Thread: Driving a 4.5A Pump from a 2KVa Generator?

Ohms law only holds for DC systems or in the case of AC the instantaneous values of A and V (or pure resistive loads, not motors). There's this thing called the phase angle between the voltage and current waveforms that messes entirely with that simple calculation.

Anyway a 2kVA genny won't deliver 2kW for the same reason (power factor), and the only way to find out if the genny will start the pump is to get an idea of the pump's starting current and compare it with the genny's maximum output current. You could of course try by plugging said pump into said genny. It'll either work, pop a breaker or stall. It depends also on what kind of motor it is, a variable speed one will be OK but a capacitor start one likely wouldn't.

Thanks bee utey, much appreciated. I knew Ohm's Law wasn't entirely applicable in AC, but I thought it might give a rough approximation.
I suppose the 4.5A cited on the pump's specification plate would be the RMS figure, once the pump was running?
Anyway, giving it a go isn't an option as I don't yet have said genny. I was thinking of buying one for other purposes, and wondered if there was some way I could find out in advance if my cunning scheme would work before outlaying the cash. Or perhaps buying a larger one (if necessary) so it could cope with this job as well.
Would an electrician's meter be able to tell me what the starting current might be? ie do those meters that they use have a "peak reading" feature?
Cheers

C00P

Hi C00P ... Your 1080watts calc is correct as that is the input power (P1) .... To give you an output power (P2) of 750watts. About a 70% efficiency.

The EU20i Honda inverter generator will go 50% over it's rated maximum output for 1 - 3 seconds for motor starting duties. It will start and run your Onga without any issues. I have been able to start and run 1500watt pump sets from an EU20i.

I would leave the Eco throttle setting at OFF so the EU20i is running at full speed when the pump starting load is applied. With Eco throttle on, the time it will take for the EU20i to respond to the starting demand the pump is asking for, and get up to a suitable operating speed, may cause the EU20i to go out on fault. There's no issue with the Honda Inverter generators if you overload them as they will just disconnect the power output, requiring a stop/restart.

How do I know this ? I have a Honda Power Equipment dealership, and the other half of my business is pumps, irrigation and plastic plumbing supplies .... Been doing this for 28yrs.

If you don't have the EU20i yet, suggest you look at the EG3600 unit. It's bigger, a bit noisier, but is 3600watts output for around the same money, and will will leave plenty of extra capacity to run other appliances simultaneously ... Also runs a large long range tank.

Cheers
Kevin

Sent from my iPad using Braille

Hi COOP,


In regard to the starting current, the rule of thumb is six times the full load current for the motor. In this application the generator will not be able to supply this starting current. The actual current supplied is limited by generator.


So it is then a question of is the limited current available from the generator sufficient to start the motor, only testing will confirm... For fire fighting duties I would recommend using a bigger generator.


Cheers
Ron

If you have access to the genset, then try it. There are a lot of calculators on line to check these things, and my own one I use for work (which is based on 20 years of real world work) says its borderline...

It could work, it may not. If you are still deciding on what size genset to get, go bigger.

It'll do it but it won't start the motor under full load.

You might get lucky with it being a centrifuge non positive displacement pump that the. "slip" is enough to let it start and run without issue.

My.math says the inverter will be running at about 70% capacity with the pump under nominal max load and will spike to around 190% for start up. Running the Honda on Mon eco mode will help it get you a start. The honda may still pull it off as most inverters have a max rating (which is your 2kv) a momentary limit which is usually 1.5 times normal max for up to five seconds and max instamt limit anything up to 5x normal max for about 1/10th of a second.

I regularly run an 800w drill on a 600w inverter. It will trip out every time.if I try to start it under load or if I try to drive a big bit at a high feed rate.


My gut feeling is that. If the pump is gravity Fed and you have some way to unload the discharge side during start up that the Honda will get it going. It may not run it for normal.domestic duties where the pump will be expected to. Start and stop against. pressure

The last thing you. red to.sorry about is heat. Those inverter type gennies are only.hit their advertised specs at a certain ambient conditions, I'm going to guess that in the middle of a bushfire is somewhat out of those specs.

Many Thanks to you all, the feedback has been exactly what I was looking for.
The pump sits alongside the main tank and is connected to a common line that links three other tanks. Most of the time the tank level is well above the pump level, so input pressure should be positive most of the time and only slightly negative (about a 300mm lift) if the tank is really low.
In common with most household pumps, this one senses a drop in output pressure which triggers it to turn on. There's no pressure vessel. If the output line comes up to pressure (ie the tap is turned off) it runs for about another 7 seconds, and then switches off, maintaining the pressure in the line. So I guess it is starting under some load. If we are about to fight a fire, then we will open the tap onto a hose which is larger than normal (1") so the output would have little resistance to begin with,and then turn the pump on manually and let it run.
Seems you all reckon it would work, but that it is likely to be marginal. Given Blknight's comment about the heat (if there's a nasty fire, it will probably mean 35 degrees C or more) it seems like the best idea is to follow your advice and go for a slightly bigger unit to give a safety margin.
So a bigger genset it is.
What triggered this thinking (apart from the recent fires) was a deal with a local tool shop offering a Honda EU20i at $1599. They are also offering a Yamaha 2.4 kVA (EF2400IS) at $1799.
The primary use for the genset is to support our caravan in our retirement, and will rarely be asked to run very much as we like to keep things simple. Lighting will be all LED, we might have a small microwave. If things get too hot we'll park where there is power and run the Airconditioner. I don't expect to use the genset to run that, although I believe it could (if the A/C unit is small enough). So mostly it will be charging the batteries if the solar system can't keep up, and we'll be running computers, so need a smooth output. Lightness, quietness and ease of use are the primary requirements. I believe the Yamaha is about 10kg heavier than the Honda, but still pretty quiet. The EG3600 weighs over 40kgwhich is going to limit its utility for our purposes.
The fire-fighting capability is for emergencies only, and we may never use it. But like a parachute, if you actually need to use it, it really has to work!
It looks as if the Yamaha might be the way to go....

C00P

To be honest, I'd think a little sideways here. A nice big battery and a quality 2000W+ power inverter attached, and a multi stage charger and/or portable solar setup to keep the battery full. Your pump can then start in an instant, and you can run a selection of 12V LED down lights inside off the battery. It means you're essentially blackout proof. Then you can purchase a small quiet generator to run the charger off in extended blackouts. You won't need a bigger genny as the inverter and battery will supply the surge. All the components will transfer well to a camping setup. Even your vehicle can be used with jumper leads to charge the system. Very flexible all round. I've got similar at my place, it's been very helpful over the last few years.

Oh and if you want fire sprinklers to do some good, perhaps a 5hp petrol fire pump would be more suitable. 750W isn't a lot.