Interview Questions - Electrical Machines

Why the harmonic restraint feature is incorporated in transformer protection?
In transformer protection, harmonic restraint feature is incorporated to guard against magnetizing inrush current.

How no-load rotational losses in electrical machines can be determined?
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 the no-load rotational loss.

What do you mean by no-load rotational loss?
The mechanical loss and open circuit core loss combined together are caused by rotor rotation. Their sum is called as no-load rotational losses.

What constitutes mechanical loss and open circuit core loss?
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 the 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.

What is the condition for the maximum efficiency of a transformer?
Maximum efficiency occurs when the variable ohmic loss is equal to the fixed core loss.

What are the conditions for successful parallel operation of single-phase transformers?
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 the same to avoid the 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 the same for all the transformers to avoid operation at different power factors.
How the direction of rotation of a dc series motor can be reversed?
The direction of rotation of a dc series motor can be reversed either by interchanging supply terminals or by interchanging field terminals.

How a single-phase induction motor can be classified?
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.
Why the tappings are normally provided on the high voltage winding of a transformer only?
Tapping is normally provided on the high voltage winding of a transformer only because it has a larger number of turns, has to handle low currents and is also easily accessible physically.

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

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