[pvusers] RE:Question on inverting DC to AC

["Robert Warren" <robertwarren@mail.com>]

Chris,
 
 If you were building an inverter (and I know you aren't from the question you asked), then it is no big deal to step up the DC voltage with a bridge rectifier/capacitor circuit: a lot of inverter manufacturers have entirely gotten away from using big transformers. But don’t build an inverter: buy a used one if you can’t afford a new one (not from the internet, but locally, so you can check it our thoroughly).  It isn’t so much the voltage step-up, but the creation of a good approximation of a sine wave, which it the complex part of building a good inverter. Plus, you have to put in some essential safety features, such as a low battery cut-off switch so you don’t kill the batteries or the inverter (low voltage means high amps, which would kill the inverter unless everything was well engineered.) 
   In addition to the fact that the NEC Code requires a special permit for anything over 50 volts DC, a good reason for not using a 120 VDC battery circuit is that you would have a  hard time finding a PV charge controller, gen set battery chargers and other 120 VDC compatible devices. I work with high voltage DC all the time in 99% of  the grid connected inverters I install (such as the Sunnyboy inverters) and I love the fact that you can use smaller cables and connectors. We use touch safe cables and have a strict practice of never working on a live connection. The one place you should absolutely follow this rule is at the battery itself: first you install an Anderson connector, then when you are ready with all you other connections, you just plug it in.
   But I can second the previous warning about high voltage DC shocks: I used to troubleshoot and repair large UPS’s and I have been shocked--it isn’t pleasant. It can do a pretty good job of melting away the end of an end wrench.  Whenever you are hooking up battery cables, you should always wear eye protection.  I still have my eyes and am glad that I always followed this rule.
Good luck,
Robert Warren

> To: pvusers@crest.org
> Subject: [pvusers] Question on inverting DC to AC
> 
> 
> Is the process of converting DC to AC simplified by keeping the voltage
> the same? For example, converting 120VDC to 120VAC. I ask because I'd
> like to be able to save money on an inverter for a system I'm
> contemplating. If I could eliminate most of the expense of an inverter
> I'd be thrilled . . .
> 
> Thanks,
> 
> Christopher Witmer
> 
> 
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robertwarren@mail.com


Robert Warren,
Senior Engineer
Solar Century Holdings Ltd
www.solarcentury.co.uk
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