The basic construction difference between ac and dc motor is that dc motor has direct conduction current supply to rotor and ac motor has inductive supply to rotor- no direct conduct. A motor needs three basic things-A magnetic field, a current carrying conductor, harnessing the created force on current carrying conductor due to magnetic field in ways to create rotary motion hence generate torque.
On the other hand Ac (Induction) motor has- Stator that stays stand still and provides rotating field, rotor that rotates. Only AC is supplied to stator which generates rotating magnetic field, then by Faraday’s laws of electromagnetic induction this rotating magnetic field induce EMF in rotor circuit, thus produce current in it.
This no direct conduction between rotor and stator,applies only in case of induction motor. But for ac synchronous motor, separate dc power is supplied to rotor. AS rotor rotates, to supply dc power to it brush, slip ring arrangement is provided.
Advantage disadvantage of ac and dc motor.
Ac motor tend to be smaller, more reliable, and less expensive but run at a fixed speed that is determined by the line frequency. On the other hand DC motor have speed-control capability,which means that speed, torque, and even direction of rotation can be changed at any time to meet new conditions.
Also, smaller DC motors commonly operate at lower voltages (for example, a 12-V disk drive motor), which makes them easier to interface with control electronics.Disadvantage of dc motor is the mandatory use of brush and slip ring arrangement in DC motor has its own limitation.