MCQs on Electrical Machines

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01․ Which type motors are preferred for lathes?
DC shunt motors.
Squirrel Cage induction motor.
Synchronous motor.
Either A or B.

Lathes machines requires uniform torque. Hence squirrel cage induction or DC shunt motors are preferred for lathes.

02․ In the 3-phase induction motor, the resultant flux is of a constant nature and is
equal to φm, where φm is the maximum flux due to any phase
3/2 times maximum value of flux due to any phase
1/2 times maximum value of flux due to any phase
3 times maximum value of flux due to any phase

Due to the interaction of 3-phase flux, the net flux is continuously rotating at a certain speed with a constant flux value which is called rotating magnetic field. This is produced without the physical rotation of the stator poles and the resultant flux is equal in magnitude and rotating in the space axis. Resultant flux = 3/2 times maximum value of flux due to any phase for 3-phase system as is evident from the diagram shown below.

03․ The power factor of an induction motor operating at no load will have a value around
0.9 lag
0.2 lag
0.2 lead

Induction motor is use mostly in industries and mostly loads are inductive in nature so that induction motors operate at lagging power factor. At no load most of the power goes to armature loss and windage loss, they are fixed losses, giving very small useful watt component. So, at no load the power factor is very low near to 0.2 lag.

04․ The speed of a 3-phase induction motor operating in its stable region
increases with increase in the load torque.
decreases with increase in the load torque.
decreases with decrease in the load torque.
remains constant.

When the motor is in stable region, the power delivered is constant. Therefore speed of induction motor is inversely proportional to the load torque.

05․ In a 3-phase induction motor, torque and supply voltage are related as
T ∝ V
T ∝ 1/V
T∝ V²
T ∝ 1/V²

06․ Semi closed slots or totally closed slots are used in induction motors, essentially to
improve starting torque
increase pull out torque
decrease starting torque
reduce magnetising current and improve power factor

In semi closed and closed slots, the air gap between the stator and rotor is small as compared to open slots. As the air gap is small, the requirement of magnetising current to establish the flux in the air gap is less. This will improve the power factor.

07․ If an induction machine is run at above the synchronous speed it acts as
an induction generator
an induction motor
a synchronous motor
none of the above

Induction motor acts as generator when its speed increases than the speed of its stator rotating magnetic field. If a prime mover is connected to the shaft and the stator is connected to ac supply, when it is rotated at a speed greater than the speed of rotating magnetic field, slip becomes negative(-1 < S < 0), torque becomes negative along with the rotor current and power. Therefore motor starts supplying current or power to the supply lines where it is connected.

08․ Which type of motors uses slip rings?
DC Shunt Motor
Synchronous motors
Both Synchronous and Induction motor
Induction motors

Slip rings are used in induction motors to increase the starting torque. A suitable value of external resistance is added equally into the rotor 3-phase winding through stationary sliding contacts known as brushes which are placed on the slip rings. The motor is started with suitable external resistance which produce the high starting torque, when the motor accelerates or under running condition, the brushes are lifted and the slip rings are joined together in order to form a closed circuit through metal collar arrangement. Under running condition slip ring induction motor is equivalent to squirrel cage induction motor. In Synchronous motor the field winding is connected in series with the slip rings to connect with a DC exciter.

09․ The rotor slots are slightly skewed in squirrel cage induction motor to
provide ease of fabrication.
reduce the magnetic hum and locking tendency of rotor.
increase the strength of rotor bars.
decrease the amount of copper to be used.

In squirrel cage induction motor there are no slots which are projected like in slip ring induction motors. Squirrel cage induction motors are not designed for any number of poles on the rotor. As the poles are automatically formed in equal number, due to low starting torque, if the rotor and stator poles are quite horizontal to each other a strong alignment force (magnetic attraction) occurs. Due to this locking tendency the rotor refuses to start which is called cogging. To eliminate this effect squirrel cage rotor slots should be angled or skewed with respect to stator slots.

10․ The armature resistance of a permanent magnet dc motor is 0.8 Ohm. At no load, the motor draws 1.5 A from a supply voltage of 25 V and runs at 1500 rpm. The efficiency of the motor while it is operating on load at 1500 rpm drawing a current of 3.5 A from the same source will be

Since the machine is in no load condition the no load power constitutes the friction and windage losses.