S of the current relay. The coil is connected across terminals, L and M. Notice that the current relay coil wire is large, because it must carry the current of the run winding all the time.
How a Current Relay Works

How a Current Relay Works

  1. Power is applied to the system.
  2. The RUN and the relay coil are at locked rotor amps (high amperage).
  3. You will notice that the relay coil is in series with the run winding.
  4. The start winding is not subject to the LRA because the contacts between L and S are normally open. 
  5. The LRA creates a strong magnetic field. This magnetic field closes the contacts between L and S
  6. The closed L and S contacts energize the start winding. 
  7. The motor is now in operation.
  8. Once the motor is up to speed the run windings current decreases
  9. The decrease in current causes the magnetic field to be much weaker and the L and S contacts will open (either gravity or spring assisted).
  10. When the L and S contacts are open, the start winding is out of the circuit.
  11. When the motor is de energized, it coasts to a stop and the current relay is all ready for the next cycle.
Current Relays

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Ron Walker
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.