Re-wiring a three phase generator
GENERATOR SET, DIESEL ENGINE DRIVEN, TACTICAL
SKID MTD., 60 KW, 3 PHASE, 4 WIRE, 120/208 AND 240/416 VOLTS
|DOD MODELS||CLASS||HERTZ||N S N|
|MEP006A||UTILITY||5 0 / 6 0||6115-00-118-1243|
|MEP105A||PRECISE||5 0 / 6 0||6115-00-118-1252|
426 cu in turbo charged
HP roughly 135-150 at the flywheel
1985 is the last year of Allis Chalmers engine production
There were only a couple of industrial applications that the 426 went in.
Most parts marked with a CAGE code:
Company Name: LIMA ELECTRIC CO INC THE
CAGE Code: 36156
ZIP Code: 45801
Address: 200 E CHAPMAN RD
ZIP: 45801 - 2012
Generator ID plate looks like this:
|DEPARTMENT OF DEFENSE|
|NOM.||GENERATOR, ALTERNATING CURRENT|
|SER.||xx-xxx-xxx||NO. OF WIRES||12|
FRAME AND SUBSYSTEMS
Company Name: ENGINEERED ELECTRIC COMPANY DBA FERMONT
CAGE Code: 93742
ZIP Code: 06606
Address: 141 NORTH AVE
ZIP: 06606 - 5195
Company Name: ELECTRIC MACHINERY DRESSER INDUSTRIES INC
CAGE Code: 19899
ZIP Code: 55413
Address: 800 CENTRAL AVE
ZIP: 55413 - 2403
This is a graphic of the component location for the 12 wire board.
Studying the wiring diagram for similar 12-wiregenerators, I have determined that the generator is wired in a 12-wire WYE or STAR configuration.
See related story UNDERSTANDING HOW GENERATORS WORK
The wiring board has built in capability for HIGH 240/416 or LOW 120/208 wiring.
The blue squares are interconnects. Lugs are labeled similar to the winding wires, ie: T1, T2, ect...
Windings are shown in red, they were inferred by the changes that take place when the interconnect board is moved.
Subsequent testing with a meter proved the diagram to be correct.
The CURRENT CURRENT EXCITER are the items from the first illustration that the generator leads loop through on their way to the interconnect board. The longer loop-through is labeled "Exciter" and diagrams state that the other loops are for load measuring.
Working from this configuration, it seems possible this unit can be re-wired for single phase 120/240 in a ZIG ZAG.
In this a ZIG-ZAG configuration you would have 120v from L1 to N andfrom L3 to N, 240v from L1 to L3.
This is the final output with the ZIG ZAG configuration.
The phase sum can be visually calculated by visiting:
and entering 60 cycle 100 amplitude for each wave, 120 phase for first and 240 phase for second.
Some people seem to think that the final result is "out of phase" and therefore not 240v.
I hope this graphic puts their fears to rest.
The genset is obviously now DE-RATED.
What does this mean? Well, I used to have a three phase generator with 60kw total rating - 20kw per phase. I no longer have three phases, so I obviously don't have 20 x 3 = 60kw anymore. What do I have?
Some people think I only have 20kw, since I only have one phase now. This is a misconception, a misunderstanding of residential power. Residential power is often referred to as single-phase. That's because the power company generates and transmits power as three phase, and taps a single phase with a transformer to drop to a residence. What's missing from the equation is the nature of the transformer: it has a center-tap. Residential power has 240v from leg to leg, and 120v from either leg to the center tap, or "neutral".
It would be more accurate to refer to residential power as SPLIT-PHASE.
This is what we are generating with the zig-zag configuration: split-phase power. Two hot legs, or phases, that are anti-phase to each other and share a common or "neutral" connection. We are emulating the output of a center-tap transformer.
To DE-RATE the generator, we could simply say we have two 20kw phases, or 40kw. This is oversimplistic, but it works: when re-wiring a three-phase generator for split-phase residential power, de-rate the generator to 2/3 the original power.
A more complex analysis tells us there are three major factors involved when rating a generator.
The first is the power factor of the prime mover - in this case the horsepowerof our engine. This is usually the limiting factor in a generator set. In this case, we know we have sufficient power to generate 60kw, so it is not a limiting factor.
Ditto for the second factor: magnetic field strength.
The third factor, which is the capacity of our windings and leads to carry current, can be quite complicated. (calculations of the effect of field and flux on the windings are tremendously difficult) We know there must be an upper limit, and we know what each phase can carry safely, but we don't have enough information to calculate the true upper limit. If we load the generator to it's original rated capacity, we could burn out our windings - or our output leads. Without further information we have to de-rate the genset by 33% for safety.
My genset has LMUs (Load Measuring Units) that will automatically shut down the exciter if the load on any phase exceeds 110% of the rated load. So in my case I can rate the generator at 22k x 2 = 44kw, or about 25% of original rating. 25% is usually a good de-rate measure, but don't take that as blanket permission to overload YOUR genset! If you are not sure your set has overload protection, connect it to your load with a line breaker rated at 75% maximum of your original load rating - or 66% for ultimate safety.
Here is how I converted my genset.
First I cut the buss bar from T12 on the back of the reconnect board. I left it laying there so you could see what I cut off.
Then I added a battery cable to choose which position I would connect T12.
If I connect T12 to the N lug, I have WYE in LOW or HIGH.
If I connect T12 to T2, I have ZIG ZAG in LOW or HIGH.
(Yes, it's connected to T8 and the moment, but since I have no use for 240/480 I won't be moving the selector plate up anyway.)
I have perfect output and run my entire house with no problems. This particular genset de-rates to 44kw (183amps) wired zig-zag, but since my main breaker is 150a (36kw) I can't even use that much!Top