Short Answer:

Rotor resistance is the electrical resistance present in the rotor winding or conductor bars of an induction motor. It plays an important role in how the motor starts and how much torque it can produce, especially at low speeds.

Higher rotor resistance improves the starting torque but reduces the efficiency due to more power loss as heat. In squirrel cage motors, rotor resistance is fixed and low, while in wound rotor motors, external resistors can be added to increase the resistance and control motor performance during start-up and speed variation.

Detailed Explanation:

Rotor resistance

Rotor resistance is a key electrical property of the rotor circuit in an induction motor. It represents the opposition to current flow in the rotor conductors and is typically denoted as R₂ in motor analysis. Though it may seem like a small component, rotor resistance significantly influences the starting behavior, torque characteristics, heating, and efficiency of the motor.

Rotor resistance varies depending on the type of induction motor:

• In squirrel cage motors, the resistance is built into the aluminum or copper rotor bars and is fixed.
• In wound rotor motors, resistors can be externally added to the rotor circuit through slip rings, allowing control over the resistance and motor performance.

1. Role of Rotor Resistance in Starting Torque:

When an induction motor starts, the rotor is stationary, and the slip is 100%. At this moment, a high relative speed exists between the rotating stator field and the rotor, inducing a strong rotor current.

According to the torque formula:

Torque ∝ (Rotor EMF × Rotor Current × Rotor Power Factor)
And rotor current depends on:
I₂ = E₂ / √(R₂² + (S × X₂)²)

• A higher R₂ increases the rotor power factor at start
• This improves starting torque, making the motor suitable for heavy-load conditions

So, increasing rotor resistance helps in achieving better torque during starting, which is why wound rotor motors are used in cranes, elevators, and mills.

2. Rotor Resistance and Motor Efficiency:

While increased resistance helps with torque at low speed, it also causes more I²R losses (power lost as heat). This lowers the efficiency of the motor, especially during prolonged operation.

• Low rotor resistance = high efficiency (less heating)
• High rotor resistance = good starting but poor efficiency

That’s why in practical applications, wound rotor motors have variable resistances that can be reduced once the motor reaches its normal running speed.

3. Rotor Resistance and Speed Control:

In wound rotor motors, external resistance can be adjusted to control speed. By increasing the rotor resistance, we can increase the slip and reduce the rotor speed under load.

This method provides smooth speed control, especially useful for applications requiring variable speed.

However, this method is not energy-efficient because the additional power is wasted in the resistors as heat.

4. Rotor Heating and Performance:

Higher rotor resistance causes more heat in the rotor windings. In squirrel cage motors, if the rotor gets too hot due to prolonged load or imbalance, it may reduce motor life.

Therefore, rotor resistance must be optimized for a balance between torque, speed control, and efficiency.

Conclusion:

Rotor resistance is the resistance present in the rotor winding of an induction motor. It directly affects motor starting torque, speed control, and efficiency. Higher resistance improves starting torque but causes greater heat loss, reducing efficiency. Understanding and managing rotor resistance, especially in wound rotor motors, helps achieve the desired motor performance in different applications.