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Re: ganging up cheap generators

Original poster: Frank <fxrays@xxxxxxxxxx>

In the real world, trying to get 3 small generators to run as a 3 phase system is virtually impossible. There is not enough mechanical momentum to allow each gen set to maintain proper phase, let alone ever determine how each gen could maintain a proper phased relation to the next one.

I do not know of any scheme to generate a multi phase power from single phase generators. I am sure it could be done but the control electronics would be unreal as well as you would need large sets, probably over 100KW to have some mechanical momentum to allow for fluctuations so one gen set will not try and trying drive the others.

even Edison found out the hard way that paralleling simple DC generators was not just connecting the outputs together.

The only way to get a phased output would be to mechanically couple each generator to be driven from a single shaft.

If the goal is to get larger power from smaller, cheaper gen sets, then have each set run its own supply and then parallel the outputs of each supply, assuming they are DC. The rectifiers would block any voltage differences from getting back to the AC side and you could parallel the output to a single energy storage source.
You would use the same components, just 3 transformers instead of one.

Thanks, Frank

At 04:45 PM 1/10/2007 -0700, you wrote:
Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 11:07 PM 1/9/2007, Tesla list wrote:
Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Tesla list wrote:

Original poster: <davep@xxxxxxxx>

 > Actually, with a suitable zigzag transformer, apparently
 > synchronization is no worse than just paralleling them.
 > Frequency/phase takes care of itself (the generator that wants to run  a
 > bit fast takes more load, slowing it down).
     Which means it takes more of the load, so there is less than 3X
     power available.
     Also the underspeed.voltage units will tend to _absorb_
     power, robbing the external load.
 > There IS an issue when throwing the switch to connect them.
 >  Amplitude is a matter of regulators and throttles.
     An current sruging baack and forth as the throttles
     run the units up and down.  And the fact the voltage
     regulation is less important than frequency regulation
     when paralleling AC sources.  And the fact that 'identical'
     regulators AND governors on three 'identical' units
     won't be.  cf any text book on power system stability.

The above may be true when attempting to synchronize AC outputs. However, both of the above problems should go away if each generator's output is first rectified before being combined to supply a common DC bus... especially if the bus also has a substantial energy storage cap.

The idea was to make three phase power to run into things like HV power supplies that have 6 or 12 pulse rectifiers (so the ripple is lower)

I suppose you're right. If you had three single phase HV transformers and the rectifier, the generators only need to need to be "close" to 120 degrees apart, and, the actual voltage regulation isn't as critical. Might well be that the load impedances seen by the generators would tend to pull them into approximate phasing. (i.e. if two generators were exactly in phase, the current pulses through the rectifiers would be simulataneous, and lower in amplitude, causing BOTH generators to speed up, but they'd be different.)

Also, I'm not sure how a standard three leg transformer (E core with a set of windings on each leg) would work with imbalanced phases. But that's a pretty standard problem.