Dear all. I am Frank, energy engineer.
I found this website as I am facing a problem.

I have a 3000kVA - 4 pole - 440V alternator

Output voltage is stable(440V) at No load

The problem I face is excitation voltage hunting between 11V DC and 15V DC at no load while output voltage being stable. I have not tried loading the alternator yet.

The rotating speed is stable.

I can improve the situation by using a stability potentiometer. Excitation voltage is a little bit more stable (Hunting between 11VDC and 12VDC).

Anyone would have an idea of the causes of this issue?

Thank you very much in advance!

 

Energy engineer Frank,

<b>When did this problem start?</b>

That should be the <i>first</i> question that is always asked when something begins to exhibit signs of a problem. Then, the next question should be: What changed just before the problem(s) started?

These two questions can really help isolate the causes of a problem. For example, in this case, if some maintenance work was recently done on the exciter then it's possible there is a wire terminal which was not properly re-connected.

That's the next thing, or, the thing which can be done while trying to understand when the problem started and what might have triggered the problem: A quick check of all the terminations in the related circuits to make sure that all the terminations are proper and tight.

If you can't relate the start of the problem to some event (maintenance; trip; high vibration; etc.), and you've confirmed all the terminations are secure, the next thing to do is to consult the manufacturer's documentation. Since you haven't told us anything about the exciter/generator manufacturer and model, it's virtually impossible to provide any unit-specific information to be of help. So, you are going to have to review the manufacturer's instructions and information to determine more about the installation at your site to see if there are troubleshooting tips and hints that might be applicable.

Generator exciters generally have two main components: the "manual" or "DC" regulator, and the "automatic" or "AC" regulator. The DC regulator uses field voltage or current as the reference and feedback for control of field voltage or current. The obvious things to look at here would be the feedback (usually a shunt resistor or something similar) in the field circuit to be sure it's working correctly and properly connected with secure terminations.

The AC regulator uses generator terminal voltage as the feedback and the reference, so the obvious thing to check is the generator terminal voltage feedback circuit.

Some units start in DC regulator, and then at some point, usually at or near rated speed but sometimes when the generator breaker is closed, automatically switch to AC regulator. Some exciters are always in AC regulator unless manually switched to DC regulator. The manufacturer's instructions for the model at your
site will tell you more about the configuration and operation of the exciter at your site.

In my experience, if you have just noticed this problem after months or years of operation without a problem, I would just close the generator breaker and monitor the terminal voltage to see if there is any instability. Sometimes, exciters are not very stable at low or no loads--not usually, but sometimes. I would, of course, only do this after trying to understand what might have caused the problem ("When did this start?") and checked and confirmed all of the circuit terminations for tightness and had a read of the manufacturer's instructions for any troubleshooting tips and hints.

Hope this helps!

 

Dear all. I am Frank, energy engineer.
I found this website as I am facing a problem.

I have a 3000kVA - 4 pole - 440V alternator

Output voltage is stable(440V) at No load

The problem I face is excitation voltage hunting between 11V DC and 15V DC at no load while output voltage being stable. I have not tried loading the alternator yet.

The rotating speed is stable.

I can improve the situation by using a stability potentiometer. Excitation voltage is a little bit more stable (Hunting between 11VDC and 12VDC).

Anyone would have an idea of the causes of this issue?

Thank you very much in advance!

Hello Frank
I think you don't have a big problem
It's stability response of AVR in very low voltage excitation. After starting loading your excitation voltage increase and you will not notice this problem

Anyway solution is by using faster response AVR, that what you do by increase stability resistor in AVR

I think you have two solutions
A- replace AVR by new one another model working in lower excitation voltage like R448 or VR6 or any other models (take care about 1 max excitation voltage not exceed shunt alternator specification 2 excitation resistance not lower than minimum resistor of excitation in AVR specification)

Let we go to solution B
By adding a resistor in series to excitor (it's look strange solution haha but it's OK) resistor value is about (30 to 40)% value of excitor resistor and power of resistor is large enough to Handel max voltage and max current
Ruffly expecting about 6 ohm, 200 W
You need to calculate it


I prefer solution A more than crazy solution B
The big problem in solution B is that by adding resistor you increase time invariation (resistor to inductive value) and this cause lower response because time increase


Your problem not very big but if you have synchronize system then you have a real big problem and only solution A is helpful

Have a good time friend