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Q.1. Why harmonic restraint feature is incorporated in transformer protection?

DRex Electronics

Answer: In transformer protection, harmonic restraint feature is incorporated to guard against magnetizing inrush current.

Q.2. How no-load rotational losses in electrical machines can be determined?

Answer: No load rotational losses can be determined by running the electrical machine as an unloaded motor at the rotor speed and with armature voltage equal to the normal generator emf. The total power input to the unloaded motor minus the no load armature ohmic loss gives the magnitude of no-load rotational loss.

Q.3. What do you mean by no-load rotational loss?

Answer: The mechanical loss and open circuit core loss combined together are caused by rotor rotation. Their sum is called as no-load rotational losses.

Q.4. 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.5. What is the condition for maximum efficiency of a transformer?

Answer: Maximum efficiency occurs when the variable ohmic loss is equal to the fixed core loss.

Q.6. 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 fulfil 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.7. How the direction of rotation of a dc series motor can be reversed?

Answer: The direction of rotation of a dc series motor can be reversed either by interchanging supply terminals or by interchanging field terminals.

Q.8. How single-phase induction motor can be classified?

Answer: Single-phase induction motor is classified based on their method of starting-

  • Resistance split-phase motor.
  • Capacitor-start motor.
  • Permanent capacitor motor.
  • Two value capacitor motor.
  • Shaded pole motor.

Q.9. 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.10. Why is armature control is superior to field control scheme in the case of a dc shunt motor?

Answer: Armature control is superior to field control scheme, due to following reasons-

  • It provides a constant-torque drive. In the shunt motor case by keeping the field current at maximum value, full motor torque can be obtained at full-load armature current at all speeds.
  • Since the main field ampere-turns are maintained at a large value, flux density distortion caused by armature reaction is limited.
  • Unlike field control scheme, speed reversal can be easily implemented here.

Q.11. What is “Hunting” in synchronous machines?

Answer: The synchronous machine oscillates about the operating point, due to-

  • A sudden change in load. A fault in the supply system.
  • A load containing harmonic torques.
  • A sudden change in field current.

This oscillatory behaviour of the synchronous machines is known as hunting.

Q.12. What are the effects of hunting in synchronous machines?

Answer: The effects of hunting are as follows-

  • It produces severe mechanical stress and fatigue in the shaft.
  • It causes great surges in current and power flow.
  • It increases machine losses and thus the temperature rise of the machine.

Q.13. Does “Hunting” occurs in synchronous generators only or in synchronous motors also?

Answer: Hunting occurs in both synchronous generators and synchronous motors.

Q.14. What are the conditions under which a shunt excited dc generators may fail to self-excite?

Answer:  A shunt excited dc generator may fail to self-excite for any of the following reasons-

  • Residual magnetism is absent. This difficulty can be overcome by exciting the field winding from a separate dc source for a few seconds with the armature at rest.
  • The field connection to the armature is such that the induced emf due to the residual magnetism tends to destroy the residual magnetism. This condition can be remedied by reversing the field connection to the armature.
  • The field circuit resistance is more than the critical value. For critical resistance Rf line coincides with the linear portion of the magnetization curve.
  • Speed is less than critical speed. At critical speed, the open-circuit characteristic is tangential to the Rf line.

Q.15. 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.16. Where capacitor-start motors are generally used?

Answer: Capacitor-start motors are used where high starting torque is required and where high starting period may be long.

Q.17. What is the use of interpoles in dc machines?

Answer: In practice, the interpoles of appropriate polarity are strengthened so that in interpolar zone, the armature crosses flux is neutralized and in addition some flux is produced there. This additional flux in the interpolar zone induces rotational emf in the commutated coil in such a direction as to oppose the reactance voltage. So, the resultant emf in the commutated coil would be zero and therefore zero current in that coil would amount to sparkles commutation. This is the reason why interpoles are designed to provide more mmf than the armature mmf in the commutating zone. In practice, the inter pole mmf may be  1.2 to 1.3 times the armature mmf per pole.

Q.18. Why the air gap between the yoke and armature in a dc motor is kept small?

Answer: To achieve a stronger magnetic field, the air gap between the yoke and the armature is kept small in rotating machines.

Q.19. 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.20. What is the cause of crawling in the induction motor?

Answer: The crawling in the induction motor is caused by harmonics developed in motor.

Q.21. Why the performance of single phase induction motor is poorer as compared to a three phase induction motor?

Answer: In a three phase induction motor the maximum torque is independent of the rotor resistance, while the slip at which it occurs increases with the rotor resistance. No such neat result is possible in a single phase induction motor as the backward rotating field thereby reducing the forward torque. As a result the maximum torque in a single phase induction motor reduces as the motor resistance increases.

Q.22. What is the main function of a starter in a three phase induction motor?

Answer: Main function of starter is to limit the high starting current to reasonable value and to provide overload and no-voltage protection.

Q.23. Why squirrel cage induction motor is preferred to the slip ring induction motor?

Answer: The rotor circuit of a squirrel cage induction motor cannot be tempered with and the machine has a low starting torque, while it has excellent running performance.

Q.24. Which method gives more accurate result for determination of voltage regulation of an alternator?

Answer: Synchronous impedance method i.e. emf method is pessimistic method i.e. VR by this method is much higher than the accurate value given by zpf method or potier triangle method. The VR by mmf method is less than the accurate value; therefore this method is termed as an optimistic method. American standard association method is also accurate method but is slightly less accurate than the potier triangle method.

Q.25. What is the shape of armature mmf waveform of a dc machine?

Answer: Triangular.

Q.26. In which type of dc machine, wave winding is employed?

Answer: Wave winding is employed in a dc machine of low current and high voltage rating.

Q.27. 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.28. Comment upon the torque produced if a d.c series motor is accidentally connected to single phase a.c supply?

Answer: In d.c series motor, the ac currents through the field and armature windings will always be in the same direction. So torque will be unidirectional but pulsating due to ac.

Q.29. How the speed of a d.c shunt motor can be varied?

Answer: Speed of a dc shunt motor may be varied by field flux control, armature voltage control and armature resistance control.

Q.30. What do you mean by the terms air-gap power, internal mechanical power developed and shaft power and how these terms are correlated to each other?

Answer: Air-gap power (Pg) – The power that is transformed from the stator to the rotor via the air-gap magnetic field, is known as the air gap power. Subtracting the rotor copper loss from air-gap power gives the internal mechanical power developed (Pm).

Shaft power (Psh) is the power available at shaft after subtracting mechanical losses from internal mechanical power developed.

Q.31. What could be the worst type of load on a supply system?

Answer: Arc furnace load is the worst type of load on a supply system.

Q.32. What will be the effect of armature mmf on the main field flux and its consequences?

Answer: The armature mmf produces two undesirable effects on the main field flux and these are –

  • Net reduction in the main field flux per pole.
  • Distortion of the main field flux wave along the air gap periphery.

Reduction in the main flux per pole reduces the generated voltage and torque, where as the distortion of the main field flux influences the limits of successful commutation in d.c machines.

Q.33 What do you mean by armature reaction?

Answer: When the armature of a d.c machine carries current, the distributed armature winding produces its own mmf (distributed) known as armature reaction.

Q.34. What are the methods for starting of 3-phase squirrel-cage induction motors?

Answer: There are primarily two methods of starting squirrel cage induction motors-

  • Full-voltage starting,
  • Reduced-voltage starting.

The full-voltage starting consists of DOL starting, only reduced voltage starting is the most popular method of starting 3-phase squirrel-cage induction motors and consists of-

  • Stator impedance starting.
  • Auto-transformer starting.
  • Star-delta starting.

Q.35. Which characteristic of a dc series motor make it to be used in electric locomotives, cranes etc?

Answer: Due to high starting torque dc series motor is used in electric locomotives, cranes etc.

Q.36. What are slot wedges in a dc machine, made of?

Answer: Slot wedges are used to reduce the air gap so made of silicon steel.

Q.37. 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.38. State Faraday’s law.

Answer: Faraday’s law states that a time varying magnetic field produces an electromotive force (emf) which may establish a current in a suitable closed circuit.

Q.39. When a slip ring induction motor will run at super synchronous speed?

Answer: A slip ring induction motor run at super synchronous speed when an emf is injected in the rotor circuit in phase with the rotor induced emf.

Q.40. 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.41. What will be the power factor if the field of a synchronous motor is under excited?

Answer: The power factor will be lagging.

Q.42. Why damper bars in case of salient pole rotors of hydro-alternators are usually interested in pole faces?

Answer: Damper bars in case of salient pole rotors of hydro-alternators are usually interested in pole faces to damp out the rotor oscillations during transient state owing to sudden change in load conditions. The damper winding comes into play only during rotor hunting, when rotor speed departs from synchronous speed.

Q.43. What do you mean by switched reluctance motor?

Answer: Switched reluctance motor means a stepper motor with closed loop control and with rotor position sensor.

Q.44. What is the use of capacitor in a single phase induction motor?

Answer: The capacitor is used to introduce a phase difference of 90 degree between auxiliary and main winding to start the motor. Single phase induction motor is not self started.

Q.45. What is the main advantage of distributing the winding in slots?

Answer: The main advantage of distributing the winding in slots is to reduce the harmonics in the generated emf wave, thus making it approach a sine wave, the other two advantages are –

  • Full utilization of the armature iron and copper.
  • Adding rigidity and mechanical strength to the winding.

Q.46. What will be the effect on starting torque and efficiency of a slip ring induction motors, if the rotor resistance is increased?

Answer: For a slip ring induction motor, if the rotor resistance is increased, then starting torque increases but efficiency decreases.

Q.47. Which type of alternator is used in hydro-electric power stations?

Answer: Salient pole alternator is used in hydro-electric power stations.

Q.48. Where capacitor run motors are generally used?

Answer: These motors are used where the required starting torque is low such as air- moving equipment i.e. fan, blower and voltage regulators and also oil burners where quite operation is particularly desirable.

Q.49. 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%.

Q.50. Define cross-magnetising.

Answer: Armature reaction in a d.c shunt generator, running at full load with the brushes not shifted from the geometrical neutral plane and saturation neglected, is cross magnetizing.