Understanding the Maximum Voltage and Frequency in a Locked Rotor

Have you ever wondered why the rotor frequency and the largest voltage are induced in the rotor when it is locked? Let's break it down in simple terms without any complex mathematical jargon or equations, suitable for a 'tradie' or a layman.

Why the Rotor is Stationary?

When an induction motor's rotor is locked, it means that the rotor is not rotating. This can happen due to various reasons, like a jam in the motor, a broken shaft, or any mechanical failure. In a locked rotor scenario, the rotor is stationary, meaning it does not generate any counter electromotive force (EMF).

What is the Slip in a Locked Rotor?

Slip is a measure of how much the rotor is lagging behind the rotating magnetic field produced by the stator. The slip (s) is given by the equation:

Slip (Synchronous_speed ? Rotor_speed) / Synchronous_speed

When the rotor is stationary, the rotor speed (N) is zero. So, slip at this point is the maximum it can be, which is 1. This is because the rotor is not moving relative to the rotating magnetic field, causing the slip to be 100%.

Induced EMF in the Rotor

The induced EMF (Er) in the rotor is related to the slip and the synchronous EMF (Es) by the following equation:

Er s × Es

Substituting the value of slip (s 1) when the rotor speed is 0, we get:

Er 1 × Es Es

This means that when the rotor is locked and not rotating, the induced EMF in the rotor is equal to the synchronous EMF of the stator. This results in the largest possible voltage and frequency in the rotor.

Conclusion

In a locked rotor condition, the rotor is stationary, which means it is not generating any counter EMF. The slip is the maximum (1), leading to the largest voltage and frequency being induced in the rotor. This is a crucial aspect to understand for maintenance and troubleshooting of induction motors.

Key Takeaways

The slip is maximum (1) in a locked rotor condition. Counter EMF is not generated when the rotor is stationary. The induced EMF in the rotor is equal to the synchronous EMF of the stator.

Additional Resources

For a deeper understanding, you can refer to the following resources:

Electric Arc - Understanding Locked Rotor Induction Motor Magazine - Locked Rotor Condition

Keywords

#locked rotor, #induction motor, #rotor frequency, #slip, #counter EMF