Switch Reluctance Motor ( SRM ) Interview Question Answer - II

Short Question on Switch Reluctance Motor

Explain the term : Torque ripple
Torque ripple
  • It is defined as the difference between the maximum and minimum instantaneous torque during steady state condition. 
  • It is expressed as percentage of the average torque.

Give reason : There is no shoot through fault in the SRM.

Shoot Through Fault

  • The SRM power converter consists of one power switch is connected in series with phase wining across DC supply. 
  • If the power fails or misfiring of switch occurs, there is significance delay time for rise of current which allows the protection circuit operates and isolates and faults. 
  • Therefore, there is no shoot through fault in the SRM in spite of failure of power switch.

What are main components of the SRM drive?

  • Power converter, control logic circuit, position sensor and switch reluctance motor (SRM)

Whether the SRM drive depends upon the excitation current polarity?


On which factor the rotor position of the SRM depends?

SRM – Rotor Position

  • The rotor position of the SRM depends upon stator phase commutation and position sensors. 
  • The position sensor may be hall sensor, magnetic sensor, pulse encoder or photo transistor.

State the advantages and disadvantages of the SRM.


  • There is no winding on the rotor therefore saving of copper.
  • The field winding is concentrated around the pole as compared to distributed winding as in the ac machine. There is no active winding resulting low resistance.  Therefore, low copper loss as compared to distributed winding.
  • Main source of heat is only stator winding so cooling is easier.
  • No cogging / crawling problem
  • The rotor does not consist any winding therefore high-speed operation is possible
  • Due to small rotor, large acceleration
  • As the windings are electrically separated, the fault in one phase does not affect the other phases.


  • High acoustic noise
  • High torque ripple (But it can be reduced by overlapping consecutive phase currents)
  • High friction and winding losses particularly at high speed (It can be reduced by making rotor surface smooth.)
  • It requires power converter for control operation.

Write the application of the SRM.

Application of the SRM

  • Automotive
  • Under water marine drive
  • Air craft actuators and generators
  • Actuator in the defense
  • Traction and electrical vehicles
  • Hand tools, power tools, kitchen appliances

On which factor the torque in the SRM depends?


  • The torque in the SRM is given by       T = I2 ( dL / dθ )
  • The torque depends upon magnitude of the phase current not the direction of the phase current. This requires only unidirectional current.

How the direction of the SRM reversed?

  • By changing phase sequence of stator phase current

State the advantage of unidirectional current property in the SRM.

  • The unidirectional current in the SRM reduces the number of power switches in the converter therefore it makes the power converter drive simple.

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