Inverter for off-grid system - square wave vs sine

[nmdave]

How does one decide whether to spec a square wave vs. sine wave inverter for an offgrid system?

The sine wave units tend to be quite a bit more expensive, but also have many valuable features in terms of charge control, managing a standby generator, etc. These issues I can understand and explain. Where I can't find good data is the impact of a square wave AC vs an approximate sine wave on equipment in the home. There's lots of anecdotal info on burned out appliances and such, but I can't seem to find any hard data on what types of equipment are likely to suffer and how bad.

Most confusing is that many of the items that are claimed to be vulnerable are really DC devices that rectify the AC anyway!

Pointers to good reading would be much appreciated.

 

[Guest]

Re: Inverter for off-grid system - square wave vs sine

I asked a similar question a while back and would be very interested in the reply. I have been to APC training seminars where the question was raised as many computer and computer peripheral manufacturers will void their hardware warranties if they find out their hardware has been hanging on an AoSW UPS. APC dodged the questions like the Governor in The Best Little Wh**ehouse in Texas (an old Dolly Parton movie).
http://www.apc.com

 

[roger]

Re: Inverter for off-grid system - square wave vs sine

With the very nature of the grounded AC power system we use in the U.S., (as well as other countries) why do electronic manufacturers think someone besides themselves should fix the problems they create?

Dave, if a square topped wave will cause problems beyond your device, I would think your question is already answered.

Final question for you, where are you proposing this distortion will ocure in a residential service?

Roger

 

[hbiss]

Re: Inverter for off-grid system - square wave vs sine

Well, I can understand that SOME switching power supplies might not like an AoSW input but I have never myself witnessed a problem least of all damage. This after providing APC backup UPS's for many hundreds of telecommunications systems as well as my own and other computer systems.

Keep in mind also that SOME manufacturers will use ANY excuse to get out of their responsibility to provide a warranty. Dirty power and lightning is #1 on the blame list so why shouldn't this follow.

Manufacturers should know full well that most popular and "affordable" UPS's provide a AoSW output. Nobody is going to use a $900 UPS for their $1200 computer. If there is a AoSW problem it's poor power supply design, which the manufacturer usually doesn't even manufacture anyway.

As for APC skirting the issue, what would you expect them to say. "Yes you are right- don't buy our products?"

Has there EVER been any substantiated evidence of a problem or is this from a bunch of computer "consultants" who will believe anything?

-Hal

 

[dereckbc]

Re: Inverter for off-grid system - square wave vs sine

nmdave, what are connecting to the UPS? There is more to UPS than AoSW and true sinewave.

 

[nmdave]

Re: Inverter for off-grid system - square wave vs sine

The situation I'm facing is an off-grid PV powered home, so the answer to what will be connected is "everything" - the fridge, TV, clocks, shop tools, dishwasher, ... If it plugs in to a receptacle, it will be running off the inverter.

One friend in a similar situstion said the only two pieces of equipment producing problems were the dishwasher when it had a fancy electronic control (instead of a plain old fashioned rotary knob) and the battery charger for his cordless drill. I've heard other stories of random pieces of equipment not working, sometimes from a basic square wave inverter, sometimes from the fancier "sinewave" units.

AoSW doesn't ring a bell - translation please.

 

[dereckbc]

Re: Inverter for off-grid system - square wave vs sine

AoSW=Approximation of Sine Wave. Your inverter generates a square wave, and uses an output transformer to approximate a sine wave by the filtering characteristics. This is one method of AoSW. There are other methods that use digital techniques, but you get the idea.

[ April 04, 2004, 02:28 PM: Message edited by: dereckbc ]

 

[hbiss]

Re: Inverter for off-grid system - square wave vs sine

In this situation if it were me I would opt for the method that provides power as close to what a utility provides as possible since you will be powering just about anything. I also don't think I like motor loads running on anything but a sine wave anyway.

-Hal

 

[hurk27]

Re: Inverter for off-grid system - square wave vs sine

Triplet has several inverter systems that will have a full 120 VAC @ 60Hz sine wave output Instead of the 110 VAC 60Hz square wave output that most low cost inverters have I have notice that when using any motor driven appliances on one of these low cost units the motor will run at a much lower speed, even using a resistance load with the motor as the instruction says, the motor will still run slow. Now I have tried a small 1 to 1 transformer in-line with the motor and it will run a full speed again after looking at both sine waves (with and without transformer) on a ocilloscope, I found the transformer will flywheel the square sine wave back into a full sinusoidal wave and give me the full 120 vac. I remembered this from my electronics class's.

 

[big john]

Re: Inverter for off-grid system - square wave vs sine

Originally posted by hbiss:
...I also don't think I like motor loads running on anything but a sine wave anyway...

Aren't a lot of motor VFDs pulse-width-modulators? Those loads certainly aren't running on a pure sine wave and they seem to operate nicely. Or does a motor have to be specially rated to run on a VFD?

-John

[ April 04, 2004, 04:41 PM: Message edited by: big john ]

 

[big john]

Re: Inverter for off-grid system - square wave vs sine

Sorry. Double Post.

[ April 04, 2004, 04:43 PM: Message edited by: big john ]

 

[nmdave]

Re: Inverter for off-grid system - square wave vs sine

the sine wave inverters targeted to the off-grid PV market are the digital type. Outback (a major vendor) claims <1% THD, which satisfies UL1741 for grid-tied systems on net-metering.

 

[hbiss]

Re: Inverter for off-grid system - square wave vs sine

Aren't a lot of motor VFDs pulse-width-modulators... does a motor have to be specially rated to run on a VFD?

Generally yes.

-Hal

 

[iwire]

Re: Inverter for off-grid system - square wave vs sine

Originally posted by big john:
does a motor have to be specially rated to run on a VFD?

From what I know that is a yes, no, maybe question.

It depends on what you need from the motors, fast speed up or a lot of short cycles and you may well need a tougher motor.

I have changed a few rotary switch/resistor bank set ups to VFDs and the original motors worked fine.

For those the mechanical loads the motors where driving had a lot of inertia to overcome so we ramped the speed up slowly to keep the current low.

 

[bgudgel]

Re: Inverter for off-grid system - square wave vs sine

It's interesting how many things have trouble with modified sine wave inverters. Usually, they're more of a modified square wave but the industry has called them mod sine wave for years.

Some appliances like most microwave ovens rely on the higher peak voltage of a true sine wave and will not cook as fast on modified square waves.

Also, a lot of things expect the smooth slope of the sine wave so that their triac-light dimmer type circuitry will work properly. I think this is why some electric blankets will not work on mod square wave. They used to just turn on and off as I remember to regulate temperature.

Bread makers sometimes won't work. Some coreless drill chargers will break (De-Walt comes to mind) when plugged into mod-square wave sources.

Some RCA small dish receivers won't work. Square wave inverters make noise too lots of times... Audible acoustic and electric noise. Hi-Fi's and audio equipment can be noisey. Some computers will not work either although I haven't personally seen one that didn't.

Motors usually like sine waves better. Motors can run warmer on mod-square.

Usually the battery chargers built into mod-square inverters , if they have a charger built in, will normally be of the Triac light dimmer type and will have a low power factor and draw big current peaks, requiring larger than normal generators for battery charging as well as causing lots of electrical noise...

There's a few differences that come to mind today.

Have a day and 1/2 !
bob gudgel
OutBack Power
Arlington, Wa.