[RE-wrenches] Quantifying power quality

[Exeltech]

Wrenches, 

Be careful about connecting *any* capacitors across an inverter's AC output.  It's not a smart thing to do.

While a capacitor may help in some cases when a load has a substantially poor power factor due to significant inductance, there are two serious concerns here:

1. The power factor seen by the inverter will be equally bad in the opposite polarity when the inductive load turns off, and IF the capacitor is still connected across the lines.  Larry implied the capacitor he connected was disconnected when the load turned off.

2. Larry "shot-gunned" a solution.  He guessed, and got lucky.  We don't know how much actual capacitance was needed in this specific situation.  Capacitors connected across an inverter's AC output can destabilize its voltage control loops, leading to erratic AC voltage, resulting in possible damage to the inverter, the load(s) .. or both.

Power factor is one of the most misunderstood aspects of electricity.



Dan
(Professional inverter design engineer .. among other things...)

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On Sun, 10/12/14, Starlight Solar Power Systems <larry at starlightsolar.com> wrote:

 Subject: Re: [RE-wrenches] Quantifying power quality
 To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
 Date: Sunday, October 12, 2014, 1:33 PM
 
Hi William,

If you recall a couple years ago we had some discussion here about a problem with a specific make appliance being powered by a Magnum inverter. The problem, it was discovered, was the appliance input PF was creating current demands that the transformer based inverter could not keep up with. When I powered the load with a low cost, high frequency inverter, it worked fine. Apparently the HF inverter can keep up with the current changes faster than the Magnum.

To fix the problem, a 50mf run type capacitor was installed in the appliance so that it was applied across the AC input when powered on. Everything worked fine then because the cap was taming the wild current created by the low PF. 

Try looking at the current and voltage waveform together (AC shunt, dual trace scope) at the UPS input. If the current is displaying large spikes and/or is out of phase, you may have found the problem is with a non linear load from the UPS. It may be the SI, like the Magnum, can not keep up and so the UPS input circuits are throwing a fit.

Or, something else you can try, connect a high frequency inverter and try to power the UPS. This can be a portable 12 volt battery and inverter. If the problem disappears, well, you take it from there.

Larry