on 3/26/2002 12:41 AM, Bruce Hebbard at [EMAIL PROTECTED] wrote: > ... could someone please explain why serious > power equipment often uses VA (kVA, MVA, etc.) as distinct from W ?
That is done in working with AC (alternating current) electricity. The power (in watts) is the voltage (in volts) multiplied by the current (an amperes) ONLY if the voltage and current are constant. (Let's face it: if they are NOT constant, you would not know what value to use?) AC power is found by calculating the product of the instantaneous current multiplied by the instantaneous voltage, moment by moment over a whole cycle of oscillations. If the current and voltage are in step (rising and falling, and reaching peaks and valleys at the same time), then the average power is found by multiplying the average* voltage by the average* current. BUT, often (usually?) the current and voltage don't rise an fall together in step. Then, the average power is NOT equal to the average* voltage times the average* current. (If they are exactly one quarter cycle out of phase, the power will be ZERO!) Yet, sometimes, the product of the average voltage and the average current is of importance to electrical engineers (I don't know the details of this). So they multiply the two together and get a value in volt-amperes WHICH IS NOT EQUAL TO THE POWER, so they do not express it in watts. Instead they call it volt-amperes. Regards, Bill Hooper retired physics professor, Florida, USA *The "average" I refer to above is the root-mean-square (RMS) average, sometimes called the '"effective value". -------------------------------------- "Simplification" begins with "SI" --------------------------------------
