Question

What is the locked rotor current in an induction motor?

Answer 1

The locked rotor current in an induction motor is the current drawn by the motor when the rotor is not rotating and the motor is applying full voltage. This condition typically occurs during startup when the motor is at standstill and trying to overcome its inertia. The locked rotor current is significantly higher than the motor's rated current and can be several times greater, often ranging from 5 to 8 times the full-load current, depending on the motor's design.

This high current is necessary to generate the torque required to start the motor but can cause substantial stress on electrical components and the power supply system.

Answer 2

The locked rotor current (LRC) in an induction motor is the current that flows when the rotor is stationary and the motor is energized. This situation occurs because the motor's rotor is effectively locked or prevented from turning, and the only way for the motor to draw current is through the resistance of the windings and the reactance of the stator.

### Key Points About Locked Rotor Current

1. **Definition and Nature**: The locked rotor current is the maximum current the motor draws when the rotor is not moving. It is significantly higher than the full-load current of the motor. This high current is due to the high impedance of the motor windings in a stationary state, which causes a large amount of current to flow through the stator windings.

2. **Relationship with Motor Design**: The magnitude of the locked rotor current depends on the motor's design, including the winding configuration, number of poles, and overall impedance. Typically, the locked rotor current can be 5 to 7 times the full-load current of the motor.

3. **Calculating Locked Rotor Current**:
   - The locked rotor current can be approximated using the following formula:
     \[
     I_{LR} = \frac{V_{s}}{Z_{LR}}
     \]
     where \( V_{s} \) is the stator voltage and \( Z_{LR} \) is the impedance of the motor when the rotor is locked.

4. **Implications for Motor Starting**:
   - **Starting Torque**: The high locked rotor current is a crucial factor in determining the starting torque of the motor. It provides the initial torque required to overcome inertia and start the rotor turning.
   - **Electrical Supply Impact**: The high starting current can impact the electrical supply and may necessitate special considerations for circuit protection and power factor correction.

5. **Measurement**: In practice, the locked rotor current can be measured using a clamp-on ammeter or by referring to the motor's datasheet, where it's usually specified as a multiple of the rated full-load current.

6. **Thermal Considerations**: Prolonged periods of high locked rotor current can lead to overheating and potential damage to the motor. This is why the duration for which the motor can stay locked is usually limited by its thermal capacity.

### Summary

The locked rotor current is an important parameter in motor design and operation, indicating how much current the motor will draw when it is not rotating. It is crucial for understanding starting behavior, power supply requirements, and motor protection needs.