## 30/03/2020

### Switch Reluctance Motor ( SRM ) Interview Question Answer - 1

What do you mean by term ‘Reluctance motor’?

Reluctance motor

• It is an electrical motor in which torque is produced due to position of rotor such that inductance of the excitation winding becomes maximum.

Describe the stator and rotor construction of the SRM.

• Stator : It is salient pole type and excited by field winding.
• Rotor : It is salient pole type.

Give reason : The reluctance motor  is also called as double salient motor.

Double salient motor

• The stator and rotor of the switch reluctance motor has salient poles therefore it is also called as double salient motor.

State the configuration of the SRM.

Configuration of the SRM

• Linear SRM
• Rotary SRM : Radial field and Axial field

• Lower copper losses due to flux do not reversal
• Higher mutual inductance and higher uneven magnetic pull on the rotor

State the disadvantage of the single phase SRM.

• High torque ripple
• Noise

On which parameter the inductance of the stator winding depends in the SRM?

• The stator winding inductance depends upon stator current and position of the rotor.

Explain aligned and unaligned position of two pole SRM.

Aligned

• When rotor poles are at 00 or 1800
• Maximum inductance
• Zero torque
• Magnetic circuit likely to saturate

Unaligned

• When the rotor poles are at 900
• Minimum inductance
• Zero torque
• Magnetic circuit does not saturate

Explain the torque characteristics of the SRM.

Torque characteristics ( Inductance – Rotor position )

• When the stator and rotor poles do not overlap, the inductance is minimum and constant. It will not contribute any torque.
• When the rotor poles move towards stator poles in aligned position, the motoring torque is produced and the inductance is increasing.
• The movement of the rotor poles does not alter the complete overlap of the stator pole. The inductance becomes maximum and constant. It will contribute zero torque.
• When the rotor poles move away from the stator poles from overlapping position, the inductance decreases and it produces negative torque.

Give reason : The ideal inductance profile is not possible in the SRM.

Inductance Profile

• The saturation of the motor causes inductance profile to curve near the top and therefore reduces the torque constant.
• The large torque ripple produces due to rectangular current pulses. It will result in increase audible noise, fatigue of the shaft and speed oscillations.

How to eliminate negative torque in the SRM?

• The negative torque in the SRM is eliminated by switching off current when the poles are separating.

How the torque ripples can be minimized in the SRM?

Torque ripple

• The torque ripples can be minimized in the SRM by designing the machine such that the inductance profile of the two consecutive phases overlap during end of one phase and starting of another phase.

Explain the term : stroke

Stroke

It is torque cycle due to one current pulse in the SRM.

Explain the term : Phase leg

Phase leg

• There are two power switches connected in series with DC source and center point of two switches is connected to one end of the phase winding. This is called as phase leg.

Explain the term : Shoot through fault

Shoot through fault

• There are two power switches connected in series with DC source and center point of two switches is connected to one end of the phase winding. This is called as phase leg.
• Only one switch is on in phase leg at any time. If the second switch fails, it causes short circuit of the DC source. This type of fault is called as shoot through fault.

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