Locked Rotor Test

At the restrained-rotor condition, slip equals one, so the induction motor operates at its starting point rather than its normal running point. This makes the measured impedance represent the combined stator and rotor effects under starting conditions. Engineers can therefore use the test data to characterize how the motor behaves when it first receives AC power.

Reduced voltage is typically applied because the test examines starting behavior while allowing current and input power to be controlled. The applied voltage, resulting current, and input power form the measured electrical response, while torque supplies a mechanical starting-performance indicator. Together, these observations support estimates of starting current, starting torque, and associated voltage drop.

The equivalent-circuit parameters derived from the test are valuable because the locked-rotor measurement includes both sides of the motor's electromagnetic behavior. Rather than treating the stator response alone as the starting model, engineers use the combined impedance and power response to estimate parameters that support performance prediction and validation of analytical or simulation models.

A basic test workflow is to mechanically restrain the rotor, apply a controlled AC voltage to the stator, and record the resulting current, input power, and torque. The rotor must remain stationary so slip stays at one throughout the measurement. Engineers then use these readings to estimate starting characteristics and equivalent-circuit parameters.

Each measured quantity contributes a different part of the engineering assessment. Current helps determine the motor's starting-current demand, input power supports loss estimation, and torque indicates starting capability. Because the readings are taken at the same locked condition, they can be interpreted together to estimate the starting response and the voltage drop expected during motor starting.

Results from the Locked Rotor Test support decisions beyond parameter identification. Engineers can use them for motor selection, performance prediction, protection design, and validation of analytical or simulation models. The results are especially relevant when a design must account for the motor's initial electrical demand, available starting torque, losses, or the voltage drop associated with starting.