Electrical Machines Interview Questions and Answers
Q.1. Why harmonic restraint feature is incorporated in transformer protection?
Answer: In transformer protection, harmonic restraint feature is incorporated to guard against magnetizing inrush current.
Q.2. What are the conditions for successful parallel operation of single-phase transformers?
Answer: The satisfactory and successful operation of transformers connected in parallel on both sides requires that they fulfill the following conditions:
- The transformers must be connected properly as far as their polarities are concerned so that the net voltage around the loop is zero. A wrong polarity connection results in a dead short circuit.
- The voltage rating and the voltage ratio of the transformers should be same to avoid no-load circulating current.
- There should exist only a limited disparity in the per-unit impedances of the transformers.
- The ratio X/R should be same for all the transformers to avoid operation at different power factors.
Q.3. Why the tappings are normally provided on the high voltage winding of a transformer only?
Answer: Tapping are normally provided on the high voltage winding of a transformer only because it has larger number of turns, has to handle low currents and is also easily accessible physically.
Q.4. What is the purpose of providing dummy coils in the armature of a dc machine?
Answer: The purpose of providing dummy coils in the armature of a dc machine is to provide mechanical balance for the rotor.
Q.5. When the motor is said to be crawling?
Answer: An induction motor when started on load does not accelerate up to full speed but runs at 1/7th of the rated speed, then motor is said to be crawling.
Q.6. A dc motor operated from a type A chopper is switched to type B chopper. How does the motor operate when type A and B are in operation respectively?
Answer: In chopper-A voltage and current both are positive so motor operate motoring and in chopper-B it will be reverse direction due to negative current and motor operating as regenerative braking.
Q.7. Why sparking occurs at the brushes during commutation in a dc machine running with a heavy load and with the brushes located on the geometrical neutral axis?
Answer: When zero-crossing of the flux density wave shifted from GNA to MNA, the coils undergoing commutation do not have zero emf induced in them. The induced emf in the commutated coils delays the reversal of armature current in the short circuited coils; this may results in detrimental sparking.
Q.8. How starting torque can be obtained in the case of a single phase induction motor with identical main and auxiliary windings?
Answer: Starting torque can be obtained in the case of a single phase induction motor with identical main and auxiliary windings by connecting a capacitor in series with the auxiliary winding.
Q.9. How a star-delta switch is equivalent to an auto transformer of ratio 57.7%, for the purpose of starting an induction motor?
Answer: Star-delta starting reduces the starting torque to one-third that obtainable by direct-delta starting and also the starting line current to one-third. It just acts like auto transformer starting with x=1/ √3 * 100 = 57.7%.
In other words, when the rotor speed exceeds synchronous speed, the slip becomes temporarily negative, induction generator action takes place and rotor is retarded.
Q.10. How you can compare the starting performance of single cage and double cage induction motors?
Answer: As compare to double cage rotor, single cage have less starting torque so at starting less heating loss occur.
A double cage motor develops higher starting torque for low starting current and its rotor heating loss is more.
Q.11. Why centrifugal switch is used in a single-phase induction motor?
Answer: Centrifugal switch can be used to disconnect either starting winding or starting capacitor. But mostly it s used to disconnect starting winding.
Q.12. What constitute mechanical loss and open circuit core loss?
Answer: Mechanical loss – This loss consists of bearing friction, brush friction and windage losses. The windage or wind friction loss includes the power required to circulate air through the machine and ventilating ducts and is approximately proportional to square of the speed.
Open circuit core loss – It consists of hysteresis and eddy current losses. These losses occur in case of stator and rotor carrying an alternating flux.
Q.13. What happens when a rotor speed, in a synchronous machine becomes more than the synchronous speed during hunting?
Answer: When the rotor speed becomes greater than synchronous speed, in a synchronous machine, induction generator torque (against the direction of rotation) is produced, which tends to retard the rotor and make the two speeds equal.
Q.14. What is the angle between the induced voltage and supply voltage of a synchronous motor under running condition?
Answer: For synchronous motor supply Vt is ahead of induced voltage Ef. under normal running condition means there is some load hence angle between Vt and Ef cannot be zero and beyond 90 degree. So angle should be greater than zero and less than or equal to 90 degree.
Q.15. What are the various losses which occurs in the d.c shunt machine?
Answer: Various losses occur in the d.c shunt machine are –
- No-load rotational loss: This loss is made up of two losses, namely
- The iron loss at working flux and speed.
- The mechanical loss, i.e. friction and windage losses at the operating speed.
- I2R loss:
- Armature circuit loss Ia2ra : where ra includes the resistance of brush contacts, armature winding inter-pole winding and compensating winding, if present.
- Field ohmic loss VfIf : This loss includes the field-rheostat loss.
- Stray load losses: They are produced by –
- The distortion of the air-gap flux due to the armature reaction.
- The currents in the commutated coils.
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