Motor starting current; genset capacity

I've got a 3/4 HP pump motor that's rated for a 32 amp starting, 9 amp
nominal current req'mt at 230 VAC, single phase. The manufacturer recommends
a 20 amp breaker.
The essential question is what the minimum watt/amp reqm't of a generator
needs to be to start and run this pump. Am I correct that at surge capacity
of at least 7.5 Kw is req'd to meet the starting current reqm'ts of the
motor or am I thinking about this wrong ? Would a 2.5Kw genset work, for
example?
I'm not sure I understand how electric motors start and why a 20 amp circuit
breaker would work when the starting current exceeds the breaker rating and
also how a genset would respond to a momentary surge-draw.
As always, TIA ...

Reply to
N7RX
Loading thread data ...
starting, 9 amp
manufacturer recommends
of a generator
surge capacity
reqm'ts of the
genset work, for
a 20 amp circuit
breaker rating and
the breaker will take the surge.... the generator capacity is the generator capacity and if you ask more if it, you wont much... a short blip.. thats it... the drive motor is maxed...it has no surge capacity.
If you want ot close off the pump discharge or suction (yes discharge if is a single stage single impeller pump, most comon except for wells) the pump will start with much less amperage draw and free wheel. So even with the small generator it might get rolling..and come up to speed, then you can open the valve.
Phil Scott
Reply to
Phil Scott
Not enough information to really answer the question. Motors can draw 6 times FLA on start 9X6 =54 amps. Not knowing what kind of pump or head on the pump there is no way guide you.
the 2.5kw might start the motor, with out an load for a while. Your pushing it really hard.
The 20 amp is all that is needed for the motor as the starting current is only a few cycles. Most generators will try to carry the load. Sometimes they run into a brick wall and then the genset shorts out and your left with an noise maker. Look into an softstart for your application. Might be cheaper than an larger genset than you need. Especially when you consider the fuel and maintenance.
Oversizing the wire when on the genset will help a little. Voltage drop is very important when starting an load. I.E. run #10 gauge stranded to the motor instead of the #12.
Gensets have an finite amount of power. The utility is almost and infinite amount.
Reply to
SQLit
The pump is rated for 5 gpm at 400' head. It's set at a depth of 210' and the elevation difference b/t the well-head and the house is 0'.
Reply to
N7RX
My simple mind wants it to be true that 1/2 the head equals 1/2 the load equals 1/2 the starting current.
Reply to
N7RX
depth of 210' and
0'.
Thats a multi stage high pressure pump.. you will need the full locked rotor KW gen set to start that baby...if its a gasoline fired gen set the gas available these days may not work to produce the max rated HP... also as the gen set ages it will not produce max rated KW.
For example my brothers camping generator and lawn mower quit running well or at all.. he drained the gas and filled with motorcycle race gas and they ran perfectly. the gas people are screwing with us these days.
Go with a propane or natural gas fuelled gen set if you want stable stand by capability go one size larger...Genrac might make one that small.
Phil Scott
.
starting, 9 amp
manufacturer
reqm't of a generator
at surge
reqm'ts of the
genset work, for
why a 20 amp
breaker rating
surge-draw.
knowing what kind
while. Your
starting current is
run into a
noise maker.
cheaper than an
the fuel and
Voltage drop is
stranded to the
almost and infinite
10/8/2004
Reply to
Phil Scott
1/2 the load
thats because you havent considered two other things... inertia, accelerating the rotor and head of water up to speed at start...and inrush current resulting from a stalled rotor under such a load. so locked rotor will remain high (starting current)... half the head will reduce the *duration of the starting current...but not the peak locked rotor amperage significantly, and yer generator set *dies at its peak surge load rating... and you do not want to buy a gen set that has to run at 100% load regardless... you want it to run easy so it lasts longer.. ... 60 to 70% of full load as a rule. not 100%
Manufactures tend to be optimistic on thier ratings as well.
Phil Scott
depth of 210' and
is 0'.
starting, 9 amp
manufacturer
reqm't of a
that at surge
reqm'ts of the
genset work, for
why a 20 amp
breaker rating
surge-draw.
knowing what kind
while. Your
starting current is
they run into a
noise maker.
cheaper than an
the fuel and
little. Voltage drop
stranded to the
almost and
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Reply to
Phil Scott
Well, I was planning on converting my gas generator to propane, but I'm not sure that this solves anything. The rated capacity of the genset is twice the nominal current reqm't of the pump but only half of the starting current.
Reply to
N7RX
propane, but I'm not
genset is twice
starting
I dont think it will work but its a half hour job to try it.
message
is
a
not
ages
quit
with
people
want
might
that
current
2.5Kw
and
the
they
an
consider
little.
gauge
formatting link
.> >> > Version: 6.0.775 / Virus Database: 522 - Release Date:
Reply to
Phil Scott
You need a soft start controller such as:
formatting link

Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz
Reply to
Bill Kaszeta / Photovoltaic Resources
snipped
snipped again
Sorry dude with that much of an head your talking seconds of locked rotor current on the motor. Your genny will never handle it, if it does it will not for very long. With that much head the soft start will have to be pushed out a lot farther in time than I would want for my pump. Before looking into the softstart better check with the manufacture of the pump to see how the pump will handle the situation. As a wild assed thought have you considered an 2 speed motor? Start on low, get the water moving then move to high. Again check with the pump manufacture about doing this before doing it.
Bottom line you need an bigger gen set. Unless you really into creating an monster that no one but you will be able to handle. What about the SO when your not around and this needs to work?????
Reply to
SQLit
| Go with a propane or natural gas fuelled gen set if you want | stable stand by capability go one size larger...Genrac might | make one that small.
Smallest Generac on NG or LP is 7 kW. But I think he needs at least that big, anyway. Next size up is 10 kW commercial model and 12 kW home model. Kohler models start at 10 kW.
A neighbor has a 5.5 kW gasoline generator, and his big screen TV won't run on it at all. But on utility power, that TV blinks half the lights almost to zero for a very brief instant to get the TV on. I think he needs a bigger genset. I'm trying to talk him into a 25 kW NG model. Then he can have some hot pizza while watching TV in the big blizzard (if his dish doesn't ice up).
Reply to
phil-news-nospam

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