# Electrical Drives and Control Question Bank: Set-2

Switched Reluctance Motor related Interview Questions:

In this website, series of posts have been released for interview questions on various topics. In the post we had collected various important questions from switched reluctance motor drives. If you are preparing for a technical interview for drives industries, then this post is written for you…

[1] What is Switched Reluctance Motor?
The switched reluctance motor is a double salient, singly excited motor. This means that it has salient pole on the rotor and the stator, but only one member carries windings. The rotor has no windings, magnets or cage windings. It works on variable reluctance principle.

[2] What are the two types of control techniques in SRM?

1. Hysteresis type control
2. PWM type control

[3] What is meant by energy ratio?

Energy ratio = Wm/[Wm + R] = 0.455
Wm = mechanical energy transformed
This energy ratio cannot be called as efficiency. As the stored energy R is not wasted as a loss but it is fed back to the source through feedback diodes.

[4] What is phase windings?
Stator poles carrying field coils. The field coils of opposite poles are connected in series such that mmfs are additive and they are called phase windings of SRM

[5] What are the major difference between SRM and stepper motor?

 SRM Stepper motor SRM is designed for continuous rotation Stepper motor is designed to rotate in step by step rotation SRM requires a rotor=position sensor It does not require rotor position sensor.

[6] What is hysteresis current control?
This type of current controller maintains a more or less constant current throughout the conduction period in each phase. This controller is called hysteresis type controller.

[7] When do you go for chopping mode of control in switched reluctance motor?
The chopping mode of control in SRM is applied only low-speed. Here each phase winding gets excited for a period which is sufficiently long.

[8] What is the working principle of Switching Reluctance Motor?
The switched reluctance motor basic principle is based on Faraday’s law of electro magnetic interaction.

• Whenever a current carrying conductor is placed in a magnetic field, it experiences a force. The direction of force given by Fleming’s left hand rule.
• SRM is a single excited and double salient pole electric motor. Due to the variation of reluctance the motor should be operated.
• The SRM develops a torque due to variable reluctance.
• When air gap is minimum, the reluctance will be minimum; hence inductance will be maximum.
• So the rate of change of inductance is zero.
• When the reluctance various in the motor, there will be a change in inductance so when a particular stator winding of SRM is energized,the rotor pole comes in alignment with that stator pole and thus the rotor rotates.

[9] Why rotor position sensor is essential for the operation of Switched Reluctance Motor?
It is normally necessary to use a rotor position sensor for commutation and speed feed back. The turning ON and OFF operation of the various devices of power semiconductor switching circuit are influenced by signals obtained from rotor position sensor.

[10] List the disadvantages of a Switched Reluctance Motor?

• Stator phase winding should be capable of carrying magnetizing current.
• For high-speed operation developed torque has undesirable ripples is a result develops undesirable noises or acoustic noises.
• For high-speed current waveform has undesirable harmonics to suppress this effect large size capacitor is to be connected.
• It requires position sensors.

[11] What are the advantages of switched reluctance motor?

1. Construction is simple and robust.
2. Rotor carries no windings, no slip rings, no brushes, less maintenance.
3. There is no permanent magnets.
4. Ventilating system is simpler as losses takes place mostly in the stator.
5. Power semiconductor switching circuitry is simpler.
6. No shoot through fault likely to happen power short circuits.
7. Developed torque does not depends upon the polarity of current in the phase winding.
8. The operation of the machine can be easily change from motoring mode to generating mode by varying the region of conduction.
9. It is possible to get very high-speed.
10. Depending upon the requirement T-w characteristics can be achieved.
11. It is the self-starting machine.
12. Energy stored in the phase winding is fed back to the supply through the feedback diodes during off period.

[12] Whar are the applications of SRM?

1. Washing machines
2. Vacuum cleaners
3. Fans
4. Future auto mobile applications
5. Robotics control applications

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