Standby Generator Unable to Start a Motor

[CuriousElectron]

Hi Guys,

Has anyone had a chance to witness an undersized generator unable to bring connected motor up to speed? I'm thinking of a situation where the required starting KVA of the motor was higher than the max output generator could provide at adequate voltage..

Just looking for any practical experience on this.

Thanks,
EE

 

[waross]

I have found that the minimum reserve KVA for DOL motor starting is 2.5 times the motor KVA.
If there are other loads already connected the minimum available KVA goes up to 3 times the motor KVA.
Generator sizing software from Cat or Onan use a more conservative voltage drop and ask for a higher ratio.

When a generator is overloaded by the starting current of a motor several things happen.
The AVR adjust the excitation to hold up the voltage.
A PMG excitation system is better than a self excited system in this regard.
Motor starting current is at a low power factor but the kW component of the starting KVA is still greater than rated running current.
This kW load causes the prime mover to slow down.
As the set slows and the frequency drops, the Under Frequency Roll Off feature of the AVR starts dropping the voltage set point to maintain the Volts per Hertz ratio.
This has two effects:
It lowers the voltage and thus lowers the KVA required by the motor.
It reduces the starting slip frequency and may move the motor characteristics up the torque curve and actually provide stronger acceleration with most but not all motors.
Using one generator and one motor at the 2.5:1 ratio will start most motors but the voltage drop will be severe.
Lighting on the same generator will often dim to unacceptable levels.
starting a motor with 3:1 available KVA capacity will result in noticeable but generally acceptable dimming of lighting.
If you have the budget, use the Cat sizing software and buy a bigger generator.

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

 

[waross]

And by the way, those figures are based on standby rated sets.
For a prime power rated set multiply the set capacity by 1.1 and use that capacity for your evaluation.
eg: a 100 KVA prime power set will be the same basic set as a 110 KVA standby rated set.

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

 

[BamBam.a]

I have a 100HP motor with a VFD and soft start. I can bypass the soft start and VFD mode too.
So one day, I had to run the motor on generator power and the VFD soft start was shot.
When I connected the load to the generator, the prime mover slowed down significantly and was not able to
catch up and within 10-15 seconds the CB on the Generator tripped and all 3 fuses on the motor was blown.

The generator has no problem running the pump and full speed and load when I use the soft start, but the lock
rotor amps was too much.

I dont know if there was some other underlying problem, but fixing the soft start (and replacing the fuses of course)
fixed the issue.

Another issue, I had, which was, I guess similar, was a generator powered CentraVac Chiller.
The Engine was misfiring due to a combination of high intake air temp and faulty spark plugs. But the engine was still running
but of course at lower HP. so when the Chiller loaded up the generator was unable to maintain the 60 hz. and was just hovering
around 56-57hz. Even at WOT.

 

[waross]

Until someone shares some numbers, Motor HP, generator KVA, generator rated Ampsand is the generator prime power or standby,
we are all spitting into the wind.

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

 

[waross]

BamBam.a
When your frequency drops to 57 Hz the chiller will be running a little slower and taking less power.
At below 57 Hz the UFRO on the AVR will become active and start to drop the voltage to reduce the KVA demand on the set.
(Under Frequency Roll Off on the Automatic Voltage Regulator)

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

 

[BamBam.a]

Waross,

See, there was something I didn't know. Cool!

It hasn't happened since that time, but I remember being worried that it might damage something somehow.

I have to admit I am not super adept in electrical theory.

 

[waross]

My rule of thumb for sizing is the generator rated current must be three times the motor full load current on a standby generator for DOL starting.
(Direct On Line.)
In a pinch, a generator rated at two and a half times the motor full load current will start a motor but the voltage drop for other devices on the generator will often be unacceptable.
The voltage drop when starting the motor will be dangerously close to dropping out a contactor based ATS. (Automatic Transfer Switch)
When that happens, the generator, with no load, speeds up and the voltage recovers.
When the voltage recovers the transfer switch will re-close and often the motor will not be able to gain sufficient speed for the current to drop and the cycle repeats.
It only takes a minute or so of "rinse and repeat" to burn the contacts open in the ATS.
I have repaired one such ATS and have heard about several more similar failures.

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

 

[BamBam.a]

Waross,

My standby generator run numerous pumps and auxiliary equipment, I am inclined to think that it was running pretty much
everything else at full churn when the last pump then kicked on thus causing the dip.

Nevertheless, this is good data, I need look into the site load vs generator size based on what you are saying.

Thanks a bunch.

 

[waross]

Minimum generator ampacity = sum of all loads in amps plus three times the Ampere rating of the last motor to start.
Sometimes you may stop a couple of smaller motors to get the capacity needed to start a large motor and then re-start the smaller motors.

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

 

[BamBam.a]

Waross,

Yes, the second time around, I did exactly as you described with no issues.

Thanks.

 

[waross]

Has anyone had a chance to witness an undersized generator unable to bring connected motor up to speed?

Actually, yes.
Years ago I was in a Central American country, doing some volunteer work and just hangin out when the country was beset by a power crisis.
The entire National Grid was on 50% rationing, 6 hours on, 6 hours off.
I Found myself installing standby generators as fast as I could.
I installed about 6 for one extended family. The sets had been sized and bought and were onsite.
They were all too small.
All of the owners had a lot of air conditioners.
The sets could not pick up the load.
At first it was trial and error. Turn off all the A/C and restart one at a time.
six sets and a couple of dozen A/Cs later I had a very accurate idea as to how big a motor a standby set could and could not start.
Then came the work-arounds.
My basic solution was an extension cord, a 6" x 6" x 4" junction box a normally open push button and a two or three pole power relay.
One for each A/C.
When you pushed the button the relay pulled in and sealed itself in.
When the power failed all the relays dropped out and all the A/Cs stopped.
The A/Cs had to be restarted individually.
Each customer knew how many A/Cs he could run without stalling the genset.
Within about five years I had replaced all of the small sets with larger sets that I had sized properly.

Has anyone had a chance to witness an undersized generator unable to bring connected motor up to speed?

Yes. Many times.
Bill
--------------------
"Why not the best?"
Jimmy Carter