Demonstrative Video
Principle of Operation: Overview
Synchronous machines are a doubly excited machine, i.e., two electrical inputs are provided to it.
Its stator winding is given three-phase supply and DC to the rotor winding.
The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux.
At a particular instant rotor and stator poles might be of the same polarity (N-N or S-S) causing a repulsive force on the rotor and the very next instant it will be N-S causing attractive force.
But due to the inertia of the rotor, it is unable to rotate in any direction due to that attractive or repulsive forces, and the rotor remains in standstill condition.
Hence a synchronous machines is not self-starting.
Some mechanical means which initially rotates the rotor in the same direction as the magnetic field to speed very close to synchronous speed.
On achieving synchronous speed, magnetic locking occurs, and the machines continues to rotate even after removal of external mechanical means.
Field winding supplied with a DC excitation
Rotor is mechanically rotated at \(N_s = \dfrac{120f}{P}\)
RMF produced by \(I_f\) induces voltages in stator winding, whose \(f\) in synchronism with the rotor speed
Two RMF: rotation of rotor & MMF of stator winding
Magnetic fields of stator and rotor combines to produce the resultant air-gap flux
Rotor driven by a prime mover pulls stator field along with it. The rotor field and the resultant field are separated by
Power Angle
\(S\)-pole of resultant flux lags behind \(N\)-pole of rotor field.
The rotor field drags resultant flux along with it and torque is created by separation of the fields, due to the magnetic attraction of the opposite poles.
As we apply more load, the torque angle gets bigger and the generator delivers more electrical power, but if it gets too big (\(>90^{\circ}\)) the generator will lose synchronism
As the load changes, the phase currents, stator field and power angle changes
In order for the angle between the fields to change, the speed must change at least momentarily
In fact, the speed tends to oscillate about synchronous speed, a process called
To reduce hunting, an additional winding has to be added