2002-03-26
----- Original Message ----- From: "Barbara and/or Bill Hooper" <[EMAIL PROTECTED]> To: "U.S. Metric Association" <[EMAIL PROTECTED]> Sent: Tuesday, 2002-03-26 15:28 Subject: [USMA:19081] Re: watt seconds ?? > 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?) You lost me here! What do you mean by constan?. The power formula for DC (P=EI) is modified when used with AC. The factor of cosine of the angle between the current and voltage must be taken in account. If the voltage and current are in phase, then the power is the equal to the voltage times the current as in a DC circuit becaue the difference in angle is zero and the cosine of zero is one. If the voltage and current are pi rad out of phase, then the cosine of pi is zero and the power is zero too, even if we were talking about a megamp time a megavolt > > 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. Not the average, but the RMS (Root Mean Square). The RMS value of the voltage and current , as opposed to the peak to peak will produce the same heating as DC when applied to a resistive load. The average value of an AC sine wave over one cycle is zero. > > 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. The use of volt-amps, as opposed to watts is important in AC. Because even though the current and voltage are out of phase, the power company still has to produce the power and use the resources as if the voltage and current are in phase. If there is a large inductive load on the line, capacitors are added to bring the power factor close to unity, otherwise the power company will lose money. > > 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". Average and RMS can not be mixed. They are both valid terms and mean something different. If you mean RMS, you have to say it, or someone who knows better will think you are talking about something else. John > -------------------------------------- > "Simplification" begins with "SI" > -------------------------------------- > > >
