Synchronous Generator: Increase of Field Current & Torque Angle

[ffp]

Hi, I'm a electrical engineering student and i have a question conserning synchronous machines, more precisely a sinchronous generator.

Considering that's no change in load, the increase of the field current of the rotor would imply in a change in the torque angle?

I need a detailed explanation e if possible a reference.

Thanks!

 

[dlgoff]

Wouldn't a greater field excitation increase the torque?

 

[gerbi]

In this case it's obvious, that changes of excitation field affects torque angle..
P=const. U=const. If=variab. Q=variab. -> variable reactive power means angle between stator and rotor fields must change -> theta=variab.

refereces ? use google, please..

Wouldn't a greater field excitation increase the torque?

constant torque on synchronous speed (from turbine or else) means constant real power. In this case the real part of stator current is constant, imaginaris part of stator current changes.

 

[ffp]

Thanks for the help, you confirmed what i was thinking. But i need to prove to a friend of mine with strong arguments.

How exactly reactive power affects the angle between rotor and stator field?

What is U?

Mathematically this could be proved with this formula:

P=3 Vt Ea sin(δ) / Xs

P=const. ; Ea increase with If; Vt increase with Ea; Xs= const. Then sin(δ) must decrease to maintain P constant and therefore, δ must also decrease.

Is that correct?

This could be proven with phasors diagram?

 

[gerbi]

How exactly reactive power affects the angle between rotor and stator field?

It would be rather, how angle affects power.. angle between fields is strictly connected with power. Angle between fields is the same angle as between stator current and voltage.

What is U?

U is voltage

Mathematically this could be proved with this formula:

P=3 Vt Ea sin(δ) / Xs

P=const. ; Ea increase with If; Vt increase with Ea; Xs= const. Then sin(δ) must decrease to maintain P constant and therefore, δ must also decrease.

Is that correct?

Make it simpler.. look for diagram that shows Torque angle vs Real Power for diffrent excitation currents..

If increases with Ea (exct. voltage), not vice versa ;)

This could be proven with phasors diagram?

It could be shown on phasor diagram, I have some but it's on paper and not in english.

 

[ffp]

I don't know how the phasors diagram of Ea and Vt will change. Since the real power is constant, the component of Ea cos(δ) cannot change, right?

And how Vt behave?
When i said Ea increase with If i meant that Ea will increase because If increased...

There's any book that explains this? Didn't find on Chapman...

 

[ffp]

In this case it's obvious, that changes of excitation field affects torque angle..
P=const. U=const. If=variab. Q=variab. -> variable reactive power means angle between stator and rotor fields must change -> theta=variab.

refereces ? use google, please..



constant torque on synchronous speed (from turbine or else) means constant real power. In this case the real part of stator current is constant, imaginaris part of stator current changes.

I didn't understand the marqued sentence. Could you explain it better?

Or post the phasor diagram?

 

[gerbi]

Guess You can't wait till Monday..

First one, phasors for steady state (constant voltage, constant real power). The line A1-A-A2 is constant real power line. You move from A1 (overexcited) to A2 (underexcited). Theta is torque angle.
http://img818.imageshack.us/img818/3142/unledduw.jpg

Uploaded with ImageShack.us


Second one. Changes of torque angle for different excitation currents (Iw). Stator voltage is constant. Assume constant torque or power (M (P)) and move from one characteristic to another (angle changes).
http://img833.imageshack.us/img833/7599/unled2jh.jpg

Uploaded with ImageShack.us

As for the marked text in post above: as I've said, angle between fields in machine is connected with angle between stator voltage and current (power factor). When you go from under excited to overexcited (sign for reactive power "Q" changes) the angle changes, power factor changes.

I hope that's clear now. As showed above, generator connected to the power grid is generating constant real power (supplied by constant torque from eg. steam turbine) on constant voltage and frequency. We achieve changes of reactive power generation by control of excitation current.

 

[ffp]

Thank you very much.I finally understood. Could you tell me the source of the images?

 

[gerbi]

Thank you very much.I finally understood. Could you tell me the source of the images?

A.M.Plamitzer, Maszyny elektryczne (Eng.:Electric machines), WNT, Warsaw 1982