I was reading the introductory chapter of my textbook on machines and when the author comes to a little introduction on synchronous machines, I was stuck on a statement midway. It goes:
"When balanced 3-phase currents are allowed to flow in the armature winding, these produce a synchronously rotating field, stationary with respect to rotor field as a result of which the machine produces torque of electromagnetic origin."
First of all, the book follows an approach of never mentioning motor or generator it mentions just 'machine' which is fair enough cos the schematic diagrams of motor and generator are the same with reversed energy flows.
But my question here is:
That in my amateur opinion electromagnetic torque is produced when there is an interaction between two magnetic fields with a relative speed between them. If they are stationary with respect to each other, how can a torque be generated? Please clarify.
P.S: if you could also explain separately the cases of motor and generator.
Best Answer
Well, you cannot deny there is a force between rotor and stator. It's exactly the same force between two fixed permanent magnets. They are fixed but if let loose would collide with each other. This can't happen in a motor because the rotor is constrained radially. Does torque also exist? Yes it does - to overcome windage and friction exhibited by the rotor. That's the no load scenario and clearly an increase in torque exists when mechanical load is applied.
It's also interesting to note that if the rotor was mechanically lossless, the rotating mag field would be physically coincident with the exact centre of the rotor pole aligned with it. As load is applied there becomes a misalignment that inevitably produces a hight force between rotor and rotating field. Maybe this is a more satisfactory explanation?