Current Excitation Unstable

[Gunawan perkapalan]

Dear All,
Please advice me,I have problem with my generator.the current excitation hunting around 2-7 A, I 'am already inject the exciter using 12 V battery and disconected source exciter from AVR but the voltage remains unstable
help me please

 

[davidbeach]

Forget the excitation current; are there any other signs of trouble? What caused you to look at the excitation current? Depending on many things, the excitation current will be much more volatile than either output voltage or output power.

 

[Gunawan perkapalan]

Thnks David,
Because when no load& with load condition the voltage main stator/output Unstable and i see the current and voltage excitation unstable

 

[waross]

What David said.
A problem with fuel flow or with a governor may cause those symptoms, even on a battery.
A generator requires much more exciter current when the generator is loaded compared to when it is unloaded.
If there is a mechanical issue causing the generator to pick up and drop the load, then the AVR will be changing the output to keep the voltage stable.
When you replace the AVR with a battery, the generator output voltage will vary as the load varies.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Gunawan perkapalan]

Thnks Waross,
I 'am Already check for the Prime mover factor
when running test without load ,the RPM Stable
AVR replace using Battery it should be stable for the output voltage,
but Not with my generator

 

[ScottyUK]

So with a battery feeding the field the output voltage is still unstable with no load on the generator?

It would be useful to know whether this is a slipring machine or a rotating rectifier machine when diagnosing the problem.

 

[Gunawan perkapalan]

Thanks Scotty
I'am Already Check for the rotating dioda 6 Pcs ,all good condtion

 

[waross]

Probably a defective/failing brushless exciter.
Disconnect the rotating diodes and megger the exciter windings.
Also check the field of the brushless exciter.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Gunawan perkapalan]

Thnks waross,
need to know,Winding Exciter already rewinding and insulation resistance is 100 M Ohm, whats normal value for resintance rotor winding and stator winding Exciter ?

 

[waross]

That depends.
Does the generator have problems when it is started cold or do the problems develop when the generator has been working and is warmed up?
What is the maximum voltage output of the AVR and what voltage did you megger at?
I have seen installations pass a 250 Volt megger test but fail a 500 Volt megger test.
Was the test done hot or cold?
I had an exciter that would fail when it got hot in service. It would not fail when heated in an oven.
Have you checked the main field winding?
Normally 100 M Ohms is adequate if the test is at a sufficiently high voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Gunawan perkapalan]

Thanks waross,
every start up the current exciter up down 2-7 A samotimes until maxium current 10 A,but after running maybe around 10 Minutes the hunting Slowly up down,
i megger winding using voltage 500 V,

Thnks

 

[waross]

What is the load on the generator?
Is this set islanded or paralleled with other sources?
Is there a surge suppressor of some type on the rotating diode plate and have you checked or replaced it?
Did you megger both the stator and the rotor of the brushless exciter?
Does the current output of the main generator carry with the field excitation?
Does the kW output of the main generator vary with the excitation?
What size is this generator?
What type of prime mover?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Gunawan perkapalan]

Thanks Waross,
These my generator nameplat

Marine Generator Type,Varistor and Rotating dioda has not been replace because already check and good condition,
any 3 Unit Generator same type,parralel operation.

Thnks for Your information