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Solved Problems on 3 φ Induction Motor - Electrical Engineering
By Prof Sunil Kumar, Electrical Engineering, IEC College, Greater Noida, Uttar Pradesh

Ques1: A 3 φ 4 pole 50 hz induction motor runs at 1460 r.p.m. find its %age slip.
Solution
N s = 120f/p = 120*50/4 = 1500r.p.m.
Running speed of motor = n= 1460r.p.m.
Slip S=( N sN)/ N s*100 =(1500-1460) x 100 / 1500 = 2.667%



Ques2: A 12 pole 3 φ alternator driver at speed of 500 r.p.m. supplies power to an 8 pole 3 φ induction motor. If the slip of motor is 0.03p.u, calculate the speed.
Solution
Frequency of supply from alternator, f=PN/120
=12*500/120 = 50hz

where P= no of poles on alternatev
N=alternator speed is r.p.m.
Synchronous speed of 3 φ induction motor
N=120f/Pm
=120*50/8 = 750 r.p.m.
Speed of 3 φ induction motor N=Ns (1-s)
=750(1-0.03) = 727.5 r.p.m.

Next - Ques 3-4



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