Generator voltage problems

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mull982

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We had a power outage at our plant this weekend, and like ususal went to fire up our generator. For some reason however we had problems with the generator and were not able to bring it online right away. The generator is a 480V generator.

The first issue that we had was that the breaker on the generator was tripped, and therfore power was not supplied to our system right away. I am not sure why this breaker was tripped. When we reset the breaker it kept tripping and we could not figure out why. Finally we took some volatge readings off of the generator and found the output voltage to be only 314V. After seeing this one of the electricians turned a dial on the side of the generator which increased the voltage to 385V, and finally the breaker stopped tripping. This 385V lead to voltage problems with other equipment in the plant however we were never able to get the volatge any higher.

What would cause this voltage to be such a low value? I'm assuming that this low voltage was what was tripping the breaker maybe on an UV trip however I'm not sure. I'm not familiar with generators so I'm not sure what the knob on the the side was for or why it increased to voltgge when adjusted. Does increasing the rpms of the generator have anything to do with the voltage or is this something else?

I have a tecnician coming to look at the generator however I was hoping to get some explanations of theoretical answers from you wonderful people.
 
Mull -
I know you already know this, and I know you guys were taking appropriate safety precautions. But, just for the rest listening in that may not completely understand, it's not a really good idea to reclose a CB if you don't know why it tripped.

carl
 
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mull982 said:
...What would cause this voltage to be such a low value? ...
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Bad voltage regulator
Switch line up wrong (VR set to "manual")
Underspeed (freq will be less than 60hz)
Bad Exciter
Bad field diodes
bad exciter slip rings (not brushless)

mull982 said:
...I'm not sure what the knob on the the side was for or why it increased to voltgge when adjusted. ...
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Probably a manual field current adjust - more field current, then the higher the output voltage

mull982 said:
... Does increasing the rpms of the generator have anything to do with the voltage or is this something else? ...
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Some, but within the range the gen should be turning - not much.

There have been previous posts on gen theory. I'll look for one.

carl
 
It sure would be handy if the generator had either a frequency meter or a tachometer. Anyone working on one should have a frequency meter.
 
a standby generator is useless if its not available for immediate use. Do yourself a favor, after its serviced by a qualified technician, cycle it once a week. Even better, get a nice ats that does it for you, and has remote. Could be anything from water in the fuel to animals living in the intake if its never run except when you need it.
 
Any new load on it? Is is very old? Even though I'm not psychic, the mere fact that it didn't shut down on undervoltage makes me guess it's an oldie that's never seen any qualified maintenance other than maybe changing the oil.
 
hillbilly1 said:
I'm supprised that the undervoltage circuit didn't shut the genset down. Was it a dual voltage rental unit?
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I don't think I have ever seen a gen that shuts down on UV. Open the breaker - yes, but not shut down the engine. How would you ever get it started?

carl
 
coulter said:
Probably a manual field current adjust - more field current, then the higher the output voltage


carl
Click to expand...

How does the current that is produced create a voltage in the generator? I understand the the current is created by a coil passing through a magnetic field but then cant figure out what happens with that current to produce the voltage and thus make your statement above be true.
 
Check..or get it checked by the representative of generator manufacturer,

a) Rotating rectifier assembly.

b) AVR

c) Voltage trim potentiometer.

ALso please ensure that you generrator is running at rated speed and the frequency is 60Hz.

Cheers.
 
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mull982 said:
How does the current that is produced create a voltage in the generator? I understand the the current is created by a coil passing through a magnetic field but then cant figure out what happens with that current to produce the voltage and thus make your statement above be true.
Click to expand...

mull -
http://www.amazon.com/Electric-Machinery-Fundamentals-Stephen-Chapman/dp/0070119503

This is a pretty good reference. Recommend to get a copy. (Oh yeah, you have to read it as well:rolleyes: )

Short course on alternators:
Two parts, first the stator: Three sets of coils (three phase) wrapped in slots cut in the steel around the outside of the motor. The output leads of these coils is where one gets the electrical power.

Second, the rotating field: This is a single coil fed with DC (just assume an adjustable DC power supply for now). So, as the field rotates, its magnetic field sweeps past the stator coils. That's what induces an AC voltage in the stator coils.

The magnitude of the induced voltage is porportional to the strength of the mag field, number of turns in the stator, the rotational speed of the mag field:

The rotational speed is pretty well fixed - 2 pole machine turning 3600rpm =>60hz. The number of turns is fixed., So this leaves the rotor mag field. The mag field is porportional to the turns on the field coil, and field current (ampere-turns). And of course, the number of turns is fixed.

So if one wishes to increase the voltage, then the current through the field must be increased. Normally, the Voltage Regulator senses the output voltage of the stator, and increases or decreases the field current to maintain the voltage setpoint.

Sometimes generators will also have a manual knob that can be used to set the field current.

I know this is pretty simplistic, but the physics are pretty close.

Q: Did you read that thread I referenced earlier?

carl
 
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Coutler

Yes I read your post from earlier Thank you. It gave a good explanation of how these knobs are used. The technician is now On-site so I am curious to find out what the problem is when he completes his checkout. I'll keep you posted.

Thanks for your response.
 
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