# Synchronous Generator: Increase of Field Current & Torque Angle

• ffp
In summary, the author discusses the relationship between reactive power and torque angle in synchronous machines. He provides a mathematical derivation for this relationship and explains how reactive power affects the angle between the rotor and stator fields. He also provides a diagram to illustrate his points.
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!

Wouldn't a greater ﬁeld excitation increase the torque?

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.

dlgoff said:
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.

Last edited:
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?

ffp said:
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.

ffp said:
What is U?
U is voltage

ffp said:
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 ;)

ffp said:
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.

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...

gerbi said:
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.

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?

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

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

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.

Last edited by a moderator:
Thank you very much.I finally understood. Could you tell me the source of the images?

ffp said:
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

## What is a synchronous generator and how does it work?

A synchronous generator is a device that converts mechanical energy into electrical energy. It works by using a rotating magnetic field to induce a current in the stator windings, which are connected to an external electrical circuit. This creates a flow of electricity that can power various devices.

## What is the importance of increasing the field current in a synchronous generator?

Increasing the field current in a synchronous generator is important because it helps to increase the strength of the magnetic field, which in turn increases the amount of electrical energy that can be produced. This can help to meet higher power demands and improve the overall efficiency of the generator.

## What is torque angle and how does it affect the operation of a synchronous generator?

Torque angle is the angular displacement between the rotor and stator magnetic fields in a synchronous generator. It affects the operation of the generator by determining the amount of torque needed to maintain synchronization between the rotor and stator fields. A larger torque angle can lead to increased losses and decreased efficiency.

## How can the torque angle be adjusted in a synchronous generator?

The torque angle in a synchronous generator can be adjusted by changing the load on the generator or by changing the field current. Increasing the field current can decrease the torque angle, while decreasing the field current can increase the torque angle. Properly adjusting the torque angle is important for maintaining the stability and efficiency of the generator.

## What are the risks of increasing the field current in a synchronous generator?

Increasing the field current in a synchronous generator can lead to increased losses and heat generation, which can cause damage to the generator. It can also result in a higher voltage output, which can be dangerous for connected electrical devices. It is important to carefully monitor and control the field current to avoid these risks and ensure safe and efficient operation of the generator.

• Electrical Engineering
Replies
37
Views
3K
• Electrical Engineering
Replies
1
Views
904
• Electrical Engineering
Replies
3
Views
1K
• Electrical Engineering
Replies
2
Views
2K
• Electrical Engineering
Replies
18
Views
2K
• Electrical Engineering
Replies
46
Views
4K
• Electrical Engineering
Replies
6
Views
1K
• Electrical Engineering
Replies
2
Views
1K
• Electrical Engineering
Replies
14
Views
2K
• Electrical Engineering
Replies
14
Views
3K