Genset strggling to start motor on soft start
Genset strggling to start motor on soft start
(OP)
We upgraded a water pump to 50hp and added a solid state soft starter. We have an existing 60kW Cummins gas genset that will start up the pump most of the time, but often surges once the pump is almost up to speed, struggling to keep a constant speed. Sometimes the generator trips on overspeed.
The generator is has a PMG alternator and 3-phase rectifying regulator. We have tried playing with the soft start settings (Current ramp w/ different current limits, power ramp, etc.) but without success.
I suspect that replacing the genset with a larger one will fix this issue but I am open to suggestions for an easier fix.
The generator is has a PMG alternator and 3-phase rectifying regulator. We have tried playing with the soft start settings (Current ramp w/ different current limits, power ramp, etc.) but without success.
I suspect that replacing the genset with a larger one will fix this issue but I am open to suggestions for an easier fix.





RE: Genset strggling to start motor on soft start
There may be tuning of the generator's voltage regulator and governor that could help as well, but a VFD should solve the starting problem.
RE: Genset strggling to start motor on soft start
RE: Genset strggling to start motor on soft start
Set the ramp time to ZERO, or as close as it will get, and then set the Current Limit to 275-300% so that it goes into CL immediately Sometimes when you have a sever limitation in available power, RAMPING into anything just causes the system to lag and it can't recover as the pump load increases with flow. Going immediately into Current Limit can help with the acceleration time. If that doesn't work, inch up the Current Limit settings a few times. Make sure you don't exceed the starts-per-hour limits of the motor, so don't be in a hurry with this. Also if a centrifugal pump doesn't start with 350%, it probably isn't going to so don't go beyond that in this scenario.
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RE: Genset strggling to start motor on soft start
RE: Genset strggling to start motor on soft start
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Genset strggling to start motor on soft start
Alternately, just go Direct On Line. The load will pull the frequency down, the Under Frequency Roll Off feature of the Automatic Voltage Regulator will drop the voltage to maintain the Volts per Hertz ratio.
If there are other loads on the set as well as the pump, think VFD.
Bill
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"Why not the best?"
Jimmy Carter
RE: Genset strggling to start motor on soft start
Accordingly, perhaps align the fluid system to draw near maximum power. For example valves near wide open to draw maximum flow on radial flow centrifugal pump (assuming you can do so without creating other problems like water hammer, or motor accelerates too slowly causing overload trip). That way when the machine nears full speed the real load on the generator doesn't suddenly drop so dramatically... there is still the pump load. This is of course opposite of the way we would normally think about arranging the fluid system to assist motor start.
Just a thought.
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(2B)+(2B)' ?
RE: Genset strggling to start motor on soft start
"If the pump is the only load on the gen set, try connecting the pump directly to the genset and then starting the genset."
Does this have any detrimental effect on the pump motor? I have used this method often, mostly with very small motor load relative to the generator size (eg 15kw cooling fan motor on a 1500kva generator), without any problems, but what if the motor rating is closer to the generator rating?
RE: Genset strggling to start motor on soft start
RE: Genset strggling to start motor on soft start
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Genset strggling to start motor on soft start
First of all, contrary to what my understanding was initially, the genset would surge throughout the entire pump start.
The default gain setting on the governor was too high and this was causing the hunting/surging, just as IBRCAN suggested. We reduced the gain though the generator control panel settings, and the genset now has no problem starting the pump up smoothly.
Thanks for the input guys.
RE: Genset strggling to start motor on soft start
RE: Genset strggling to start motor on soft start