Which type of relay is NOT used for induction motor protection?

Loss-of-field relays are primarily utilized in synchronous motors where they monitor the DC field current to ensure that the motor remains synchronized with the power supply. When there is a loss of field excitation, these relays can send signals to protect the synchronous motor from damage due to excessive slip or instability. Induction motors do not rely on a field winding for their operation; instead, they function using a rotating magnetic field produced by alternating current in the stator winding, making the loss-of-field protection irrelevant in their context.

In contrast, overvoltage, underfrequency, and overcurrent relays are common protective devices for induction motors. Overvoltage relays protect against damage from excessive supply voltage, underfrequency relays guard against operating below acceptable frequency levels which may affect performance, and overcurrent relays safeguard against excessive current that can lead to overheating and failure. Therefore, it is clear that loss-of-field relays are not applicable for the protection of induction motors.