Email of the day, Question about Generator Stator Windings

From: Thomas

Subject: generator windings in parallel


I read the post and still confused. You rewire the head or the receptacles on a generator.

My wiring is just like the Coleman schematic. Why can’t I rewire the head to go from series to parallel before the breakers?

George’s Answer: 

YOU CAN…… BUT should you ?

We don’t want to exceed the current rating of the ‘winding’. We protect that..

Consider a possible fault: a stator power winding goes open when the generator stator windings are wired in parallel. now we have one winding doing all the work, where is your breaker, and is it still adequate to protect the winding?

In the case of a common plug in, many if not most today are rated at 15amps. Exceed that by too much, and you can damage the connections in the plug itself.

The potential problem can come from that higher rated plug, and sometimes we see 30 Amp 120VAC plugs in use on the generator or attached to it.  If we have such a thing as part of our generator, AND it’s protected with a 30 Amps fuse (an example), then we are set up to fry our winding via overload.

I talk in general terms, as there’s no way to keep up with the changes in generators, and exactly what you have. But I’ll attempt to share a potential difference in generator performance.

If you have a simple kiss generator head with a loose connection right where the two stator windings were tied together, and only one coil was providing power, (current) you’d see the voltage ‘droop’ far lower than expected as compared with two coils WHEN you increased the load. If we have no AVR at all, the operator of the generator might give pause, and wonder why?


With an AVR in use, might it see the voltage too low and attempt to boost field current, and if poorly designed, could it cause you even more grief? I think so fuse it or breaker it best.

If we open our household breaker boxes and see how they ‘breaker’…. it makes no difference how an American standard 120VAC supply is used.  The breaker is at the head of it all, and if it’s used to help supply a 240vac supply, it returns to another breaker in the box, and makes use of another 120VAC supply that comes from that commercial transformer on the street, and this secondary is very much like our two stator windings, they rightfully choose to protect each of the 120VAC supplies, and should we emulate what they do?

This transformer we see on the pole, it’s designed  similar to generator stators with the assumption that you will not need more current in any single 120VAC circuit than what one side of the center tapped secondary can deliver.

So… if you place that breaker and size it for the individual stator winding, you can save yourself a lot of trouble if things happen just wrong.


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As a side note:   There are some folks that are in areas unique, or they have been exposed to three phase, and they think those two hots delivered to your house are no more than two phases of the three. This is not the case, and we need keep it straight.  Your house is normally supplied by a single phase, and that transformer secondary winding and the center tap confuse people.

As for the Professionals: They don’t always get power right. I remember one Telecom office that had a number of air handlers that were effectively used to keep the building at a slightly higher air pressure than outside. About 5 years after install, Switching Tech Jim Swanson, noticed that one air handler was running backwards! He didn’t know why. But he did know that the HVAC guy had changed the filters and checked on things a number of times. Fact is, it was more than one guy that had done the maintenance and found it AOK.  I never ever noticed, Jim did! He didn’t know why, I did having a 3 phase AC back ground. I wrote a memo and sent it off to the power engineer to have two of the three leads swapped, it wasn’t our world, as most of what we did was -48 DC.

Swanson noticed a lot of things not done quite right, the type of man every crew needs, and always worth his weight in hard currency. And that reminds me.. Minimum wage was $1.25 cents when we were kids.  That’d be $26.50 in silver melt value today..







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5 Responses to Email of the day, Question about Generator Stator Windings

  1. Ronmar says:

    I wonder sometimes if some of this knowledge isn’t dying off. Had the same thing happen on a patrol boat I work on. Had cooling and ventilation problems. A large handfull of people worked on it and even swapped the exhaust blower motor(which was running at proper RPM) once with no positive results. I was interested as this blower drew hot air out of a console that contained some of our electronics equipment which was running warm. A little looking, and I determined they had a phase swapped on the 3 phase blower motor. Electricians swapped 2 wires and we were back in buisness. Come to find out, they had been living with vent problems since a failed motor was swapped out early LAST year… This was looked at by some pretty knowledgable people, even to the point of running a bore scope thru all the ductwork looking for an obstruction…

    • George B. says:


      Great example! In one technology I worked in, we had a few circuit packs that cost $30,000 each! Imagine frying those just because an electrician failed to use a phase rotation meter before he started a job, or to note if it were sucking or blowing. And of course, we don’t know how much of that electronics on your boat might die an early death… all because..

      One thing we do know, electricians always make the effort to check phase rotation when they tie up, and ready the boat to switch to shore power 🙂

  2. George B. says:

    Not everyone leaves a comment, I get emails that contain comments:

    Message Body:
    I have a 20Kw ST generator, and have been reading your latest Email of the day and that got me thinking… Oooops.
    When I remove the existing circuit box on the gen, I’m thinking that is where I should put the circuit breakers to protect the windings. My question is, I should put circuit breakers on what legs of the winding If I am in a 120/220 volt configuration?

    Thanks from Chuck!

    This mail is sent via contact form on UtterPower

    George’s Answer:

    I don’t want anyone to think I’m recommending that the breakers need to be in or on the gen head or generator frame. I really believe that anything we can mount off the head or gen frame should be mounted there. If it’s a stationary install, we have the luxury of doing so. The reason is after many hours of running, we see light bulb filaments (indicators) stressed by vibration, and a few of us have seen Analog meter movements fall apart over time, and other things happen too.

    My suggestion is to place that breaker at the head of what you do, the first connection other than an extension lead to bring it to your first panel box.

    But here you bring an example of where we draw the line, the conversation prior was about small generators, and how we can easily overwhelm one stator winding, and the danger of damaging one. The mention of the potential to add a DPDT switch where we might configure the stator windings to allow the 120VAC only setup was adopted by Honda on their popular 5000 watt construction generator, and I saw that on Makita’s, and a few Japanese brands not so well known but respected by the trades.

    At this power level, there are loads like chop saws, and portable table saws that tax one winding. While we mention it, smarter guys run shortest cords on air compressors for nailers, and they run long hoses.

    Burning up a nice saw is easiest to do on a long cord, the cheaper the cord, the easier it is to do..

    But where were we? Oh yes… we’re discussing your 20 KW head, that’s well beyond the size I’d consider setting up for 120 VAC only unless I had a very special circumstance. You likely know that Chuck, but let’s say it for readers..

    Murphy says.. as soon as you are convinced you’ll never have a need for 240VAC, you’ll have one.

    But.. where were we? Oh yes! I can’t think of where you can go wrong by putting those breakers in as the first thing after the wire and the statorwinding. But this gets us into other issues. There are precision breakers, and then the standards, and if we go with the standards we need know that they are often rated to carry 80% of their rating without tripping out, so do size for that max continuous load no matter what you use..


  3. Jetguy says:

    Actually, I am seeing a clear definition between a DIYer and average Joe. We have this ability to see what could happen. In our mind, we look at a machine, we follow the power and motion through the entire machine and then look for thing that could go wrong. We then step through each one and see how that affects every other part in the system. I can actually see the winding glowing red when the shunt connecting the 2 windings for load sharing fails and the full load is carried by one winding. You can imagine the AVR cranking up the current and voltage to wide open in the field. Almost able to smell the buring varnish and see the smoke. You hear the governor on the engine kick it up a notch to maintain RPM as the windings are now beginning to short and then the molten copper falls off the spinning rotor, jams between the field poles and the ensuing trainwreck now finds the weak link the the driveline. Belts would smoke, lovejoys could get interesting and hopefully the “event” doesn’t break the crank in the process. Worst case is a frame failure in that the engine breaks free and continues to run possibly rolling through the walls the structure in the process.

    Not everyone should be bashed for not having this “gift” but those of us who do can clearly see who does and doesn’t think that way.

  4. George B. says:


    Great comment..

    We don’t make fun of them, but when we see a man rant on… that you are only stupid if you don’t see the value in a voltage regulator, question if he’s really one of us.. Perhaps he’s just a salesmen?

    DIYers often define their need before they buy or build..

    Sometimes our needs are simple, and we KISS it at less cost and with higher reliability for what we do..

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