How to Synchronize Two Generators

Principle:

 

The theory is to synchronize two generators so that they can both supply the same load. Their output voltage, frequency, and phase are matched so they can operate in parallel which will add the capacity of the second machine to the first. Once in sync, they will tend to keep each other in sync and if one generator engine shuts off or fails to keep up, the other will run that generator as a motor, quite possibly damaging the alternator and engine.

 

Use generators that have manual controls to adjust engine speed (frequency) and voltage.

 

The generator receptacle panels should each have a 120-volt convenience outlet to use as the alternator leads to sync the generators. The larger 120/240 volt 30 or 50 amp receptacle is used to parallel the generator outputs and supply the load and must have its own breaker to turn the output on and off. This is breaker referred to below.

 

120/240 volt generators have two hot leads and one neutral. One lead is L1, the other is L2. When making the sync circuit, connect to L1 on both generators, or connect to L2 on both generators. If you can’t figure it out, then don’t so this.

 

Beginners Method:

This is sometimes called The Dark Lamp method. Its used primarily for two generators that are the same size and is an inexact method. You probably don’t want to try this with generators of more than few kilowatts in size.

 

Decide which generator is the lead and which one is secondary. Turn off the 120/240-volt breaker on the secondary generator. The main breakers on both generators should be on.

 

Connect the 120/240-volt receptacles in parallel. Turn off the 120/240 breaker on the secondary generator.

 

Connect the neutral leads from the convenience receptacles together with a single wire.

 

Connect an incandescent, 240 volt lamp in series between the hot leads of the convenience outlets.

 

When either or both of the generators are running, there is current present on the neutral and hot connection, the wires, the switch, and the lamps regardless of the breaker positions—use all due care not to touch or come into contact with this wire or any connections.

 

Check connections and setup. 120/240-volt breaker off on secondary. Hot leads connected in series with a 240-volt lamp. Neutral leads connected together. 120/240 volt receptacles connected in parallel.

 

With all connections in place, start the lead generator, then the secondary. The lamp will either turn on and flicker or stay on. It is possible, although very unlikely, it will not turn on and in that case the generators are perfectly in sync. Consider your chances of winning ten Powerball lottery jackpots over ten consecutive weeks far higher than this actually happening - well maybe not that high, but this is a very rare happenstance.

 

Slightly alter the engine speed (frequency) of the secondary generator either faster or slower to slow the flickering to a slow blink. A good on-off cycle is about one-half second if you can manage it. The light is out, then grows bright as it turns on, dims as it goes out, then goes dark again.

 

When the light is dark, the two AC-cycles are in sync. As the light goes dark, turn on the 120/240 volt breaker of the secondary generator. The secondary generator will adjust to the cycle of the lead generator.

 

At this point, whatever phase difference existed between the two generators will almost instantly correct itself. Make sure you turn on the secondary breaker when the phases are as close as possible (Dark Lamp) or you could seriously damage either or both engines or alternators or both. On larger generators, the sudden correction can cause catastrophic damage and injure anyone close by. This is why this method is only appropriate for small generators of perhaps 5000 watts or less.

 

Good Theory Put into Practice:

 

Once you understand the principle and have successfully synced a pair of small generators of the same size, with the right equipment you can sync larger generators or multiple units to work in parallel similar to the way the power grid works and you can connect multiple generators of different sizes to act as a single source.