1 phase from 3 phase gen

[dougyes]

We were checking out a military generator. The specs on the side have various voltage options, all 60 htz and all 3 phase. There were 4 lugs to hook up to for power and one ground lug on the chassis. How can we get 1 phase from this unit?

[Geoff]

You just have to change the wiring from the generator.  But you will lose output-- for instance, I rewired a 5kW 3-phase generator for 120 and the out put went down to 3500 kW.  There is simple wiring diagram showing Delta , Wye, and Zig-zag wiring references for 12 lead wiring.

[kevink1955]

Looks like he is looking at a 4 lead unit that would not be reconectable for single phase. You can use 2 of the 3 phase leads to get single phase power at two thirds of the generator rating. Being it is a mil spec unit you may even be able to get more than the two thirds with out damaging the gen end.

Be aware that most 3 phase sets are 120/208 volt output not the 120/240 that the single phase unit would be. Your 240 volt loads may not like the lower voltage, anything with a heater will accept it but heat slower. Motor loads need to be checked on a case by case basis.

[gus]

My old Onan was originally a building standby 3-Phase rated at 7.5KW.

It was converted to single phase at 5.5KW.

[Ray D]

If the military unit can not be changed, maybe you would have a dedicated second breaker box for the 3rd phase.

Ray D

[Sean]

... The specs on the side have various voltage options, all 60 htz and all 3 phase. There were 4 lugs to hook up to for power and one ground lug on the chassis. How can we get 1 phase from this unit?

That depends on how many wires come out of the head and connect to those four lugs.

If you have 12 wires there in total, then the head can be re-strapped for either straight 120 (which I recommend for sets up to 6kw in size) or for 120/240 split phase (which I recommend for larger sets) with a de-rating of 2/3.

If you have fewer wires than that coming from the head, then the set can not easily be re-strapped and you would need to re-wire the stator.  Probably not worth it -- just use two of the legs as Kevin said.  You can connect a buddy plug or something similar to the third leg.  On the two legs you use, you can get 2/3 the rated power if all your loads are 120.  You'd get just 58% of rated power for line-to-line loads.  Remember that resistive loads such as heaters will have just 75% of the output that they would have on 240 when run on 208.  All this is why surplus three-phase units are often not worth the trouble.

If you have a model number I might be able to tell you more.  A photo of the junction box would be the next best thing.  Failing that, the number of actual leads coming off the head and a description of how they are labeled would help.

-Sean
http://OurOdyssey.BlogSpot.com

[dougyes]

Thank you all!
Here is the model number: MEP 004A, mfg date 9/76
12.5 KW
options on the tag:
240/416v 60 htz 3 phase
120/208v 60 htz 3 phase
197/240v 60 htz 3 phase
I think it was made by White co.
It has 73 hrs on the meter

[Sean]

Well, I have good news and bad news.

I downloaded the manual, all 493 pages of it.  The good news is that it is, indeed, a 12-wire head and could, in theory, be re-connected to a zig-zag output for 120/240 split phase at 2/3 of the full rating.

The bad news is that several of the stator leads pass through current transformers which provide feedback to the regulator, as well as another set of transformers for current boost, and yet another set of transformers for metering and paralleling purposes.  Merely re-connecting the 12 leads to the proper configuration could lead to problems with the regulator circuit.

Someone with way more time and possibly more generator expertise than I have would need to pore over this manual for quite a few hours to discern what, if any, impact reconnecting the stator leads would have on the regulator circuit before you could proceed.

The 12 leads are in a standard configuration, with adjacent loops being on T1-T4/T7-T10, T2-T5/T8-T11, and T3-T6/T9-T12.  I would guess that the moving link connections in the existing output J-box, which would seem to also have a three-pole load contactor inside, reconfigure from parallel to series on each winding pair.

Like all things military, this is documented to a fare-thee-well, so you should be able to find someone who can check the regulator circuit for you and determine if the leads can be reconnected to a zig-zag configuration.  All that said, I have to say this set is a very complicated piece of equipment compared to the typical RV generator and could pose a real challenge should it happen to break down somewhere along the way.  I prefer sets without so many complicated circuits and parts.

FWIW.

-Sean
http:\\OurOdyssey.BlogSpot.com

[Oonrahnjay]

(snip)  I have to say this set is a very complicated piece of equipment compared to the typical RV generator and could pose a real challenge should it happen to break down somewhere along the way. 

    Added to that is the "'76 Year of Manufacture" label.  I'm not sure that I'd want to rely on a 35-year-old piece of electrical/electronic equipment that's been operated for 2 hours a year during that time.  Military stuff is usually preserved pretty good, but taking this kind of risk on an item that would require the special kind of info and knowledge that Sean describes here ... I dunno.  "Are you feeling lucky"?

[dougyes]

Thank you all for the replies. Even if it changing to 1 phase worked, we would not want to loose 1/3 of the capacity. And your advice on simplicity is well taken.