Synchronous motors are electric motors that work at a fixed and synchronized speed with the applied voltage frequency. They are popularly used in various industrial and commercial applications due to their unique characteristics and abilities. These motors are known for their ability to maintain a constant speed, making them suitable for applications requiring precise speed control even with load fluctuations. Synchronous motors are commonly used as synchronous generators in power generation to convert mechanical energy into electrical energy. They enhance efficiency and reliability in various industrial processes, maintaining power quality, achieving precise control, and supporting power generation.
In this post Electrical Engineering Online shares Objective Electrical MCQ Questions on Synchronous Generator.
The statement that is true regarding a synchronous motor
The statement that is true regarding a synchronous motor
- It operates at a speed lower than the synchronous speed
- It operates at a speed higher than the synchronous speed
- It operates at the same speed as the synchronous speed
- It does not have a fixed speed of operation
Correct Answer: 3. It operates at the same speed as the synchronous speed.
Explanation: A synchronous motor operates at a speed that is synchronized with the frequency of the applied voltage.
The power factor of an ideal synchronous motor is
The power factor of an ideal synchronous motor is
- 0
- 1
- -1
- It depends on the load
Correct Answer: 2. 1
Explanation: An ideal synchronous motor operates at unity power factor, meaning the current is in phase with the voltage.
In a synchronous motor, hunting refers to
In a synchronous motor, hunting refers to:
- The oscillation of rotor speed
- The overheating of stator windings
- The loss of synchronism between the rotor and the stator
- The excessive vibration of the motor
Correct Answer: 1. The oscillation of rotor speed
Explanation: Hunting in a synchronous motor refers to the small oscillations in rotor speed due to variations in the load.
Which type of synchronous motor does not require DC excitation
Which type of synchronous motor does not require DC excitation
- Salient pole synchronous motor.
- Cylindrical rotor synchronous motor.
- Hysteresis synchronous motor.
- Permanent magnet synchronous motor.
Correct Answer: 4. Permanent magnet synchronous motor.
Explanation: In a permanent magnet synchronous motor, the permanent magnets provide the necessary magnetic field, eliminating the need for DC excitation.
In a synchronous motor, armature winding is placed on the
In a synchronous motor, armature winding is placed on the:
- Rotor
- Stator
- Shaft
- Frame
Correct Answer: 2. Stator.
Explanation: The armature winding in a synchronous motor is placed on the stator, while the field winding is placed on the rotor. In electrical machines, two terms are often used about windings. These are field windings and armature windings. Field winding refers to the winding that produces the main magnetic field in machines. In contrast, armature winding refers to the winding where the main voltage in induced.
The torque developed by a synchronous motor is directly proportional to
The torque developed by a synchronous motor is directly proportional to:
- Armature current
- Field current
- Speed
- Power factor
Correct Answer: 2. Field current.
Explanation: The torque developed by a synchronous motor is directly proportional to the field current, assuming other factors remain constant.
The power factor of a synchronous motor can be improved by
The power factor of a synchronous motor can be improved by:
- Decreasing the field current
- Increasing the field current
- Decreasing the load
- Increasing the load
Correct Answer: 2. Increasing the field current.
Explanation: By increasing the field current of a synchronous motor, the power factor can be improved.
What is the effect of overexciting a synchronous motor
What is the effect of overexciting a synchronous motor?
- It increases the power factor
- It decreases the power factor
- It increases the speed
- It decreases the speed
Correct Answer: 1. It increases the power factor.
Explanation: Overexciting a synchronous motor means increasing the field current, which improves the power factor of the motor.
The pull-out torque of a synchronous motor is
The pull-out torque of a synchronous motor is:
- The maximum torque it can develop without losing synchronism
- The minimum torque it can develop without losing synchronism
- The torque required to start the motor
- The torque required to stop the motor
Correct Answer: 1. The maximum torque it can develop without losing synchronism.
Explanation: The pull-out torque of a synchronous motor is the maximum torque it can develop without losing synchronism with the rotating magnetic field.
Which of the following is an advantage of a synchronous motor over an induction motor
Which of the following is an advantage of a synchronous motor over an induction motor
- High starting torque
- Simple and robust construction
- Better efficiency at full load
- Lower cost
Correct Answer: 3. Better efficiency at full load.
Explanation: Synchronous motors typically have better efficiency at full load compared to induction motors.
Synchronous motors are commonly used in applications that require
Synchronous motors are commonly used in applications that require:
- Variable speed control
- High starting torque
- Continuous operation at constant speed
- Low power consumption
Correct Answer: 3. Continuous operation at constant speed.
Explanation: Synchronous motors are suitable for applications that require continuous operation at a constant speed, such as generators and power factor correction.
The synchronous speed of a 3-phase synchronous motor with 4 poles and a 50 Hz power supply is
The synchronous speed of a 3-phase synchronous motor with 4 poles and a 50 Hz power supply is:
- 750 rpm
- 1000 rpm
- 1500 rpm
- 1800 rpm
Correct Answer: 3. 1500 rpm.
Explanation: The synchronous speed of a synchronous motor can be calculated using the formula: Ns = (120 * f) / P, where Ns is the synchronous speed, f is the frequency, and P is the number of poles.
What happens if the field winding of a synchronous motor loses its excitation while the motor is running
What happens if the field winding of a synchronous motor loses its excitation while the motor is running
- The motor continues to run at the same speed.
- The motor stops immediately
- The motor starts to run at a higher speed
- The motor starts to run at a lower speed
Correct Answer: 2. The motor stops immediately.
Explanation: If the field winding of a synchronous motor loses its excitation, the motor loses synchronism and comes to an immediate stop.
What is the primary disadvantage of a synchronous motor compared to an induction motor
What is the primary disadvantage of a synchronous motor compared to an induction motor?
- Lower efficiency
- Higher cost
- Lower power factor
- Limited speed control
Correct Answer: 2. Higher cost
Explanation: Synchronous motors are generally more expensive than induction motors, which is a primary disadvantage in certain applications.