Special Electrical Machines Interview Questions

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Special Electrical Machines Interview Questions: Electrical Interview Questions

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Q.1. Do stepper motors have internal or external fans?


Ans. No. Because the heat generated in the stator winding is conducted through the stator iron to the case which is cooled by natural conduction, convection and radiation.

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Q.2. Why do hybrid stepping motors have many phases sometime more than six ?


Ans. In order to obtain smaller step angles.


Q.3. Any disadvantage(s) of having more phases?


Ans. Minor ones are: more leads have to be brought out from the motor, more interconnections are required to the drive circuit and more switching devices are needed.


Q.4. What is the main attraction of a multi-stack VR stepper motor?


Ans. It is well-suited to high stepping rates.


Q.5. You are given a VR motor and a hybrid stepper motor which look exactly similar. How would you tell which is which?


Ans. Spin the rotor after short-circuiting the stator winding. If there is no mechanical resistance to rotation, it is a VR motor and if there is resistance, then it is a hybrid motor.


Q.6. How do you explain it ?


Ans. Since VR motor has magnetically neutral rotor, it will not induce any e.m.f. in the short-circuited winding le, the machine will not act as a generator and hence experience no drag on its rotation. However, the rotor of a hybrid motor has magnetic poles, hence it will act as a generator and so experience a drag.


Q.7. Will there be any harm if the rotor of a hybrid stepper motor is pulled out of its stator?


Ans. Yes. The rotor will probably become partially demagnetized and, on reassembling, will give less holding torque

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