# How does a Synchronous condenser shift the phase

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1. Nov 29, 2015

### tim9000

Hi,
So when you over-excite the rotor of a synchronous motor, or under-excite the rotor of a synchronous generator, how is the flux actually changing the power factor of the armature to leading? (something to do with the magnitude of the armature reaction flux vector?)
That is to ask: by what mechanism does this modify the relation of the current to the voltage waveforms?

Pictures welcome.

Cheers!

2. Dec 7, 2015

### cnh1995

The current flowing through the motor (Iph or Iaph) is due to the resultant emf i.e. vector sum of stator voltage applied(Vph) and back emf Eb.

When the excitation is changed, length of Eb vector changes (and Vph is constant throughout). This changes the magnitude as well as phase of the resultant emf. You can see it in the phasor diagram. There is a phase difference of θ(≅90°) between Er and Iph. So, as the Er vector changes its phase, Im vector is dragged along with it, thereby changing the power factor angle. Magnitude of Im also changes as magnitude of Er has changed (Er=Im*Zs) and it changes in such a way that its active component remains constant i.e. Im*cosΦ is constant since the loading condition is unchanged.
So, to summarize,
Change in excitation changes the phase angle of the opposing emf→Hence, the angle between resultant emf and supply voltage(Vph) changes. →As Im and Er have fixed phase difference, Im is modified as per the magnitude and position of Er→This changes the angle between Im and Vph(Vph is fixed)→This changes the p.f.

Last edited: Dec 7, 2015
3. Dec 7, 2015

### cnh1995

I believe armature reaction flux vector only determines the magnitude of Zs.

Last edited: Dec 7, 2015
4. Dec 8, 2015

### Staff: Mentor

The secret to understanding synchronous machines is to shift your reference frame to the D-Q (direct and quadrature) axes. Everything becomes magically simpler and easy to understand in that frame.