What principle does the operation of a potential relay rely on?

The operation of a potential relay is based on the principle that motors develop counter electromotive force (emf). When a motor is running, it generates a voltage opposite to the applied voltage due to the motion of the rotor within the magnetic field. This counter emf is a key indicator of the motor's operational state.

In more detail, the counter emf increases as the motor speeds up, and when the motor reaches a certain threshold of speed, the potential relay is activated. The relay uses this counter emf signal to determine when to disconnect the start winding of the motor, allowing it to operate efficiently without overheating or causing damage to the start winding. This function is critical in systems where the motors are designed to start with a higher torque, then switch to a lower operational load.

Other options do not adequately describe the function of a potential relay. For example, while motors do develop excess pressure in some applications, this does not relate to the relay's operational mechanics. The notion of motors operating at constant speed or requiring constant voltage is more about motor performance than the relay's activation method, which specifically hinges on recognizing the counter emf as an indicator of speed.