1. The frame of an induction motor is usually made of
(A) Silicon steel(B) Cast iron
(C) Aluminum
(D) Bronze
Answer (B) Cast Iron
Explanation:
The frame provides the structural support for all other motor components.
The frame must be made up of the 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.
2. In an induction motor, no-load the slip is generally
(A) Less than 1%(B) 5%
(C) 2%
(D) 4%
Answer : (A) Less than 1%
Explanation:
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.
3. The shaft of an induction motor is made of
(A) Stainless steel(B) Carbon steel
(C) Cast iron
(D) Aluminium
Answer : (B) Carbon Steel
Explanation:
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 must be of
(A) Solid(B) Hollow
(C) Flexible
(D) Any of the above
Answer : (B) Hollow
Explanation:
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.
The advantage of hollow shafts:
High tolerance to the rotational imbalance due to low weight.
Low rotational inertia
Lightweight comparable to the solid shaft of the same diameter
Due to low specific mass hollow shafts can tolerant to high-temperature
Cheaper in cost
5. Which of the following component is usually fabricated out of silicon steel?
(A) Stator core(B) Bearings
(C) Shaft
(D) None of the above
Answer : (A) Stator core
Explanation:
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.
6. Slip ring of an induction motor is usually made up of
(A) Aluminium(B) Copper
(C) Phosphorus Bronze
(D) Carbon
Answer: (C) Phosphorus Bronze
Explanation:
A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure.
It is used in case of AC supply, where AC is required.
With slip ring, the voltage in the external circuit varies like a sine wave and the current alternates the direction.
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 provide good electrical conductivity and low thermal conductivity
Since slip ring induction motors used to accelerate heavy loads over a 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 4% slip. The frequency of rotor current will be
(A) 50 Hz(B) 25 Hz
(C) 5 Hz
(D) 2 Hz
Answer : (D) 2 Hz
Explanation:
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
(A) Full-load torque
(B) Slightly more than full-load torque
(C) Low
(D) Negligible
Answer : (C) Low
Explanation:
Starting torque is directly proportional to the rotor resistance of an induction motor.
The resistance of squirrel-cage induction motor can not be varied as compared to the slip ring induction motor.
In the 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
(A) 100%(B) 80-90%
(C) 50-60%
(D) Less than 50%
Answer: (B) 80-90%
Explanation:
Induction motors are often compared to Transformers because both the machines work under the principle of “mutual induction”. An induction motor is also called as 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
(A) Two series winding in stator(B) Two parallel windings in stator
(C) Two parallel winding in Rotor
(D) Two rotors moving in opposite direction
Answer: (C) Two parallel winding in Rotor
Explanation:
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 have 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 high cross-section area and it is made up of low resistance material like copper.
The upper and lower cage bar is separated by 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 starting period, it gave high resistance means high starting torque whereas it gives low resistance during running condition.