Where do I find starting current for subm. pump?

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Old 08-19-09, 11:21 PM
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Where do I find starting current for subm. pump?

I was recently disappointed that my 3.5kW generator won't start my well pump. Didn't trip any breakers--it instantly stalled the generator
I thought 25A would be more than enough to start an 8A rated pump. There were no other loads on the gen at the time.

I was considering a generator for my cabin but unless starting current is way less for a 3/4hp shallow well pump than a 1/2hp submersible, I won't bother.
 
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Old 08-20-09, 08:01 AM
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Originally Posted by guy48065 View Post
I was recently disappointed that my 3.5kW generator won't start my well pump. Didn't trip any breakers--it instantly stalled the generator
Yes that's disappointing, but not unexpected. The minimum I've ever seen a well pump run on was a 5kW genny, but usually something upwards of 8kW is better.

I thought 25A would be more than enough to start an 8A rated pump. There were no other loads on the gen at the time.
Of course it's all related, but you have to consider more than just the starting current. A huge part of it is the rotational inertia of the generator which is basically a function of mass of the unit. A small engine simply doesn't have enough inertia to start anything but tiny motors without stalling.

I was considering a generator for my cabin but unless starting current is way less for a 3/4hp shallow well pump than a 1/2hp submersible, I won't bother.
It's unlikely that it will be less. If you know the manufacturer and model number of the pump sometimes you can look up the specs on the website or call their customer support number.
 
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Old 08-20-09, 12:11 PM
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The pump owner's manual has a section on running off a generator & a recommended output. The recommended genny size for a 1/2hp 2-wire split phase pump is 3kW. Mine is a 3.5kW (4kW surge) so I thought I was covered.

It handles other inductive loads OK--what's special about a well pump? Is it the weight of 90 feet of water sitting on top of the pump? That's why I was wondering if it would be different for a suction pump.

If I can't find the surge rating do you think I could catch it with an amp clamp & the Peak Hold feature on a good DMM?
 
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Old 08-20-09, 12:28 PM
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Originally Posted by guy48065 View Post
The pump owner's manual has a section on running off a generator & a recommended output. The recommended genny size for a 1/2hp 2-wire split phase pump is 3kW. Mine is a 3.5kW (4kW surge) so I thought I was covered.
It surprises me that they only recommend a 3kW. Perhaps is the pump a long distance from the generator? Do you know the AWG of the wire?

what's special about a well pump? Is it the weight of 90 feet of water sitting on top of the pump?
That's it. Water exerts 0.5 PSI of head pressure for each foot of the water column, so based on the 90' depth you've got about 45 PSI of head pressure on the pump.

That's why I was wondering if it would be different for a suction pump.
Yes it would be better, but that is offset by the fact that it's a 3/4 HP instead of 1/2 HP. What you gain in head pressure you might lose to the bigger motor.

If I can't find the surge rating do you think I could catch it with an amp clamp & the Peak Hold feature on a good DMM?
Yes you could catch it with a DMM that has an inrush feature, however what you can't really capture is the frequency and voltage lag when the generator bogs down. Both of these contribute to an additional increase in start-up current beyond what you would see when grid connected. So if you do measure it with the meter, make sure there's an additional margin above what you measure.
 
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