It is commonly used in commercial medium-sized applications where start and run capacitors are present in the compressor connection diagram. The voltmeter relay has normally closed contacts. The relay coil is connected in parallel with the starting coil of the compressor.
The voltage at the starting coil increases as the speed of the motor compressor increases until it reaches the specific pickup value, at which point the relay armature is attracted by opening its contacts and disconnecting the starting capacitor from the circuit. After opening, there is enough induced voltage on the starter coil to continue attracting the armature and keeping the relay contacts open.
