What size generator do I need?

[Avocet]

I want to run a 2hp compressor (single phase) from a petrol generator. I borrowed a 2kW generator from a mate, but it won't start the compressor. The compressor motor starts to turn and buzzes a bit, then the generator's overload light starts flashing.

I'm guessing that the startup current is a bit more than the 2kW the generator can supply?

Does anyone know what sort of size of generator I'd need to be able to allow the compressor to cut in and out?

Ta!

 

[xr4x4]

A motor will pull a lot more on initial start up, your genny will need to prove around 3kw peak for start up but this will not do the genny anygood, as the compressor will do this a lot.

Much better to get a petrol driven compressor.

 

[ricicle]

Does the compressor have a dump valve or is it trying to compress air straight from start-up ?

 

[Avocet]

Thanks both. I think it does have a dump valve, (each time it stops there's a slight hiss of air escaping for a fraction of a second), but to be honest, I don't think it really gets as far as trying to compress any air in any case. I did wonder about cracking off the union from the exhaust port for a few revolutions, but I won't be able to do that each time it cuts-in. I have another compressor (an old Hydrovane) which definitely has a dump valve and runs continuously, so it doesn't have the frequent big spikes of a piston compressor, but it has a 3hp motor rather than the 2hp of the other compressor, so the generator still can't handle it. If I could be confident that the small (2hp) compressor would start adequately from a (say) 4kW generator (or 5kVA), I'd just look out for a second hand one of those.

 

[xr4x4]

Only example I can give...

I used to work in a power tool shop and we sold gennys, compressors etc and we used to get these questions all the time.

We had a couple who (without doing the home work) decided to join a bouncey castle franchise, franchise owner sold them a 2kw blower, all the kit and said 'just buy a genny to power it'

So they came in, brought a 2kw 'peak' genny and left (we didn't know the story at this point)

They came back because it didn't work, we told them they would need a much bigger one, 4kw plus but they couldn't afford it and wasnt expecting this problem.

We had one genny that had a output of 2400 kw and 2600 kw peak, they insisted that we try this one to see if it would work, so we did.

It did work but the fan struggled and the genny laboured hard when it started up, once it was running, it was fine.

It worked, so we sold it, what could we do..!

 

[SimonH2]

You will need a genny a LOT larger than the compressor. As mentioned, motors take a huge inrush of power as they start up (may be as much as 5-10 times the normal running current). Then once they start turning, they'll only do (at best) a rev or two of the compressor before you start getting back pressure* while the motor is still running up to speed.

My observation is that many generators "just collapse" and lose their output when overloaded by a motor of a similar nominal power to the nominal power of the genny. I've witnessed a similar farce watching someone attempting to weld away from mains. As I predicted, not one combination of genny (several) or welder (also several) was capable of striking an ark - or at least of striking an ark that melted anything, and this was to weld a heavy fitting to a heavy door. In the end they got the building supplier to weld it with his portable set when he was round to sort out one or two other snags (it was one of his factory welds that had failed and let the fitting fall off after all).

I suspect that you may get better results if the genny is oversized, but still not enough to start the compressor by "pre loading" it with something like a fan heater. I've not tried this, but I suspect this may increase the field (to produce the power to run the load), and mean it can droop more before collapsing or shutting down the welder. As I say, it's just speculation.

* Soft start valve help here by bleeding air until the pressure reaches a certain level. These (or at least the ones in my mate's compressor) are small brass plugs with a "pip" in the middle. When the compressor starts up, they hiss wildly until the pressure rises - at which point the pip pops out and they close off.

 

[bernardgreen]

This may not be an option for you as you already have the compressor.

The compressor's air pump is connected to the drive motor by a centrifugal clutch that operates to mechanically connect pump to motor when the motor has reached normal speed. Using a soft-start control on the motor will avoid the motor's start up load tripping out the generator. A fly wheel on the motor will supply the mechnical energy to get the pump pumping with out slowing the motor and increasing the load on the generator.

 

[Avocet]

Thanks all. I think you've saved me from making an expensive mistake! As it's only for one job, I think I'll go for a paint brush instead. I was hoping to paint some railings, but they're a good 100m from the nearest power.

I must admit, I'd been thinking of a genny (was without power for 7 hours after the gale on Thursday!) but that's a pretty infrequent occurrence. Also, I'd need it (mainly) to power laptop, router, lights and a few other bits and pieces at home. Would I be right in thinking the output from a cheap genny will be pretty "dirty" for micro-electronics? Up until now, I've run those from an inverter connected to a car battery - although I guess I could use the genny to power a battery charger?