MEP-002/003 engine servicing

[rickf]

OK, We all know about idling a military generator. BIG No-No. I have a question. Is there any way of disconnecting the critical generating components without disconnecting the engine operating system? Example #1, Can I unplug the generator controllers from the control box but still have the starting and power to the fuel pumps available? Example #2, Can I unplug ALL of the wiring going to the control box and rig jumpers for the pumps and injection pump solenoid and use a simple starter button for the starter? I have a pesky engine and I am trying to get it straightened out but I do not want to fry the controls because it is not running fast enough.

Rick

 

[LuckyDog]

Hey Rick...

Here is what I did on mine while trying to get the motor running correctly.

J12 on the control box will disconnect the field winding from the VR. It is the little one above the three big ones. It only has two wires in it. Sort of like that is why its there, in my minds eye.

With the cable disconnected, the VR can't/won't over current and burn up.
Worked for me.

Paul

 

[Harleyd315]

I asked the same thing back in Aug/2012. Starting a diesel engine at 10 deg F and letting it run at 1800 rpm without any warm up is against everything I have learned working around trucks and heavy equipment. I know never to let these gen sets idle. But this is my question, if the damage that occurs is on the A/C side of the unit, can't there be a way to not excite the field or keep the generator from working? If the generator master switch excites the generator, then could you remove the wires that excite it and conncet them to a spring loaded toggle switch and excite the field manually after a few minutes of warm up? I am no electrical wiz so I was hoping some of the electrical experts here would take a look at this. Thanks Chris

 

[storeman]

Harley315,
From Luckydog's post, it seems you just got your answer on what to interrupt and where to place a switch.
Jerry

 

[Triple Jim]

I got to thinking about this no-idle rule for the MEP-002A and MEP-003A. The regulator controls the exciter's field by applying current to the control windings of CVT1. When current is applied, CVT1 reduces its output to the rectifier bridge A4, and current to the exciter windings is in turn reduced. If the engine is run at idle, or speed lower than 1800 RPM, the AC output of the generator is reduced, and the regulator board will try to bring it up. It does this by reducing its output to control windings of CVT1 so that CVT1 is at maximum output. This means that the output transistor of the regulator board is working the least when trying to increase the output of the generator, and is least likely to overhead and fail.

Maybe other generators with a different scheme of regulation do indeed stress their regulators if the engine is run slowly, and maybe the recommendation of not idling them for more than a few seconds is valid for them, but it looks like that problem is not present on the 002 and 003. I have no doubt that output transistors fail on the 002 and 003 regulators, but it would be more likely for that to happen from running the generator unloaded, forcing the regulator to work hard to keep the output down.

Sometime soon I plan to run my MEP-003A at idle for a while, shut it off, and feel the regulator's output transistor. I'll let you know what I learn, but I have no doubt that it will be cool.

 

[glcaines]

In the -12 manual there is a specific warning about not idling the engine. Why would the military put this warning in place if it wasn't true?

2-4. STARTING GENERATOR SET (SEE FIGURE 2-3).​

CAUTION​

Do not idle engine.Operate generator set only at rated RPM.Do not use either to aid starting generator set.Do not operate in excess of 200 hours continuouswith JP-4, MIL-J-5624 bulk fuel.a. When starting a set with a dry fuelfilter drain or change1) Push in the DC circuit breaker.​

system

 

[rickf]

This is about to get interesting!!

 

[Triple Jim]

That's a good point, glcaines. It could be that there are other reasons not to idle the engine. For one, it's an air cooled engine, and gets little cooling at idle. I was thinking only of the warning I've read on this board several times that said that the output transistor of the regulator is in danger if the engine is idled.

 

[storeman]

Triple Jim,
Sounds 180 out from conventional wisdom on this forum and the TMs, which indicate throttle is to be left at run setting. I'm really interested in your results.
Jerry

 

[Triple Jim]

I have been known to be a bit of an iconoclast, Jerry. I'll see if I can run the test soon.

 

[storeman]

Well you aren't that far from me, and I'm always interested in parts, as you know.
Jerry

 

[Triple Jim]

I hope I don't go from a functioning generator to parts in a short idle test, but if I do, you'll be at the top of the list.

 

[storeman]

Who knows? Its the ultimate "no-no"

 

[Triple Jim]

In the -12 manual there is a specific warning about not idling the engine. Why would the military put this warning in place if it wasn't true?

Gary, I checked the -12 manual for both the 002 and the 003. It's interesting, or odd, that the warning is only in the 002 manual.

I'm going to do the test shortly.

 

[Triple Jim]

OK, I just ran my MEP-003A. First I disconnected the output of the main transistor at terminal 17 of the regulator board. This terminal sinks the current through the exciter field control transformer CVT1 supplied from terminal 14, which is in turn supplied by rectifying diodes CR6 and CR7 (maybe CR8 and CR9 in some boards), which are supplied AC by T1. I connected a Fluke 87 meter between wire X39C16 and terminal 17 so that it reconnected the wire and read the current that the main output transistor was carrying.

Then I started the generator as usual, running at 1800 RPM. The current was about 230 mA to keep the output at 240 Volts. Then I reduced the engine speed with the governor cable as low as it would go. This really isn't what I'd call idle speed, but it's considerably slower than 1800 RPM. The current went down to 130 mA. After letting it run like this for a couple minutes, I stopped the engine and felt Q3, which is the big transistor on the board. It was about the temperature of everything else in the control box... cold.

This is not a recommendation that everyone go out and idle his MEP-002A or MEP-003A. I do hope it provides some information and dispels a myth. As I said above, there may be other reasons that it's not good, or even bad, to idle these generators, but so far all I've thought of is that it wouldn't be good to do it while a load is connected.

Also, as I said above, only the MEP-002A's -12 "OPERATOR AND ORGANIZATIONAL MAINTENANCE MANUAL" has the warning against idling, the MEP-003A's comparable manual does not.

I have no doubt that Q3 sometimes fails, and I'm sure that's not the only component on the board that does, but at this point, I do not believe that Q3 or anything else in the regulator will be damaged, or even stressed, by idling.

 

[rickf]

Thanks for risking your equipment for this test. I am not in the slightest bit a generator engineer so I get to sit back and soak this all in. Is it possible to do damage on the DC side by running to slow? Also, another thought, If there were a load on the generator at the time you idled it down that would change things wouldn't it? Beside burning up what was attached. But if it were a resistive load like a heating element what effect would that have overall if you slowed down the generator.

Rick

 

[Triple Jim]

Well, you're certainly welcome Rick, but I don't feel that I risked my equipment. Logic said there would be no harm, and the meter immediately confirmed that when I slowed down the engine. I don't know of any reason that the battery charging system would be harmed by running below 1800 RPM any more than a vehicle's charging system is harmed when the engine is idling.

As I said above, I wouldn't slow the engine if a load were on the generator. First and most obviously, the frequency would no longer be 60 Hz. Also, who knows what the voltage would be (?). And of top of that, cooling of the engine and generator are reduced at low speeds because both are cooled by fans running at engine speed. The only reason I can think of to ever run below 1800 RPM would be if it were necessary when working on things like the governor.

As I tried to say, I have probably not thought of every possible problem that might result from running below 1800 RPM, but at this point I'm not afraid to do it if I have a reason to.

 

[edgephoto]

Well, you're certainly welcome Rick, but I don't feel that I risked my equipment. Logic said there would be no harm, and the meter immediately confirmed that when I slowed down the engine. I don't know of any reason that the battery charging system would be harmed by running below 1800 RPM any more than a vehicle's charging system is harmed when the engine is idling.

As I said above, I wouldn't slow the engine if a load were on the generator. First and most obviously, the frequency would no longer be 60 Hz. Also, who knows what the voltage would be (?). And of top of that, cooling of the engine and generator are reduced at low speeds because both are cooled by fans running at engine speed. The only reason I can think of to ever run below 1800 RPM would be if it were necessary when working on things like the governor.

As I tried to say, I have probably not thought of every possible problem that might result from running below 1800 RPM, but at this point I'm not afraid to do it if I have a reason to.

Good news Jim. Although I was looking forward to picking up some parts from you.

 

[screejunk]

Recommendations in the manuals, assumes a load (and a variable load at that) is being applied to the unit. So the bridge rectifier and voltage regulator are both 'in action' and working in concert. Running a test with no load isn't very useful or realistic. A test should be run with a fixed load and a variable load. So the general rule to always run at 60 hz and 1800 rpms should still be headed. Military manuals are always written from a practical standpoint (not a theoretical standpoint), so they would assume various fixed and variable loads being applied to the generator and the entire electrical system (bridge rectifier, voltage regulator, phase selector switch, etc.) are all doing something...and often a dynamic-something. I am not an electrical engineer (We partner with experts that are licensed!). This might not be a detailed enough response, but just adding my $.02 about how/why the manual has the warning which should be headed. This is one of the few things that can go wrong with these units AND that can be avoided easily. GMG/Bob

 

[steelandcanvas]

Thank you Gentlemen, for a very interesting and enlightening tread.