i want to know What is the difference between synchronous and asynchronous motor?
A synchronous motor runs at a speed which is synchronised to the frequency of the AC voltage that supplies it. For example if the motor is supplied at a frequency of 60Hz then it will run at either 60, 30, 20 etc revs/second, depending on the number of magnetic poles (2, 4, 6 respectively) that it has (by design).
An asynchronous motor generally runs slower than the above with a speed that usually varies somewhat with the amount of mechanincal load that it is driving.
Induction motors are asynchronous because the motor torque is due to the stator magnetic field interacting with currents induced in the squirrel cage (of copper bus bars) in the rotor. In order to induce currents in the squirrel cage, the rotor has to slip with respect to the synchronous frequency. If you have never seen a squirrel cage, take a 1/4 HP induction motor apart and pull the rotor out.
may dc motor can be asynchronous??
Most dc motors, including both brush motors (series connected, shunt connected, permanent magnet) and brushless motors are asynchronous, in part because there is no periodic time signal. There are many types of stepping motors that are inherently synchronous to a clock. Also, Hall-Effect sensor-type brushless motors can be synchronized to an external clock. Other dc motors can be controlled by feeding back an RPM signal to a controller.
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