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Three phase synchronous motor will have
No slip ring
One slip ring
Two slip rings
Three slip rings
Two slip rings
A three-phase synchronous motor requires a constant DC supply to its rotor field windings to create a stationary magnetic pole system. This excitation current is provided through two slip rings mounted on the rotor shaft, connected to brushes which carry the DC supply from an external source.
A three-phase synchronous motor requires a constant DC supply to its rotor field windings to create a stationary magnetic pole system. This excitation current is provided through two slip rings mounted on the rotor shaft, connected to brushes which carry the DC supply from an external source.
NsтАЛ=P120fтАЛ тАФ Synchronous speed formula where f is frequency and P is poles
IexтАЛ=RfтАЛVdcтАЛтАЛ тАФ Field excitation current calculation
The synchronous motor operates by the interaction between the stator's rotating magnetic field and the rotor's DC-excited magnetic poles. The stator is fed with a three-phase AC supply, while the rotor is fed with a DC supply via two slip rings and brushes. The constant polarity of the rotor poles allows them to lock onto the rotating magnetic field, enabling the motor to run at synchronous speed (NsтАЛ=P120fтАЛ).
The rotor of a synchronous motor is an electromagnet requiring external DC power.
Slip rings facilitate the transfer of electrical power from a stationary source to a rotating shaft.
Synchronous motors are 'doubly fed': AC on stator, DC on rotor.
The number of slip rings is determined by the need to complete the DC circuit loop.
Constant speed operation irrespective of load.
Can be used for power factor correction (over-excited mode).
Not self-starting (requires auxiliary starting method).
Requires complex DC excitation system.
Large industrial compressors.
Power factor improvement banks.
Brushless synchronous motors replace slip rings with a shaft-mounted exciter (rectifier bridge) to eliminate maintenance.
Slip rings are not used in squirrel-cage induction motors, which is why Option A is incorrect for this machine type.
C is correct тАФ A three-phase synchronous motor requires two slip rings to provide a DC path for the rotor field windings.
Always remember that slip rings are for excitation or rotor resistance control; if you see '3-phase', it refers to the stator supply, while the slip rings refer to the rotor interface.