The correct answer is: A. no slip-rings.
A slip ring is a type of electrical contact that allows for the transfer of power and/or signals between a rotating and stationary part of a machine. Slip rings are typically used in three-phase induction motors, which require a rotating magnetic field to operate. The rotating magnetic field is created by three windings that are arranged in a star or delta configuration. The windings are connected to slip rings, which are mounted on the shaft of the motor. The slip rings allow current to flow from the stationary stator windings to the rotating rotor windings.
Synchronous motors, on the other hand, do not require a rotating magnetic field to operate. Instead, they use a permanent magnet rotor that is aligned with the stator field. This means that synchronous motors do not need slip rings, and they are therefore more efficient than induction motors.
Here is a brief explanation of each option:
- Option A: no slip-rings. This is the correct answer, as synchronous motors do not require slip rings to operate.
- Option B: one slip-ring. This is incorrect, as synchronous motors do not have slip rings.
- Option C: two slip-rings. This is incorrect, as synchronous motors do not have slip rings.
- Option D: three slip-rings. This is incorrect, as synchronous motors do not have slip rings.