apc inverter output

[richerdick32]

Hi, I want to measure the current output from a UPS. I seem to remember that there is a reason that you can't use a standard current clamp on inverter outputs as it will not give a true reading. I am assuming that the output from the ups will be some sort of PWM waveform? could somebody please remind me of the reasons and indicate how inaccurate the readings are likely to be.[highlight #FCE94F][/highlight]

 

[itsmoked]

Older meters assumed sine waves and gave bad results. Newer "True RMS" meters have no difficulty with inverter outputs. VFD outputs are another story though. Their output is so complex that even True RMS can't do it reliably.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sibeen]

The output of a UPS will be sinusoidal, or at least quasi sinusoidal. APC did produce some real crap over the years where there would be a stepped square wave style output, but even these get themselves into the quasi sinusoidal area. Basically, a UPS has some form of filtering to the PWM, something that VFDs don't bother themselves with. So if your meter is true RMS it won't have an issue with any form of UPS output current reading.

 

[jraef]

Besides, it's reading VOLTAGE from an inverter output that cheap meters have more trouble on, current is usually pretty good if you have a modern True RMS meter or an old fashioned analog clamp-on meter. The problem is, the value of that reading is something worth questioning. The current you read on the output of any inverter is going to be inclusive of distortion current and reactive current. In other words the meter will read ALL current coming from the inverter, but not all of it will be useful for some types of loads. Depending on the sophistication of the inverter output algorithm, the distortion current can be anywhere from 0.3% to 18% in one report I read (which was outlining the justification for employing a better algorithm, which meant more expensive components).

Distortion current will not do much in the way of useful work, unless maybe to a heating element, because for the most part all it does is make things run a hotter. So if your inverter is shown by your ammeter to be putting out 10A, but it is inclusive of of 18% distortion current, the amount of amps you REALLY have to work with may be only 8.2A, because when your loads consume 8.2A of real current, they might ALSO be consuming another 1.8A of added current that is just heating up things with windings in them, such as transformers on power supplies, motor windings etc. The inverter however still has to put it ALL through its transistors, whether it is useful to you or not.

From my experience, APC has never been known for aggressively pursuing quality over price points, so I would not be as hopeful that they would spend the extra money to research the better ways to do it. I may be wrong on that because things can change, but I worked for Sola for a couple of years a long time ago and at that time, we got our butts handed to us by APC, who was selling absolute junk UPS systems, but at prices so low people put on their blinders to buy them at "big box stores" and hope for the best.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington