# 100+ Synchronous Motor MCQs – Top Most Important Synchronous Motor Objective Multiple Choice Questions

Electrical Engineering XYZ Multiple Choice Questions on Synchronous Motor. A synchronous motor is an electric motor that rotates at a constant speed and remains synchronized with the frequency of the alternating current (AC) power supply. It has a rotor with permanent magnets or electromagnets that align with the rotating magnetic field generated by the stator. This synchronization ensures that the motor maintains a fixed speed and is used in applications where precise speed control is required, such as in industrial machinery, power plants, and electric vehicles. Synchronous motors are efficient, have a high power factor, and are commonly used in situations where consistent speed and accurate control are essential. In this article Electrical Engineering XYZ shares top 100+ objective multiple choice questions on Synchronous Motor.

## Sort The advantages of amortisseur winding

Sort The advantages of amortisseur winding:

1. It increases stability
2. It develops torque which is useful for fast machine
3. It develops torque which is useful for slow machines
4. All of these

Correct answer: 4. All of these

Explanation: 4. All of these is correct answer

1. It increases stability: The presence of an amortisseur winding helps improve the stability of the synchronous motor. It reduces the chances of oscillations and enhances the motor’s ability to maintain synchronism with the power supply.
2. It develops torque which is useful for fast machines: The amortisseur winding generates torque in a synchronous motor, and this torque is particularly beneficial for fast machines. It assists in achieving and maintaining high rotational speeds efficiently.
3. It develops torque which is useful for slow machines: In addition to fast machines, the torque developed by the amortisseur winding is also valuable for slow machines. It helps overcome inertia and provides the necessary starting torque, enabling the motor to reach and maintain the desired speed.

Therefore, the correct answer is: All of these.

## The correct statement about the speed of synchronous motor

The correct statement about the speed of synchronous motor:

4. It is pulsating and change at irregular intervals

Explanation: A synchronous motor is designed to operate at a fixed speed determined by the frequency of the AC power supply and the number of poles in the motor. It maintains synchronization with the power supply frequency, resulting in a constant speed. Unlike induction motors, which exhibit slip and slight speed variations, synchronous motors do not experience significant changes in speed from no load to full load. This characteristic makes them suitable for applications where a precise and consistent speed is required, such as in power plants, industrial machinery, and synchronous clocks.

## The synchronous motor is not a self starting motor because

The synchronous motor is not a self starting motor because:

1. It uses damper winding
2. No stator is present
3. The slip is zero in the motor
4. The rotor’s instantaneous torque reverses after half cycle

Correct answer: 4. The rotor’s instantaneous torque reverses after half cycle

Explanation: In a synchronous motor, the rotor’s instantaneous torque reverses after each half cycle of the AC power supply. This means that the direction of the torque changes periodically, which prevents the rotor from continuously rotating in a single direction and starting on its own. The rotor’s torque is dependent on the interaction between the rotating magnetic field of the stator and the magnetic field of the rotor. Since the rotor’s torque changes direction, it cannot develop the necessary torque to start and maintain rotation without an external means of bringing it to synchronous speed. Therefore, a synchronous motor requires external starting mechanisms.

## The overexcited synchronous motor is used to

The overexcited synchronous motor is used to:

2. Support loads which have low torque
3. Supply reactive power
4. None of these

Correct answer: 3. Supply reactive power

Explanation: An overexcited synchronous motor refers to a synchronous motor whose field winding is supplied with a DC current that produces a magnetic field stronger than the synchronous speed requires. This causes the motor to operate at a leading power factor, which means it generates reactive power and can supply it to the electrical system. By supplying reactive power, the overexcited synchronous motor helps to improve the power factor of the system, reduce line losses, and stabilize the voltage. Therefore, the correct answer to the multiple-choice question is “3. Supply reactive power.”

## The method which can be used to start a synchronous motor

The method which can be used to start a synchronous motor:

1. Use an external prime mover
2. Reduce the stator frequency to safe point
3. Use Amortisseur windings
4. All of these

Correct answer: 4. All of these

Explanation: All above mentioned methods can be used for starting synchronous motor.

1. Using an external prime mover: This involves using an external motor or engine to drive the synchronous motor to its synchronous speed.
2. Reducing the stator frequency to a safe point: By decreasing the stator frequency, the synchronous motor can start and gradually accelerate to synchronous speed.
3. Using Amortisseur windings: Some synchronous motors are equipped with Amortisseur windings, also known as squirrel cage windings, which can aid in the starting process by providing initial torque.

## Speed regulation of the Synchronous motor is

Speed regulation of synchronous motor is:

1. 0
2. 1
3. 10
4. 100

Explanation: Speed regulation refers to the ability of a motor to maintain a relatively constant speed even under varying load conditions. In the case of a synchronous motor, its speed is determined by the frequency of the AC power supply and the number of poles in the motor. The speed of a synchronous motor remains constant (synchronized) with the supply frequency.

Unlike asynchronous motors (induction motors), synchronous motors do not experience significant changes in speed when subjected to changes in load. Due to their inherent design and magnetic field synchronization, synchronous motors exhibit very low speed regulation, often close to 0%. This means that even with varying loads, the speed of a synchronous motor remains almost constant.

## The maximum pullout torque in a synchronous motor occurs at

The maximum pull out torque in a synchronous motor occurs at:

1. 45º
2. 60º
3. 90º

Explanation: In case of synchronous motor the torque vary as a sine of the torque angle δ

Mathematically,

Torque (T) = Tmax sin δ

From above equation once can easily understand that the maximum torque occurs at a torque angle of 90 electrical degrees. This torque is called the pull-out or pull out torque, that implies the maximum value of torque that a synchronous motor can develop
without pulling out of synchronism.

## Synchronous motors are self-starting, the statement is

Synchronous motors are self-starting, the statement is:

1. True
2. False

Explanation: Synchronous motors are not self-starting. Unlike induction motors, which can self-start due to the interaction of the rotating magnetic field with the rotor currents, synchronous motors require external means to initiate rotation. This is because synchronous motors rely on maintaining precise synchronization between the rotating magnetic field produced by the stator and the rotor.

To start a synchronous motor, a rotating magnetic field must be established in the stator. This can be achieved through various methods such as using a separate induction motor to initially rotate the synchronous motor, or by providing a starting mechanism like a pony motor or a prime mover. Once the synchronous motor reaches a certain speed and synchronizes with the stator field, it can continue to operate without external assistance.

In summary, synchronous motors are not self-starting and require external means to initiate rotation and establish synchronization with the stator field.

## The power factor of under excited synchronous motor is

Power factor of under excited synchronous motor is:

2. Lagging
3. Unity
4. Zero

Explanation: In a synchronous motor, the power factor is determined by the relative phase angle between the stator current and the rotor magnetic field. An under-excited synchronous motor refers to a condition where the field current is adjusted to create a lagging reactive power component.

When the synchronous motor is under-excited, the field current is adjusted to produce a magnetic field that lags behind the stator current. This creates a lagging reactive power component in the motor, resulting in a lagging power factor.

Therefore, the correct answer is: Lagging

## The percent slip of synchronous motor is

The percent slip of synchronous motor is:

1. 0
2. 0.5
3. 99.5
4. 100