# Start up draw for motors



## Windy in Kansas (Jun 16, 2002)

Often the start of draw of motors is spoken of in this forum and this if for information, no question.

A couple of weeks ago I had occasion to look up the draw and the start up draw needed for my well pump. I was shocked at the difference. About 1100 watts for running and pumping, but a huge 4900 for starting. Figures rounded some. 

I expect the high start up is needed to begin pushing the water in the well pipe and actually get it flowing.


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## PD-Riverman (May 24, 2007)

I would feel that "most" of the start draw is the "start-up" getting the motor from a stand still up to several 1000 RPM's. My shallow well pump pulls around 1100 running to some where around 2000 on start up(according to the meter on the inverter which could be wrong". 





Windy in Kansas said:


> Often the start of draw of motors is spoken of in this forum and this if for information, no question.
> 
> A couple of weeks ago I had occasion to look up the draw and the start up draw needed for my well pump. I was shocked at the difference. About 1100 watts for running and pumping, but a huge 4900 for starting. Figures rounded some.
> 
> I expect the high start up is needed to begin pushing the water in the well pipe and actually get it flowing.


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## Ross (May 9, 2002)

That's a lot of power to change states of inertia. If you have a tach in your car you can see it when you accelerate. It takes extra amps to get an electric motor up to speed and sure if it's also got a load to wind up too there's even more. Once it's at rated speed it drops the amount of electricity it needs to its rated draw.


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## Explorer (Dec 2, 2003)

Windy in Kansas said:


> Often the start of draw of motors is spoken of in this forum and this if for information, no question.
> 
> A couple of weeks ago I had occasion to look up the draw and the start up draw needed for my well pump. I was shocked at the difference. About 1100 watts for running and pumping, but a huge 4900 for starting. Figures rounded some.
> 
> I expect the high start up is needed to begin pushing the water in the well pipe and actually get it flowing.


It is very similar for air compressors.


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## ace admirer (Oct 5, 2005)

in rush current,,,,,the first few thousands of a second up to 1000% of loaded current can flow just to build up the magnetic field before the motor even begins to work against inertia..luckily the line losses keep current as low as the numbers you quoted.....

this is why motors starters with overloads are used to protect larger motors...

and this is why we do not want to cycle motors on and off without a period of cooling....even cooling during the running period one reason we need "deadband" on controlled devices.


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## Guest (Jul 18, 2011)

I had a 20 gallon compressor (sorry, don't remember the HP) that I tried to run off a 3000 watt genny to power an air nailer. The genny could start and run the compressor if the compressor had no pressure in the tank. If the compressor hit it's high pressure and shut off, it couldn't start back up when the pressure fell untill I came down off the roof and bled out the remaining pressure. It made horrible noises in the mean time so I designated a person to turn off the compressor after it charged up and stopped and bleed off the pressure when the nailer didn't have enough pressure before starting it up again. It seems to me that trying to start working against the pressure took more energy that starting with no pressure to buck.


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## deaconjim (Oct 31, 2005)

When a motor is de-energized, it is basically a long piece of copper wire with very little resistance. When voltage is applied to the windings, current draw is determined by dividing the voltage by the resistance of the copper wire. Given the fact that a straight piece of copper wire has a very low resistance, you can see that starting current would be pretty high. Once the electro-magnetic field builds up in the moter, the motor starts turning, which in turn creates counter electromotive force (counter EMF) which looks like resistance in the windings. This counter EMF limits the current flow to what we consider to be running amps.

The rule of thumb for starting current in a three phase motor is 6-7 times the running current. I'm not sure how that would apply to a single phase motor.


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## Jim-mi (May 15, 2002)

Nimrod that "3000" watt gen typically will put out only 3500 watts for "surge"
What you are doing there will shorten the life of those units.


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## Windy in Kansas (Jun 16, 2002)

I thought most compressors today had unloading valves so that they didn't have to start against the pressure from the tank. Guess that may not be the case.


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## Energy Rebel (Jan 22, 2011)

Windy in Kansas said:


> I thought most compressors today had unloading valves so that they didn't have to start against the pressure from the tank. Guess that may not be the case.


They do.
There are still a few that are made without unloading valves, but most do.


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