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Wednesday, 5 October 2011

THREE PHASE INDUCTION MOTOR INTERVIEW Questions 3

What are the different Ratings used for Induction Motor?
The nameplate of a 3-phase induction motor provides the following information:
(i) Horsepower
(ii) Line voltage
(iii) Line current
(iv) Speed
(v) Frequency
(vi) Temperature rise
What do you mean by horsepower rating and does it shows motor synchronous speed?
The horsepower rating is the mechanical output of the motor when it is operated at rated line voltage, rated frequency and rated speed. Under these conditions, the line current is that specified on the nameplate and the temperature rise does not exceed that specified.
The speed given on the nameplate is the actual speed of the motor at rated full- load; it is not the synchronous speed.
Why double-cage construction is used in Motors?
In order to provide high starting torque at low starting current, double-cage construction is used.
As the name suggests, the rotor of this motor has two squirrel-cage windings located one above the other.
What is the function of outer winding of double cage construction?
The outer winding consists of bars of smaller cross-section short-circuited by end rings. Therefore, the resistance of this winding is high. Since the outer winding has relatively open slots and a poorer flux path around its bars so it has a low inductance. Thus the resistance of the outer squirrel-cage winding is high and its inductance is low.
What is the function of inner winding of double cage construction?
The inner winding consists of bars of greater cross-section short-circuited by end rings. Therefore, the resistance of this winding is low. Since the bars of the inner winding are thoroughly buried in iron, it has a high inductance [See Fig. (8.35 (ii))]. Thus the resistance of the inner squirrel- cage winding is low and its inductance is high.
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What do you mean by synchronous speed of a 3-phase induction motor ?
The speed at which the flux produced by 3-phase stator windings of induction motor rotates is called the synchronous speed of the motor. It is given by :
Ns = 120 f/p
where Ns= synchronous speed in r.p.m.
f = supply frequency in Hz
P = number of poles
Why is the field producing winding of a 3-phase induction motor made stationary ?
A 3-phase induction motor has two windings viz stator winding supported by the stationary part of the machine and the rotor winding placed on the rotor. So far as the basic operation of the motor is concerned, it makes no difference which winding is located on the stator. The machine will function equally well with field producing winding as either the stationary or rotating element. Making the field winding stationary element eliminates the use of slip rings and brushes and, therefore, will result in a very trouble-free construction.
why does the rotor of a 3-phase induction motor rotate in the same direction as the rotating field ?
When three-phase stator winding is fed from 3-phase supply, a rotating magnetic field is set up which cuts the rotor conductors. Since the rotor circuit is closed, currents start flowing in the rotor conductors. Now, rotor conductors are carrying currents and are in the magnetic field. Therefore, mechanical force acts on the rotor, tending to move it in the same direction as the stator field. The fact that the rotor is urged to follow the stator field (i.e. rotor moves in the direction of stator field) can be explained by Lenz's law. According to Lenz's law, the direction of rotor currents will be such so as to oppose the cause producing them. Now, the cause producing the rotor currents is the relative speed between the rotating field and station­ary rotor. Hence to reduce this relative speed, the rotor starts running in the same direction as the stator field and tries to catch it.
Why cannot 3-phase induction motor run at synchronous 'speed ?
The rotor follows the stator field. In actual practice, rotor, can never reach the speed of stator field (i.e. synchronous speed). If it did, there would be no relative movement between the stator field and rotor conductors and, therefore, no torque to drive the motor. Hence 3-phase induction motor can never run at synchronous speed.
Why is the air gap between the rotor and stator of a 3-phase induction motor kept as short as possible ?
The air gap between the rotor and stator of a 3-phase induction motor is made as small as possible in order that :
(i) mutual flux may be produced with a minimum exciting current.
(it) leakage reactance are as small as possible.
How is magnetizing current kept small in a 3-phase induction motor ?
The magnetizing current drawn by a 3-phase induction motor is very large (30-50% of full-load stator winding current) owing to the presence of air-gap between the stator and rotor of the



































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