MCQs on Electrical Machines

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01․ The noise resulting from vibrations of lamination set by magnetic forces, is termed as

The hum is generated by the magnetic field that happens due to the continuous reversing of the frequency of the supply or it is due to magnetostriction. The magnetic field in the AC machine or transformers has coils which are still able to move slightly due to the vibration. The laminations of the armature are treated in a similar way but also vibrate at the line frequency and it is almost impossible to stop. We can only reduce it by good design.

02․ Autotransformer makes effective saving on copper and copper losses, when its transformation ratio is equal to
very low
less than one.
greater than one.
approx to one.

To quantify the saving the total quantity of copper used in an auto transformer is expressed as a fraction of that used in a two winding transformer as, This means that an auto transformer requires the use of lesser quantity of copper given by the ratio of turns. Hence, if the transformation ratio is approximately equal to one, then the copper saving is good and the copper loss is less.

03․ Counter emf of a DC motor
exceeds supply voltage
aids applied voltage.
helps in energy conversion.
regulates its armature voltage.

When the motor armature continues to rotate due to motor action, the armature conductors cut the magnetizing flux and therefore emf are induced in them. This emf is known as counter emf or back emf. This back emf or counter emf makes the DC motor a self-regulating machine i.e. it makes the DC motor to draw armature current which is just enough to develop the required load torque. Thus it helps in energy conversion and it is always less than applied voltage.

04․ The armature of DC motor is laminated
to reduce the mass.
to reduce the inductor.
to reduce the eddy current loss.
to reduce the hysteresis loss.

The armature is of cylindrical or drum shaped made with high-grade silicon steel in form of stamping or lamination. By using laminations, the path of eddy currents is divided into several small loops and hence area of the eddy current loops is minimized. So the laminations reduce the eddy current losses.

05․ Width of a carbon brush should be equal to
less than the width of commutator segment.
the width of the 1 to 2 commutator segment.
the width of the 2 to 3 commutator segment.
the width more than 3 commutator segment.

Width of carbon brush should be equal to the width of the 2 to 3 commutator segments. With only one coil undergoing commutation and width of the brush equal to one segment width, the reactance voltage, and hence the sparking, increases as the slot width decreases. Hence the brush width is made to cover more than one segment. However, if the brush is too wide, then those coils which are away from the commutating pole zone or coils not coming under the influence of inter pole flux and undergoing commutation leads to more sparking. Hence, brush width greater than the commutating zone width is also not advisable.

06․ Rotating part of DC motor is known as
Carbone brush.

Armature is rotating part of a DC motor and is built up in a cylindrical or drum shape. It is also known as house of conductors.

07․ Speed of DC motor can be controlled by varying
applied voltage.
its flux oer pole.
resistance of armature culture.
all of these.

Speed of a DC motor is expressed as, So, speed can be controlled by adjusting any one of the three factors appearing on right hand side of expression -
  1. applied voltage to the armature circuit
  2. flux per pole
  3. resistance of the armature circuit

08․ Eb/Vt ratio of a DC motor is an indication of
speed regulation.
starting torque.
running torque.

Electrical motor efficiency is defined as the ratio of mechanical power developed to the total electrical power input. In case of DC motor total electrical power input = Vt × IL. Where Vt is supply voltage and IL = Load current. The mechanical power developed = Ta × ω. Where, Ta = Torque in armature and ω is the speed of rotation in rad/sec. So mechanical power in armature = Eb × Ia. So efficiency is given by

09․ In DC shunt motor if load is increased, the speed
increased slightly.
reduce slightly.
remains constant.
increase proportional.

In case of DC shunt motor, the flux per pole is considered to be constant, torque increases with the increase of load current. If the load current increases then the armature current also increases and the speed slightly falls due to the increase in voltage drop in armature.

10․ A large series motor is never started without some mechanical load on it because otherwise it will
produce sparking at brushes.
open fuse or circuit breaker.
draw too much current.
develop excessive speed and damage itself.

When the motor is connected across the supply mains without load, it draws small current from the supply mains which flows through the series field and armature. The speed tends to increase so that back emf may approach the applied voltage in magnitude. The increase in back emf weakens the armature current and hence the field current. This again causes an increase in speed and in back emf. Thus the field continues to weaken and speed continues to increase until the armature produces such centrifugal force that it may come out from its shaft and gets damaged.