MCQs on Electrical Machines

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01․ What is the frequency of rotor current of 50 Hz induction motor operating at 4% slip?
1 Hz

Frequency of rotor current = Slip × supply frequency = 0.04×50 ⇒ 2 Hz

02․ A 3-phase 6 pole 50 Hz induction motor is running at 5% slip.What is the speed of the motor?
1000 rpm
900 rpm
950 rpm
850 rpm

03․ Which of the following is/are not a self starting motor?
3-phase induction motor
1-phase induction motor
both 1 and 2
none of the above

When a 1-phase supply is given to the stator containing 1-phase winding, a pulsating or alternating magnetic field is produced in the air gap which links with the rotor according to the mutual induction, produces emf and current as it is closed. The rotor also experiences a torque but does not start or rotate. This is analysed according to double field revolving theory. According to this theory a 1-phase alternating flux contains two components rotating at Ns in opposite directions in equal magnitudes. The forward flux rotating at Ns rotates the rotor at N with forward slip Sf producing a forward torque. Similarly the backward flux rotating at Ns rotates the rotor at -N with backward slip Sb producing a backward torque. Forward torque and backward torque are equal during starting. Therefore the net torque when the motor starts is zero and the motor is not self starting.

04․ The breakdown torque of a synchronous motor varies as
applied voltage V

Therefore torque T ∝ P As power varies as applied voltage V, torque of synchronous motor is also varies as applied voltage V.

05․ A synchronous motor is supplying a load at unity power factor. If the load on the motor is increased keeping its excitation and terminal voltage constant, the power factor
will remain the same
will become leading
will become lagging
none of the above

Power P = (EV/X) × sinθ P = VI cosθ When excitation and terminal voltage are maintained constant and load is increased, to get the constant power, power factor will decrease and become lagging power factor.

06․ In a synchronous motor electromagnetic power varies
directly as applied voltage
directly as square of applied voltage
inversely as applied voltage
none of the above

Power P = VI cosθ Therefore, the electromagnetic power varies directly as applied voltage.

07․ An alternator is feeding an infinite busbar. Its prime mover is suddenly shutdown. The alternator will
continue to work as alternator but the direction of rotation will reverse.
come to stand still.
continue to work as synchronous motor with same direction of rotation.
start working as induction motor.

In case of failure of prime mover of the generator and if the excitation is still present, then generator draws power from the other parallel generators and starts working as a motor with same direction. It will absorb power from the busbar.

08․ When the excitation of normally operating unloaded, salient pole synchronous motor suddenly gets disconnected, it continues to run as a
variable reluctance motor.
switched reluctance motor.
induction motor.
DC motor.

The torque developed in reluctance motor is due to variation in reluctance. It is similar to the operation of salient pole synchronous motor in unexcited condition. A salient pole synchronous motor is running at Ns, when its excitation is made zero, it runs at synchronous speed only. Due to reluctance torque, motor acts as a variable reluctance motor without any excitation.

09․ A 3- phase synchronous motor can operate over a wide range of power factors i.e. from lagging to leading power factor. This is achieved by
varying the speed.
changing the field excitation.
varying the applied voltage.
changing the load.

Synchronous motors having outstanding electrical features in this particular area. The motor will act as a resistor or a pure inductor or a pure capacitor with varying excitations. Therefore, by varying excitation we can operate the motor at different power factors as follows,
  1. Critical excitation - unity power factor
  2. Under excitation - lagging power factor
  3. Over excitation - leading power factor

10․ Which of the following is not a necessary condition to be satisfied for synchronising an incoming alternator to an already operating alternator?
same voltage magnitude
same frequency
same prime mover speed
same phase sequence

The conventional power generation is through synchronous generators which are operating in parallel across the common terminal known as bus bar. The necessary conditions are 1. Same voltage magnitude 2. Frequency should be same 3. Same phase sequence