Compressor Power is not Measured by Amps Alone

Use kilowatts or characterize the motor curve to determine operating conditions.

By Hank van Ormer for Plant Services

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Too often, and most incorrectly, amperage alone is thought to be an accurate reflection of kilowatts (kW), which is then used to calculate an air compressor’s full-load specific power (scfm/kW) or to estimate flow (scfm). It's equally incorrect to use this estimated kW to calculate the percent of full load in the capacity-control cycle.

Of course, kilowatts are a function of amperage (Eqn 1).

kW = (A x V x 1.732 x pf)/1,000 Eqn 1

where kW = input kilowatts
A = motor current (amperes)
V = line voltage
pf = power factor

But, you can calculate kW from measured amperage accurately only if you simultaneously measure the voltage and power factor. Or use a kW meter or motor analyzer, which gives a direct kW reading.

To read the rest of this article from Plant Services, visit: http://www.plantservices.com/articles/2009/065.html.
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