Genset strggling to start motor on soft start 2

[X49]

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.

 

[dpc]

You might consider replacing the soft starter with a VFD. Assuming this is some type of centrifugal pump, if the generator is large enough to run the pump at normal speed, it should be able to run it up on a VFD.

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.

 

[X49]

Unfortunately space is very limited and the cost of adding drives would also be quite significant since there are two pumps in duty-standby. However, this may still be preferable to replacing the genset.

 

[jraef]

Before you do that, see if you can program the soft starter like this.

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.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[LionelHutz]

It sounds like the governor has stablility issues. It starts to overshoot when it backs off the throttle once the motor starting inrush is done?

 

[jraef]

A long time ago, old AVRs could not handle the "chop" created when SCRs are firing and it translated to instability. Then soft starters that used Phase Locked Loop sensing for the zero-cross would go nuts trying to track the moving target, and the entire process would result in the soft starter never allowing the motor to fully accelerate and it would trip on overload. That was a big problem 10 or 15 years ago and the soft starter mfr I had worked for actually came out with a retrofit filter package for the AVRs on CAT generators because it was so common. But I would have thought that all generator mfrs have encountered it by now and dealt with it by adding more filtering into the AVR sensing circuit. I haven't seen this happen for a long time, but maybe this is a really old genset? If not, a really crappy one perhaps?

"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

 

[waross]

If the pump is the only load on the gen set, try connecting the pump directly to the genset and then starting the genset.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Maybe a contributor to the transient is the sudden transition from high generator real load during start to very low load as the motor reaches full speed.

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.





=====================================
(2B)+(2B)' ?

 

[genhead]

Waross
"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?

 

[IBRCAN]

I recommend you have a look at the generator engine's governor, as others have suggested. I don't have much experience with gaseous fuelled engines, however a diesel genset should be able to go from 100% load to no load instantaneously with minimal overshoot ie. approx. 2-3 hz. Would expect a gas engine to operate similarly. The governor gain setting may be too high, which can cause prime mover instability after load transients.

 

[waross]

genhead; Think of it as the poor man's VFD. The load has pulled the frequency down and the AVR has dropped the voltage to maintain close to the same V/Hz ratio. The slip frequency is lower than DOL starting on the grid. You have a good chance of starting a motor this way that may not be successfully started DOL on an undersized generator.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[X49]

I thought I would bump this thread up now that the issue has been solved.

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.

 

[dpc]

Thanks for the update - it's nice when the solution is simple.

 

[IBRCAN]

Good to hear you got it sorted out.