Almost all, if not all generators at the nominal size for an 'emergency' set these days will have under frequency roll off (UFRO) capability in the AVR. This drops the excitation until the machine is within operating speed.
What Freddy said.
A long time ago, AVRs did not have the UFRO function.
The old AVRs would generally withstand running out of fuel, but running the set slow without first turning off the AVR would toast the AVR in a minute or less.
This often happened when a mechanic ran the set at an idle to adjust the valves.
That was so long ago that all of these sets would have been repaired with a newer AVR with UFRO.
UFRO typically starts dropping the voltage at 47 Hz or 57 Hz for 50 Hz and 60 Hz sets respectively.
Bill
--------------------
"Why not the best?"
Jimmy Carter
The primary function of UFRO is to reduce loading when an overload has dropped the frequency.
Reducing the voltage to resistive loads drops the kW demand.
Reducing the voltage to motors when the frequency drops maintains the V/Hz ratio and avoids magnetic saturation.
Many loads have a reduced kW demand at reduced speed.
All of this helps the generator to recover from overloads and block loading.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Yes, but the normal procedure is to start, stabilize and then close the switch.
Some loads may not like to be brought up slowly.
I would not be surprised by a lot of refrigerator failures.
A typical small hermetic unit bleeds off the differential pressure and typically starts up unloaded.
Once running it has a lot of dependency on the flywheel effect to carry it through the reciprocating torque pulses.
With a slow start there is a danger that the diff pressure will build and stall the motor before it is going fast enough for the flywheel effect to become effective.
Of course the internal thermal protection will ALWAYS protect a stalled motor, until it doesn't.
A caveat, if the only load is a motor that is too large for the gen-set to start, it may be possible to start it by connecting it to the set and then starting the set.
This will provide an effect similar to a VFD. Both the voltage and frequency rise together with very little starting surge.
This may require the controls to be fed from a UPS, and a PMG may have to be bypassed with a non PMG AVR powered from a UPS.
If there are other loads in addition to the motor they may be connected after the motor starts.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Good to know. I have extreme scenarios in mind- running out fuel, an ATS that has failed to transfer to normal, someone hitting the "E" stop, starting the gen backup afterwards with every ATS still in emergency.
With the monumental lack of focus it's not a good example of anything.
But, for a 50 Hz generator the setting that I am used to is 47 Hz.
Seeing a voltage drop below 45 Hz is reasonable.
Bill
--------------------
"Why not the best?"
Jimmy Carter
thread237-389842
I think that the UFRO function was probably added in the 90s.
Due to the failure rate of the non UFRO AVRs many of them have been replaced with newer models.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Beware of low fuel and automatic start.
I had a set run out of fuel and then repeatedly try to start until the battery was dead.
This was a light tower providing job-site light at a remote intersection.
The fuel truck had filled the tank but was unable to start the set.
We did not have a heavy jumper set. We had to wait while our truck put some charge back into the set battery before getting it going.
Not the most fuel efficient use of a pickup truck.
If this had been a standby set in a building with the utility power still out the challenge may have been greater.
Something to consider.
ps. I have no idea why a light tower had automatic start. It is not normal for light towers.
Bill
--------------------
"Why not the best?"
Jimmy Carter
