This type of relay is used with high starting torque motors. It operates in a similar manner to the current relay except that the switch contacts are normally closed. The solenoid coil, once energized, maintains a magnetic force strong enough to open the switch contacts and keep them open whilst the compressor is running.
The relay has a much higher design voltage rating than the supply voltage. As the motor approaches its design speed, the voltage across the coil can sometimes be more than twice that of the supply voltage.
When power is supplied to the circuit, the relay contacts are closed. Both motor windings are energized and starting is achieved. As the motor increases speed, the voltage in the start winding increases to cause an increase in both voltage and current passing through the coil. When the design voltage of the coil is reached, the current creates a strong magnetic force to pull in the plunger and contact bridge to open the start circuit, but allows the compressor to operate on the run winding.
When the relay contacts open, the voltage and current across the coil will decrease but will maintain a magnetic force strong enough to keep the contacts open until power is disconnected. The contacts will then return to the closed position ready for a restart.
This relay is wired in parallel, unlike the current type which is wired in series. A voltage relay circuit is shown in Figure 37.