Generator frequency decreasing when overloaded
- Thread starter philly
- Start date
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ptonsparky
Tom
- Occupation
- EC - retired
The rpm of the driving motor drops and the gen itself is no longer turning fast enough to maintain hz.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
The output frequency is a direct function of generator RPM. Simply, the greater the load on a generator, the physically harder it is to turn. The energy has to come from somewhere.
The same thing happens in your car. Watch the tachometer or listen carefully to the engine idle speed when switching on and off the headlights, rear defroster, or other large load.
Years ago, in a science class, the teacher had a hand-cranked PM phone-ringer generator. If the output terminals were shorted, the generator could not be turned by hand at all.
The same thing happens in your car. Watch the tachometer or listen carefully to the engine idle speed when switching on and off the headlights, rear defroster, or other large load.
Years ago, in a science class, the teacher had a hand-cranked PM phone-ringer generator. If the output terminals were shorted, the generator could not be turned by hand at all.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
An engine powered generator has a device on it called a "governor" (not a politician) that looks at the speed of the generator and controls the fuel going to the engine in order to maintain speed. As the load increases, the engine starts to slow down, the governor increases the fuel and keeps it from drifting too far out.
Think of it the way you experience the Cruise Control on your car. If you have even rested your foot on the accelerator when in Cruise Control, you can feel it getting sucked down when you start to go up a hill. But no matter what, it is still limited by what the car engine can handle, and if the hill is steep enough and the pedal is all the way to the floor, the car still slows down. So it is with a generator governor; if the demand for electrical power is greater than what the engine can deliver, the governor can't make the engine put out more HP that it's capable of, so it slows down. In the case of an electrical generator, that translates to a change in the frequency, which is something easier to measure than engine output HP.
Think of it the way you experience the Cruise Control on your car. If you have even rested your foot on the accelerator when in Cruise Control, you can feel it getting sucked down when you start to go up a hill. But no matter what, it is still limited by what the car engine can handle, and if the hill is steep enough and the pedal is all the way to the floor, the car still slows down. So it is with a generator governor; if the demand for electrical power is greater than what the engine can deliver, the governor can't make the engine put out more HP that it's capable of, so it slows down. In the case of an electrical generator, that translates to a change in the frequency, which is something easier to measure than engine output HP.
- Location
- Massachusetts
Not that I disagree with the other answers, but .......... stop overloading the generator and the problem will go away.
Thanks for the responses guys.
Jraef your car example is a very good example and paints a pretty clear picture of what is happening.
Just for the record this is not an actual problem I'm having with overloading a generator just simply a topic I was reading about and became curious.
Jraef your car example is a very good example and paints a pretty clear picture of what is happening.
Just for the record this is not an actual problem I'm having with overloading a generator just simply a topic I was reading about and became curious.
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
Yeah, as the load gets beyond the generator, the frequency drops. And as noted the govener tries to maintain RPM to maintain frequency. When you have many generators interconnected, all their governers work to keep the frequency constant, and if it drops then they all try to speed up.
in a large interconnected system, if the frequency goes too low, or the rate of change of frequency is too great (depending on relay capability and setting), then relays will operate to shed load, to attempt to stop the decline in frequency.
If frequency dropping continues then an underspeed cutout will operate, and take the generator off-line.
In a large interconnected system, if that happens a lot you get a cascading blackout.
The opposite effect can also happen; if a large load goes offline then the frequency increases, and the generators need to back off, reducing the prime mover input.
We had a good excursion like that a while ago here in New Zealand, when the earthquake hit, pretty much all of Christchurch dropped off line, causing a large over-frequency excursion. The 'quake caused the transformers to shake, causing the oil to slash about inside. The large transformers have oil sensors, and they thought the transformers were out of oil, and commanded the transformer be shut down to prevent damage. Couple of seconds later the whole city is powerless.
Then the destruction to the underground cables began...
in a large interconnected system, if the frequency goes too low, or the rate of change of frequency is too great (depending on relay capability and setting), then relays will operate to shed load, to attempt to stop the decline in frequency.
If frequency dropping continues then an underspeed cutout will operate, and take the generator off-line.
In a large interconnected system, if that happens a lot you get a cascading blackout.
The opposite effect can also happen; if a large load goes offline then the frequency increases, and the generators need to back off, reducing the prime mover input.
We had a good excursion like that a while ago here in New Zealand, when the earthquake hit, pretty much all of Christchurch dropped off line, causing a large over-frequency excursion. The 'quake caused the transformers to shake, causing the oil to slash about inside. The large transformers have oil sensors, and they thought the transformers were out of oil, and commanded the transformer be shut down to prevent damage. Couple of seconds later the whole city is powerless.
Then the destruction to the underground cables began...
The generator loading determines the torque on the motor or engine that drives it.
Substituting freq. for RPM in the formula
5252xhp/torque= rpm
will tell you how hp, torque and freq. will vary.
This formula assumes the engine/motor hp is constant into the generator.
Substituting freq. for RPM in the formula
5252xhp/torque= rpm
will tell you how hp, torque and freq. will vary.
This formula assumes the engine/motor hp is constant into the generator.
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