Excitation Question, Diesel Generator


Thread Starter


We have four diesel generators that supply both "hotel power" at 440v, and a short time 600v for a large motor that we need to start occasionally.

The 600v is obtained by over-exciting the generator with an additional 48vdc for a short time (typically 6 seconds). After which, the generator goes back to supplying the normal 440vac.

As mentioned, each of the four generators do their jobs independently. Each has a "hotel load" and a short time motor start load. Three of the four generators are working correctly with the 600vac short-time output.

This is the issue, we have one generator that has recently seen a need to increase the 6 second over-excitation to 12 seconds. I'm at a loss as to why we need to double the excitation time.

Anyone familiar with this problem?

Thanks in advance.
You could be seriously damaging the generator stator, core, and field by overexciting it.

Please provide the complete nameplate information on the generator, excitation system, diesel engine, and the large motor.

What happens if you try to start the motor without overexciting the generator?
>Are you seriously telling us that you over excite a 480V
>Gen to 600V and now wonder why it isn't working?

I am "seriously" telling you the manufacturer is over exciting the generator.

We simply use it, per their design, in our facility.


These four generators have been doing the same motor start, 4 times a day, everyday, for 15 years.

This is a new issue that I thought might be able to be solved through this forum, I guess I am mistaken.

Thanks anyway.
OK. Lets start all over. Sorry we offended you, but this is a very unusual situation. Most of us are experienced with much larger generators with tight margins that heavily punish the user for exceeding design parameters. But if you have been doing 4 starts per day for 15 years, it sounds like your generator was properly designed for this unusual application.

So each of the 4 generators drives a large 600V motor. We need more information to help you.

1. We need nameplate information on the diesel, generator, motor, and the load the motor is running. Also the starting current.

2. Are all 4 generator system identical?

3. What happens if you don't give the excitation system the extra time?

4. Is the load accelerating at a slower rate on the questionable generator vs. the good ones?

5. Do you have access to recording equipment to record generator 3 phase output voltage, motor current, motor speed?

6. What load is the motor running - a fan, compressor, pump?

7. Is there something that controls the load on the motor - fan dampers, unloading valve, discharge valve? Is it possible that the motor is not unloaded during the start when it is supposed to be?

Lots of people would be happy to help you with this very interesting application, but we need a lot more detail.