(or)
b)A 4 pole three phase induction motor operates from a supply whose frequency is 50HZ Calculate
(i)the speed at which the magnetic field of the stator rotating
(ii)the speed of the rotor when the slip is 0.04
(ii)the frequency of rotor current when the slip is 0.03
(iv)the frequency of rotor current at stand still
Answers
Answer:
Electrical Machines ≫ Three Phase Induction Motor
Question:
A 4-pole, 3-phase induction motor operates from a supply whose frequency is 50 Hz. Calculate the speed of the rotor and frequency of the rotor currents when the slip is 0.04.
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Options:
N = 1450 rpm and frequency is 4 Hz
N = 1440 rpm and frequency is 48 Hz
N = 1460 rpm and frequency is 4 Hz
N = 1440 rpm and frequency is 2 Hz
Correct Answer: Option 4 (Solution Below)
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Solution:
Concept:
The synchronous speed is given by
Where,
f is the supply frequency in Hz or C/s
P is the number of poles
The induction motor rotates at a speed (Nr) close but less than the synchronous speed.
Slip of an induction motor is given by,
Where,
Ns is the synchronous speed
and Nr is the rotor speed
Rotor current frequency (fr) = (s)(f)
The slip is negative when the rotor speed is more than the synchronous speed of the rotor field and is in the same direction.
Calculation:
Given -
Number of poles (P) = 4
Frequency (f) = 50 C/s
Slip (s) = 4 % = 0.04
Now rotor speed is
Nr = Ns (1 – s) = 1500 (1 – 0.04)
Nr = 1440 rpm
Rotor current frequency (fr) = 0.04 x 50
fr = 2 Hz