# 100 Most Important 3-phase Induction Motor MCQ with Explanation| Polyphase induction motor MCQ With Explanation|objective type question of an induction motor

#### 1. The frame of an induction motor is usually made of

Correct! Wrong!

• The frame provides the structural support for all other motor components.
• The frame must be made up of rigid material that can absorb vibration as well as noise in the induction motor.
• The frame of the induction motor is made either of welded sheet steelÂ or fabricated steel, cast iron or cast aluminum alloy.
• The material is selected based on cost, heat transfer characteristics and manufacturing capability considering the size of the machine.
• The frame of small AC induction motor is made up of cast iron but the frame of medium size induction motor is made up of welded sheet steel.

#### 2. In an induction motor, no-load the slip is generally

Correct! Wrong!

• Slip, s, is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm, or in percentage or ratio of synchronous speed.

s = ns - nr/ns

where

nsÂ  = synchronous speed

nr= Â running speed

• The torque developed by an induction motor is proportional to the slip.
• At synchronous speed, the slip is zero and hence torque developed by the motor is zero.
• In the case of an ideal motor having zero friction and zero losses, a motor can rotate at synchronous speed, because there is no need for a motor to develop any torque to sustain rotation.
• But for the practical motor, there will be some friction and other losses, which the motor has to overcome by developing some torque. And to develop that torque, the motor must have slip. Therefore Motor can not run at synchronous speed but at a slightly lower speed.
• At full rated load, slip varies from more than 5% for small or special purpose motors
• For large motors, the slip isÂ less than 1% Â because an induction motor consists of short circuit rotor winding having small resistance therefore small slip will induce the large current in the rotor which can produce sufficient torque.

#### 3. The shaft of an induction motor is made of

Correct! Wrong!

• A shaft is a metallic, rotating and oscillating rod that rotates continually to transfer movements or torque in the machine.
• Shafts transfer movement and torque in machine hence they are subject to torsion and shear stress, therefore they must be strong enough to bear the stress.
• For an Induction motor shaft, we need a material with high torsional rigidity as well as material that can take a lot of stress.
• Carbon steel is the appropriate choice for making the shaft of an induction motor.
• Steel is a mixture of iron and carbon. The more we add carbon in the steel, the more it becomes harder and stronger.
• The hollow shaft can resist more torsion load with the same resisting area (same weight) as compared to the solid shaft. Therefore the shaft of the induction motor must be of the hollow type.

#### 4. The shaft of an induction motor must be of

Correct! Wrong!

• The hollow shaft can resist more torsion load with the same resisting area (same weight) as compared to the solid shaft. Therefore the shaft of the induction motor must be of the hollow type.
• The advantage of hollow shafts:
1. High tolerance to the rotational imbalance due to low weight.
2. Low rotational inertia
3. Lightweight comparable to the solid shaft of the same diameter
4. Due toÂ  low specific massÂ hollow shafts can tolerant to high-temperature
5. Cheaper in cost[/bg_collapse]

#### 5. Which of the following component is usually fabricated out of silicon steel?

Correct! Wrong!

• The stator core is made of silicon because it possesses high magnetic permeability and low magnetic hysteresis.
• Adding silicon to steel increase its resistivity and strength and it is easier to magnetize silicon added steel.
• It is laminated to reduce eddy-current losses.[/bg_collapse]

#### 6. Slip ring of an induction motor is usually made up of

Correct! Wrong!

• A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure.
• Phosphor bronze is an alloy of copper, tin, and phosphorus. The tin increases the corrosion resistance and strength of the alloy. The phosphorus increases the wear resistance and stiffness of the alloy.
• Phosphorous bronze provideÂ good electrical conductivity and low thermal conductivity
• Since slip ring induction motors used to accelerate heavy loads over a Â long period of time and to withstand excessive heat phosphorous bronze slip is used.

#### 7. A 3-phase 440 V, 50 Hz induction motor has a 4% slip. The frequency of the rotor current will be

Correct! Wrong!

• Frequency of the rotor current is given as fr = sf fr= 0.4 x 50 = 2 Hz

#### 8. The starting torque of a squirrel-cage induction motor is

Correct! Wrong!

• Starting torque is directly proportional to the rotor resistance of an induction motor.
• The resistance of the squirrel-cage induction motor can not be varied as compared to the slip ring induction motor.
• In theÂ squirrel-cage induction motor, the rotor has very low fixed resistance, therefore, the starting torque is low due to low power factor and high reactance.
• The starting torque could be increased by increasing starting resistance but due to high current in the rotor during starting will increase the copper loss which will decrease the motor efficiency.

#### 9. The efficiency of an induction motor is about

Correct! Wrong!

• Induction motors are often compared to Transformers because both the machines work under the principle of "mutual induction".Â An induction motor is also called a rotating transformer with secondary winding short-circuited.
• As we know that the transformer's efficiency is greater than 95%.
• An Induction motor includes friction and windage(i.e., mechanical losses) losses addition to the no-load and copper losses, hence efficiency comes down to 80% to 90%.

#### 10. A double squirrel-cage induction motor has

Correct! Wrong!

• In squirrel cage motor there is no provision made for adding external resistance because the end rings are permanently shorted with the rotor conductor, therefore, the starting torque of squirrel cage induction motor is very poor due to low rotor resistance.
• The starting torque could be increased by increasing starting resistance but due to high current in the rotor during starting will increase the copper loss which will decrease the motor efficiency.
• As the name suggests the double squirrel-cage induction motor has two sets of parallel squirrel-cage winding in the same rotor.
• The outer cage bar has a smaller cross-section area as compared to inner cage bar
• The outer cage bar is made up of high resistive material such as aluminum, brass, etc.
• The outer cage has less leakage flux linkage because of their relatively open slots hence it has lower reactance.
• The Lower cage bar has a high cross-section area and it is made up of low resistance material like copper.
• The upper and lower cage bar is separated by a narrow slit or construction.
• The inner cage has high leakage flux linkage due to the presence of slit hence it has high self-inductance.
• Therefore during the starting period, it gave high resistance means high starting torque whereas it gives low resistance during the running conditions.

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3. vignesh says:

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4. vignesh says:

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Email dharmkumar2268@gmail.com

Question no 5 .answer no 2 point Adding silicon to steel increase its resistivity ,this line is correct or not Adding silicon to steel increase its permeability

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13. pratyusha says: