MEP003a Unstable Voltage

[Lukes_deuce]

Ive been repairing my mep003a recently. It was producing electricity as it should and then I moved the unit from the truck to trailer. I went to start up the unit the other day and now while having the selector on 120/240 volts, the voltage gauge was reading 140 volts. I tested the lugs and L1 showed 70 volts and L3 showed 70 volts. Convenience outlet also showed 70 volts. So I dug into the TMs and followed the procedure on paragraph 8-2 in TM5-6115-585-34 to supply 12 volts at TB3-5 and TB3-6. After a few seconds of the unit running with the 12 volts supplied, the voltage meter jumped up and then started to swing from 190V to 260V. I let it run for 15 seconds and then shut it down. Looked over everything and tried again. Voltage still swings from 190 to 260. The link shows a video of the gauge swinging. No audible sound of loading and unloading of the motor. I checked the lugs to confirm the reading and my fluke meter did bounce back and forth. Just want to see what this could be? I am not an electrical engineer, so Im trying to learn as I go.




Video - https://www.youtube.com/watch?v=SCrPaRGQuBs

 

[jamawieb]

Check the connections on TB3 and make sure they are tight. Then check the Voltage regulator. I'm beating that is the problem.

 

[Triple Jim]

It's possible that turning the adjustment screws on the regulator back and forth several times will fix a problem with dirty contacts in the potentiometers. Just count turns and be sure you get them back where they were when you're done. Really, either one could be causing the instability. (sorry I didn't think of this when you PMd me)

 

[glcaines]

I would bet that the voltage regulator has failed if moving the adjustment screws don't fix the problem. These are the same symptoms that I had when my VR failed. A transistor burned out. I purchased another VR board and then repaired my original with a replacement transistor. I'm assuming that you know that you should never idle the engine on either an MEP003A or MEP002A? The engine should be shut down while at normal operating RPM. Running the engine at low RPMs (idling) will damage the VR.

 

[fa35jsf]

I haven’t looked at the TM myself. That being said, as a point out that if the TM said to supply 12V, it may of meant a regulated 12V. It appears in your pictures that you hooked up to a battery, which if it was fully charged could have been putting out roughly 13.5V. Just saying.


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[Triple Jim]

I haven’t looked at the TM myself. That being said, as a point out that if the TM said to supply 12V, it may of meant a regulated 12V. It appears in your pictures that you hooked up to a battery, which if it was fully charged could have been putting out roughly 13.5V.

The application of 12v to the exciter field is just a set to see if the generator itself is OK. It it causes output power somewhere in the ballpark of correct, you can rule out a main generator problem. It's usually done with a battery, and that should be stable enough to give a reasonably steady output voltage. Apparently in this case something is wrong like a loose connection somewhere.

Luke, was the video made with 12VDC applied to the exciter field, or with the connections back to normal? It's possible it's caused by an unstable voltage regular. Did you wipe the pots on the regulator board back and forth a few times as I suggested above?

 

[Lukes_deuce]

The application of 12v to the exciter field is just a set to see if the generator itself is OK. It it causes output power somewhere in the ballpark of correct, you can rule out a main generator problem. It's usually done with a battery, and that should be stable enough to give a reasonably steady output voltage. Apparently in this case something is wrong like a loose connection somewhere.

Luke, was the video made with 12VDC applied to the exciter field, or with the connections back to normal? It's possible it's caused by an unstable voltage regular. Did you wipe the pots on the regulator board back and forth a few times as I suggested above?

Jim,

The video was with the battery connected. After the video, I did reconnect the wires as they should be on T3. The gen gave The same results with the voltage swinging.

I have not been back outside to work with the unit anymore. It's been 20 degrees or less lately so I haven't been motivated to work on it. Hopefully it will warm up in a few days. In the mean time, I will continue to look over the TM and diagrams to familiarize myself with the unit.

Thanks to all who answered.

 

[Lukes_deuce]

I would bet that the voltage regulator has failed if moving the adjustment screws don't fix the problem. These are the same symptoms that I had when my VR failed. A transistor burned out. I purchased another VR board and then repaired my original with a replacement transistor. I'm assuming that you know that you should never idle the engine on either an MEP003A or MEP002A? The engine should be shut down while at normal operating RPM. Running the engine at low RPMs (idling) will damage the VR.

When I first bought the unit, the throttle cable was broken. In the process of getting her running and then replacing the cable, the unit did idle. Not for extended times but a few minutes here and there. My question is, do the transistors fail imediately or take time to fail? The unit ran properly after the throttle cable was installed. So could of the idling started the damage and then using the unit finished it off?

 

[Triple Jim]

The 002A and 003A use a magnetic amplifier (CVT1) that is controlled by the regulator. Regulator output is sent through the control windings of CVT1 when the regulator wants to reduce generator output. This means that if you run the engine below 1,800 rpm, the regulator will see the low output and attempt to increase generator output by reducing its output, to let CVT1 increase the exciter field current. Therefore, running the engine below 1,800 rpm does not stress the regulator, and in fact lets it loaf along doing little or nothing.

Even so, there's no reason to run the engine at any speed other than 1,800 rpm. I just wanted to point out that unlike other generators that do stress the regulator when running under speed, the design of the 002A and 003A does not.

 

[Lukes_deuce]

The 002A and 003A use a magnetic amplifier (CVT1) that is controlled by the regulator. Regulator output is sent through the control windings of CVT1 when the regulator wants to reduce generator output. This means that if you run the engine below 1,800 rpm, the regulator will see the low output and attempt to increase generator output by reducing its output, to let CVT1 increase the exciter field current. Therefore, running the engine below 1,800 rpm does not stress the regulator, and in fact lets it loaf along doing little or nothing.

Even so, there's no reason to run the engine at any speed other than 1,800 rpm. I just wanted to point out that unlike other generators that do stress the regulator when running under speed, the design of the 002A and 003A does not.

Thanks Jim for the info. That's good to know. So when running the engine below 1800rpm, what does that affect? Also is there any other way to tell what rpm the motor is running besides the frequency gauge?

 

[Guyfang]

Luke,

If your set makes power, use a multi meter. Other then that, timing light. No other way.

 

[Isaac-1]

Thanks Jim for the info. That's good to know. So when running the engine below 1800rpm, what does that affect? Also is there any other way to tell what rpm the motor is running besides the frequency gauge?

The cheapest tool to measure output frequency (and voltage) is likely a Kill-A-Watt P-4400, under $20 on Amazon, etc.

 

[Triple Jim]

Thanks Jim for the info. That's good to know. So when running the engine below 1800rpm, what does that affect?

The cooling fan will run slower, but with all the cooling capacity available, I doubt if that would normally be a problem. Obviously the frequency output will be way off. The generator may not be able to maintain normal output voltage.

Running higher than 1,800 rpm will make the regulator work extra hard though. I've run my 003A at various speeds and loads while monitoring the regulator output current. Here are a couple a post that described what I learned.

https://www.steelsoldiers.com/showt...02-003-engine-servicing&p=1288686#post1288686

https://www.steelsoldiers.com/showt...zed-your-MEP-Generators&p=1312903#post1312903

A few posts after that second one I describe the actual regulator current range I found.

 

[Lukes_deuce]

Finally had some daylight hours to continue the diagnosis of the genset. I went into the control area and tightened down every screw connection I could see. Also when to the distribution box and tighten down T3.

After starting up the unit, I was reading 140 volts on the 110/220 setting. The variable voltage was gone. I had 70 volts at the convience outlet. Honestly, I had to figure out what the "pots" ment, as at first, I didnot see any screws to adjust on the voltage regulator board. Eventually I found R 16.& R17. I turned them back and forth a few times to clean the contacts. Ended up getting 0 AC volts now at the meter and with my own meter at the output posts. I was really scratching my head now. I thought I burnt something up. So I went and followed the TM again in chapter 8 to by pass the VR. Hooked up the battery and low and behold she was making 225 Volts on the meter and confirmed with my handheld meter. It was a little unsteady but not bad. I used a shop vac from the front outlet and all seemed well. I then hooked up the wires back to T3 and no voltage at all. I'm guessing it's the VR??? Any thoughts would be great.

Thanks

Luke

 

[Guyfang]

When you start the set up, hold the start switch in the start position. Does the voltage come up? If not, the start switch may not be giving the Excitor the 24 volts that is needed to light off the main gen. The way to check it is to start the set, hold it in the start position, and check if you have 24 volts at pin 7 on the S1 start switch. What kind of S1 do you have, if you don't know, post a picture.

 

[jamawieb]

Guy he has a 003a and I believe the Pin on the starter he needs to check is 27 but I could be wrong since I'm not in front of my schematics.

 

[Lukes_deuce]

When you start the set up, hold the start switch in the start position. Does the voltage come up? If not, the start switch may not be giving the Excitor the 24 volts that is needed to light off the main gen. The way to check it is to start the set, hold it in the start position, and check if you have 24 volts at pin 7 on the S1 start switch. What kind of S1 do you have, if you don't know, post a picture.

I will start the set this afternoon and test the starter switch. I usually hold it for a second or two after the unit is running. I'll also take a picture of the switch. Only have a video that shows the switch and I can't upload videos off my phone.

 

[Guyfang]

OOOOOPS, sorry. trying to read tooooooo many threads. Don't worry about holding the switch up to long. There is a switch, S7 that keeps the starter from clashing with the flywheel. It it does happen, you need to adjust the S7.

Yes, pin 27 sounds right to me. Check it for 24 volts.

 

[Lukes_deuce]

Sorry for the long wait time. I went out a tested pin 27 on the starter switch. While holding the switch to start after the set was running, I had 23.7 volts on the pin. I had no AC voltage. I tried turning the switch to start after the set was running and it did not change the AC voltage. An interesting thing that did happen, was since it was dark out side, I saw some type of spark of electricity. I saw it from a reflection off of the wall next to the generator. It looked like it was from the inside of the generator. It was only for a second, while I was holding the switch to start and then went off on its own while I was still holding the switch to start. Then I release the switch and let the genset run. No AC voltage.

 

[Guyfang]

Actually, that's an old trick I used. Turn off the lights and look for a spark. You need to find out what's sparking. From the sound of it, you might have a broken Excitor wire. I say might, because I can't look over your shoulder. But if it sparks when you held up the S1, then that's a good clue.