Tom, Excellent questions! The first is easy: Residences and light commercial occupancies have meters that measure real (true) power only. That's because only real power does work, and that's what you are paying for. The classic kWh meter with the spinning aluminum disk was perfected by Ferraris and Shallenberger more than a century ago, and millions are in service today. The most recent improvement is a magnetically-levitated disk that nearly eliminates any errors due to bearing friction. A revenue-grade kWh meter is extremely accurate, and very seldom requires service.
True power is consumed only when the applied voltage and the resulting current are in phase. In this specific case, volts times amps equals watts. When the current is not in phase with the applied voltage, we enter the mysterious world of apparent power, which is expressed as volt-amperes. Nearly all apparent power seems to be consumed by inductive apparatus such as motors and transformers, except that such currents are returned to the source as the magnetic field collapses. Suffice it to say that the current actually flows in the circuit, but it does no real work. When the power factor (PF) is poor, a lot of current flows in the circuit that must be provided by the utility through larger diameter wires and bigger transformers. Since PF is the ratio of true power to apparent power, the utilities are always looking for ways to keep the PF close to unity, so that they can put off installing thicker wires and bigger substations. As an incentive for heavily-motored industries to increase the PF, some substantial penalties are levied on those who don't correct their PF. The easy way to increase the typical PF is to add parallel capacitance so that the inductive reactance is compensated by capacitive reactance. The goal is to get the PF above 0.95. Another reason for adding capacitors across the line is for voltage regulation. In rural areas with long distribution lines- usually at 12,000 volts- the voltage drop due to reactive currents can be significant. The utility will add capacitor banks every few miles to help keep voltage drop within narrow limits. This is as good a time as any to state that the standard nominal utilization voltage in the United States is 120/240 VAC on single-phase systems, and 120/208 VAC on three-phase systems. There hasn't been "110" or "220" in this nation for more than half a century, but some (usually older) folk still use those terms. The standard kWh meter does its magic by using a simple principle. The torque on the aluminum disk is caused by two coils (or sets of coils) that create a combined magnetic field. One coil creates a magnetic field proportional to the applied voltage, while the other coil creates a magnetic field proportional to the line current. When both voltage and current are present, the disk spins. A small permanent magnet acts as an eddy-current brake, and ensures that the speed of the disk's rotation is exactly proportional to the product of voltage and current that are in phase- true power. The disk is geared to a register that records the revolutions over time, resulting in power times time- energy. The rotating-disk kWh meter is being replaced with all-electronic meters in many areas. Such meters can record reactive power usage, and also record the times that peak demands occurred. Some really fancy electronic meters can be remotely polled with a wireless system, so the meter reader can drive by the house or business and get the data on the fly. Carrier-current data transfer systems are now in use that allow the utility to read meters from a central office that is miles away. Not only does the utility not have to set meter readers out into rural areas, but any power outage is immediately revealed due to the loss of data. Back to your original question about your customer's motors. If he is not being penalized by the utility for excessive reactive power demand (i.e., low PF) then he will save nothing. However, the judicious application of capacitors may improve the voltage regulation within his building. Some mountaintop repeater sites have a power feed that is many miles long, and voltage drop caused by air conditioning and power supplies can be significant. Some older Motorola and GE station supplies are not very efficient and have low PF when lightly loaded. The typical site owner/manager is usually not a power engineer, and may be ignorant of the significant inefficiencies of a low-PF power system. This may exist for years without anyone doing anything about it, leading to recurring station problems and poor power quality. The national standard for nominal utilization voltage is 120 VAC +/- 5%. If the receptacle voltage goes below 114 VAC or exceeds 126 VAC, something needs to be done. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Thomas Oliver Sent: Sunday, February 08, 2009 11:43 PM To: repeater-builder@yahoogroups.com Subject: [Repeater-Builder] OT Power Factor Question for any electrical engineers out there. Are the meters on the side of buildings metering real power or apparent power? Is power factor correction worth doing if the power company is not dinging the customer for low power factor? This article http://powerelectronics.com/power_management/motor_power_management/705PET23 .pdf <http://powerelectronics.com/power_management/motor_power_management/705PET2 3.pdf> talks about residential power factor correction and my conclusion (from this article) is the savings would never be recouped. Second conclusion is the only benefit with correction is the wires between the source and load don't heat up as much. What about the wires in the motor or transformer? do they also heat less? I would think so. Third conclusion is by correcting power factor you are helping the utility company more than yourself because these phase differences "standing waves" exist all the way back to the power generation source therefore the utility lines have more loss due to their greater length than the customers building wiring has. The reason I am researching this is a customer of mine has roughly 50 hp of total motors in his shop and wanted to know if he could save 30% on his electric bill like some salesman of power factor correction black boxes told him he could. I realize I am going to have to look at his energy bill to see if there is a charge for low power factor and maybe call the utility company to see if he will get a lower rate if he adds PFC devices tom (\__/) ... (='.'=) (")_(")
