Answer / sanjeet singh

if we talk about the single phase induction motor.they are not self start because no motor can run on single phase.so to start such motors we required an auxiliary winding ,which providing the another phase to starting a single phase motor.such as,in a ceiling fan a capacitor providing the starting phase. so single induction motor is not self start.

Answer / yogesh

Single phase induction motor or synchronous are motor not self starting motors. In case of 3 phase, induction motor is self starting one. Synchronous motor is not a self starting one. Reason: if 3 phase supply is applied to stator, the rotating magnetic field(RMF) is produced with sync speed which will induce the emf on the rotor and produce flux. The rotor flux interacts with stator flux and rotor will rotate to catch the speed of stator(rotating magnetic field speed i.e. Sync speed). In sync motor the rotor is either in salient or non salient pole. If 3 phase supply is given to stator, RMF is prodused. Apply DC supply to the rotor(LET Salient pole rotor with 2 poles). Now N , S poles are induced in rotor. If at one instance stator N pole is nearer to rotor S pole, at that time the rotor tries to move in clock wise direction (for eg.). Suddenly the stator poles(rmf) will move to the another portion of the stator. Let stator S Pole is nearer to rotor S pole, at that time rotor will try to move in anti clk wise direction because stator N pole is at exactly in opposite direction to that of rotor axis. Since RMF is rotating in sync speed the rotor can not move in both clk wise and anti clk wise direction. The result is both clk and anti clk torques get cancelled each other. The rotor does not rotate. To make the sync motor self starting we can provide damper windings on the pole faces of rotor that could act as induction motor at starting then after running of the rotor switch on the DC supply to rotor. Now magnetic locking takes place between stator poles and rotor poles. Now the motor runs in sync speed.

why the rotor is outside in case of ceiling fan?

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