Potential relays and Back EMF

The Potential Relay

It is used in place of a current relay to deliver greater torque to a motor.
As single phase compressors get larger , the cannot have winding's from a current relay causing voltage drops to the main winding where we need higher motor power and torque.
The potential relays solves this problem by disengaging the start capacitor with back emf caused by motor induction effect.
The relay coil is rated at a much higher voltage , even as much as 400V AC.
During start-up. the start winding is already engaged giving the phase shift to start the motor, when we hit around 75% of the motors full speed the back emf has risen enough to disengage the start capacitor by inducing a higher voltage into potential relay coil which opens the start switch and remains open while the compressor is running. This way the main or run winding is not impinged by a second resistance from a relay coil which uses current to isolate the start winding.

The compressor is a split phase type and as such like RSIR types the start winding is not disengaged and remains in operation with the main or run winding similar to a PSC motor.  This allows greater running efficiency, especially for system which have more compression ratio such as low temperature or even larger medium temperature systems.

It is a very simple device but it does seem to be complicated by the way it is perceived in unit wiring diagrams.
It is a great way to see motor back emf in these images and video , this is responsible not only for the relay disconnection but also the opposing current to limit the motor amps too.

Check out my YouTube video