Which of the following single-phase motor has lowest starting torque?

Explanation:

Shaded Pole Induction Motor

Definition: A shaded pole induction motor is a type of single-phase induction motor that is known for its simple design and construction. It is called a shaded pole motor because it has a shading coil wrapped around a portion of each pole to provide starting torque.

Working Principle: The shaded pole induction motor works on the principle of creating a rotating magnetic field. The stator of the motor is constructed with salient poles, and each pole is divided into two unequal halves by a shading coil (usually a copper ring). When the single-phase AC supply is given to the stator winding, it produces an alternating magnetic field. Due to the shading coil, the magnetic field in the shaded part of the pole lags behind the magnetic field in the unshaded part. This creates a rotating magnetic field that induces a current in the rotor, generating a torque that starts the motor.

Advantages:

• Simplicity in design and construction, making it cost-effective and easy to manufacture.
• Reliable operation with minimal maintenance required.
• Suitable for low-power applications where starting torque is not critical.

Disadvantages:

• Low starting torque compared to other types of single-phase induction motors.
• Relatively low efficiency due to the power losses in the shading coil.
• Limited to low-power applications due to its low starting torque and efficiency.

Applications: Shaded pole induction motors are commonly used in small household appliances, such as fans, blowers, hairdryers, and other low-power devices where high starting torque is not necessary.

Correct Option Analysis:

The correct option is:

Option 4: Shaded pole induction motor

This option correctly identifies the single-phase motor with the lowest starting torque. The shaded pole induction motor, due to its simple design and the use of a shading coil, produces low starting torque. This makes it suitable for applications where high starting torque is not required.

Additional Information

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

Option 1: Capacitor start induction motor

The capacitor start induction motor is designed to provide high starting torque. It uses a capacitor in series with the starting winding to create a phase shift and produce a strong starting torque. This motor is commonly used in applications requiring high starting torque, such as air compressors, pumps, and refrigeration units.

Option 2: Capacitor start and capacitor run induction motor

This type of motor also has high starting torque. It uses a capacitor for starting and another capacitor for running to improve the efficiency and power factor. The motor provides a high starting torque and smooth operation, making it suitable for applications like air conditioners, conveyors, and heavy-duty machinery.

Option 3: Split phase induction motor

The split phase induction motor has moderate starting torque. It uses two windings, a starting winding, and a running winding, to create a phase difference and produce starting torque. While it has higher starting torque than a shaded pole motor, it is lower compared to capacitor start motors. It is used in applications such as washing machines, small fans, and blowers.

Conclusion:

Understanding the differences between various single-phase induction motors is essential for selecting the appropriate motor for specific applications. The shaded pole induction motor, with its low starting torque, is suitable for low-power applications where simplicity and cost-effectiveness are important. In contrast, capacitor start and split phase motors provide higher starting torque and are used in applications requiring more power and efficiency.