Induction Motor Objective Questions With Explanation Part-01

1. Which of the following component is usually fabricated out of silicon steel
A. Bearings .
B. Shaft
C. Stator core
D. None of the above

Answer: C. Stator core

  • 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.

2. The frame of an induction motor is usually made of
A. silicon steel 
B. Cast iron
C. aluminium
D. bronze

Answer: B. Cast iron

  • The frame provides the structural support for all other motor components.
  • The frame must be made up of a 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 steel 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.

3. The shaft of an induction motor is made of
A. stiff
B. flexible
C. hollow
D. any of the above 

Answer: C. hollow

  • 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 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 shaft of the induction motor must be of the hollow type.

4. The shaft of an induction motor is made of
A. high speed steel
B. stainless steel
C. carbon steel 
D. cast iron

Answer: C. carbon steel 

The frame of an induction motor is made up of closed grained cast iron.
 The shaft of the Induction motor is made up of mild steel/carbon steel.

Various parts of the  induction motor are
  • Stator
  • Rotor
  • Fan
  • Bearing
  • It consists of a 3 phase winding with a core and metal housing.
  • Windings are such placed that they are electrically and mechanically 120° apart from in space.
  • The winding is mounted on the laminated iron core to provide a low reluctance path for generated flux by AC currents.
  • The air gap between the stator and rotor is uniform and symmetrical.
  • It is the part of the motor which will be in a rotation to give mechanical output for a given amount of electrical energy.
  • The rated output of the motor is mentioned on the nameplate in horsepower.
  • It consists of a shaft, short-circuited copper/aluminum bars, and a core.
  • The rotor is made of non-insulated copper conductors in rod or bar formats & short-circuited with end rings. So it is also called a short-circuited rotor.
  • Therefore in the case of the squirrel cage induction motor, it is not possible to add any external resistance in series with the rotor circuit for starting purposes.
  • The rotor core is laminated to avoid power loss from eddy currents and hysteresis.
  • It does not require a slip ring and brush assembly. 
  • Conductors are skewed to prevent cogging during starting operation and give a better transformation ratio between stator and rotor.
  • The fan is attached to the backside of the rotor to provide heat exchange, and hence it maintains the temperature of the motor under a limit.
Bearing and shaft assembly:
  • The bearings support the rotor.
  • It maintains the air gap and transfers the shaft load to the motor casing.
  • The rotor core-winding assembly is mounted over the shaft.
  • The shaft extends outside the casing and transfers the mechanical energy from the rotor to the load.
  • In case of maintenance of motor, Bearings require maximum attention as they are more exposed to wear & tear.
  • Bearings acts as a medium between stator body and rotor due to which rotor is able to rotate 
5. In an induction motor, no-load the slip is generally
A. less than 1% 
B. 1.5%
C. 2%  
D. 4%

Answer: A. less than 1% 

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 ns  = synchronous speed
nr  =  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 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 slightly lower speed.
  • At full rated load, slip varies from more than 5% for small or special purpose motors
  • For large motors, the slip is less than 1%  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.

6. In medium sized induction motors, the slip is generally around
A. 0.04%
B. 0.4%
C.  4%
D. 14%

Answer: C.  4%

  • A rotor of a three-phase induction motor rotates at a speed close to the synchronous speed but not equal to the synchronous speed.
  • The difference between the synchronous speed and the rotor speed is known as slip speed.
  • It is given by Slip speed = Ns - Nr
  • Slip is defined as the ratio of slip speed to the rotor speed.
  • In a three-phase induction motor, the slip is generally 2% to 5%.
  • Range of slip motoring mode is 0 < s < 1
  • Range of slip for generating mode is 1 < s < 2
  • In medium sized induction motors, the slip is generally around 4%

7. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to
A. reduce windage losses
B. reduce eddy currents
C. reduce accumulation of dirt and dust 
D. reduce magnetic hum

Answer: D. reduce magnetic hum

In Squirrel cage rotor, slots in lamination or rotor core is not made parallel to the rotor shaft. A slight angle is maintained due to some advantages. This is called the rotor Skew.

Rotor of an Induction Motor skewed due to following reason…
  • With the bar skewed, the amount of the bar cutting the field line grows continuously and the next bar starts cutting the field lines as the first finishes. Due to this, we get Uniform Torque.
  • To run quietly by reducing the magnetic hum , reduce rotor locking tendency. Rotor locking tendency occurs when rotor teeth remain directly under stator teeth thus they might be magnetically attracted.
  • Primarily to prevent the cogging phenomenon. It is a phenomenon in which, if the rotor conductors are straight, there are chances of magnetic locking or strong coupling between rotor & stator.
  • Increase effective Magnetic Coupling between Stator and Rotor Fluxes.
  • Rotor bars are skewed to prevent the cogging effect.
  • When an induction motor refuses to start even if the full voltage is applied to it, this is called cogging.
  • Starting torque of an induction motor depends on the product of the magnitude of stator and rotor current and sine of the angle between both.
  • If the conductors remain linear, the angle between stator and rotor current will be 180 degrees. As sin(180)=0, the starting resultant torque will be zero and thus motor will fail to start. This phenomenon is called cogging.
  • Skewing makes the rotor conductor longer with the reduced cross area. This increases the rotor conductor resistance hence starting performance and the torque of an induction motor are improved.

8. In case the air gap in an induction motor is increased
A. the magnetizing current of the rotor will decrease
B. the power factor will decrease
C. speed of motor will increase
D. the windage losses will increase 

Answer: B. the power factor will decrease

If the air gap of an induction motor increases,
  • The permeability of the magnetic circuit rotor-to-stator will decrease.
  • The magnetizing inductance of the motor thus decreases.
  • The magnetizing current will increase; This will cause a poor power factor at all loads.
  • The magnetic flux in the air gap will decrease and leakage fluxes will increase; This will cause a reduction in the maximum available torque.

9. Slip rings are usually made of
A. copper
B. carbon
C. phosphor bronze
C. aluminium

Answer: C. phosphor bronze

Slip Rings:
  • A wound rotor motor uses three slip rings, typically made of high-quality phosphor bronze, mounted on the shaft with brushes resting on them.
  • Each slip ring is connected to one of the three phases of rotor windings.
  • The slip ring brushes, made of graphite, are connected to a resistive device, such as a rheostat.
  • As the slip rings turn with the rotor, the brushes maintain constant contact with the rings and transfer the resistance to the rotor windings.
  • Slip rings are provided in a three-phase slip-ring induction motor to insert the external resistance in the rotor circuit of the motor.
  • The frame of an induction motor is made up of closed grained cast iron.
  •  The shaft of the Induction motor is made up of mild steel.

10. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be
A. 200 Hz
B. 2 Hz
C. 50 Hz
D. 0.2 Hz

Answer: B. 2 Hz

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

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