# modified sine wave to true sine waveform conversion



## Pharon (Jan 20, 2014)

What kind of UPS do you have? If it's a double conversion type, it shouldn't care what the input sine wave looks like -- it's converting it to DC anyway.

If it's standby or line interactive, then you have a problem.


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## telsa (May 22, 2015)

1manElectricalShow said:


> I am the lone electrician at a chemical manufacturing facility and I've been asked to repurpose a 30 amp solar system to supply backup power (in case of a utility outage) to our server rack. I have a 750 watt PV array feeding 6 2volt deep solar storage batteries. I've successfully managed to get the batteries charging from the array, but I have now run into a very new and somewhat stumping problem. I'm reusing the magnum 3112 modified sine wave inverter (30 amp single phase 120 inverter). I've installed a buck transformer to step the line voltage to 113V and small transfer switch that is normally passing the line in from the controlled distribution panel and back through to the lugs. When testing this equipment everything went swimmingly, however when we did our "final" failover tests (controlling the actual load ie server rack, complete with rack mount UPS) I discovered that the UPS units DO NOT like modified sine wave power. I am now in search of something, anything, that would easily and simply convert modified sine waveform to a pure or true sine wave, ANY IDEAS? I have considered purchasing either a pure sine wave inverter, as well as possibly using a VFD that takes in single phase and outputs three phase to clean up the waveform. Any help would be greatly apprechiated.


https://en.wikipedia.org/wiki/Uninterruptible_power_supply#Ferro-resonant

The traditional solution for your power range. ^^

However, testing is 'all.'


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## 1manElectricalShow (Aug 30, 2012)

the rack mount UPS are definitely* not accepting* the modified sine as their relays are cycling and quickly discharging. I've spoken with the inverter manufacturer's tech support line and they do not provide any sort of device to clean up and smooth out the staircase sine wave, all they can recommend is buying a pure sine inverter. In my investigations I have learned that I would need two inverters in parallel reach our current supply needs (30 amps), otherwise I would need to purchase six more batteries to attain 24v on the dc side of this system.


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## 1manElectricalShow (Aug 30, 2012)

What I am looking for in a solution to this problem is simply a method for force feeding the modified sine wave (staircase sine) to a true mathematical sine wave, thus avoiding changing out the UPS units or the inverter. Any suggestions along these lines would be GREATLY appreciated.


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## 1manElectricalShow (Aug 30, 2012)

Pharon said:


> If it's standby or line interactive, then you have a problem.




yes the batteries are APC Smart UPS, line interactive and very specifically list sine wave as required, so in short, I do have a problem.


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## Pharon (Jan 20, 2014)

Sounds like you need a double conversion voltage regulator. I'm not sure there's an easier fix.


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## 1manElectricalShow (Aug 30, 2012)

This issue has been SOLVED. The trick was using the UPS interface and changing the config for power quality allowable from GOOD to FAIR. This solution was a bit of a shot in the dark but testing has proven it to be SOLVED. Thanks for the help gentlemen


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## triden (Jun 13, 2012)

Gotta love the software tweak solutions. Next time use a SunnyIsland - they make quality inverters http://www.sma-america.com/products...y-island-4548-us-6048-us.html#Overview-137003


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## 1manElectricalShow (Aug 30, 2012)

@ triden, yes I do appreciate that these ups units are customisable, however one of the older units detected a "surge",so not out of the woods yet as the main LAN switch was plugged into it during final testing this afternoon, luckily it was right after lunch (not many users on the network)and email as well as a database server were temporarily down as a result. I am still pursuing a method of cleaning the sine wave up a bit so any help with a device to do this would be of great help. Also, the Sunny Island brand looks robust and very promising as a last resort of purchasing a new inverter ,thanks


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## dmxtothemax (Jun 15, 2010)

You would be much better off just gettinga pure sine wave invertor !


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## Pharon (Jan 20, 2014)

If you're just feeding servers, you don't need a perfect sine wave. Those computers are just rectifying it to DC anyway. As long as the UPSs no longer trip offline you should be good to go.


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## JRaef (Mar 23, 2009)

You can get Sine Wave Filters that consist of relatively large inductors, resistors and caps, that could clean it up quite a bit. TCI (Trans Coil Inc.) and Schaffner are a couple of ones that come to mind. But it would possibly still give you trouble if the equipment you are feeding relies on the sine wave for internal clock timing of other things. It would also likely cost you more than just replacing the 6-step inverter with a PWM based sine wave inverter.


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## riverpc100 (Dec 2, 2012)

The inductor type filter was what first hit me too.
30 amps would make for a big inductor and could cost a good bit. But, a 30 amp pure sinewave inverter can't be cheap.
if you're not pulling much of a load could it be tested using a few lower amperage VFD reactors?

JRaef could probably answer that.


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## 1manElectricalShow (Aug 30, 2012)

So far, the software/UI setting adjustments have been tested as a reliable fix on this issue, I did look into the inductor as a solution. However, so far all tests on this system as it currently stands have proved successful.


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## splatz (May 23, 2015)

If the software fixes it that's great. I have never been a big fan of APC UPS's. At one point some or most of their products did not produce a true sine wave output. I didn't realize they were now configurable to tolerate marginal input power but it's good to know. I wonder what it does internally to make the change, and if it will be reliable and long lasting. (Why make it a configurable option rather than automatic or a default?) 

The Best brand, which I believe is now part of Eaton, makes the FerrUPS which is an online dual conversion UPS and has the ferroresonant transformer built in. The product has been around a long time and has a great reputation for being tried and true in environments with marginal power quality. Eaton says their newer lines are just as good or better with different technology, but they keep making the FerrUPS because it has a following. 

If you can't change the equipment, we have used these a long time, I think they might really improve the reliability of the connected equipment as the sales literature claims: 

http://www.solahevidutysales.com/power_conditioning.htm


I think it's worth putting these in with sensitive loads with backup generators, rather than trusting the generator to make perfect power.


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## JRaef (Mar 23, 2009)

Duplicate posting.


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## JRaef (Mar 23, 2009)

The problem with 6-step inverters feeding other power electronics is usually that the other devices are now using an SMPS (Switch Mode Power Supply) style input as your server UPSs likely are. In an SMPS they go through a simple diode bridge rectifier to a DC bus, then a DC/DC chopper to regulate voltage, then a PWM inverter to recreate the AC output. In the rectifier stage, the diodes will only conduct at the peak voltage, which on a 6-step inverter output only takes place in the top step. So ALL of the current required by the load has to pass through in those peaks, but on the 6-step waveform, the peaks are foreshortened so the normal "gulping" of current through the diodes becomes worse. The rectifier was designed assuming a sine wave input, so the diodes are sized accordingly and the DC bus caps are selected to smooth out the predicted ripple from rectifying that sine wave. What happens when that diode conduction is foreshortened is that the DC bus in the SMPS becomes too rippled for the bus caps to handle and the transistors on the PWM inverter section start to misfire, or it shuts down to protect itself.

What the "software" solution often does is to current limit the output of the SMPS device, which lessens the DC bus ripple. So in this case your UPS for the server rack is being hobbled a little in recognition of the poor "power quality" of the input coming from that 6-step solar inverter. That often works fine for a while because the batteries back up the output, but only to a point because now they are essentially contributing all of the time instead of only on an outage or dip, so they can over heat. At the same time, the diodes in the rectifier are being stressed by having to conduct at higher peaks, even though for a shorter time and at some point, they will over heat too. The UPS internal protection system often just boils all these issues down into a simplistic failure alarm called "Surge", because the details are too complex to go into on a tiny little screen. 

A ferroresonant transformer, like the Sola CVS units shown, will smooth out the sine wave, but at a price. They are only about 85% efficient, plus they TOO will current limit and flat-top the sine wave when feeding a diode bridge rectifier which might recreate the problem you are having if you are not careful. So to avoid that you must seriously over size it, which makes the inefficiency worse. When I worked at Boeing Computer Services where we used those a lot to clean up harmonics (another benefit), we had to over size them by 50%. That gets expensive, and hot, and it costs more to operate, so in the long run it might end up being less expensive to just bite the bullet now and replace the solar inverter.

The sine wave filters I mentioned earlier should work better, because they will not flat-top, and their internal caps will help supply the peaks that the UPS input is looking for. Still not cheap, but less operating cost than the ferro based units. 

Forget any ideas involving VFDs, it would not solve anything here.


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