In which of the following motor resistance is added in rotor circuit to change its speed?

Explanation:

Slip Ring Induction Motor

Definition: A slip ring induction motor, also known as a wound rotor induction motor, is a type of induction motor where the rotor is connected to external resistances through slip rings. This arrangement allows for the addition of resistance to the rotor circuit, which can be varied to control the motor's speed and torque characteristics.

Working Principle: In a slip ring induction motor, the rotor winding is connected to external resistors via slip rings and brushes. When the motor is started, the external resistors are initially included in the rotor circuit. As the motor speeds up, the resistance can be gradually reduced to achieve the desired operating characteristics. This method provides better control over the starting current and torque compared to squirrel cage motors, where such adjustments are not possible.

Advantages:

• Improved starting torque: The external resistances allow for a higher starting torque, which is beneficial for applications requiring a high initial torque.
• Reduced starting current: The resistors limit the inrush current during startup, reducing the mechanical and electrical stress on the motor.
• Speed control: By varying the external resistance, the speed of the motor can be controlled, making it suitable for applications where variable speed operation is required.

Disadvantages:

• Complexity: The presence of slip rings and brushes makes the motor more complex and requires additional maintenance compared to squirrel cage induction motors.
• Higher cost: The additional components and complexity result in a higher initial cost and maintenance requirements.

Applications: Slip ring induction motors are commonly used in applications requiring high starting torque and variable speed operation. Examples include cranes, hoists, elevators, and conveyors.

Correct Option Analysis:

The correct option is:

Option 2: Slip ring induction motor

This option correctly identifies the type of motor where resistance is added to the rotor circuit to change its speed. The slip ring induction motor is designed to allow external resistances to be connected to the rotor, providing control over the motor's speed and torque characteristics.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Squirrel cage induction motor

This type of induction motor has a rotor constructed with bars short-circuited by end rings, forming a cage-like structure. The squirrel cage rotor does not allow for external resistance to be added to the rotor circuit, making it impossible to control the speed in the same manner as a slip ring induction motor. Therefore, this option is incorrect.

Option 3: Single phase induction motor

Single phase induction motors are typically used for smaller loads and do not have provisions for adding external resistance to the rotor circuit. The speed control methods for single phase motors are different and do not involve changing the rotor resistance. Hence, this option is incorrect.

Option 4: Synchronous motor

Synchronous motors operate at a constant speed determined by the supply frequency and the number of poles. They do not use rotor resistance for speed control, as their speed is not variable once synchronized with the supply frequency. This makes this option incorrect for the given statement.

Conclusion:

Understanding the characteristics and operational principles of different types of motors is crucial for correctly identifying the appropriate motor for specific applications. The slip ring induction motor stands out in its ability to use external resistors in the rotor circuit for speed and torque control, making it suitable for applications requiring high starting torque and variable speed operation. This feature sets it apart from other types of motors, such as squirrel cage induction motors, single phase induction motors, and synchronous motors.