[RE-wrenches] Quantifying power quality, 50 mf capacitor

[Exeltech]

Hello Ray,

The "capacitor across the load" trick works in some cases where mod-square-waves are involved, and where high harmonic content is present.  You're effectively creating something of a "low pass filter", and taking a bit of the edge off of the square waves.  A capacitor isn't needed for a true sine wave inverter because the harmonic content is very low already.

As Larry mentioned in his post, he wired the capacitor across the load in a manner that when the load was disconnected, so was the capacitor.  If you're going to do it .. that's the best way, but be careful just the same.  Leaving a capacitor of that size connected at all times is a bad idea.  I won't go into detail as to the "why?", but suffice to say such explanation is complicated.  In simple terms, if an inductive load causes a lagging power factor of X, and a capacitor of appropriate value connected in parallel with the inductive load results in an [essentially] unity power factor, then when the inductive load is disconnected, and the capacitor isn't, you end up with a leading power factor of X .. which is just as bad as the inductive load by itself.  That said, a capacitor connected across a mod-square wave inverter will tend to "soften" the square edges somewhat, and may make certain loads "happier" .. but the effect on the inverter isn't
 predictable, particularly where the AC output voltage control circuits are concerned.

To say you've been amazingly lucky would be appropriate.


Dan




-----Original Message-----
From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Ray Walters
Sent: Sunday, October 12, 2014 16:21
To: RE-wrenches
Subject: Re: [RE-wrenches] Quantifying power quality

Hi Dan;

> I, like Larry, must be amazingly lucky then, because we've solved power
> quality issues for many years with 50mf caps.  Usually, its been front
> loading washers not running on Trace SW inverters, but also Grundfos CP
> pumps, too.  Never had a problem, and some of these have been running that
> way for over 15 years.
> 
> I never had a power quality issue with an Exeltech, so I've never used this
> trick with your inverters.
> 
> R.Ray Walters
> CTO, Solarray, Inc
> Nabcep Certified PV Installer,
> Licensed Master Electrician
> Solar Design Engineer
> 303 505-8760


> On 10/12/2014 12:58 PM, Exeltech wrote:
> 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...)
>