Understanding Starting Torque in Motors

Starting torque is the torque produced by an electric motor when it starts from rest. It is a crucial factor, especially when the motor needs to start under load. Different types of motors have different starting torque characteristics.

Starting Torque in Induction Motors

Induction motors are widely used in various applications. There are two main types:

• Squirrel Cage Induction Motor
• Slip Ring Induction Motor

Let's examine their starting torque.

Squirrel Cage Induction Motor Starting Torque

A standard squirrel cage induction motor is known for its simple and robust construction. However, it typically has a relatively low starting torque compared to its full-load torque, especially in standard designs. The starting current is usually high. While modifications exist to improve the starting torque (like using double-cage rotors), the basic squirrel cage motor offers moderate starting torque.

Slip Ring Induction Motor Starting Torque

The slip ring induction motor has a wound rotor, where the rotor windings are connected to external resistors via slip rings and brushes. This external resistance is variable. During starting, external resistance can be added to the rotor circuit. Adding resistance to the rotor circuit at starting has a significant effect on the starting torque.

The torque developed by an induction motor is given by the formula:

\( \text{Torque} \propto \frac{s E_2^2 R_2}{R_2^2 + (s X_2)^2} \)

Where:

• \(s\) is the slip
• \(E_2\) is the rotor induced EMF
• \(R_2\) is the rotor resistance
• \(X_2\) is the rotor reactance

At starting, the slip \(s = 1\). The starting torque becomes:

\( \text{Starting Torque} \propto \frac{E_{2,start}^2 R_2}{R_2^2 + X_{2,start}^2} \)

For a slip ring motor, the rotor resistance \(R_2\) can be increased by adding external resistance. By increasing \(R_2\), the starting torque can be significantly increased. In fact, the maximum torque occurs when \(R_2 = s X_2\). At starting (\(s=1\)), maximum starting torque can be achieved when the total rotor resistance (internal + external) is equal to the rotor reactance (\(R_2 = X_{2,start}\)). This control over rotor resistance allows the slip ring induction motor to achieve a much higher starting torque compared to a standard squirrel cage induction motor.

Alternators and Starting Torque

Options 3 (salient pole Alternator) and 4 (non Salient pole Alternator) refer to alternators. Alternators are synchronous machines primarily used to generate electrical power when driven by a prime mover. They are not typically started as motors in this context, and the concept of "starting torque" as applied to induction motors is not relevant to their normal operation as generators.

Conclusion

Based on the characteristics of the different machines listed:

• Slip ring Induction motors allow for the addition of external rotor resistance, which can be used to achieve high starting torque.
• Squirrel cage Induction motors typically have moderate starting torque in their standard form.
• Alternators are generators, not motors in the context of discussing starting torque.

Therefore, high starting torque can be obtained in a slip ring Induction motor.