Repeat AVR failures

[badservo]

I have a 200kW 480V Onan generator that has been installed for a few years and the voltage regulator (MX321 AVR) keeps failing (no output). I checked the rotating diodes and all were good, checked all coil resistance and all were good, checked everything for shorts to ground with DMM and all seem ok. There is an isolation transformer board to supply a voltage signal to the AVR and I did find a bad solder joint (this has been fixed) on a prior inspection when the first AVR failed. The generator is connected with an open transition automatic transfer switch and solidly grounded neutral supplying lighting, motors (largest 60HP), and other misc. small loads. The AVR has not failed while in use under load but seems to fail either start up, shutdown, or when the generator disconnects from load (ATS transition). This problem started to happen a while (maybe 2 years ago) after the 400A output breaker melted due to a loose connection between generator and breaker which also melted a few low voltage wires (signal and battery charger from ATS) without damaging the AVR. The replacement AVR will work for a while and fail like the others. Anyone else have an issue with similar repeated AVR failures? What else can cause this issue?

Thanks

 

[waross]

Do you have a bad surge suppressor on the rotating diode board?

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

 

[ScottyUK]

Short on the field winding caused by copper movement at operating speed? Happens on big sets, I'm sure it can happen on little ones.

 

[catserveng]

How long does a new AVR last before it fails? A few minutes, hours, days?

As I remember an MX321 is for PM type generators, is the PM power ok, within specs of the AVR manual?

What is the field volts at both no load rated volts and after load is applied?

You said you used an ohm meter to make checks, have you done any megger testing? Have you measured field resistance?

Do you have the aux breaker used with these type AVR's sometimes? Usually the trip coil is wired to B0 and B1 and the contacts wired to K1 and K2, have seen these breakers have problems and turn off field output. A lot of times there is just a jumper across K1 and K2, but make sure its in good condition.

Is a droop CT installed and is it the correct type? Most of the Newage AVR's use a 1 amp secondary CT, have dealt with a number of failed Newage AVR's over the years that were the result of an improper droop CT installed.

Hope that helps, Mike L.

 

[ScottyUK]

Is this a slipring machine or does it have shaft-mounted rectifier? I guess the latter from the size.

 

[badservo]

waross:
The suppressor looks ok and is not shorted but could be failed open, will replace to be sure.

ScottyUK:
That is possible, would a megger test have a chance at finding this type of fault?

catserveng:
The generator runs several hours a day and fails between 8 and 30+ days of use. There may be a month or more where it is not used at all. Used for backup or to limit peak demand during abnormal production schedule.
Yes it has a PMG output is 210V unloaded which is in spec.
Field volts no load: 10V at .1A (waiting for surge suppressor before I do a load test)
I do not have megger equipment
field resistance 20ohms
It does have a breaker on K1-K2 which has never tripped but does not have a trip coil on B0-B1
It does not have a droop CT.

ScottyUK: Correct, no sliprings, it has shaft mounted rectifiers.

I will do a load test in a day or two and fill in the missing information. Thanks for the help.

 

[ScottyUK]

If it was a slipring machine then you could use a megger to look for an earth fault on the field winding while the shaft is rotating. On a rotating rectifier type it is bordering on impossible to do this - not totally impossible, but even on a big set it is far from easy. Hence the question!

You could also have an inter-turn fault on the field winding, which wouldn't show up as an earth fault with the megger. This fault would effectively reduce the gain of the generator so that you needed more field current for a given voltage output due to there being fewer field turns effectively in circuit, so your AVR has to work harder. If you have factory test results and accurate instruments then you could possibly detect such a fault, but if it is only present during rotation then you are going to struggle. It can sometimes be detected as a vibration signature which varies in sympathy with the field current because the shorted turn causes asymmetric heating of the rotor and induces a thermal bend in it, which in turn causes vibration. This type of fault can be hard to diagnose with certainty, even with all the instruments fitted to a utility-class generator.

Rectifier diodes are known to fail, typically at the internal wirebond connection to the silicon die on types where there is such a connection. That brings the next question: what type of diode is used - stud-mounted or hockey puck? The hockey puck type are pretty good at surviving rotational stresses - normally they fail because of bad assembly practices. Stud types are more prone to failure due to their internal construction. Should reasonably easy to test the diodes using a megger in reverse and a tungsten lamp in the forward direction.

 

[redlinej]

To add to what the guys said,you should lower the excitation current trip set-point and see if it trip,this will let you know if the exciter stator is drawing excess current.this should help,also if have a scope-meter you can detect a fault diode from the ripple in waveform.

 

[redlinej]

this is a doc on faulty diode Link

 

[ScottyUK]

The generator OEM should be able to give you the open circuit characteristic of excitation v's terminal voltage. If you have lost a diode or have a bad field winding then the actual curve for the damaged machine will lie below that of an undamaged one. Drive the field with a variable low voltage DC supply in place of the AVR output. 0-30V and a couple of amps should be adequate for most small machines.