I found synchronous motor used as simple AC squirrel cage induction motor. the rotor of syn motor got damaged. I replaced the rotor by an equivalent rotor of squirrel cage induction motor. To my surprise I found motor drawing no load current as 60-65% of its rated current. What must have gone wrong?
Because, not all the energy supplied to the stator is available to the rotor. The stator core, the leakage inductance and the rotor core consume quite an amount of energy which is called Reactive Power or Reactive energy. In Synchronous motors, this is overcome by supplying the reactive power required, separately from an excitation system. And, thus, a synchronous motor can rotate at synchronous speed. As this external excitation cannot be given in an induction motors - (it would be more appropriate to say, Aysnchronous Induction Motor, as Synchronous Motors are also Induction Motors, in fact) - they can never achieve synchronous speed.
The rotor of an induction motor is not identical to that of a synchronous motor.As you can see for a 2pole synchronous motor the rotation is 3600 rpm while an induction motor has a speed of 3540 rpm only at full load.The number of conductors in the induction motor rotor may be lesser than that of the synchronous motor.
If you fit an armature from an induction motor into the stator of a synchronous motor with the same number of poles, then you will end up with an induction motor (which will run always slower than a synchronous motor; about 2800 rpm as opposed to 3000). But if the number of poles does not match, then you will end up with a rather inefficient machine. Moral of story: Replace a faulty part with the proper part.
