Electric machine interview question

1.The unit of the rate at which energy is expended?

  • Watt

2. What is a tap‐changing transformer?

  • It is a transformer that has one or more leads wire output so the switch can be used to change the output voltage from a reference wire.

3. Different Tests conducted on Transformers?

  • Polarity Test: Polarity test on the transformer helps to determine the same instantaneous polarity of the transformer. The relative polarities of the primary and secondary terminals at any instant must be known for connecting the windings of the same transformer in parallel

  • Open Circuit Test: This test is also called as no load test. This test helps to determine the core losses in the transformer.

  • Short Circuit Test: Short Circuit test of a transformer helps to determine the full load copper losses for a given transformer.

  • Sumpner’s Test: This is also called back to back test. In open circuit and short circuit tests, no information is obtained about the temperature rise.

4. What are the advantages of a star-delta starter with an induction motor?

  • The main advantage of using the star delta starter is a reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online starting.

  • Hence the starting current is reduced; the voltage drops during the starting of the motor in systems are reduced

5. What are CT and PT?

  • CT is the current transformer and PT is the Potential transformer both are instrument transformers. CTs and PTs are used with voltmeters and ammeters where measurement of a large value of current may damage the measuring device.

The working of CT and PTs are based on the electromagnetic flux linkage as common transformers.

6. Why Delta Star Transformers are used for Lighting Loads?

  • For lighting loads, neutral conductor is must and hence the secondary must be star winding. And this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta/star transformer is used for lighting loads.

7. What is the difference between MCB & MCCB, Where it can be used?

  • MCB is a miniature circuit breaker that is thermally operated and used for short-circuit protection in a small current rating circuit. The MCCB molded case circuit breaker is thermally operated for current overload and magnetic operation for instantaneous trip in short circuit condition. Low voltage and low frequency can be incorporated. Normally used where the normal current is more than 100A.

8. What is the difference between Isolator and Circuit Breaker?

  • The isolator is an offload device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. It is manually operated and does not contain any solenoid, unlike circuit breaker. it should not be operated while it is having a load.

  • Circuit breaker is on load automatic device used for breaking the circuit in case of abnormal conditions like short-circuiting, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current.

9. What is SF6 Circuit Breaker?

  • SF6 is Sulpher hexa Fluoride gas… if this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.

10. What are the types of cooling system it transformers?

  • ONAN (oil natural, air natural)

  • ONAF (oil natural, air forced)

  • OFAF (oil forced, air forced)

  • ODWF (oil direct, water forced)

  • OFAN (oil forced, air forced)

11. Difference between megger test equipment and contact resistance meter test instruments?

  • Megger test equipment used to measure cable electric resistance, conductor continuity, phase identification whereas contact resistance meter test instruments used to measure low resistance like relays, contactors.

12. What is a 2 phase motor?

  • A two phase motor is a motor with the starting winding and the running winding have a phase split. eg: AC servo motor.

13. What is meant by armature reaction?

  • The effect of armature flux to main flux is called as armature reaction. The armature flux may support main flux or opposes main flux.

14. What is Automatic Voltage regulator (AVR)?

  • AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

15. Why use the VCB at High Transmission System? Why can’t use ACB?

  • Actually the thing is vacuum has high arc quenching property compare to air because in VCB, the die electric strength is equal to 8 times of air. That y always vacuum used as in HT breaker and air used as in LT.
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