Typically, a refrigeration, air conditioning compressor, or pump motor needs high starting torque. A start capacitor, wired in series with the motor’s start, or auxiliary, winding, provides additional current to give the motor more starting torque when the circuit is energized. Start capacitors, unlike run capacitors, are designed for intermittent duty.
Capacitor start motors require an electrical device to disconnect the start capacitor as soon as the motor attains the minimum speed needed. The most practical, reliable, accurate and economical device for this purpose is a potential relay. A potential relay senses voltage across the start winding. When this voltage reaches a sufficient level, the start capacitor, which is in series with the start winding, must be disconnected from the circuit.