electric meter rotor rpm vs power

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larussell
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electric meter rotor rpm vs power

Post by larussell » Mon Jul 28, 2003 8:15 am

Does anybody know how to take the rpm of the rotor on an electric meter and convert it to kilowatts? I have found general info on the net that says the meters are calibrated with a strobe, but nothing specific that says how you know many RPMs = 1kW (or how many rotations = 1 kWH).

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Externet
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Re: electric meter rotor rpm vs power

Post by Externet » Mon Jul 28, 2003 9:05 am

Hi.
I would say every power meter manufacturer can do it their ways.
Try counting how many disc turns takes to change the least significant digit one count; if that digit is tenths of a KWH, multiply by 10 to get the number of revolutions per KWH.
Miguel
- Abolish the deciBel ! -

russlk
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Re: electric meter rotor rpm vs power

Post by russlk » Mon Jul 28, 2003 9:35 am

I asked my friend, who worked for the power company, how many turns of the wheel for 1 KWH. He said there is a number on the front, something like KH=xxxx, which is the number of turns for 1/1000 of a KWH. I checked my meter which has Kh7.2. I counted 50 turns and there was very little motion of the first digit, so 7200 turns/KWH may be right.

larussell
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Re: electric meter rotor rpm vs power

Post by larussell » Mon Jul 28, 2003 11:03 am

Thanks; I'll check it out and see if I get something reasonable on mine.

rosborne
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Re: electric meter rotor rpm vs power

Post by rosborne » Mon Jul 28, 2003 12:20 pm

I think you have to know what your load is and go from there or go from what you know your motor is drawing.<p>You might have a 1 HP motor (that's about 746 Watts), but you can't know whether the motor is suppling full load. The rpms ought to be about the same whether turning a full load or spinning freely (no-load), the difference is how much current is drawn. So you need to know the current your drawing, the voltage is given. If you've never taken a current reading, be safe. Make sure you know what to do and get a sanity check from someone.<p>Useful conversions for you.
1 Horsepower equals about 746 Watts.
Power = Voltage(rms)*Current (not true for three phase, I rember that 3phase was confusing for me).
Energy = Power*time<p>If you knew you had a 1HP motor and that the motor liked to rotate at 1000 rpm and that when you had the load connected you drew 5 Amps at about 115 VAC(rms), you would know that
5*115 = 575 Watts and that if that same load were continually supplied with power that in one hour you would have used 575 Watt*hours and if you knew that your loaded rpm was 950 you would know that at 575 Watt*hours your motor rotates at 950 rpm. You could experiment and come up with a correlation between load and rpm but I honestly don't remember if it's a terribly linear thing (I think so though). I seem to remember that there is motor statistic that gives the fall off in rpm from no load to full load that would make your life a lot easier. I think it comes as a percentage, not sure. Not much help, huh?
-Rick

rosborne
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Re: electric meter rotor rpm vs power

Post by rosborne » Mon Jul 28, 2003 12:21 pm

Sorry about that, I just realized that had nothing to do with your question. :-\
Rick

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