re-wiring my MEP-004a for different voltages, including single phase

deuce bigalow said:

Good thread.

Sewerzuk how do you have the 2 services hooked to the generator? Do you use 2 wires to each L1 & L3 terminals and not use L2?

I will eventually be doing the same thing, powering both house and shop with mine as well. For now it's just the shop and I hooked up L1 & L2 and didn't use L3.

I was thinking once I added a second service (when I build the house) that I would need to get a transformer to take 416v 3ph and deliver 240v 1ph to both. Then an electrician friend told me that would be a waste of money, not sure what his reasoning was.

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I have one service wired to L1, L2, L0 and the second service wired to L2, L3, L0. This means that I'm using all 3 phases and I can get close to the rated power out of the genset, but it is never balanced. I tried to set it up so that most of my constant 120v loads were on L1 (house) and L3 (shop), since L2 is shared. If I were to turn on everything in the shop and the house at the same time, the overload on L2 would be the first to trip. But, because of how I have the set loaded, I can get the full 15kW out of it.

Transformers don't change 3 phase to single phase; they only step voltage up or down. So, you could take 1 phase of 416, and get a transformer to step it down to 120/240, but doing that wouldn't gain you anything over just pulling 120/208 out of the generator. You would still be limited by the overloads on whatever phase you pull the 416 off of.

The best solution, IMO, is to do the low zigzag conversion I am planning. That's the best way to get a full 15kW single phase 120/240 out of the generator.

The advantage I see to leaving one of these generators 3 phase, is that it allows you to power some 3 phase equipment AND power your single phase service. Lots of people I have sold these do do just that; they don't want to pay for 3 phase service, so the generator provides single phase backup power for their shop and lets them run their lathe/milling machine/whatever. That is the whole reason I initially set mine up.

I hope to have a video up this weekend, showing the conversion process, load testing, waveform analysis with the oscilloscope, etc. I plan to do the CT re-wiring next week. I am working on repairing my "test" -004a this morning....

Low zigzag reconfiguration is complete! Just finished a bit of testing tonight; its works as predicted. 120/240v single phase on L1-L3. Voltmeter's other positions become rather useless, but it still works properly when selected to the L1-L3 position. I haven't done any load testing on it yet; tomorrow I'll pull my load bank into the shop and run it through some heavy testing to make sure no issues will pop up.

I took a video of the entire procedure; I'll edit it tonight and upload it to youtube soon. I'll post the link here as soon as it is finished.

Now that I've proved that 120/240 low zigzag works, the next step will be to test how the overload trips react to high loads on each phase, and then re-wind the CT's to adjust that setpoint (and indicate the proper % current).

If I can make that work, I'll move on to 240v 3 phase delta.

Here's the video describing the conversion:
EDIT: This video has been superseded with another video, describing additional steps necessary for proper meter and VR operation. Available at the end of post #1.

[media]http://www.youtube.com/watch?v=Mb8pcERIMgo[/media]

Very nice! I think that will clear up a lot of misunderstanding. A note- to put the generator back into 3-phase 120/208 all that you have to to is move the jumper wire connection point from T2/T8 back up to either N/T10/T11 and will be back to the factory original electrically.

Ken_86gt said:

A note- to put the generator back into 3-phase 120/208 all that you have to to is move the jumper wire connection point from T2/T8 back up to either N/T10/T11 and will be back to the factory original electrically.

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Yep! This means that this conversion to 120/240 single phase low zigzag isn't a permanent mod...a person could shut the set down and, in less than 1 minute, reconfigure it back to 120/208 3 phase.

I will be playing around with the CT's to allow for 100% load and accurate current readings on the set...after that is done, converting back to 120/208 3 phase will be more time consuming (removing a winding from the CT involves removing all 12 leads from the bottom of the voltage reconnection board).

can you draw 15 kw off the two phases set for 120/240 single phase? i had seen a conversion before for 120/240 but it derated the gen.

Good question- In this case its going to be all about if the windings and leads use a large enough wire to handle the current. The overload protection is there to protect the winding from too much current. I believe the over-current devices are set to 120% of rated load, so without changing anything you could get close. Look at the write up in a previously posted link for a better explanation. I believe that no one has tried to see where the point of failure actually is? While it is possible to override or change the protection, would that be wise? At some point the engineers who designed the machine decided these things for reliability, longevity, and safety of the generator. I can garrantee that this time of year, while it is colder outside the generator has a much better chance of running at full capacity. But in the summer when it is 90°F plus outside will it still be able to do this?

jbk said:

can you draw 15 kw off the two phases set for 120/240 single phase? i had seen a conversion before for 120/240 but it derated the gen.

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The answer is a little complicated...
The overloads will trip at 130% rated current (or about 56A, which is 13.4kW). 100% rated current, or 43A, is about 10.3kW. So...the short answer is no, you can only get about 13kW out of it, operating just below the overload trip setpoint.
Because of how the windings are set up, I don't believe we are overloading any of the 12 windings or their leads in the generator. We ARE passing more current through the leads that go from the reconnection board, through the main breaker, and to the load terminals. I wouldn't be concerned with the added current through the main breaker, but I am concerned with the small-ish gauge wire. I think it should be one size larger if a person intended to run the set at 15kW all of the time.

I intend to do additional load testing on this set in both single and 3 phase, pull a winding out of the CT to adjust the power and amp meters, and adjust the overload trips to roughly 80A (equivalent to 130% of rated current in single phase). I'll post my findings here as I get the time.

This is all experimental; what I'm doing was never designed into the set. I believe that these MEP's won't have any problem with it (or I wouldn't be doing it), but, as the saying goes, your results may vary. I'm not guaranteeing that any of this will work for anybody else, or is even safe to do. Wrench at your OWN risk

That is Excellent !! Thank you for the video and your efforts

Had an hour or so to do some testing tonight. It is about 10 degrees below freezing outside, so for the first time I set up the load bank and generator inside of my closed shop and plumbed the exhaust outdoors. Normally my waste oil furnace does a fine job of keeping the shop warm, but it gets REALLY hot in there with a generator pumping 15kW into a resistive load bank...might have to move the load bank outside for tests on warmer days. Can't imagine what it would be like with my -006A pushing 75kW into it.
I haven't done any 3 phase tests on it yet, so I haven't had the chance to verify that the meters all read properly. Some of the meter responses might be because of the single phase mod, some might be because the meters themselves are out of whack.
I hooked up my load bank single phase and began doing some load tests on the generator. Basically, I just wanted to see what the set was capable of in single phase, and if my predictions were correct. I'll post a complete list of my findings and observations in my initial post once all of my testing is done, but I thought I would sum up a few of my results from tonight:

- The set's voltmeter only seems to read properly when selected to the L1-L3 position (240v), and L3-L0 position (120v). I expected L1-L0 to also read 120v, but it reads closer to 200v. I don't know why yet...
Edit: problem solved with some reconnections, described in post # 40

- The overloads function as predicted. I loaded the set to about 65A, and it tripped at about 9 minutes. I set it to about 58A and pulled that load for about 30 minutes with no trip. Seems my prediction was correct for those

- The % rated current meter reads the exact same thing for L1, L2, and L3. I missed the prediction on that one...I had guessed that it wouldn't read properly except in 1 position
Edit: it SHOULD read the same...provided the generator is wired properly in a zigzag configuration

- The % power meter doesn't move at all. I suspect that it is not functioning. I'll test and repair it when I do a 3 phase load test.
Edit: it was broken and required replacement

-I used an infared thermometer to check for hot spots in the wiring...none found.

The big issue:
-There appears to be more voltage droop from zero to full load that I am used to. I was getting 120v at no load, and about 100v at a full 15kW single phase load. I'll see if this is a problem with defective excitation circuitry, or if the issue is related to the single phase mod.
Edit: It was related to the single phase mod. Problem solved with some reconnections, described in post # 40


Overall, things went as expected. It seems that this modification is a viable one for the person who wants 120/240 split phase out of their machine.

Next step (which probably should have been the first step) is to set the generator back up for 3 phase and do some load testing to check for proper operation of all of the generator's components.

Thanks for the well documented description of the easy way to rewire the voltage board to get single phase. I have always known that this 12-wire generator could be reconfigured for single phase, same any 12-wire generator. But you have shown the simple way to do it on the MEP series generators, and in a way that it can easily be changed back to 3-phase if desired.

Please let us know if you find a problem in the voltage regulation circuitry on your unit. The droop from 120 Volts unloaded to 100 Volts fully loaded seems excessive to me. Could there be something amiss in the voltage regulator?

Watched your video, fantastic work. Great audio, nice production values. Thanks, I really appreciate it.

I have a couple of 016b generators sitting here in Tualatin, was trying to figure out how to get 240vac ~and pull out a neutral. Will give the Low Zigzag connection a try.

RichardR said:

Please let us know if you find a problem in the voltage regulation circuitry on your unit. The droop from 120 Volts unloaded to 100 Volts fully loaded seems excessive to me. Could there be something amiss in the voltage regulator?

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Yep...a 20v droop is very excessive and means that the set can't be used for a situation that varies between zero and full load. A 5kW variance from something like 10kw up to 15kw would be fine, as the voltage would only move 5-6 volts. But the issue still remains. I have a feeling that it is connected to the way the regulator/exciter circuitry senses voltage. Perhaps its sensing on the now "floating" L2 is affecting its output. May also be connected to the odd reading I'm getting on my voltmeter.
Edit: problem solved with some reconnections, described in post # 40

Before I go any further into figuring out why, I'm going to re-wire for 3 phase again and do the normal operational checks I do just to make sure the set works properly in its "normal" configuration. No sense chasing problems related to the single phase swap if some of the sets components are faulty to begin with...

I hope its in your regulator card because this is a very tempting setup.

I think a heavy duty switch to allow swapping back and forth between single and three phase shouldn't be too hard.
Good work, and keep up the progress.

Hammer said:

I think a heavy duty switch to allow swapping back and forth between single and three phase shouldn't be too hard.

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I guess it depends on how many things need to be switched over
I do know what the switch inside of the MEP-002 and -003 looks like (I had to remove, disassemble, and repair one once). It's a nightmare....


On another note, I did manage a few minutes to reset the generator for 3 phase and do some load testing on it. Everything worked properly except for the % power meter (I'll replace it before doing any further testing). I was really hoping to see the same voltage droop issue that I was having when running single phase, but it wasn't so. It maintained a steady voltage over the entire load range.

This voltage droop issue is the biggest "problem" I see with the conversion so far, so I intend to tackle it first. I have a feeling it will involve some fairly simple wire swaps for the VR/exciter. I hope to get an hour or so this weekend to study the schematics and figure out what to do.

Hi Sewerzuk....

I've been following this with HUGE interest. I was looking at doing this....either double delta or the low zigzag. Decided that it probably wasn't worth the risk of melting my PU794 down if I mucked it up.

In my travels to learn what I needed to do, I looked long and hard at the exciter/regulator circuit.

It looks to me that the Static Exciter circuit receives 208 volts on pins L and M of J13. I don't know what the IC marked U1 on the schematic is, but I am guessing that it helps control the voltage delivered to the exciter.

Perhaps it is this piece of things that is responsible for the voltage sag. If it saw 240 instead of 208, it might not respond to a sag in the load........

Just guessing....wish I knew what the IC is....I haven't taken the cover off to see if the military contractor left the markings on it. looks kinda like a pulse maker that might be getting integrated to provide the exciter voltage..........

Thanks for exploring this......you are braver than me.

EDIT: After poring over the schematic, it looks to me that this circuit is getting its 208 voltage from T8 and T9 which is the weird leg in the zig zag....perhaps switching to T7 instead of T8? Even then perhaps it might be necessary to use a step down xformer to drop the 240 down to 208.......

UnReconstructed said:

Hi Sewerzuk....

I've been following this with HUGE interest. I was looking at doing this....either double delta or the low zigzag. Decided that it probably wasn't worth the risk of melting my PU794 down if I mucked it up.

In my travels to learn what I needed to do, I looked long and hard at the exciter/regulator circuit.

It looks to me that the Static Exciter circuit receives 208 volts on pins L and M of J13. I don't know what the IC marked U1 on the schematic is, but I am guessing that it helps control the voltage delivered to the exciter.

Perhaps it is this piece of things that is responsible for the voltage sag. If it saw 240 instead of 208, it might not respond to a sag in the load........

Just guessing....wish I knew what theIC is....I haven't taken the cover to see if the military contractor left the markings on it. looks kinda like a pulse maker that might be getting integrated to provide the exciter voltage..........

Thanks for exploring this......you are braver than me.

Click to expand...

Should be specified in the -24P TM.

Getting to this voltage droop, remember Ohm's Law. You have twice the impedence in the zig-zag leg (ohmic+inductive, for the most part, you can assume Ohm's Law).

If this generator's VR uses voltage of a single winding for output measurement, if that winding is in the zig-zag, there could be issues. In other words, you may have to change the reconnection so the single leg (neutral to 120V) feeds the VR.

Yes, this can make the reconnection a bit of a bear. If it were me, I would make a reconnection mock-up, feeding it off the panel through 10k ohm resistors and looking at the o-scope traces before applying the full reconnection/load.

UnReconstructed said:

EDIT: After poring over the schematic, it looks to me that this circuit is getting its 208 voltage from T8 and T9 which is the weird leg in the zig zag....perhaps switching to T7 instead of T8? Even then perhaps it might be necessary to use a step down xformer to drop the 240 down to 208.......

Click to expand...

I had hoped for this kind of collaboration...thanks!

I had an hour or so to sit down and read the principles of operation for the VR/exciter, and study the schematics. It looks like the VR makes 3 connections to the generator leads; T8, T9, and T12. That gives it 2 separate legs of 120v, and a 208v input when it is still wired 3 phase. It gets the same input regardless of where the voltage recon board is.
When I reset it for zigzag, those connections gave it 120v on what used to be 208, a second leg of 120v, and 0v.
I don't know what it does with those voltages...but since it "mostly" worked with that HUGE variance in voltage input, I'm not too worried about changing things around.
My plan is to move the lead that was on T8 to T11. This will change the VR inputs back to something more normal; 240v, 120v, 120v.

Before I do that, I'm going to double check the overvoltage trip...just so I don't accidentally have some kind of runaway and create lighting instead of steady single phase

Got finished with work a little early today, so I spent some more time reading over the schematics. I don't believe the difference between 208v and 240v is significant, as 240v is within the normal adjustment range of the VR during normal 3 phase operation.
I believe I identified and corrected all of the issues:


1. The voltage regulator senses voltage from between T8, T9, and T12. While I'm not sure what it does with these voltages internally, my low zigzag mod changed the voltages that it saw from this:
T8 and T9: 208v
T9 and T12: 120v
T8 and T12: 120v

To this:
T8 and T9: 120v
T9 and T12: 120v
T8 and T12: 0v

To correct this, I moved X8HH16B from T8 to T11. This results in the VR seeing the following voltages:
T11 and T9: 240v
T9 and T12: 120v
T11 and T12: 120v

As you can see, this brings its voltage inputs back to something more normal.


2. The thermal watt converter receives inputs from T7, T8, and T10. My low zigzag mod changed the voltages it saw from this:
T7 to T8: 208V
T7 to T10: 120V
T8 to T10: 120V

to this:
T7 to T8: somewhere around 208V. It is in the middle of the now floating L2
T7 to T10: 120V (right leg)
T8 to T10: 120v, depending on current flow, load, etc. Probably drops as load goes up.

To correct this, I moved X8A16B to T3. This results in the thermal watt converter seeing the following voltages:
T7 to T3: 240V
T7 to T10: 120v
T3 to T10: 120V

This brings the voltage inputs to the thermal watt converter back to normal.


3. This is probably the most important one; and looking at it I believe I'm a little lucky that I didn't burn anything up during my initial testing. I should have thought about/caught this issue right away when I first did the modification...but I caught it now and no harm is done. There is a lead that goes to T12 on the recon board that provides the L0 (neutral, or zero potential) to many components in the special relay box, the thermal watt converter, the voltage regulator, exciter, LMU, overvoltage relay, and the voltmeter (might even be more). When I cut the buss bar that connected T10, T11, and T12 together and ran the jumper from T12 to T3, I created a floating neutral inside of all of those components. This explains the strange readings I was getting on the voltmeter, and was likely the biggest reason for the large voltage droop. I moved the lead (X12EG16N) from its original position on T12 to T13, restoring the neutral.



After all of the reconnections were made, I fired up the generator and duplicated the load test. Sure enough, the voltmeter worked properly and the excessive droop was gone.
I only ran the set for a few minutes during the testing; this weekend I hope to do some extended load/overload tests, and then jump on the CT re-wiring to correct the ammeter and overload trip issue.