A three-phase induction motor with a built-in thermal relay (thermal protector). A bimetallic (or PTC) sensor embedded in the stator winding responds to its temperature. On overload the current rises and the copper losses heat the winding; when the temperature reaches the trip setpoint, the protector opens the stator circuit and the motor is de-energised and stops. After the winding cools below the reset temperature the protector recloses (automatic reset). This protects the motor from overheating caused by overload, a locked rotor or a lost phase. The model shows the torque M, the winding temperature and the protector state (closed/open).
This page is a utility for simulating induction motor with built-in thermal protection (3-phase) online with specified initial values.
Using the model. Start the motor with the three switches and watch the winding temperature in the caption (T). Raise the load with the "load torque" slider: the current rises, the winding heats up more, and when it reaches the setpoint the thermal protector trips (protector OPEN) — the motor stops. Once it cools it resets automatically and the motor restarts. The "supply voltage" slider changes the supply. The scopes show the line voltage U(L1‑L2) and the phase current I(L1) — you can see the current vanish when the protector trips.
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