[RE-wrenches] Modified sine deterioration

[Exeltech]

Hello Jason,

Aside from a peek at the mod-square inverter waveform now and then (with an oscilloscope) .. there's nothing more you can do in terms of "troubleshooting".  Your interval could be a year or more, especially when the product is new.  In terms of "preventive" .. install inverters in a clean, dry, and as "room temperature" a location as possible.  This will help extend the life of ALL of the components.

High quality electrolytic capacitors, used within their specifications in well-designed circuits, will wear out very slowly - as in many decades.  Drake's mention below is a perfect example.  He's working with a 20-year old unit, and parts have improved considerably in 20 years.  The "electrolytic capacitors will always die early" mantra is an urban legend that just won't die.  Cheap low-quality parts will die young.  So will any other parts used outside of their specifications.  Will "film" capacitors outlive "electrolytic" capacitors?  Yes.  Will it matter to you if the electrolytic-based product lives only 40-70 years compared to a "film capacitor" version wearing out in [say] 70 or 80 years?  High quality versions of either part, properly designed into a circuit and used within their respective specifications, will serve the purpose well.

Any changes you'll observe in the waveform due to capacitor "wear-out" will tend to be very gradual, as in years and years.  Take a good photo of the 'scope screen, print it, and keep it on file as a before/after reference.  Check the wave shape and "amplitude" (voltage) under identical conditions each time.  A simple 100-watt incandescent lamp will help.  Turn off all loads except the lamp.  Plug in the lamp, attach your 'scope, and take a picture.  Fact is, you should do this step for ALL of your installations when new.  Keep the printed image with your customer's records.  Make a digital backup copy for future use.  Ink fades.  Refer back to it as needed.

While this won't reveal everything that could possibly go wrong .. it's a quick check that may provide some indication of pending trouble.


Dan


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On Tue, 5/20/14, Jason Lerner <wapalco at rockisland.comwrote:

Subject: Re: [RE-wrenches] Modified sine deterioration
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org
Date: Tuesday, May 20, 2014, 10:55 AM

Hi Dan,

I still have 4 Trace U series inverters in the field and have told these customers that they should be saving up for a new inverter for the last 10 years.  Is there some preventive troubleshooting I could do before they potentially have the problem Drake is describing,  or just die of old age?   I have an O scope,  but have never actually used it on Modified Sine Wave....

Thanks,

Jason Lerner
Waldron Power and Light Co.

On May 20, 2014, at 7:54 AM, Exeltech wrote:

Hello Drake,

As a professional electrical design engineer in the solar energy industry, I can say without hesitation, the simple answer to your question is "Yes".  Waveforms can deteriorate as components age.  Lower-quality capacitors will dry out in 20 years' use (or less!), and this in turn can indeed alter the waveform.  You'd need at a minimum a good oscilloscope to observe the waveform.  If it's really ragged, and no longer stair-stepped, the inverter is clearly suspect.  If it resembles a "Pyramid", there may be another issue afoot.

Motors connected to modified square wave power turn at lower RPM and operate [sometimes much] hotter than their sine-fed brethren.  Over time, the increased heat takes its toll on the wire insulation in the motor, causing an eventual breakdown of the insulation and a short-circuit of the windings.  Presto!  Motor BBQ (sometimes inverter too).


By the way .. to use "quality" in context with reference to a modified square wave is quite an oxymoron .. but in this case, it fits.


Dan

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On Tue, 5/20/14, Drake <drake.chamberlin at redwoodalliance.orgwrote:

Subject: [RE-wrenches] Modified sine deterioration
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org
Date: Tuesday, May 20, 2014, 9:24 AM

Hello Wrenches,

We are diagnosing a system with a 20 year old Trace DR 1512 inverter. Recently a motor burned out that had been running on the system. I do understand that modified sine (modified square) wave inverters are hard on motors in general.

My question is, "does the quality of the Mayan Pyramid sine wave deteriorate with time as the capacitors dry up, or for any other reason?  Would the power quality be expected to have declined on an old unit like this?

Thanks,

Drake

Drake Chamberlin
Athens Electric LLC
OH License 44810
CO License 3773
NABCEP Certified Solar PV
740-448-7328
http://athens-electric.com/