50 Top Synchronous Motor Interview Questions

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50 Top Synchronous Motor Interview Questions

A synchronous motor is an AC motor that runs at a constant speed specified by the power supply frequency and the number of poles. Synchronous motors, unlike induction motors, operate at synchronous speed with no slip.

Synchronous motors are used in a variety of applications, including 

  • Industrial drives, 
  • Power generation 
  • Power factor correction synchronous condensers, and 
  • Precise motion control systems.

Synchronous motors sustain synchronism since the rotor rotates (rotation) at the same rate as the stator’s rotating magnetic field.

 synchronous motor ROTATION

A synchronous motor’s speed is proportional to the frequency of the power source and inversely proportional to the number of poles in motor.

Because of their set synchronous speed characteristics, synchronous motors remain less efficient at handling varying loads than induction motors.

When a synchronous motor lose synchronism with the power source, it stops working efficiently and may need to be re-synchronized before it can be restarted.

Auxiliary devices such as damper windings (or) starting motors are commonly used to get synchronous motors up to synchronous speed prior synchronizing with the power source.

In comparison to induction motors, synchronous motors provide greater 

  • Power factor correction, 
  • Higher efficiency under constant load, and 
  • Precision speed control.

To run at varying speeds, synchronous motors need external control systems such as variable frequency drives (VFDs).

A synchronous motor is made up of a stator, a rotor, an excitation system, and, in some conditions, a damper winding or a starting mechanism.

The excitation system generates a magnetic field by supplying direct current to rotor windings. 

This field synchronizes with the rotating magnetic field of the stator, allowing motor to run at a synchronous speed.

Synchronous motors, on the other end, rely on the excitation system to provide the magnetic field required for synchronism with stator field.

There are two types: 

  • DC excitation systems – which use DC power to power the rotor, and 
  • Permanent magnet excitation systems – which use permanent magnets in the rotor to generate the magnetic field.

By adjusting the excitation current, synchronous motors can modify their power factor. 

The power factor of the motor can be improved or corrected by altering the excitation.

The damper winding assists the motor in starting by allowing for some early slip prior to attaining synchronous speed. 

It also assists in the motor’s stability amid unexpected load fluctuations.

50 Top Synchronous Motor Interview Questions

Synchronous motors normally function at synchronous rates and may not be the best solution for applications demanding high torque at low speeds unless additional gearing (or) modifications are used.

In synchronous motors, speed regulation frequently involves managing the excitation (or) modifying frequency of power supply via variable frequency drives (VFDs).

Synchronous reluctance motors utilize the reluctance torque principle & feature a simplified rotor shape without windings or magnets. When compared to typical synchronous motors, they have the potential for improved efficiency and simpler construction.

Because of its capacity to maintain synchronism with the power source, synchronous motors are functioning above synchronous speed can operate as generators, transforming the mechanical energy into the electrical energy.

The RPM formula for synchronous speed is

Synchronous Motor Speed (RPM) = (120 X Frequency) / Number of Poles

Synchronous Motor Speed (RPM) = (120 X f)/P

The number of poles in synchronous motor is governed by the motor’s design and construction. It is a fixed feature that the manufacturer specifies.

In applications requiring consistent speed and excellent efficiency, such as 

  • Industrial pumps, 
  • Compressors, fans, and 
  • Certain types of industrial machinery, 
  • Synchronous motors 

are chosen.

The load angle is the angular difference between the magnetic fields of the stator and the rotor. 

Proper load angle adjustment is critical for the motor’s efficiency and synchronism.

Synchronous motors, known as synchronous condensers, can function in driving mode while also generating electrical power to grid as generators in specific conditions.

To minimize hunting or instability in synchronous motors, several control systems and stabilizing technologies, such as 

are employed.

Optimizing excitation control system effects the efficiency, stability, and reaction to varying loads of the motor, as well as its power factor.

The 

  • Required speed, 
  • Torque characteristics, 
  • Power factor requirements, 
  • Efficiency, and 
  • Capacity to control the motor’s operation 

are all important considerations for choosing the right synchronous motor.

Power analyzers (or) meters can be used to measure the power factor of a synchronous motor. It is a vital characteristic that indicates how efficient the motor is at converting electrical power into practical work.

When synced to the same frequency and phase, synchronous motors can work in parallel with the other synchronous motors (or) power sources.

Maintaining synchronism entails dealing with difficulties like as load variations, transient situations, and effective regulation of the excitation system to avoid synchronism loss.

The inertia, ability to endure mechanical loads, and efficiency of a motor are all affected by the rotor design & material. High magnetic permeability materials are frequently employed to improve performance.

synchronous motor  performance

Permanent magnet rotors outperform traditional rotor designs in terms of efficiency, power density, & potentially lower losses, resulting in improved motor performance.

In comparison to induction motors, which self-start, synchronous motors often require external help such as damper windings, starting motors, (or) extra devices to get them to synchronous speed before linking to the power source.

A synchronous motor’s torque-speed curve is relatively flat at the synchronous speed & rapidly declines while operating below synchronous speed. 

It has a strong torque at the rated speed, making it perfect for constant-speed applications.

While asynchronous motors can operate on single-phase power, synchronous motors need three-phase power due to the need for synchronism with a spinning magnetic field.

Cooling systems, such as air (or) liquid cooling, are installed in synchronous motors to remove the heat generated during the operation, guaranteeing ideal temperature & preventing overheating.

Synchronous motors can have a variety of rotor shapes, such as 

  • Cylindrical rotors, 
  • Salient pole rotors, or 
  • Interior permanent magnet (IPM) rotors, 

each with their own set of performance characteristics.

To maintain stability and control, the excitation system, which consists of field windings (or) permanent magnets, is controlled by regulating the excitation current (or) magnetic field strength.

synchronous motor stability and control

Synchronous motors can be built to resist extreme conditions by combining robust construction, insulation, & environmental protection techniques.

The correct phase sequence is essential for synchronizing numerous motors (or) connecting them to a power supply that ensures they rotate in the desired direction without harming their performance.

Synchronous motors perform best under steady loads, but their efficiency may suffer when subjected to variable (or) fluctuating loads.

Yes, in applications requiring accurate speed control and consistent rotational speed, which include precision machining (or) motion control systems, synchronous motors are favored.

While both work at synchronous speeds, PMSMs feature permanent magnets integrated in the rotor, which provides more efficiency and torque density than conventional synchronous motors.

The formula for calculating torque in a synchronous motor is as follows:

Torque = Power (watts) / (2 x П x Speed)

Under-excitation can result in reduced power production and overheating, whereas over-excitation can result in excessive reactive power & resulting motor instability.

Synchronous motors are designed to run at synchronous speeds, and exceeding that speed may necessitate modifications (or) additional equipment.

The phase angle among the magnetic fields of the stator and rotor governs the motor’s torque production and influences its operating parameters.

The selection between synchronous & induction motors is influenced by factors such as needed speed, torque characteristics, control needs, and efficiency considerations.

To prevent damage during faults, synchronous motors incorporate preventative measures such as 

  • Overcurrent protection, 
  • Temperature monitoring, 
  • Vibration analysis, and 
  • Automatic shutdown systems.

Synchronous motors can be used in regenerative braking systems, which transform the mechanical energy into the electrical energy while slowing or stopping machinery.