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Military MEP and Aircraft Gen-Sets

MEP-005a No AC output after overload, TM confusing on field flashing


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  #1  
Old 10-01-2012, 02:10 PM
abloy abloy is offline
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Default MEP-005a No AC output after overload, TM confusing on field flashing

So, I have a new to me, 1200hrs, MEP-005a, that seemed to be working OK, after filing the fluids, and cleaning some gunk out of the injector pump checkball.

In otherwords, I could start it, hold the start switch in the start position, see AC on the gagues, etc.

Then, I hooked it up to a significant test load, and accidentally shorted put a minor short on between two phases (line to line). The 15A breaker in my test load for the circuits in question popped pretty instantly, but after that, I can't get AC output from the genset.

Essentially, all the AC gauges are dead, and don't come to life when holding the OFF-ON-START switch in start like they used to. I started looking in the TM at the voltage regulator/excitier circuit, but honestly the trouble shooting seems a bit vague. I am confused as to whether to pull the VR box and try to bench test it (though the TM shows a diagram for this, they don't seem to list actual numbers), look at Relay R5, or whether I can manually flash the field somehow?
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Old 10-01-2012, 04:39 PM
25Eagle34 25Eagle34 is offline
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Quote:
Originally Posted by abloy View Post
So, I have a new to me, 1200hrs, MEP-005a, that seemed to be working OK, after filing the fluids, and cleaning some gunk out of the injector pump checkball.

In otherwords, I could start it, hold the start switch in the start position, see AC on the gagues, etc.

Then, I hooked it up to a significant test load, and accidentally shorted put a minor short on between two phases (line to line). The 15A breaker in my test load for the circuits in question popped pretty instantly, but after that, I can't get AC output from the genset.

Essentially, all the AC gauges are dead, and don't come to life when holding the OFF-ON-START switch in start like they used to. I started looking in the TM at the voltage regulator/excitier circuit, but honestly the trouble shooting seems a bit vague. I am confused as to whether to pull the VR box and try to bench test it (though the TM shows a diagram for this, they don't seem to list actual numbers), look at Relay R5, or whether I can manually flash the field somehow?
abloy, I have the same problem with my MEP-005A except mine was set for 416 volts and put it on my load bank which is rated at 220 AC. Burnt up the load bank and hen set licked off on overload. Please share what you find out with me.
Thanks, Don
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  #3  
Old 10-03-2012, 08:17 AM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

abloy - are you still looking for help, or have you resolved this?
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Old 10-03-2012, 09:51 AM
abloy abloy is offline
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

I'm definitely still looking for help.

I haven't messed with this much since I posted, but I did figure out where in the TM the bench test instructions are. I have a Variac, so generating the voltages necessary isn't a big deal. I started to pull the VR box out, but I apparently need a 12" 1/4' socket extension to get the last few bolts, so that's not done yet.

I did measure the voltage on the Terminal block that has the field leads: with a crappy VOM, I saw 1-2V DC when holding the stop-run-start switching into start position, so its definitely not sending the appropriate voltage out of the VR box. I am afraid that the transformer in the VR box might be toast if the voltage is that low, but I guess it will have to wait until I can pull it apart.

Ideas would be GREAT!
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Old 10-03-2012, 08:36 PM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

I'd disconnect the 2 field wires from the output of the V/R, start the set, run the engine up to about 1800 rpm, and apply 6-12V DC to the field. This should give you somewhere in the vicinity of nominal AC voltage.

The field only needs an amp or so, so it doesn't take much to power it up - even a 6V lantern battery would do it...

Doing this test will tell you conclusively if the generator itself is OK...
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Old 10-04-2012, 05:12 PM
abloy abloy is offline
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Before I read your response, I tried using 9V batteries to do this, though I left the VR connected (I was hoping that the if the manual flashing feature was dead, the VR would take over once the generator was making AC.

It didn't work, even with 2 9V batteries in series, touching them to the terminal block with alligator clips seemed to cause a tiny, tiny twitch in the AC volts meter, and when the leads were pulled away from the terminal block, I noticed a spark jumping from the positive battery lead each time. I probably wont get a chance to mess with it any more for a few days, but I was planning on pulling the VR and bench testing it next using the instructions from the TM.

1. Are 9V "transistor radio" batteries simply too limited in current capacity to excite the field? Do the wires from the VR have to be off for this to work?

2. If 9V battery should have worked, what should I be thinking about? Measusring insulation resistance on the coils? Rotating diodes?

Thanks for all the help!!!!
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Old 10-04-2012, 06:08 PM
Triple Jim Triple Jim is offline
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

I would be very surprised if 9V transistor radio batteries could supply enough current for this test.
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Old 10-04-2012, 09:36 PM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Quote:
Originally Posted by abloy View Post
Are 9V "transistor radio" batteries simply too limited in current capacity to excite the field?
probably not. Assuming they were fresh, and they were alkaline batteries, they can deliver about an amp of current, which should have caused a noticeable AC output.

Quote:
Do the wires from the VR have to be off for this to work?
No, it might work if you leave them connected, but the result will be ambiguous. IMO, it's pointless to run the test without disconnecting the V/R. Because if you leave it connected, you don't know where the battery's current flows (through the field as intended, or backwards through the V/R??. ) Also, you risk damaging the V/R itself.

Quote:
If 9V battery should have worked, what should I be thinking about? Measusring insulation resistance on the coils? Rotating diodes?
Rotating diodes would be my first guess.

I stand by my suggestion in post 5 as the most efficient way to proceed. Bench testing the V/R takes a lot longer.

Quote:
Originally Posted by Triple Jim View Post
I would be very surprised if 9V transistor radio batteries could supply enough current for this test.
Why would that surprise you?
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Old 10-05-2012, 12:17 AM
Triple Jim Triple Jim is offline
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

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Originally Posted by Jim McIntyre View Post
Why would that surprise you?
Because I don't think they can supply enough current for the test. From Ray-O-Vac's site, referring to lithium 9v batteries:

"The maximum continuous current rating for the Rayovac 9V Lithium battery is 120 mA. The maximum pulse current at room temperature is 400 mA."

Just to check, I just used my Fluke 87 meter in max/min mode to test the short circuit current of a new Alkaline 9V battery. I put the leads across the battery terminals, and the max reading was 197 mA.

That seemed low so I tried another one and got a couple Amps at short circuit, so the first one must have been older than I thought. I still doubt a 9V battery can supply enough current at anywhere near 9V to supply the exciter field. I haven't tried it, so maybe I'm wrong...
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Old 10-05-2012, 11:11 AM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

I measured 3.8 A max on my Fluke 179. But, it dies out fast. That's why I recommended a lantern battery for this test.

Still, I think this bodes well for the OP - If he really observed a small twitch in the AC output with application of a 9V battery - maybe the generator itself is OK.
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Old 10-05-2012, 02:51 PM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Well, I pulled the field wires and tried the lantern battery (with F1 to positive, F2 to negative).

I got about 22V Line to Ground, AC output.

I also, while I was at it, checked the field resistance: 2Ohms

And, since I had a good meter ( portable scope +DVOM) I checked the VR output. It actually was putting out 22VDC when the run switch was held into the start position with the generator running, using the meter's frequency feature to set engine RPM for 60hz. So, I put the field wires back on the terminal block, and tried running it, and got about the same results as with the battery (though with a little higher AC voltage, only when in the start position.

So is my best hope rotating diodes? Any hints here? I'd like to order them from Digi-Key or Mouser if someone has a P/N, and just change all of them If I'm going to have to pull the entire thing apart. Any hints on changing them?

Anyone know if the MEP-004 instructions and p/ns (1N1190 /1N1190R) listed in this thread are applicable:

http://www.smokstak.com/forum/showthread.php?t=68838

I'm guessin that the 1N1190 isn't the right part, since its only rated at 35A. Since the MEP-004a is 15KVA and the MEP-005A is 30KVA, I would think I'd be looking for a 70A capacity part? I can't find an NSN for the diodes in the technical manuals I have, so I'm a little lost.
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Old 10-05-2012, 03:45 PM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Can't say for sure what to do next. 22V AC is in the grey area. Had you seen 120V, I'd say V/R. Had you seen 0 VAC, I'd say rotating rectifiers. Did you use a 6V lantern battery? I know my MEP-004a only needs about 12V to get full AC output. Maybe the MEP-005 needs substantially more. At this point I'd guess it's the V/R, but I'm not very certain.

Hopefully somone with a working MEP-005 can chime in...

Last edited by Jim McIntyre; 10-05-2012 at 06:02 PM. Reason: added stuff
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Old 10-06-2012, 08:41 AM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Quote:
Originally Posted by abloy View Post
...
[/COLOR]Anyone know if the MEP-004 instructions and p/ns (1N1190 /1N1190R) listed in this thread are applicable:

http://www.smokstak.com/forum/showthread.php?t=68838

I'm guessin that the 1N1190 isn't the right part, since its only rated at 35A. Since the MEP-004a is 15KVA and the MEP-005A is 30KVA, I would think I'd be looking for a 70A capacity part? I can't find an NSN for the diodes in the technical manuals I have, so I'm a little lost.
The power through the diodes is substantially less than 35 amps, as it is the exciton power only to the rotor assembly.
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Old 10-06-2012, 10:03 AM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

In the MEP-004 TM, they show the field voltage to be 62.5 V and the total resistance to be 2.19 ohms. (Note this is the main rotating field, not the static field that the OP is applying small batteries to) That works out to about 30 amps. (Elsewhere in the same TM, they show 53 V and 3.31 ohms - not sure which is correct...)

If it turns out to be the rotating rectifiers (and I'm not saying it is at this point), I'd just pull out the bad ones, and order the highest rated silicon rectifier in the same case that Mouser stocked...

To the OP - you need the parts manual for your set. Mine for the MEP-004 shows the diodes in an exploded view of the rotor clearly as 1N1190 and 1N1190R.
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Old 10-06-2012, 06:15 PM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Quote:
Originally Posted by Jim McIntyre View Post
In the MEP-004 TM, they show the field voltage to be 62.5 V and the total resistance to be 2.19 ohms. ...
Careful not to confuse a coil's DC resistance with its impedance. DC resistance will always be (much) lower than impedance. These diodes are being used in an AC circuit (they are rectifiers) so we need to either compute or measure the circuit's impedance to compute the current through the diodes.

It appears that typical excitation voltage in a 004/005 is under 50 volts (the test procedure says use a 50 volt meter to test) and a quick look at the exciter schematic leads me to think we're talking a few amps at most.
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Old 10-06-2012, 07:38 PM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

About using batteries to energize the field

I don't think I would use lantern batterys or 9V bats as they have very limited current output capability, if you measure the battery voltage as you connect it to the field I think you will find it drops, likely alot.

I would just use a lead-acid gelcell bat or car bat, they have low resistance and can source alot of amps without dropping their voltage.

---------- Post added at 07:07 PM ---------- Previous post was at 06:59 PM ----------

Quote:
Originally Posted by nehog View Post
These diodes are being used in an AC circuit (they are rectifiers) so we need to either compute or measure the circuit's impedance to compute the current through the diodes.
Easier to assume dc resistance is the impedence and calculate amps based on that, then look for a diode that fits and buy one with a higher amp and PIV (peak inverse voltage) rating, so you have less stress on the diode and thus better reliability.

---------- Post added at 07:12 PM ---------- Previous post was at 07:07 PM ----------

Ps.....I think the feild has very significant inductance. On my Onan 7kw 7nhm the rotor is 25ohms, I energized it with 24 VDC (2 car bats), when I disconnected it I got a FAT 1/2 inch long spark (1/2 inch = ~15KV)

---------- Post added at 07:38 PM ---------- Previous post was at 07:12 PM ----------

On mine the 24 ohm is the dc resistance., also on mine the DC voltage to the rotor ranges from 18-60VDC
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Old 10-06-2012, 09:09 PM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Quote:
Originally Posted by nehog View Post
Careful not to confuse a coil's DC resistance with its impedance. DC resistance will always be (much) lower than impedance.
Do you understand what is on the rotor assembly of this machine? Do you realize there are two field coils in this machine's 'brushless" configuration? (basically this generator has one generator whose output is rectified in the rotating rectifiers to energize the field of the main machine)

Maybe this will help:

This generator has 4 windings:

1. static field
2. rotating exciter
3. rotating field
4. stator

The static field is energized with DC by the static exciter (V/R), and draws an amp or two at about 12V. The rotating diodes rectify the AC output of the rotating exciter and apply the resultant DC to the rotating field. The rotating field is rated at about 30A on the MEP-004A, and probably more than that on the -005. And of course the stator is what develops the set's main AC output.

As I stated, I was referring to the rotating field - the 2nd field, if you will. A "quick look' at the exciter schematic (the V/R in the context of this thread) tells you nothing about the magnitude of the current that has to be supplied to the rotating field. And it is this current that flows through the rotating rectifiers. The static exciter output is miniscule in comparison.

Last I checked, the field in a synchronous machine like this is energized with current having a predominately DC component, so the DC resistance of the coil is significant in calculating the field current. Yes, of course it has inductance as Len K mentions, which impedes attempts to change the field current quickly. But a good designer would use the DC resistance as the parameter which limits the current simply because the field current could be entirely DC, in which case the reactive component of the impedance drops to zero and all that remains is resistance.
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Old 10-08-2012, 04:35 PM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Quote:
Originally Posted by Jim McIntyre View Post
To the OP - you need the parts manual for your set. Mine for the MEP-004 shows the diodes in an exploded view of the rotor clearly as 1N1190 and 1N1190R.

I found the parts manual (TM 9-6115-465-24P), and I think it lists the same diodes as the MEP-004a,

5961-00-892-0888 DIODE
JAN1N1190 81349

5961-00-901-2290 DIODE
JAN1N1190R 81349

So...Mouser appears to have two different choices:

http://www.mouser.com/Search/Refine....ord=844-1N1190

The difference a/ra versions seem to have higher current ratings (40A vs 35, 800vs 400), but they have lower reverse current ratings. Which ones should I buy?

-Bob
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Old 10-08-2012, 05:37 PM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

Lower reverse current is good, not bad, so the A/RA versions are better in all respects. I'd use that one myself. Of course, replace all of the diodes so they are reasonably matched.

I'd recommend doing as many of the diagnostics as you can to eliminate the regulator first as replacing those diodes is a 'bit' difficult.
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Old 06-10-2013, 10:31 AM
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Default Re: MEP-005a No AC output after overload, TM confusing on field flashing

I finally got around to changing the diodes Saturday, and they definitely cured the problem, I have perfectly normal output and operation now.

Its a pretty easy job to do on the MEP-005 , much easier than it sounds like it would be on a MEP-004:

1. Pull the Louvered end panel off the machine. This is the thing with doors immediately below the control panel. There are a bunch of 7/16" head bolts along the edges, plus three that go downwards at the bottom edge. A 1/4" drive 7/16" socket + extension works for all but the bottom ones, which require a wrench.

2. Pull the access door and its rubber gasket at the top of the end bell casting. It has two 7/16" bolts. There is no need to pull the bearing retainer/end casting itself.



3. (Optional) Pull the screen off the lower portion of the end casting. It has 4 7/16" bolts holding it on. You don't really need it off until you drop a diode, socket, etc.

4. Blip the starter to position a diode at the top center of the access hole. Desolder the wire going to the diode. You'll need a 11/16" socket to remove then, I wound up grinding a cheap deep well socket down slightly so it wouldn't have the beveled end, and could fit flush on the diode's hex.



5. With each diode removed, I tested with a DVOM to see if they were bad, and what polarity they were. This was faster than scraping the paint and gunk off the good ones to read them. There were two bad ones, but I replaced them all with the Mouser parts already discussed in this thread.

Honestly, this was a much, much faster job than I thought it would be, less than two hours total, and I bet I could do it in half of that again.

Thanks to everyone for their help!!
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