Sizing a small generator

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charlie b

charlie b

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I have a project that will involve photovoltaics for a small pumping station, with a small generator as the backup source. The total load, with all components operating, is about 5 amps, at 380 volts, three phase, equivalent to about 3.3KVA. There is only one motor, and it draws about 3 of the 5 amps. So it the load is 2 amps just before the motor starts, how do I make sure the generator can handle the starting current without stalling?
 
charlie b said:
I have a project that will involve photovoltaics for a small pumping station, with a small generator as the backup source. The total load, with all components operating, is about 5 amps, at 380 volts, three phase, equivalent to about 3.3KVA. There is only one motor, and it draws about 3 of the 5 amps. So it the load is 2 amps just before the motor starts, how do I make sure the generator can handle the starting current without stalling?
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I think I would be asking how small of a generator in that voltage and three phase can you get? As for starting a motor I think the rule of thumb is to add 75% to the draw for start up. So you should have a load of around 8 amps total.
 
charlie b said:
I have a project that will involve photovoltaics for a small pumping station, with a small generator as the backup source. The total load, with all components operating, is about 5 amps, at 380 volts, three phase, equivalent to about 3.3KVA. There is only one motor, and it draws about 3 of the 5 amps. So it the load is 2 amps just before the motor starts, how do I make sure the generator can handle the starting current without stalling?
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Offhand I'd say 2 to 3 times the running kVA of the motor plus the other load (genset rated 7 to 10kVA... approximately 5 to 8kW).

Here's what Kohler says:

http://kohlerpower.com/common/pdfs/107308_WP_GensetMotorStarting.pdf
 
charlie b said:
Liberia.
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Ahhhh. That's far from my 41.8 - 47.6 degree north latitude world. A friend of mine has a few liberian friends and they used his oven a few Thanksgivings ago to cook a turkey. They burned it so bad he had to repaint the place. That's all I know about liberia/liberians - well now I also know that they have 380 volt power.
 
electrofelon said:
Ahhhh. That's far from my 41.8 - 47.6 degree north latitude world. A friend of mine has a few liberian friends and they used his oven a few Thanksgivings ago to cook a turkey. They burned it so bad he had to repaint the place. That's all I know about liberia/liberians - well now I also know that they have 380 volt power.
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I think it is one of the three countries in the world that hasn't gone metric. Myanmar (was Burma) is another.
 
ActionDave said:
I always heard 1kva per horse power as the rule of thumb. No one else has mentioned that so far. Am I the only one or is it a flawed rule?
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For running it's probably OK.
Starting current can be upwards of six times full load current.
I think the issue Charlie b might have to consider is how that will drag the generator voltage down rather than stalling. The gen set inertia might cope with the latter.
 
ActionDave said:
Yes, but that is the same reason I was told you need one kva per hp; to overcome inrush.
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One HP is 0.746kW
If you take the efficiency and pf as both to be 86% and 0.86 respectively you are already at one kVA for steady state running.
And, for a motor of the size being discussed here, these might be optimistic figures.
 
ActionDave said:
I always heard 1kva per horse power as the rule of thumb. No one else has mentioned that so far. Am I the only one or is it a flawed rule?
Click to expand...

It is a fair rule of thumb for running load, but is most unlikely to suffice for starting conditions unless a soft start or inverter drive is to be used.

Can the other equipment at the pumping station be shut down, short term during pump starts ? That would allow a smaller generator.

When the generator is NOT running, presumably the pump is to be powered from a battery bank and a power inverter ?
If so, perhaps the pump can be started on battery/inverter power and then transfered to the generator ?

If the pump is normally battery powered via a power inverter, then it might be simpler to have the generator charge the battery when this be low, rather than power the pump directly from the generator.
 
broadgage said:
It is a fair rule of thumb for running load, but is most unlikely to suffice for starting conditions unless a soft start or inverter drive is to be used.

Can the other equipment at the pumping station be shut down, short term during pump starts ? That would allow a smaller generator.

When the generator is NOT running, presumably the pump is to be powered from a battery bank and a power inverter ?
If so, perhaps the pump can be started on battery/inverter power and then transfered to the generator ?

If the pump is normally battery powered via a power inverter, then it might be simpler to have the generator charge the battery when this be low, rather than power the pump directly from the generator.
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Yes, I guess it could be done that way.
Seems a bit complex though for such a small machine.

Off topic I know, but which part of London?
 
After a bit of web searching, I fall back to ceb58's post #2.
The 1st question may well be how small of a 3 phase generator can I locate.
(9.6 kw is the smallest I have haves seen referenced)
 
Many thanks for all the responses. I think I will call for a minimum of 6kW generator. If the smallest they are able to find is closer to 10, that will not be a bad thing.

To answer some of your questions, there are three separate facillities, and low-tech is the name of the game. Everything will be turned on and off by hand, and transfers will be done manually. That precludes a starting on one power source and transfer to another. The system will run on photovoltaics during the day, and might be shut down at night. That part is uncertain, and I don't know whether the generator is intended as a backup source during the day.
 
You need to know the VAR capability of the generator, the running load, and motor data including the starting pf of the motor. When the motor starts it will have a low pf, which means it will be drawing a lot of Vars and the generator needs to be able to supply that.

I venture to guess since you are in Liberia, at 380V 3phase, the motor is going to be an IEC design, which then Hp plays no factor, it is all KW rated. The LRA can vary greatly, I have seen them as high as 12X FLA. If you are specifying it than you can control that part to some degree.

So, lets say it is a 1800W motor (3A x 380V x 1.732 x 0.92pf) and LRA is going to be 6X, than on starting the current is 18A.

This translates to 11.85KVA. Assume a .28 starting pf and you need 11.38kVars on starting. We'll assume the running load is resistive so it will not need any Var contribution.

Your generator will need to supply running load plus starting load:
1.3KVA or kW (resistive) + starting load 3.3kW = 4.6kW
and starting Vars of 11.38kVars for total; (4.6kW +j11.38kVar) or 12.3KVA @ 68deg.

Looking at generator Var capability curve will determine final size of generator. You could be conservative and use rated standby size which is approx 10% greater than prime rating. The pf is always at 0.8 on prime rating which translates to 36.87deg.

Which means you would need a machine capable of minimum 19kVA @ 0.8pf (standby) or 17.3KVA (Prime) to meet the 11.38kVar starting requirement.

Ok, someone PLEASE check my numbers, its been a while since I did that by hand.
 
realistically, what is the smallest 3 phase 380V generator you can get?

I don't think i have ever seen a decent 3 phase generator of any voltage smaller than 15kW.
 
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