The voltmeter relay is usually installed when the starting torque of the compressor is quite high, taking into account the circuit and voltage fluctuation in the mains.
Voltmeter operation
This relay is designed to keep contacts 1 and 2 connected when not in use.
At the start of the work, the coil will sufficiently magnetize when it reaches the necessary voltage, for which the resultant voltage at the terminals is constructed from the end of the secondary winding with the start of the primary winding. This voltage will always be higher than the supply network, since the secondary winding and the phase capacitor will provide the differential found in these terminals, which differential with the types of compressors and phase capacitors indicated therein.
Normally, these coils work with an approximate voltage between 260 and 380 volts according to the type of compressor, even being the voltage of electrical supply of 220V.
The working voltage of the relay coil is obtained when the synchronizing speed of the compressor motor is reached. The contacts 1 and 2, which are connected, open within the preset time, disconnecting the electrolytic starting capacitor from the circuit.
Note that this operation requires only mechanical work, to activate the device to turn off the contacts 1 and 2, turning it on when the compressor is switched off.
