Rotor Resistance Starter for 3 Phase Slip Ring Induction Motors

Understanding the appropriate starter for a 3 phase slip ring induction motor is crucial for its efficient and safe operation. Among the various types of motor starters, the rotor resistance starter is uniquely suited for slip ring induction motors.

Slip Ring Induction Motor Fundamentals

A slip ring induction motor is a type of 3 phase induction motor where the rotor winding is connected to external resistances through slip rings and brushes. This design allows for the addition of external resistance into the rotor circuit during the starting period.

• Wound Rotor: Unlike squirrel cage motors with short-circuited rotor bars, slip ring motors have a wound rotor with windings brought out to slip rings.
• Starting Challenges: Induction motors, especially large ones, draw a very high starting current (5 to 7 times the full load current) if started directly on full voltage (Direct On-Line or DOL). This can cause voltage dips in the supply system and excessive heating in the motor windings.
• Low Starting Torque: Without external resistance, the starting torque of an induction motor can be low, especially if the rotor resistance is not optimized for starting.

Rotor Resistance Starter Mechanism

The rotor resistance starter is specifically designed to overcome the starting challenges of slip ring induction motors by manipulating the rotor circuit resistance. Here's how it works:

1. External Resistance Inclusion: At starting, maximum external resistance is added in series with each phase of the rotor winding via the slip rings.
2. Current Reduction: Increasing the rotor circuit resistance limits the initial inrush of current, thus reducing the high starting current drawn from the supply.
3. Torque Improvement: By adding external resistance, the rotor resistance `\(R_2\)` can be made closer to the rotor reactance `\(X_2\)` (i.e., `\(R_2 \approx X_2\)` at starting, where slip `\(s=1\)`). This condition helps in achieving maximum starting torque. The starting torque of an induction motor is given by the formula:
   \(T_{st} \propto \frac{s E_2^2 R_2}{R_2^2 + (s X_2)^2}\).
   At starting, since slip `\(s=1\)`, the formula becomes:
   \(T_{st} \propto \frac{E_2^2 R_2}{R_2^2 + X_2^2}\).
   Increasing `\(R_2\)` effectively improves the starting torque.
4. Resistance Cut-Out: As the motor gathers speed, the external resistances are gradually cut out in steps. This is because the slip `\(s\)` decreases, and the rotor frequency `\(s f\)` decreases, leading to a reduction in rotor reactance `\(s X_2\)` and a natural increase in rotor current. Removing the external resistance allows the motor to accelerate to its full speed while maintaining high efficiency.
5. Short-Circuited Rotor: Once the motor reaches near full speed, all external resistances are completely cut out, and the rotor windings are short-circuited.

This method ensures a smooth start with reduced starting current and enhanced starting torque, protecting both the motor and the power supply.

Comparison with Other Starter Types

Let's examine why other common 3 phase motor starters are not suitable for slip ring induction motors:

In summary, the unique construction of the slip ring induction motor, which allows access to its rotor circuit, necessitates a starter that can modify this circuit to control starting characteristics. The rotor resistance starter perfectly fulfills this requirement by adding and gradually removing resistance from the rotor circuit, making it the ideal choice.