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It is stated in the textbook I'm studying that if we reduce the excitation current after the rotor has reached synchronous speed , its speed will remain constant even if the excitation current becomes zero.
I know that the reason the rotor runs at synchronous speed is because the poles created by the excitation current are attracted to those of the stator. If we reduce the current to 0 though, there are no poles on the rotor to be attracted.
So how does the motor still run at synchronous speed?
Wound rotor synchronous machine tend to have a salient rotor structure. As a result they exhibit a small amount of reluctance torque.
Usually this of little use as it is not enough to facilitate breakout but can be enough to counter drag if unloaded
In my understanding this phenomenon will not change even for wound rotor (non salient) synchronous machines which produce no reluctance torque.
All you need for maintaining magnetic locking is the magnetic flux. This magnetisation will now be adjusted by the reactive power. The machine will draw reactive power from the mains to maintain the magnetic lock. This will be reflected by the change in power factor. Please check the power factor with and without excitation. Of course the (lagging) current will increase and as the first observation you will see this change in current for a constant load (active power should be the same). That should automatically explain you the reason behind this behaviour.
The same principle also applies to Induction Generators used in wind mills. Hope this helps.
If we gradually reduce the excitation of a synchronous motor when it is running at no-load, we find that the motor continues to run at synchronous speed even when the exciting current is zero. The reason is that the flux produced by the stator prefers to cross the short gap between the salient poles and the stator rather than the much longer airgap between the poles. On account of this phenomenon, the motor develops a reluctance torque. If a mechanical load is applied to the shaft, the rotor poles will fall behind the stator poles. Thus, a considerable reluctance torque can be developed without any dc excitation at all.
