I am still pissed at getting marked wrong on an exam in college. Freshman EE class.
The question was: What is Power Factor. My answer: Cosine of the Phase Angle I was right. They were wrong. I was robbed of some points. Somebody has to pay someday... From: Mark Radabaugh via Af Sent: Tuesday, October 28, 2014 9:32 AM To: af@afmug.com Subject: Re: [AFMUG] APC "load in watts" For what it's worth since most people are not used to power factor (PF) Power is measured as volts * amps * PF = Watts VA ratings are volts * amps For DC the power factor is effectively 1 and can be ignored (we are talking supply power here... you RF guys don't need to go off on a tangent here...). The current draw of a load does not always track the voltage supplied. If the load is purely resistive (no inductors, no capacitors) then rising voltage and current track linearly. Most AC loads are not purely resistive and the current may lag or lead the voltage - meaning the current rise lags behind what would be expected for the voltage applied. The power factor (PF) is the ratio between the (expected) current and voltage and it's expressed as a number between -1 and 1. Old switching power supplies have a PF around .8 A 500W, 120V power supply with a PF of 1 draws: Voltage: 120V, Current (500/120) = 4.16A A 500W, 120V power supply with a PF of 0.8 draws: Voltage: 120V, Current (500/120/.8) = 5.21A The power supply with the PF of 0.8 requires considerably more current from the UPS than the power supply with a PF of 1 ~ 25% more. UPS manufacturers rate power supplies based on VA rather than Watts since (take your pick here...) they don't have control over the power factor of the equipment you are adding, or the VA number is bigger and looks better on the box. You can get more runtime out of your UPS if you pay attention to the power factor of the computers you attach to them. Better power supplies have better power factors. They cost a little bit more but can save you money on your UPS. Mark On 10/28/14, 10:35 AM, Robbie Wright via Af wrote: Under, about click on UPS. You'll see apparent power rating in VA and real power rating in watts. Under status>ups, you should also see output voltage and amps so you can figure out watts if you want. It also logs your usage in kWH as well. Robbie Wright Siuslaw Broadband 541-902-5101 -- Mark Radabaugh Amplex m...@amplex.net 419.837.5015 x 1021
