Author Topic: Improving inverter efficiency?  (Read 3063 times)

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TDC

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Improving inverter efficiency?
« on: July 23, 2016, 05:06:14 PM »
 I've read about ozstools PowerJack overhauls and I'm wondering if similar modifications world work well on other low efficiency inverters, an APC 1500 Smart-UPS for instance?  What are the main causes of low inverter efficiency, the transformer? I'm thinking smaller toroidial transformers or cores are easier to find than the big stuff needed for a 6kw oztools inverter.

After stumbling on a youtube video, I bought an APC 1500 Smart UPS off ebay for $50. It was sold for parts/not working but it works and it even came with good batteries!  I didn't take any notes but it seems to have horrible efficiency and draws to much idle current.  The video:  https://www.youtube.com/watch?v=2HXvV30E7wQ

joestue

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Re: Improving inverter efficiency?
« Reply #1 on: July 24, 2016, 01:15:12 AM »
Adding a ferrite core inductor to the low voltage side of the transformer will work to reduce the no load idle current significantly, under both inverting and float/battery charging mode.

I used about 7 turns of wire around a core with a cross sectional area of about 1 inch diameter or .78 square inches. The core was gapped about a paper thickness on one side and none on the other.

However, those inverters push dc current through the transformer because the pwm control is generated by a custom APC manufactured 16 pin dip and it doesn't ensure zero dc volt bias. Also when it shorts out the transformer in order to use it as a boost inductor during battery recharge, (The pwm for which is generated by a different 16 pin dip.) it also does not draw a symmetrical amount of power from both sides of the line.

I have not yet attempted to wind a custom toroidal transformer for mine, I don't have a core available for that experiment at this time. I suspect the lower dc resistance of a toroidal core transformer and its much sharper saturation curve may cause problems for the inverter, as it does not have any dc current sense circuitry on the low voltage side.

far as i understand there are two current transformers on the ac high voltage secondary side. one of them is in series with the 20uF capacitor across the inverter 120vac output and it used to set the pwm frequency because the ripple current generated by the pwm shows up in that current transformer, you can change the capacitor for a smaller one (10uf is enough) to reduce the no load power draw by about 1 watt.

when you add an inductor to the low voltage side of the transformer you will notice the pwm frequency will  decrease. i think mine dropped as low as 8Khz. i believe if you increase the capacitor, or if you remove it from the board and reconnect it through a few more turns through the current transformer, you will see the pwm frequency increase, because it now sees much more current ripple so it increases the frequency to compensate.

the other current transformer is used to measure the current drawn from the inverter. you can change the resistors downstream of that ct in order to increase the output power well beyond 1500VA if you 1) increase the number of mosfets, 2) wind a custom transformer 3) install a second or third capacitor next to the mosfets (the board probably has room for a second if you only have one)

Also there is no current mode control upon recharging the battery, it will draw an incredible amount of current from the line, whatever it takes to hold the battery volts at 13.8 volts. A friend of my boss tried this with a large battery bank, the ups caught on fire.
« Last Edit: July 24, 2016, 01:20:45 AM by joestue »
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joestue

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Re: Improving inverter efficiency?
« Reply #2 on: July 24, 2016, 02:18:11 AM »
i may spend some time trying to figure out how to fool the UPS into adjusting the pwm to remove the dc current.

basically you need to sense the dc current in the primary, convert it to a voltage, then add that voltage to the feed back network that regulates the output voltage.

but such efforts are like polishing a ...

it might be easier just to build your own ups, or build a board as a drop in replacement for the two 16 pin pwm generating IC's
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TDC

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Re: Improving inverter efficiency?
« Reply #3 on: July 24, 2016, 05:34:40 PM »
 Here is an APC schematic. http://www.eevblog.com/files/apc_smart-ups_su2200_3000.pdf
I'll check the DC bias and look for a suitable E core.  The board has a spots for another capacitor and 8 more FETS for a total of 16. BTW, I'm only interested in using it as an inverter.   As for tuning up the power, one of those youtube videos shows adjusting the limits through the serial port.

What type of toroid cores are needed, steel, ferrite or?  Cross section per kw?

joestue

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Re: Improving inverter efficiency?
« Reply #4 on: July 24, 2016, 06:45:23 PM »
yes i believe i have looked at that same schematic before,  it all looks very familiar.

You need a steel toroid core for the main transformer, you can buy then from alphacoredirect for 3.50$ a pound plus shipping.. however the best deal on the internet  right now is probably this one: http://www.ebay.com/itm/281542186688 53 pounds of transformer for 110$ (from my point of view, shipping may be different for you)

as far as how big a core you need, a toroidal core with a core cross section equal to the EI transformer in those inverters will be significantly heavier and also more efficient by nearly an order of magnitude. (provided you use large enough wire) and the toroidal core will only consume about 1 watt per kilogram at 1.7T flux density at 60hz. however, the dc current that those inverters produce will certainly produce a lot more core losses in the toroidal transformer and you may not see any gains at all, except for the lower dc resistance of the windings

as far as saturation goes, you can sit and just listen to the inverter. you will periodically hear it buzz a little differently.

there are some differences between the transformers for the various models. some have a separate winding that is used for both buck and boost, others have separate windings, or a tapped winding. i don't think the exact turns ratio matters that much, when you get into winding large transformers often you can't match an exact integer ratio.



for an inductor, the yellow/white iron powder toroids that you can get out of most computer smps are worthless. you can actually increase the idle current of those inverters by adding them to the circuit.

i can help you with the inductor selection but it will be mostly guess work if you don't have a current probe for an oscope.

if you can find three ferrite cores from TV or computer monitor flyback transformers, or similarly sized cores, that will be sufficient for the inductor. how many turns also depends on the voltage, is yours 24 or 48.
« Last Edit: July 24, 2016, 07:02:05 PM by joestue »
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TDC

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Re: Improving inverter efficiency?
« Reply #5 on: July 25, 2016, 10:52:52 PM »
Thanks for the info.   I'll probably try adding the inductor, but not sure it's worth going much farther and winding a transformer for unknown results.  Maybe I'll get the book for the oztools inverter before deciding anything.  My UPS is 24v.....  any reason I couldn't wind a transformer with two 24v primaries to allow use in either a 24 or 48v inverter? 

joestue

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Re: Improving inverter efficiency?
« Reply #6 on: July 26, 2016, 06:41:26 PM »
I agree they are not worth the effort. but adding an inductor could potentially save you 10$ a year on your electrical bill if have one of them running 24/7 for whatever purpose.

yes, you should be able to wind a transformer that will work for both 24v APC smart ups' and other 48 volt inverters, the generic ratio is, iirc, 13vac:120vac.

a more efficient transformer could be wound with a 14:120vac winding because there will be less voltage drop in the copper. nothing fundamentally changes for a different voltage inverter. just double the winding(s)
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