How do I measure this electric load?

[velvetfoot]

I just got a new clamp ammeter that measures inrush current. My 240 volt deep well pump measured 60 amps on startup and 11 amps continuous. This is on one of the 120 volt legs (readings were similar for both legs). What is the peak inrush current: 60 amps, at 240 volts? That would be 14,400 watts. Or, should I be multiplying by 120 volts and coming up with 7,200 watts? My generator, which is rated at 7550/13500 continuous/surge watts, has run this load in the past.

Sorry if this is a dumb question. I'll be looking more on the web as well.

 

[Murphy2000]

Yes, 14KW inrush.

The surge on your generator and the inrush rating on your pump are two different issues. The inrush on the pump is only 500 milliseconds or so. (1/2 second)

As for the surge on the generator, it would depend on if the tripping mechanism is magnetic or thermal. If its magnetic, you're screwed. If its thermal, its an issue of heat vs. time.

 

[velvetfoot]

Thanks. The gen. handled the load in the past. It just blows my mind that it's so high compared with a running current of 11 amps. If I ever replace the well pump, I'd consider something with a softer start.

Would power factor come into play? Like maybe less watts required as a result?

 

[steam man]

I started to post a reply here yesterday but had to think about it some more. Typically motors of this type duty will have a 6-1 ratio for starting current. Some large ones go as high as 10-1. (You should see the ones at my job maxing out paralleled generators at 5000 amps. That will make you sweat). I did a little research on generator sizing at http://www.generatorguide.net/ which was very informative. The power factor works against you as generators are typically rated for unity PF while motors typically run about .8. As murphy2000 implied you seem to be up against the limits of your generator. At hard starting, a large voltage drop on your generator is hard on the motor. It can shorten its life span. Does the generator voltage take a big drop when starting the pump motor?

 

[velvetfoot]

Thanks steam man. Six times the running current seems right in my measurements. Heck, when I first moved into the place, I noticed a little light flicker when the pump started up (on utility power).

I did try to start it once with a 5kw generator-I was in the basement and the lights dimmed and then went bright. I think the pump tripped. I was sweating there. I've used the 7550/13500 watt unit during a power outage and it worked. I haven't tested it lately, but I will. I have no gauges on the unit. Can I plug my multimeter into a couple of the 120v plugs and come up with 240? I don't know if the peak hold function works on voltage, but it has a min/max as well, which I guess is slower.

A new generator at this point would be a really hard sell.


Here is a picture of the generator panel:

 

[oldspark]

How long is the run of wire to pump from gen?

 

[velvetfoot]

Off the top of my head, the pump is in a 450' hole, but I have no idea how far down the pump is. There's about 30' from the well head to the pressure switch. Not sure the gauge of that wire. I think they are separate wires going through the coduit to the well head. There's about another 30' of 10 gauge to the breaker panel (with interlock switch), and an 8 gauge wire about 35' long from the panel to the inlet box, and then about 20' or so of some flexible black heavy gauge (10?) wire (not sure of the gauge) to the genny.

 

[steam man]

I am not sure if the 2-120 outlets are wired separately, i.e. 1 on each hot side of the neutral or just paralleled. Put your meter on the hot terminal of each outlet and see what it says. If it says zero your on the same leg. 240 would mean opposite legs. Try the min/max readings. The size of the wiring makes a big difference in the voltage drop. Hopefully it is sized right.

 

[oldspark]

Thats what I was wondering about, the voltage drop.

 

[velvetfoot]

I started the water pump twice today on generator power.
It gurgled twice each time but started the pump.
I didn't measure the voltage, but it must've taken a hit.

 

[velotocht]

Velvetfoot:
Deep well pumps are notoriously hard to start for several reasons: (1) There an inductive load - there are by nature hard to start; (2) the wire run is long (up to 450' in your case) - that means the point were the system voltage is regulated (gen) is a long way from the load (pump) and (3) for the pump to start it needs to 'lift' some water. A 3/4 inch or 1" pipe 450' long holds a lot of water. At the moment the pump rotor starts to turn it has to lift all this water - straight up!

I have a 150' deep well. My 5Kw generator won't start my 240 VAC well pump.

RonB

 

[semipro]

Velvetfoot:
Deep well pumps are notoriously hard to start for several reasons: (1) There an inductive load - there are by nature hard to start; (2) the wire run is long (up to 450' in your case) - that means the point were the system voltage is regulated (gen) is a long way from the load (pump) and (3) for the pump to start it needs to 'lift' some water. A 3/4 inch or 1" pipe 450' long holds a lot of water. At the moment the pump rotor starts to turn it has to lift all this water - straight up!

I have a 150' deep well. My 5Kw generator won't start my 240 VAC well pump.

RonB

Good thread!

Just a note on lifting height. Your pump only has to lift a column of water equivalent to the height from the water surface to the storage tank. In other words the depth at which the pump is set isn't what matters, its the depth to water. Of course the pump depth does affect wire length and the voltage drop.

The pump also needs to overcome the pressure in the storage tank. If that's 30 PSI that's equivalent to to about another 70 ft of pumping height.

 

[LLigetfa]

Good thread!

Just a note on lifting height. Your pump only has to lift a column of water equivalent to the height from the water surface to the storage tank. In other words the depth at which the pump is set isn't what matters, its the depth to water. Of course the pump depth does affect wire length and the voltage drop.

The pump also needs to overcome the pressure in the storage tank. If that's 30 PSI that's equivalent to to about another 70 ft of pumping height.

True, on traditional bladder tank systems but not on snifter/bleed-back systems with air volume control tanks. They bleed off the pressure and allow air into the line so the pump starts at 0 PSI and pushes against air. Here is a good KB article on pumps.

http://greenroadfarm.com/wells.html

 

[velvetfoot]

Thanks, and nice link.

I just think that, unless going for an off-grid operation, the owner and pump installer will go for the more economical solution, rather than thinking about operating during a power outage.