Compressor Power is not Measured by Amps Alone

April 21, 2009
Use kilowatts or characterize the motor curve to determine operating conditions.
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 1where kW = input kilowatts A = motor current (amperes) V = line voltage pf = power factorBut, 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: