Amp draw on a motor at different voltages, same load

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mark32

mark32

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Currently in NJ
Hello, I used to work for a shop that built electric pressure washers. Before being shipped out we, of course, tested each machine. The shop had 120/208 so my question is this. After setting the machine up to do a certain pressure, a record was made of that as well as the amp draw. Let's say we tested a machine at 208 and the customer hooks it up to 240. At 240, would the amps go up or down? Initially I thought they would go down, but the more I think of it, I think they would go up, including the output of the motor. Any thoughts?
 
T

templdl

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Location
Wisconsin
Re: Amp draw on a motor at different voltages, same load

What will the actual voltage be that the pressure washer is being supplied by? You didn't provide the actual NP information no did you say if it's a 1 or 3ph motor. From the info that you had provided I would guess that it is 1ph from the 120v rated you indicated. The 208v is a bit odd for a motor in combination with the 120v but not something that couldn't be done.
It is of my opinion that 240 is too high for 208v motor unless the manufacturer labels it for that application. If motors are designed to the industry NEMA standard, motor should be able to operate +-10 of the NP voltage. 208v + 10%=228.8v. One thing to remember is that 120, 208, 240, 277, and 480v are all nominal. Often times 120 which may be from either a 120/240v 1ph 3w system or from a 208y/120v system my be less than 120v. Note that the 240v may be referred to as 230 or 220v where that 120 then becomes 115 or 110v. Motors designed for a 480v system are commonly nameplated 460v.
 
mark32

mark32

Senior Member
Location
Currently in NJ
Re: Amp draw on a motor at different voltages, same load

Thank you templdl for your response, sorry I wasn't as clear as I could have been. The machines in question are single phase 120 or 240, but we did three phase as well. I agree that a motor made to run on 240 shouldn't be operated on 208, we just tested it at 208 because that's what we had. Thanks.
 
infinity

infinity

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Journeyman Electrician
Re: Amp draw on a motor at different voltages, same load

The shop had 120/208
Click to expand...
I'm assuming that the 120/208 volts is the voltage of the building that you're working in.

Let's say we tested a machine at 208 and the customer hooks it up to 240. At 240, would the amps go up or down? Initially I thought they would go down, but the more I think of it, I think they would go up, including the output of the motor. Any thoughts?
Click to expand...
The current would go down in your scenario. But as Templdl said you would be operating outside of the normal range of a motor rated for 208 volts.

[ September 27, 2005, 08:30 PM: Message edited by: infinity ]
 
mark32

mark32

Senior Member
Location
Currently in NJ
Re: Amp draw on a motor at different voltages, same load

Thank you infinity for your response. Again I don't dispute the fact that the company I worked for did things oddly, such as testing a machine at a different voltage than it would be at the customer's location. The reason I bring this up is because I came across the following question in a master electrician's exam prep book (By Tom Henry). The question is this "When operated on a voltage 10% higher than the nameplate rating, an appliance will:" And the correct answers are
1. Have a shorter life
2. Draw a higher current
3. Use more power

So I just applied it to my own question. If the load remains the same, the higher voltage will push more amps through the load, thus increasing the output, yes? The motors we used were typically rated 120/230 I believe, certainly not rated for 208 though. But again, my question doesn't concern whether it's right or wrong to run the motor at a voltage it wasn't designed for. Thanks, Mark
 
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