Generator sizing, electronic loads, and load bank.

[SmithBuilt]

I have a customer that wants to put in a generator to supply his phone system and data. Not much load about 7kva.

I spoke to generac about feeding the electronic loads. He stated that i would need to multiply the electronic load by 5 in order to size the generator. I also would have to install a load bank inorder to load the generator due the harmonics.

Does this sound correct? Would a large enough generator handle the harmonic load without the load bank?

 

[zbang]

Does that 7kva include cooling and lighting? If not-
Since you've got 7kva (call it 7 kw for the moment) going in, you've also got 7kw of heat to remove. And how about lighting (more heat)? If I've done the math right, that's 2-3 tons of cooling to run, and that'll load up the generator a bit. Also, how about expansion? If the computer load is really around 7kva (measured, not assumed), I probably wouldn't consider less than a 20kva gnerator.

 

[SmithBuilt]

That number includes a mitsubishi cooling unit. It has variable speed fans and compressor so I feel its as bad as the electronic equipment.

No lighting loads are to be fed. Strange situation, cheap cheap customer. Perhaps I should run, but I do not want to lose a good customer.

 

[Dnkldorf]

Maybe I don't understand the question, he wants to use a generator to power a 30a/240v load?


Is this for back-up only?

A UPS wouldn't work for this?

 

[SmithBuilt]

Maybe I don't understand the question, he wants to use a generator to power a 30a/240v load?


Is this for back-up only?

A UPS wouldn't work for this?

It is for back up. A UPS will not last as long as he would like.

 

[charlietuna]

Smith,
Educate your customer---7 kw might run his data load but what about all his support systems. Without A/C he probably needs to shut down his data equipment. This should be engineered or at least oversize the gen set to cover your butt!

 

[Dnkldorf]

Can I at least assume, when you are talking harmonics, this is a 3ph 208V system?
Harmonics wouldn't be a problem on a 120/240V 1 ph.

If this is the case, a 3ph 20KW generator, installed is going to run what?

If this is the case, a used scalable APC like this might work?

But I have no idea what your equipment is.

Just a thought.

(add, how long does he want it to run for?)

 

[SmithBuilt]

Smith,
Educate your customer---7 kw might run his data load but what about all his support systems. Without A/C he probably needs to shut down his data equipment. This should be engineered or at least oversize the gen set to cover your butt!

The 7 kw includes the AC. He doesn't care about running any other loads. I will make sure he gets a larger generator.

Can I at least assume, when you are talking harmonics, this is a 3ph 208V system?
Harmonics wouldn't be a problem on a 120/240V 1 ph.


(add, how long does he want it to run for?)

Single phase. Harmonics is not be the correct term, I should have said inductive.

Sometimes due to weather the power may go out for a longer period than a UPS can supply. The genset may have to run for a day or so.




My question is about having to install a permenant load bank like the tech said. I have never seen this done before. And is it possible to just upsize the generator and eleminate the load bank.

 

[ron]

The A/C will act as a load bank if it will run year round. The load bank would be if it were a diesel and you needed it to load past 50% load, avoiding wet stacking.

 

[Pierre C Belarge]

The A/C will act as a load bank if it will run year round. The load bank would be if it were a diesel and you needed it to load past 50% load, avoiding wet stacking.

How is maintenance of this system to be considered? It sounds like you or someone needs a little help in preparing for what is still unknown. It would be a shame if the unit is installed, no power is lost for a year or two...then power goes out and the generator does not perform as was originally planned.

 

[don_resqcapt19]

Some generators don't work well when the load is mostly electronic devices. The frequency and voltage can "hunt". Adding a load bank will help with this problem. It is not a harmonic issue, but is related to how the the switching power supplies draw their current from the supply. The chopped up current draw is an issue for some generator control systems.

 

[SmithBuilt]

Some generators don't work well when the load is mostly electronic devices. The frequency and voltage can "hunt". Adding a load bank will help with this problem. It is not a harmonic issue, but is related to how the the switching power supplies draw their current from the supply. The chopped up current draw is an issue for some generator control systems.

So a load bank is added permanently. It seems this would be terribly inefficient. Are there other ways?

Thanks for the replies.

 

[JES2727]

Some generators don't work well when the load is mostly electronic devices. The frequency and voltage can "hunt". Adding a load bank will help with this problem. It is not a harmonic issue, but is related to how the the switching power supplies draw their current from the supply. The chopped up current draw is an issue for some generator control systems.

Don, I'm curious, could you clarify what you mean by frequency and voltage "hunting"?

So a load bank is added permanently. It seems this would be terribly inefficient. Are there other ways?

Thanks for the replies.

Smith, if the load bank is to solve issues other than wet stacking I'm not sure that there other ways to achieve this. Adding some more load should do the trick, but apparently your customer doesn't want that. A load bank is not really inefficient, running the generator with little or no load is harmful and in the long run more costly than adding a load bank.

 

[Dnkldorf]

Is wetstacking a more/less of a problem with LP or NG, or just more serious with gas/diesel generation?

 

[JES2727]

Is wetstacking a more/less of a problem with LP or NG, or just more serious with gas/diesel generation?

Wet stacking is an issue on lightly loaded diesel generators. If the engine's not working hard enough and getting good and hot, some unburned fuel goes out through the exhaust and eventually clogs up the stack.

 

[Dnkldorf]

Wet stacking is an issue on lightly loaded diesel generators. If the engine's not working hard enough and getting good and hot, some unburned fuel goes out through the exhaust and eventually clogs up the stack.

That's what I thought. Thanks

Smith's problem I think is going to be the "cheap, cheap customer part".

 

[SmithBuilt]

Smith's problem I think is going to be the "cheap, cheap customer part".

Agreed, I might as well put some real money in the bid and let him worry about it.

 

[qcroanoke]

Some generators don't work well when the load is mostly electronic devices. The frequency and voltage can "hunt". Adding a load bank will help with this problem. It is not a harmonic issue, but is related to how the the switching power supplies draw their current from the supply. The chopped up current draw is an issue for some generator control systems.

A while back we had a genset powering a UPS giving us a fit by shutting down on overspeed for no reason. I posted about it here on the forum.
It was fixed by adding a small load bank. We were advised by the UPS vendor and the Genset vendor that a resistive load would fix it. I was very skeptical but it was installed
and tested and worked like a champ. No shutdown at all. Go figure.....

 

[ron]

Often the HVAC load for the IT equipment acts as the needed load bank.

 

[don_resqcapt19]

Don, I'm curious, could you clarify what you mean by frequency and voltage "hunting"?

The generator controls are confused by the electronic loads and keep trying to change the frequency and voltage to what the generator output is supposed to be, but is always a step behind the load and the voltage and frequency output from the generator is not steady.