why ac superconducting motors are common and not dc?
It's about the superconducting DC homopolar motor. Always a solution I guess.
Certainly a DC motor made of superconductors is possible. It's true the motor itself wouldn't be superconducting when doing work, but it would be 100% efficient since there is no resistive loss. As many DC motor designs do, it would generate a back EMF when rotating. The power source would have to overcome this EMF when providing current. (Thus there would be no free lunch.) Such a motor is described in the reference. EDIT: I have to disagree with a statement in the answer following this one. A DC motor is not simply an AC motor with a commutator; it has a DC stator field. AC motors have a rotating stator field.
A direct current (dc) motor is nothing more than an alternating current (ac) motor with a commutator. It is far more efficient to electronically convert dc to ac to drive an ac motor than to use a dc motor for high power applications. - An old electric power engineer.
Its so simple AC is cheap.
