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MOOG IGV actuator replacement
GE 7FA IGV servo dithering too much, the right amount or is something else going on?
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About one year ago we replaced the MOOG IGV actuator on this machine which is controlled by a GE MARK Ve. Recently we began experiencing IGV fault trips (i.e., LVDT feedback too far from the demanded position). The LVDTs looked good with both of them producing nearly identical feedback in the R,S & T cores (or packs as they call them now) at the min position and the max position.

I replaced the servo and using the calibration program, checked the IGV's response to step increases in the manual position and the also the "POSITION" test routine that produces a full stroke ramp from min to max and back. The LVDTs tracked the demand signal very accurately.

However, I noticed that the servo current trend trace (which is a nice feature of the Ve cal program) appeared as a very thick line, unlike the premix and quat valve trend lines. It appeared to be the result of rapid oscillation of the servo current. I assumed it was dither, but it seemed unusually high. This "dither" disappears at the extreme ends of travel when the servo is saturated with pos or neg current.

I went out to the IGV and could hear the servo rapidly cycling like a machine gun in every position between min and max. The actuator shaft was rapidly moving up and down in short strokes, and I could feel the pressure pulsations. This seems quite excessive to me and could be the cause of such a short life span for the servo and possibly the actuator itself. Note that the servo spool oscillation disappeared with the IGV driven to the min or max positions.

There is a value for dither in the Regulator configuration (I don't recall it off hand), with the dither frequency set at zero, I believe. Maybe it's excessive, or maybe there's something else going on. I'm convinced that the equipment is going to have a short service life with this kind of wear and tear and I can't see the need to move actuator that much that often to prevent it from sticking. Ideas anybody?

1 out of 1 members thought this post was helpful...


we operate a 7ea in AGC mode 24/7. We also have seen short service life from the IGV actuator, Young-Franklin is now producing a "severe service" actuator for plants that load follow or otherwise see a lot of mw changes, hence lots of IGV actuation. A year or so ago I had a very experienced GE TA onsite that noted that our .m6b file had dither turned on for all gas valves and the IGV. He suggested turning this off since it is no longer needed on all of the newer electohydraulic control valves. I never experienced the pressure pulsations that you speak of, but I was never out at the actuator while stroking the valve. I would definately turn the dither off on all control valves for sure.

We had a problem a time back with control of our SRV. It was causing interstage gas pressure to fluctuate at times. By setting up a high speed trend (40ms) in the MKVI I was able to trend the individual currents from RST to the SRV servo. Turned out the "T" core was fighting the other cores at times, causing the valve to move the wrong way. Only way to spot it was the high speed trend of each individual cores current to the servo. This may not relate to what you are seeing, but might be worth a look if the "pulsations" are still present with dither turned off. Did any of this start with the MKV-E conversion? I would be curious to see the MKV-E, read about but never seen one yet!!


By john1542chupa on 28 May, 2009 - 8:12 pm
1 out of 1 members thought this post was helpful...

In my system, which was a very simple could not self destruct system, rapid oscillation of the actuator was an indicator that the gain was too high. We would lower the gain, oscillation would cease, actuator movement would be smooth, and there would be a lag between command versus effect. But the process was smooth, and made a good product.

thank you,
John Ross

I don't have any experience with the Mark Ve, and very little with the Mark VIe. Can you attribute this to any one processor or I/O Pack (<R>, <S>, or <T>) or does it appear to be common to all three outputs?

It was my understanding that if the dither frequency was set to zero, that the dither amplitude setting didn't matter, that the dither function was effectively disabled. But, I've since heard rumblings that that's not true, and I really don't know what to believe. (That's one of the problems with Speedtronic panels; it's difficult to know what's true and what's a wive's tale and what's fact and what's myth.)

If you can hear the servo "singing" that would indeed seem to be excessive. I would wonder what would happen if you changed the IGV actuator output to another unused servo output and if the same thing happened? (You would have to do this for all three processors.)

I would also wonder if there is some electrical noise being picked up on the wiring that is somehow being "magnified" by the servo output.

I don't know what the execution rate for the servo outputs is for the Mark Ve; I think some of the Mark VIe components run at 200 Hz and others run at 100 Hz.

So, if you can narrow it down to one processor or I/O Pack, or to that particular output (of all three processors--which doesn't seem likely but could happen if there's some other setting which we don't know about (there's a lot of settings in the Mark VIe!))--that would help. A single output will drive a servo, but the device won't be at exactly the reference position (because of the loss of the null bias current from the other two outputs). You could try disconnecting two outputs at a time until all three outputs have independently driven the servo to see if there's any change (better or worse).

But, a singing servo doesn't bode well for the service life of the servo. (I can just hear Moog: "Cha-ching!")

Thanks for the responses guys. I posted this question the evening I changed out the servo and having been called back out at 2am for a cluster of other problems. I was unable to work on this problem the next day. That fell on the day shift. I do know that the new chattering IGV servo caused another trip that same evening.

The day shift was at a loss to explain the chattering, too, so they placed a call to GE's remote diagnostic service support people in Salem, VA. They looked at all the trip data (they have full online access to our MARK-V's, the MARK-Ve, the EX2000's and our LCI's and our HMI's) and the symptoms exhibited by the IGV servo. They recommended replacing the "last chance" hydraulic filter on the actuator and the chattering servo.

This resolved the problem, although we don't definitively know if it was the filter, the servo itself or a combination of the two. I suspect it was the servo because the filter has only been in service for 5 months and our oil is pretty clean.

Frankly, I'm surprised this fixed the problem as we usually have good results with our reconditioned servos. I did not have a chance to verify that all three coils were being driven in the same direction after installation and I don't know if the day shift checked it prior to replacement, so reverse coil polarity may have been an issue.

The IGV servo is connected by a cannon plug so there's no way to cross wire it in the field, and it had been working properly on another IGV prior to the its being reconditioned so reverse polarity seems unlikely.

Does anybody know a way to bench test these servos for coil polarity?

I've pulled brand new servos out of the box, installed them with the same color coding, and had one or two coils reversed. Brand new servos from Moog.

I think sometime servos, especially the refurbished ones, are just a little "sensitive." It might have been that just by removing the filter that something was "un-tweaked."

GE have a paper out about new turbine lube oil formulations. These new formulations have caused lots of servo problems just because of the constituents in the oil. I don't have the publication number (sorry) but will ask around and see if I can obtain the number.

I've heard talk that GE engineering in South Carolina have been approached by Abex trying to get their servos approved again. You might ask about being a "beta" site if you're interested.

But I don't think the servos are the problem; I think the new turbine lube oil forumlations are the problem. GE's "solution" for the new lube oil formulations is to offer a centrifual separator to help remove suspeneded solids (I think that's what the jist of the paper is).

I'll write back to this thread with the number if I can find someone who knows the publication number. Or if someone else has the publication number, can you post it to this thread?

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We have Frme-VI Machine controlled with Mark-IV.

We also faced the same problem (hunting of IGV hevily) & got it resolved by replacing the Hydraulic Filter (Make:PALL) for IGV Servo Valve. We have observed that the servo fails to operate in case of oil starvation i.e low oil supply. Frequncy of chaulking of IGV filter is more may be because of high circulation of lube oil volume.

Besides one must take care to remove the air from the hydraulic line whenever the replace the filter, servo and hose pipe to IGV cylinders.

Servo polarity testing is discsussed in best way by markvguy in best thread is

With regards

Dear Sir,

We are looking for repair kit for the moog actuator (MOOG IGV ACTUATOR, MOD: 85-612B101, SN: 114) oil seals, piston rings and other o-rings. Please help us to find the repair kit.


Have you contacted Moog directly? I just read Moog have opened an office/facility in India.

It's not clear from your post if you're looking for a "rebuild kit" for the servo or the hydraulic actuator (double-acting cylinder) I've never heard of a "rebuild kit" for a servo, and certainly I would never try nor recommend trying to rebuild a servo on site.

There's really nothing special about the actuator (double-acting cylinder) and I'm sure any reputable hydraulic repair/refurbishment shop would be able to either rebuild, refurbish or even offer a suitable replacement for the actuator (double-acting cylinder).

Try contacting Moog directly, they may refer you to a local or regional supplier. I just did a quick Internet search for "Moog servo" and several suppliers popped up--including Moog.

Please write back to let us know how you fare.