Motor Inrush Current With Generator

[delectric123]

We were trying out a used portable generator we recently purchased, on a hog facility. Its a 1997 CAT with 8900 hrs. It still looks great despite all those hours. the generator size on the nameplate reads 100 KVA, 50 KW at 0.5 PF, 278 Amps at 208Y/120V . Really curious why KW is rated at 0.5 PF. All generators I've seen are rated at 0.8 PF. So its really an 80 KW.
Anyways, facility voltage is 208Y/120v 3 ph. During test run, load was reasonably balanced and average current on phases was about 115 Amps. then went inside and tried out the 20 HP pressure pump. The voltage dipped so much the lights actually went out for a second or too, then the motor ran uneven for about 3 seconds before running normally. Per NEC, 20 HP motor at 208 FLA is 59 amps, and LRA is 321. Is the inrush too much for a 80 KW generator, or is something wrong with it? we had problems with it at first with fluctuating voltage under minimum load but we changed the fuel filters and that really improved things.

 

[GoldDigger]

What you are missing is that a low PF rating for a generator is not a purely good thing.
What it is telling you is that the engine itself can only produce enough power for a 50kw load, but the generator head is overrated enough to handle the current of a 100KVA load.
That is why the engine nearly stopped when called upon not just to produce not just 70kw but extra for motor starting.

Tapatalk!

 

[Jraef]

What you are missing is that a low PF rating for a generator is not a purely good thing.
What it is telling you is that the engine itself can only produce enough power for a 50kw load, but the generator head is overrated enough to handle the current of a 100KVA load.
That is why the engine nearly stopped when called upon not just to produce not just 70kw but extra for motor starting.

Tapatalk!

Bingo. It's a numbers game played by the genset mfrs (or suppliers). The original purpose of this may have been as a lighting-only backup, for example in a big-box retail store that had to provide emergency exit lighting for longer than batteries could hold it up. Some Costco stores are like that. In general they don't use a backup generator at all, they just require that the entire store be cleared out in 10 minutes or less. But some jurisdictions don't allow that, they say that egress lighting must be xxx foot candles for yyy minutes, which pushes them into needing a backup Genny. So they buy the cheapest thing they can get to ONLY supply the egress lighting, no motors, no HID lighting, no computers etc.

 

[kwired]

Bingo. It's a numbers game played by the genset mfrs (or suppliers). The original purpose of this may have been as a lighting-only backup, for example in a big-box retail store that had to provide emergency exit lighting for longer than batteries could hold it up. Some Costco stores are like that. In general they don't use a backup generator at all, they just require that the entire store be cleared out in 10 minutes or less. But some jurisdictions don't allow that, they say that egress lighting must be xxx foot candles for yyy minutes, which pushes them into needing a backup Genny. So they buy the cheapest thing they can get to ONLY supply the egress lighting, no motors, no HID lighting, no computers etc.

Kind of makes sense, except that for egress lighting only 100 kVA seems like it would run an awful lot of lighting, surely if cheap is the idea they wouldn't have that large of a generator for egress lighting only. I have seen well lit outdoor sports complexes using less energy than 100 kVA we are only talking somewhat minimal egress lighting here, and it's not like an average Costco store is the size of a large international airport either.

 

[texie]

If this genset really has a PF of .5, then goldiggers comment would make perfect sense. But you mention this machine was built as a portable Cat, presumably a factory built portable. I have never in all my years seen a machine built for this purpose and size that did not have a power factor of .8. Now if this was originally built for some special purpose such as, maybe, to supply some type of UPS or such it would be possible to be rated at .5, but even that is unlikely.

 

[Jmckay12]

Jmack

Jmack

We were trying out a used portable generator we recently purchased, on a hog facility. Its a 1997 CAT with 8900 hrs. It still looks great despite all those hours. the generator size on the nameplate reads 100 KVA, 50 KW at 0.5 PF, 278 Amps at 208Y/120V . Really curious why KW is rated at 0.5 PF. All generators I've seen are rated at 0.8 PF. So its really an 80 KW.
Anyways, facility voltage is 208Y/120v 3 ph. During test run, load was reasonably balanced and average current on phases was about 115 Amps. then went inside and tried out the 20 HP pressure pump. The voltage dipped so much the lights actually went out for a second or too, then the motor ran uneven for about 3 seconds before running normally. Per NEC, 20 HP motor at 208 FLA is 59 amps, and LRA is 321. Is the inrush too much for a 80 KW generator, or is something wrong with it? we had problems with it at first with fluctuating voltage under minimum load but we changed the fuel filters and that really improved things.

I'm not saying that you are not pushing the limit of the generator but I had the same thing happen to me and it ended up needing a new fuel filter. Just a thought in case you wanted to give it a try.

 

[Jraef]

Kind of makes sense, except that for egress lighting only 100 kVA seems like it would run an awful lot of lighting, surely if cheap is the idea they wouldn't have that large of a generator for egress lighting only. I have seen well lit outdoor sports complexes using less energy than 100 kVA we are only talking somewhat minimal egress lighting here, and it's not like an average Costco store is the size of a large international airport either.

I just used that as an example of why someone would specify and buy a genset with a .5 PF rating on it, meaning that it was originally purchased for some specific purpose that no longer applies.

 

[delectric123]

If this genset really has a PF of .5, then goldiggers comment would make perfect sense. But you mention this machine was built as a portable Cat, presumably a factory built portable. I have never in all my years seen a machine built for this purpose and size that did not have a power factor of .8. Now if this was originally built for some special purpose such as, maybe, to supply some type of UPS or such it would be possible to be rated at .5, but even that is unlikely.

I looked about 3 times when I saw that KW rating at a 0.5 PF, but its definitely so. The unit definitely looks factory original. Its totally enclosed and runs amazingly quiet. It was previously owned and used by a POCO, maybe that would explain things.

What you are missing is that a low PF rating for a generator is not a purely good thing.
What it is telling you is that the engine itself can only produce enough power for a 50kw load, but the generator head is overrated enough to handle the current of a 100KVA load.
That is why the engine nearly stopped when called upon not just to produce not just 70kw but extra for motor starting.

Tapatalk!

I wasn't out by the generator to hear how the engine reacted. I figured it was the generator fields collapsing, if I'm saying that right.

 

[GoldDigger]

It sounds to me as if the engine at full throttle simply could not maintain RPM against the torque required by the load.
Unlike POCO, a generator cannot "brownout" to reduce the load by reducing the voltage.
Or at least most regulators do not implement that strategy.
If the result was a large enough voltage drop as the engine slowed, control relays and motor starters may have dropped out allowing the engine to recover somewhat.
I once had a small permanent magnet generator (MacCullough, I think) which was located a long extension cord away from where I used it. I am embarrassed to admit that I sometimes stopped the generator by throwing a short circuit on it.

Tapatalk!

 

[texie]

I looked about 3 times when I saw that KW rating at a 0.5 PF, but its definitely so. The unit definitely looks factory original. Its totally enclosed and runs amazingly quiet. It was previously owned and used by a POCO, maybe that would explain things.




I wasn't out by the generator to hear how the engine reacted. I figured it was the generator fields collapsing, if I'm saying that right.

I would suggest contacting Cat with the serial number and have them tell you the rating. It would be very unusual for this to be .5 PF. They sell these right "off the shelf" at .8 and it would be real unlikely for someone to have a portable built at .5 PF as you could probably buy the .8 machine production model(with the larger engine) for less money. There has to be more to this.

 

[Jraef]

... It was previously owned and used by a POCO, maybe that would explain things....

There ya go. Probably a backup supply generator for switchyard control. They use 125VDC supplied from a battery bank so that they can continue to operate breakers and switches if the power fails, so as to control how it is restored. In an extended power failure, they need backup power for that DC system. So being a battery based system, the genset had no surge load, it just has to keep the battery charger energized.

 

[Cow]

I would contact CAT with your model/serial numbers and see what they say about the load you are trying to run on it. I believe all the generator companies have specialists who are used to answering generator sizing questions.

That's where I'd start anyhow.

 

[GoldDigger]

There ya go. Probably a backup supply generator for switchyard control. They use 125VDC supplied from a battery bank so that they can continue to operate breakers and switches if the power fails, so as to control how it is restored. In an extended power failure, they need backup power for that DC system. So being a battery based system, the genset had no surge load, it just has to keep the battery charger energized.

In addition to not seeing motor surge loads, if driving a simple rectifier it would see a load with poor distortion power factor. That could justify a lower resistance, higher current generator head coupled to a smaller engine.

Tapatalk!

 

[J.P.]

How about a soft start?