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Slip ring of synchronous motor carry
DC current
AC current
No current
None of above
DC current
In a synchronous motor, the rotor is energized by an external DC supply to create a constant magnetic field, which is necessary to lock the rotor at synchronous speed with the rotating magnetic field of the stator. The slip rings serve as the sliding contacts that transfer this DC excitation from the stationary source to the rotating rotor field winding.
In a synchronous motor, the rotor is energized by an external DC supply to create a constant magnetic field, which is necessary to lock the rotor at synchronous speed with the rotating magnetic field of the stator. The slip rings serve as the sliding contacts that transfer this DC excitation from the stationary source to the rotating rotor field winding.
VfтАЛ=IfтАЛRfтАЛ тАФ Voltage drop across the rotor field winding for DC excitation
NsтАЛ=P120fтАЛ тАФ Synchronous speed equation showing the relationship between frequency and poles
The DC supply is connected to the stationary brushes, which ride on the rotating slip rings mounted on the motor shaft. Because the rotor requires a unidirectional (constant) magnetic polarity to interact with the stator's alternating flux, DC current is continuously fed into the rotor field windings through these slip rings.
Synchronous motors require a constant DC magnetic field for the rotor.
Slip rings provide a sliding contact mechanism for power transfer to rotating parts.
The stator receives 3-phase AC, while the rotor receives DC excitation.
In brushless synchronous motors, slip rings are replaced by a rotating exciter and rectifier.
Allows for independent control of the rotor flux by adjusting the DC field current.
Enables variable power factor control (over-excitation/under-excitation).
Slip rings require regular maintenance due to mechanical friction and wear.
Presence of brushes can cause sparking and sparking-related noise.
Industrial synchronous motors for heavy load driving.
Power factor correction using synchronous condensers.
| Feature | Rotor (Slip Rings) | Stator (Windings) |
|---|---|---|
Current Type | DC Current | AC Current |
Slip rings are not used in squirrel-cage induction motors because the rotor current is induced by transformer action.
Option B is incorrect because AC in the rotor would create a pulsating magnetic field rather than the steady field required for synchronous operation.
A is correct тАФ Slip rings in a synchronous motor are used specifically to feed the constant DC excitation current to the rotating field windings.
Always remember: Stator always receives AC in synchronous machines, while the rotor requires DC. If the rotor were fed AC, the motor would not maintain synchronism.