MCQs on Electrical Machines

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01․ Dynamic braking is very effective for
shunt motors.
separately excited motors.
series motors.
differential compound motors.

Dynamic braking is very effective for separately excited motors. As in separately excited motors the direction of field can be very easily altered by altering the terminals of the field.

02․ Dynamic braking is employed to brake
non reversing drive
reversing drive
both 1 and 2
none of the above

Dynamic braking is employed to brake both reversing drives and non reversing drives.

03․ DC machine is a
conduction machine.
convection machine.
both are correct.
none of above are correct.

DC machine has direct contact to conductor and controlling of current is also direct. Hence it is called as conduction machine.

04․ Generators work on the principle of production of
dynamically and statically induced emf.
statically induced emf.
none of the above.
dynamically induced emf.

DC generator and AC generator works on dynamically induced emf. Transformer works on static induced emf.

05․ Induction motor operation depends on
rotating magnetic field.
stationary magnetic field.
either of these.
none of the above.

According to Faraday's law, when a current carrying conductor is placed in magnetic field it will experience a mechanical force. To get the mechanical force, a stationary conductor should cut the rotating magnetic field. In order to produce rotating magnetic field (RMF), it requires a 3-phase symmetrically distributed winding and a 3-phase supply to be given to it. The 3-phase currents produce 3-phase flux in the 3-phases and they interact and produce a net flux which is a constant but changes its axis, which is called as rotating magnetic field (RMF).

06․ Find the number of poles required, when the frequency is 50Hz and speed of the motor is 500 rpm?

Synchronous speed is defined as the speed with which the rotating magnetic field rotates for a standard frequency and for a given number of poles.

07․ The sequence of induction motor is RYB, then the direction of induction motor can be changed by which of the following sequence?
All of the above

The flux always moves from a leading phase to lagging phase. The direction of induction motor depends on rotating magnetic field direction. Therefore by interchanging any two phases, (but not all at the same time) direction of rotating magnetic field can be reversed, and this will reverse the direction of induction motor.

08․ Which of the following pairs is/are correct?
open slot----- leakage flux is less
closed slot----- leakage flux is high
semi closed slot-----leakage flux is moderate
All of the above

Leakage flux in machine will depend on the air gap between stator and rotor. If air gap is more leakage flux is less and vice versa. In open slot type machines air gap between stator and rotor is more i.e. reluctance is high. Therefore leakage flux is less. In closed slot type machines air gap between stator and rotor is less i.e. reluctance is low. Therefore leakage flux is high. In semi closed slot type machines air gap between stator and rotor is moderate i.e., reluctance is moderate. Therefore leakage flux is moderate. So, all the combinations are true.

09․ Which of the following machines have highest power factor with respect to magnetizing current?
Open slot type machines.
Closed slot type machines.
Semi closed slot type machine.
Any of the above.

Operating power factor will depend on the air gap between the stator and rotor. If the air gap is more it will require more amount of magnetizing current, this will decrease the operating power factor and vice versa. In open slot type machines air gap between the stator and rotor is more. Therefore operating power factor is poor. In closed slot type machines air gap between the stator and rotor is less. Therefore operating power factor is more. In semi closed slot type machines air gap between the stator and rotor is moderate. Therefore operating power factor is also moderate. Therefore the operating power factor in closed slot type machines is more than the others.

10․ Which of the following induction motor has the highest starting torque?
Squirrel cage induction motor.
Slip ring induction motor.
Same in both induction motors.
None of the above.

Depending on the type of rotor used, induction motors are classified as squirrel cage induction motor and slip ring induction motor. The squirrel cage induction motors have low starting torque. For high starting torque requirement slip ring induction motors, also called as wound rotors, are used. In slip ring induction motor, the rotor core consists of slots which contain 3- phase winding similar to stator. The rotor winding is essentially connected in star and the three terminals end at 3 slip rings which are mounted on the shaft. A suitable value of external resistance is added equally onto the rotor 3-phase winding through stationary sliding contacts known as brushes which are placed on the slip rings. When the motor is started due to external resistance it produces the high starting torque. Therefore the starting torque is directly proportional to resistance value, which will increase the starting torque of slip ring induction motors.