generators

Hello, and no. Assuming we're talking about two gasoline/diesel engine-driven A.C. generators, you can't wire their outputs in parallel unless the equipments are operating at the same frequency (both engines operating at the same rpm) and have output voltages that are of equal amplitude and in phase. Even if you could manually set-up these conditions, the sets would quickly drift out-of-sync. IOW, auxiliary control circuits are required to provide this capability. Some gensets have the capability built-in and all you have to do is connect a special synchronizing cable between the two sets. Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

Hello disaster.

If they were really generators the answer might be yes but as they are more likely to be alternators then the answer is no. Is the current that these two units provide AC or DC?

You need what is called an optical isolator. Just wire half the lights to one unit and half to the other.

If you can ever get them in sync they will tend to hold each other in step but I am not really sure how you do that. I suppose if you had some large resistive load that you could connect between them, with the other phase conductors connected it would end up pulling them together. Once the voltage across the load dropped to zero you could connect them.

LOL Oh, that brightened a dull morning..

If you have individual control of the governors and voltage regulators, all you really need is a couple of light-bulbs.

If the two units are separated by a single, double-pole switch; connect one light bulb from line to load side of one pole, and another light-bulb from line to load side of the other pole. With both units running at *nearly* the same speed, the light-bulbs will each 'see' 240V when the two generators are 180 degrees out of phase and 0V when they are 0 degrees out of phase (i.e. in synch).

Adjust the unloaded unit's governor so the bulbs are rising/falling in brightness at a slow pace and shut the disconnect 'midway through the period of darkness'. Or, install a voltmeter across one of the two bulbs and wait for 0VAC.

Of course, now that they are paralleled, you have the bigger issue of keeping them paralleled and sharing the loading (both real and reactive). We could write a couple of chapters about governor droop settings, voltage regulator droop settings, instrumentation to monitor real and reactive load sharing, reactive compensation circuits, unit protection schemes, etc... etc... etc...

Getting them in synch and paralleled, that's the easy part. As Paul Harvey might say, "It's the rest of the story...."

daestrom

Sure is !!

Okay, the *short*, *short* answer.

NO!!

daestrom

The real *short* answer may be a *Short(Circuit)* if things go just slightly wrong.

I discussed some of it in a previous post. Did you read it? Here's the long answer....

In order for the two generators to be connected together, they must both reach the maximum positive voltage at the same instant. They must also reach their maximum negative voltage at the same instant. This is called being 'in synch' (short for operating in synchronism with each other).

So first, you have to hook up some 'instruments' to figure out exactly when the two are 'in-synch' in order to connect them together. If you screw this up and connect them when one generator is at its maximum positive part of the sine wave, and the other is at its maximum negative point, you have 240V of the two generators and nothing but copper wire between them. Massive amounts of current, sparks, 'magic smoke leaking out'.

But even once you have them 'in sync' and connected together, you have another set of problems. What if one generator's maximum positive voltage is just a little bit higher than the other? Generators designed to be in parallel have a special circuit built into their voltage regulators to handle this situation (it's called a 'reactive droop' circuit). If your machines don't have one, then any mismatch in the voltages between the two machines results in 'circulating currents'. Those are currents flowing out of one machine and into the other. They too can reach very high values, overheat the windings and cause the 'magic smoke' to leak out.

Next, even if you get the voltage regulators to work together and avoid circulating currents, you next have to look at the way that the load is shared between the two machines. What is there to keep one machine from supplying all the real power to the load, and the other machine to just sit there idling? Or worse, what keeps one machine from supplying power into the other machine and actually turning it into a motor? The governors of the two machines prevent this if they are set up right. By adjusting the governors of both machines to deliberately slow down the engine *slightly* when there is more load on the generator. So if they started out equally sharing the load and some fluke of nature caused one to assume a little more load, its governor would slow its machine down *very slightly* and that would cause some of the load to be shifted from itself over to the other machine. This adjustment is called 'speed droop' and its important that both machines have comparable settings.

Now, do your two gensets have reactive droop circuits in the voltage regulators to prevent circulating currents? Almost certainly not. Do your two gensets have governors that are calibrated and adjusted to provide the same amount of speed droop on the two machines? Probably not.

A few gensets out there are built to be operated in parallel. The Honda EU series is one that comes to mind. But most gensets this size are designed as backup power for some small number of loads. *Not* for parallel operation as a generating unit.

daestrom