Test: Synchronous Machines - 1 - Electrical Engineering (EE) MCQ

• Coupling Angle: The coupling angle in a synchronous motor refers to the angle between the stator magnetic field and rotor magnetic field.


• Power Developed: The power developed in a synchronous motor is given by the formula P = VIcosθ, where V is the voltage, I is the current, and θ is the power factor angle.


• Maximum Power: The maximum power developed in a synchronous motor occurs when the power factor angle is 0, i.e., cosθ = 1.


• Optimum Coupling Angle: The optimum coupling angle for maximum power transfer in a synchronous motor is when the rotor magnetic field is exactly 90 degrees ahead of the stator magnetic field.


• Angle of 90°: At a coupling angle of 90°, the power factor angle is 0, and the motor develops maximum power.

• Half the number of poles: When the stator windings are connected in such a fashion that the number of poles are made half, it means that the synchronous motor now has double the number of poles compared to before.


• Synchronous speed formula: The synchronous speed of a motor is given by the formula: 120f/p, where f is the frequency of the power supply and p is the number of poles.


• Effect on speed: Since the number of poles has doubled, the synchronous speed of the motor will also double according to the formula.


• Result: Therefore, the speed of the rotor of a synchronous motor will increase to two times the original value when the stator windings are connected in a way that halves the number of poles.

• Definition: A synchronous motor is a type of AC motor in which the rotation of the shaft is synchronized with the frequency of the supply current.


• Working Principle: Synchronous motors rotate at a constant speed called synchronous speed, which is determined by the number of poles and the frequency of the power supply.


• Load Variation: The speed of a synchronous motor remains constant regardless of the load variations.


• Reasoning: This is because the rotor of a synchronous motor rotates at the same speed as the rotating magnetic field of the stator, resulting in a constant speed.


• Applications: Synchronous motors are used in applications where a constant speed is required, such as in industrial processes, synchronous clocks, and power factor correction.

• Synchronous motor: In a synchronous motor, the stator and rotor magnetic fields rotate at the same speed, which is synchronous speed. This is achieved by supplying the rotor with a DC current to create a magnetic field that locks in step with the rotating magnetic field of the stator.


• Universal motor: A universal motor is a type of electric motor that can operate on both AC and DC power sources. It is commonly used in household appliances like vacuum cleaners and power tools. However, the stator and rotor magnetic fields do not rotate at the same speed in a universal motor.


• Induction motor: In an induction motor, the rotor rotates at a speed slightly lower than the synchronous speed of the stator field. The speed difference between the rotor and stator fields creates the torque needed to drive the motor.


• Reluctance motor: A reluctance motor operates based on the principle of reluctance torque, where the rotor rotates to minimize the magnetic reluctance between the rotor and stator. However, the stator and rotor magnetic fields do not rotate at the same speed in a reluctance motor.

• Synchronous Motor: A synchronous motor is a type of AC motor where the rotation of the shaft is synchronized with the frequency of the supply current.


• Breakdown Torque: Breakdown torque is the maximum torque a motor can produce before stalling or stopping.


• Applied Voltage: The voltage applied to the motor affects its performance, including the breakdown torque.

• For a synchronous motor, the breakdown torque is directly proportional to applied voltage.


• When the voltage applied to the motor increases, the breakdown torque also increases.


• This relationship is linear, meaning that a higher applied voltage will result in a higher breakdown torque.


• Therefore, the correct answer is option A: directly proportional to applied voltage.

• Angle between stator flux and rotor poles: The angle between the rotating stator flux and rotor poles is crucial in determining the performance of an induction motor.


• Definition of torque angle: The angle between the rotating stator flux and rotor poles is known as the torque angle.


• Importance of torque angle: The torque angle affects the torque production and efficiency of the motor.


• Role of synchronizing angle: The synchronizing angle is essential for the smooth operation of the motor and ensuring proper power generation.


• Relation to power factor: The torque angle is related to the power factor of the motor, as it influences the power output and efficiency.

• Direction of field current: The direction of the field current determines the direction of rotation of a synchronous motor.


• Reversing the field current: When the direction of the field current is reversed, the magnetic field produced by the rotor will also reverse.


• Effect on motor: The reversal of the magnetic field will not stop the motor or cause the winding to burn. Instead, it will cause the motor to continue running in the same direction.


• Reason: This is because the stator field will still be out of phase with the rotor field, causing the motor to maintain its rotational direction.

• Low field current: If the field current applied to the synchronous motor is too low, it may not generate enough magnetic flux to pull the rotor into synchronism with the stator.


• High core losses: High core losses in the motor can lead to reduced efficiency and performance, causing it to fail to synchronize properly.


• High field current: Excessive field current can also prevent the motor from synchronizing correctly, leading to issues with the magnetic field alignment.


• Low short circuit ratio: A low short circuit ratio can impact the ability of the motor to synchronize properly, as it affects the reactance of the system and the overall performance.

It is important to ensure that the synchronous motor is operating within the correct parameters and that the field current applied is optimal for synchronization to occur successfully.

• Hunting in a synchronous machine: Hunting refers to the small oscillations in speed that occur when a synchronous machine is not operating at its synchronous speed.


• Damper bars: Damper bars are used in synchronous machines to prevent hunting by providing damping torque.


• Development of induction generator torque: When the rotor speed of a synchronous machine becomes more than the synchronous speed during hunting, the damper bars develop induction generator torque.


• Induction generator torque: This torque helps to stabilize the machine and bring it back to its synchronous speed.

• Similarity to Transmission of Mechanical Power by Shaft: A synchronous motor operates on the principle of electromagnetic induction, where the stator creates a rotating magnetic field that induces a current in the rotor. This current produces its own magnetic field, which interacts with the stator's field to generate motion. This process is akin to the transmission of mechanical power through a shaft, where one rotating component drives another.


• Stator and Rotor Interaction: In a synchronous motor, the stator and rotor must synchronize their speeds for efficient operation. The stator's magnetic field rotates at a constant speed, while the rotor follows this rotation to maintain synchronization. This interaction ensures that the motor runs smoothly and efficiently, much like a well-aligned transmission system.


• Efficient Power Generation: Synchronous motors are known for their high efficiency and power factor, making them ideal for applications where precise control and stable operation are required. By closely controlling the interaction between the stator and rotor, synchronous motors can deliver power reliably and efficiently, similar to how a well-designed transmission system optimizes power transfer.


• Applications in Power Generation: Synchronous motors are commonly used in power plants and industrial settings where precise control and synchronization are crucial. By mimicking the efficient transmission of mechanical power through a shaft, synchronous motors play a vital role in generating electricity and driving machinery with precision and reliability.