Yeah, that is a big issue with those big and heavy ferroresonant power supplies from GE and Motorola. At low loads, they are incredibly inefficient, with very low power factors- meaning the volt-amperes are much higher than the watts. Proof of this statement is found in the Files section of this group, in my Power Supply Load Test folder.
You pay for electric energy in kilowatt-hours, kWh, not for volt-amperes. Regardless of what the ammeter reads, the Kill-A-Watt reading of 43 watts is what counts. If we assume that the repeater sits idle for 24 hours and consumes 43 watts, that is 1.032 kWh per day- probably 15 cents worth. I have one of those Kill-A-Watt meters, as well as some spinning-disk meters, and they agree very closely. Electricity suppliers don't like low power factors, because even though the power consumption may be low, the utility must build their infrastructure to supply those reactive amperes, meaning bigger generators, transformers, and power lines. Large industrial customers are often penalized for low power factor, to help pay for the additional capital equipment that must be installed to supply reactive amperes. That's why new commercial gear is using power-factor-corrected switchmode power supplies. The definition of "high power factor" varies, but most utilities want PF to be above 0.9. It should be emphasized that one cannot determine power consumed (watts) by separately measuring volts and amps. Separate measurements result in volt-amperes, which is also called "apparent power." To measure true power, one must use a wattmeter. A mechanical wattmeter is called an electrodynamometer, and is a meter that has two coils- instead of one coil and a permanent magnet. One coil is connected in parallel with the load, and is energized by voltage. The other coil is much heavier and is connected in series with the load, and is energized by current. The torque on the meter movement is the instantaneous sum of voltage and current in phase, result in a deflection indicative of power. An electronic wattmeter uses a circuit element known as a four-quadrant multiplier, meaning that the comparison of voltage and current is continuous through 360 degrees. I have just such an instrument, a WD-767 digital wattmeter made by VIZ. It can display true-RMS volts, true-RMS current, and true power in watts. Very handy! 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Mike Morris WA6ILQ Sent: Sunday, October 04, 2009 6:10 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Spinning disk wattmeter... A ham I know is doing some research and needs to locate a spinning disk KWH meter, with socket, cheap or free... If he turns up something interesting it will end up as a repeater-builder article. He wrote: >My concern is that the cabinet I had here last year measured at idle >1.5 amps at 120V (180VA) yet also only measured 43 Watts with >the Kill-A-Watt meter. I am looking for another device to tell me what >the electric company is actually seeing and billing. Might one of your >connections have an extra single phase KWH meter in the junk box? I suspect he has a situation involving power factor. Mike WA6ILQ
