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(a) Induction motor does not rotate at synchronous speed
The rotor of a induction motor never succeeds in catching up with stator field. If it
really did, there will be no relative speed between stator and rotor field. Hence, no
rotor e.m.f, no rotor current. Therefore, no rotor torque to maintain rotation. That is
why rotor runs at speed lower than that of stator and stator field runs at Ns speed.
Ns = 120 f /p and rotor speed N = Ns (1-s)
(b) Capacitor-start single phase induction motor
The necessary phase difference between Is and Im is produced by connecting a
capacitor in series with starting winding. The Capacitor is generally electrolytic type
and its usually mounted on the outside of motor. The capacitor is designed for short
duty service and when motor reaches 75% of full load speed. The centrifugal switch
‘S’ opens and cuts out starting winding and capacitor from supply only main winding
gregorymasila answered the question on February 22, 2018 at 16:34
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