Hi Eric, I agree with most of what you say, except the bit about generators and horsepower.
I've always understood poor power factor to be a problem because the generator and distribution system needs to carry higher currents to deliver the same energy into a poor load. That's a problem because the power companies either suffer higher resistive (and financial) losses in their systems, or they need to use heavier distribution wiring and thicker wires in their transformers and generators (increasing the infrastructure costs). To offset these costs they financially discourage anyone from using poor power factor loads. It's the same in a generator set. The difference between the 1,000 Watt/ 1250 KVA rating is that with a bad load up to 250 Watts are wasted in resistive losses in the generator windings, requiring a bit more torque from the engine to provide the extra 250 Watts and some extra energy to further cool the generator. Electronic power supplies, especially older switch modes, cause a whole new set of problems. While they show up as poor power factor loads, they also create harmonics in the supply network, and that could show up on electronic power meters very inaccurately! Cheers, Mark VK3BYY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Eric Lemmon Sent: Wednesday, 17 September 2008 11:22 AM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Volt-Amp (Re: APC UPS Charging Power) Albert, You are forgiven, because you pose an important question! The spinning aluminum disk in the kilowatthour meter found on most residential service-entrance panels measures true power in kilowatts versus time, which equals energy. Thus, your electric utility charges you for the true power you use, not for volts times amperes- known as reactive power. Although the utility must provide the capability to supply all of the amperes you need, some of those amperes are "given back" to the utility due to a lower than unity power factor. That is why many utility companies charge a "kVAR Penalty" to certain industrial power users whose volt-ampere demands far exceed their watt demands, meaning that the power factor is low. Industrial power users strive to keep their power factors at 0.95 or above, to avoid some really painful penalties! The power factor, or PF, is simply watts divided by volts time amperes. The issue of power factor is why large Diesel generator sets have ratings such as 1000 kW/1250 kVAR. In simple terms, any AC generator requires torque (engine horsepower) to meet true power demands, and excitation (field flux intensity) to meet reactive power demands. When the generator load is reactive, that is, it has a power factor less than unity, the generator must not only have the horsepower to supply the energy in watts, but it must have excess capacity to handle the additional current required by motors and other low-power-factor loads. In a nutshell, that is why a 1000 watt generator may be unable to keep running a refrigerator that uses only 900 watts; the fridge may require 1200 VA to operate because it has a low power factor, and the small generator has no ability to handle such loads. Because of its relatively small amount of spinning mass, such a small generator probably could not even handle the refrigerator's starting current- which is about 5 to 6 times its running current. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Albert Sent: Tuesday, September 16, 2008 2:13 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Volt-Amp (Re: APC UPS Charging Power) Hopefully, you will forgive me for hijacking the post, but this brings up a question I have had for a long time. What on earth is a "volt-amp"? My logic would state that is is the same as a watt, which is volts x amps, as you probably well know. So what on earth is it? Confused..... Albert ------------------------------------ Yahoo! Groups Links
