A question was posed on the Home Improvement SE... Can I buy or make an adapter to hook 2 generators together to run a 220 volt welder? Any contribution to this thread (from an electrical engineer) would be appreciated.

Essentially, the goal would be to take two (random phase) sources of 120VAC or 240VAC and combine them with a device to produce 240VAC. I assume that single phase and 60Hz is important; and conserving/combining the power/amperage would be especially important to the function of the welder.

What device(s) would be used to accomplish this (with respect to efficiency and simplicity of design)? A schematic (and maybe a flow chart) would be wonderful. Also, if the answer really is to rectify AC to DC then invert back to AC, would you (ideally) convert 240 or 120 VAC to DC? Also, would an un-transformed, full-wave bridge rectifier produce about 160VDC from 120VAC and 320VDC from 240VAC?

This "project" seems just like tying a generator to a grid (of equal power). I feel like there must be a better way to tie an AC generator to a grid than to rectify and then invert... and I'm thinking that there must be a controller-device that senses the grid-phase and adjusts the phase from an inverter anyway, right? Wouldn't it be more efficient to just use that controller-device (or something to control the phase or timing) to tie AC from a generator to AC from a grid?

Of course the motors could be mechanically synchronized, but in many circumstances that would not be convenient.

  • \$\begingroup\$ Maybe think hard about this. The generator phase is directly related to the engine shaft position. How is it possible for the phase angle to be changed without directly controlling the engine? \$\endgroup\$ – Dwayne Reid May 27 '16 at 20:58
  • \$\begingroup\$ @DwayneReid well a capacitor would certainly shift the phase... 180°, right? The phase shift would be electronic. I'm thinking a controller must be used for tying (for example: solar panels) to the grid, right? \$\endgroup\$ – Ben Welborn May 27 '16 at 20:59
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    \$\begingroup\$ Power plant generators are mechanically synchronized to be in phase at all times, using a computerized control loop. Your best best it to connect the shafts together with a shock absorbing connector and make sure their in phase. That is the cheaper solution. The AC>>DC>>AC solution will be very expensive. \$\endgroup\$ – Sparky256 May 27 '16 at 21:43
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    \$\begingroup\$ @Sparky256: the computerised control loops are a new invention and aren't required to run a grid. This is partially addressed in How do power stations maintain 50 Hz?. \$\endgroup\$ – Transistor May 27 '16 at 23:58
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    \$\begingroup\$ The idea is to use only a single generator, not two generators (which isn't quite what you asked for, but may solve the actual problem anyway.) The step-up transformer makes the 120Vac into 240Vac. The tradeoff is that the generator still has to supply the same power, so the load current seen by the generator is double. If the load draws 1200Watt at 240Vac (assuming PF=100% and no losses) then the 1800Watt(?) generator provides 120Vac at 10Amp and the load gets 240Vac at 5Amp. There will be some power loss at every step, and power factor may be less than 100%. \$\endgroup\$ – MarkU Jun 1 '16 at 19:46

I think you have misinterpreted the original question, which was only about hooking two generators in parallel in order to get enough current. Hooking them in series to increase the voltage was not mentioned.

Of course it's possible — that's exactly how all of the multiple generators attached to the national distribution grid are connected! Alternators of this type are synchronous machines, and function equally well as motors as well as generators.

The key to making it work is to make sure that they are in phase before connecting them. Once you throw the switch, they are effectively "locked" together as if their shafts were physically coupled. Each one will then add or subtract power to this "mini-grid" according to the torque on its shaft. If one tries to run slower than the other, its generator will be driven by the other as a motor, keeping it up to speed.

One simple way to check the phase is to simply connect some light bulbs across the circuit breakers. Make sure that they're rated for 2× the phase voltage, because that's what they'll be getting when they're out of phase!


simulate this circuit – Schematic created using CircuitLab

Fire up the first generator and connect it to the output grid. For each subsequent generator, you fire it up and watch its light bulbs. They will flash at a rate that's equal to the frequency difference between that generator and the grid. Adjust the speed of the second generator until the flashing slows and the light bulbs go out. At that moment, the generators are at the same frequency and phase, and you can connect the new generator to the grid.

Of course, the small DIY generators that we're talking about aren't really meant to be controlled in this way. They generally have simple mechanical governors that keep the frequency approximately right, and voltage regulators that modulate the field current to keep the output voltage approximately in the right range. These mechanisms would probably be "confused" to some degree by such a hookup.

It's also possible that the engine is over-powered relative to the generator, and has more drag than the generator (when operating as a motor) can overcome. This would force the two generators out of sync, and large currents would flow, hopefully tripping their breakers and disconnecting them.

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    \$\begingroup\$ Even if you phase synchronize the two generators, they are not going to stay synchronized for any period of time. Even with no load. \$\endgroup\$ – Eric Urban May 27 '16 at 22:42
  • \$\begingroup\$ @EricUrban: Why do you say that? Explain what's wrong with my description of how it works. \$\endgroup\$ – Dave Tweed May 27 '16 at 22:55
  • \$\begingroup\$ The generators do not run at an identical RPM. The generators also do not run at a constant RPM. When the load changes the RPM deviates briefly from the desired value. \$\endgroup\$ – Eric Urban May 27 '16 at 22:58
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    \$\begingroup\$ @EricUrban: Do you understand the concept of synchronous machines? Once connected to the grid, they do indeed run at the same frequency, and they track each other as the load changes. But the regulator on the "prime mover" (the gas engine in this case) needs to be designed to cope with this scenario, as I alluded in my final paragraphs. \$\endgroup\$ – Dave Tweed May 27 '16 at 23:08
  • \$\begingroup\$ @EricUrban Tying generators together the way Dave describes is a bit like mechanically tying the shafts together through a spring and clutch. \$\endgroup\$ – Spehro Pefhany May 27 '16 at 23:22

David Tweeds answer will work and is easy for people to visualise .Wiring could be simplified by using a single 120/240 power transformer to create a 120VAC grid.The single transformer is just as safe as David tweeds answer.If you know what you are doing and understand the single transformer you can use an auto transformer .The auto transformer wastes less power and is smaller lighter and cheaper .

  • \$\begingroup\$ There are no actual transformers in my diagram. I represented the generators as center-tapped transformers, partly because CircuitLab does not have an actual generator symbol, and partly because most DIY generators have dual windings that are connected in series to produce 240V with a center-tap neutral. The point of the original question was to produce enough current at 240V to run a high-powered welder. \$\endgroup\$ – Dave Tweed May 27 '16 at 23:31

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