MCQs on Electrical Machines

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01․ A motor armature conductor of length 0.5 m is exposed to a flux density of 4 Wb / m2. Calculate the force experienced by the conductor, when the armature current is 5 Amp?
15 N.
20 N.
Can't be determined.
None of these.

The force experienced by a current carrying conductor placed within a magnetic field is given by F = BIL. ⇒ F = 4 X 5 X 0.5 = 10 N, which is not given in the options.

02․ In case of induction motors the ratio of length to pole pitch for minimum cost, is taken as
1.5 to 2.
2 to 3.
3 to 5.

In the design of an induction motor to incorporate the design feature of minimum cost, the ratio of core length of pole-pitch should be 1.5 to 2.

03․ The use of double squirrel cage winding on the rotor provides
low noise.
reduced starting torque.
a large starting torque.
higher efficiency.

Double squirrel cage winding provides larger rotor resistance than the single squirrel cage winding. If the rotor resistance is more, then the starting torque is also more. Hence, it provides the larger starting torque.

04․ Which of the following machine is invariably provided with forced ventilation?
Fractional horse power slow speed motor.
Fractional horse power high speed motor.
Hermetically sealed motor of refrigerator.
50 MW generator in a power plant.

In the industrial motors, if the motor or machines is fully sealed, then we have to provide forced ventilation system to dissipate the internally generated heat. Hence, the hermetically sealed motor of refrigeratoris invariably provided with forced ventilation.

05․ In induction motors, there is large decrease in maximum power factor when the dispersion coefficient is ________________.
remains same.
kept constant to certain value.

Dispersion Coefficient is defined as the ratio of magnetizing current per phase to ideal short circuit current per phase. Usually, if the Dispersion Coefficient is smaller, then the power factor is higher because both are inversely proportional. Hence, there is a large decrease in maximum power factor, when the dispersion coefficient is increased.

06․ In induction motor when the length of the air gap is increased, the magnetizing current of the motor increases while the short circuit current __________________.
kept constant to certain value.
remains unchanged.

We know that the increasing the air-gap increases the need for magnetizing current. So, both are proportional to each other. Hence, it compensates the value of the power factor of induction motor. During that time, the short circuit current is always same.

07․ Hunting phenomenon is observed in which motor?
Synchronous motor.
DC motor.
Induction motor.
Both A and C.

If the load connected to the motor is suddenly changed by a large amount, then rotor tries to retard to take its new equilibrium position. But due to inertia of the rotor, it cannot achieve its final position instantaneously. While achieving its new position due to inertia it passes beyond its final position corresponding to new load. This will produce more torque than what is demanded. This will try to reduce the load angle and rotor swings in other direction. So there is periodic swinging of the rotor on both sides of the new equilibrium position, corresponding to the load. Such oscillations of the rotor about its new equilibrium position, due to sudden application or removal of load is called swinging or hunting in synchronous motor.

08․ Reluctance and hysteresis motors are the types of which electrical motor?
1 - Φ synchronous motor.
3 - Φ induction motor.
Stepper motor.
DC motor.

Basically, the Reluctance and hysteresis motors are having the permanent magnet rotor. If we give the single phase power supply to the stator winding (internally, it has one main winding and auxiliary winding), then it tries to work under the principle of synchronous motor. That means the magnetic field from the rotor is magnetically coupled with the stator rotating magnetic field. Hence, the speed is constant at all time for a limited load. Still if we apply the load, then it tries to stop the rotor running. That is why those motors are called as the single phase synchronous motor.

09․ In most condition which frame of motor will be preferred?
Open type.
Totally enclosed fan coiled.
Drip proof.
Splash proof.

Basically, the totally enclosed type enclose with fan cooled system gives the complete protection for any kind of motor. That is why this system is mostly preferred for the selection of motor enclose.

10․ The wattage rating of a ceiling fan motor will be
200 - 250 W.
250 - 500 W.
50 - 150 W.
10 - 20 W.

The load consumption by a ceiling fan is not that much high. Basically, it consumes around 50 - 150 Watts of power. The power is varied for the different types of fans.