stacked vs split-phase vs transformer

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ChrisOlson
ChrisOlson Banned Posts: 1,807 ✭✭
in Off Grid Solar & Battery Systems #1
Since we're looking at an inverter upgrade because I'm not totally happy with the overall efficiency of our power system, I tried an experiment today. I got ahold of an Outback PSX-240 transformer. The PSX-240 is much heavier duty than our Trace T240.

I wired the thing up to a single inverter to see how it compares with the stacked configuration with a balancing transformer on it. Amazingly, it's more efficient running 240 volt loads than the stacked setup. After I got it hooked up we had about 350 watts of normal loads in the house and my wife had laundry going so the clothes dryer was running. The load on the inverter was only 17 amps (with generator support so the gen was carrying most of it).

Attachment not found.

With the single inverter running at ~50% rated load it was pulling 87 amps from the battery bank, according to my clamp-on meter. The bank was at 24.9 volts under load so that figures out to roughly 94% efficiency.

While all this was going on my wife decided to make supper. She made some spaghetti on her induction cooktop range. When she turned on two induction elements to boil water for the spaghetti and cook some hamburger for the sauce, the load on the inverter went up to 31 amps. The bank voltage dropped to 24.5 volts and my clamp-on meter said 172 amps from the bank, which figures out to 88% efficiency (this is still with the generator running with the inverter in gen support mode).

The dryer got done and the load on the inverter dropped to zero amps for about 5 minutes and then the inverter shut the generator off (the induction range was still going). When the generator disconnected the load on the inverter went back up to 17 amps (no gen support now). I had the same 87 amps without gen support.

When my wife shut the range off the load dropped to 4 amps. The bank recovered to 25.2 volts and the clamp-on meter showed 18 amps from the batteries - still figures out to 94% efficiency - it doesn't seem to want to get above that no matter what.

But regardless, I've done enough measurements in the past to know that from 500 - 2,000 watts load it's about 3-5% more efficient with the single inverter and the transformer than it is with the stacked setup and a T240 balancing the legs for the 120 volt loads.

It would be really cool to compare our old SW+ against a newer XW4024 to see how it pans out. I would bet a shiny new nickle that the old Trace would beat the XW.
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Chris
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #2
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    Re: stacked vs split-phase vs transformer

    Very cool Chris. The only 240V load on my back up system is a 1 hp Septic pump. It runs off my single inverter using a PSX-240. I've always wondered what i might be giving up in efficiency doing it this way. Good to know I may be not giving up anything!
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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #3
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    Re: stacked vs split-phase vs transformer

    I don't think you are. That is one heck of a transformer. I check the amps in vs out, and voltage in vs out with close to full 25 amp load on it and that transformer is 96% efficient! Way more efficient than a second inverter. And I think that's part of the difference in the overall efficiency of using the 120V/transformer setup vs stacked.

    I'm going to continue playing with this setup for a few days and see what happens. We have loads on in our house 24 hours a day (internet satellite modem and router, digital clocks in the range and microwave, furnace blower this time of year, etc.). The loads are night are very low, and running two inverters with a balancing transformer for those lightweight 120 volt loads uses a lot of power just to keep that extra inverter lit, and basically doing nothing.

    The other area where one inverter with a transformer will probably be better is in the fact that the SW+ runs most efficient in the 300-2,000 watt range. When you have dual inverters with a balancing transformer, and the house loads are only 300-400 watts, both inverters are running below their peak efficiency, while a single one is at its prime.

    If this pans out I might use one for 120 volt loads and the other for 240 volt with the transformer. One of the details that I'd have to work out on that is that some things like my wife's range have a digital clock and electronics that draw about 2 watts. That stuff runs off one leg of the 240 and if the 240 volt inverter goes into search mode then the clock setting is lost and the electronics on the control panel won't respond to turn the appliance on.

    But I have to play with this some more to see what happens in overall power consumption using just one inverter vs two, and if the single one can meet all our loads on a day-to-day basis. When my wife turned on the clothes dryer, as an example, two inverters just play with that and then start the generator to help out. The single inverter was putting out 46 amps (13 amps over what it's rated at) for close to a minute before the generator came online and took over most of the load.
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    Chris
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  • Ken Marsh
    Ken Marsh Solar Expert Posts: 114 ✭✭✭✭
    #4
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    Re: stacked vs split-phase vs transformer

    Good work Chris,
    The difference between the sine wave and the MSW inverters is that the sine wave inverters have a transformer.
    Transformer magnetization loss is typically around 1% of full load.
    And indeed, this is the stand by loss we take when going to sine wave inverters.
    A transformer isolates the output and gives us 240 Volt with center tap, which are desirable attributes.
    But we do take a significant stand by loss for having the privilege of a transformer.
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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #5
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    Re: stacked vs split-phase vs transformer

    Ken, it looks to me like the PSX-240 transformer is using roughly 10 watts in standby. Which is about half of what a second inverter in a stacked 120/240 split phase arrangement uses. When I wired up the transformer I made sure that the leg that comes directly the inverter is the one that's powering the 120 volt circuits in the range for the clock and electronics so the transformer-supplied leg doesn't have to run that 24/7, and is only required when the range is actually used.

    Same with the clothes dryer - there's a slight imbalance in the legs when the clothes dryer is operating due the fact that the motor that turns the drum and operates the controls is 120 volt. So one leg is only used to operate the heating element in it and draws about 20 amps. The other leg is used for the heating element, motor and controls and pulls about 22 amps. I made sure the high draw leg for the dryer comes from the inverter, and the transformer leg only has to supply the other (lower draw) side of the split phase.

    Our well pump is the same on both legs so that's not an issue.

    My wife cooked breakfast this morning on her induction cooktop and it didn't even start the generator for load amps.

    What prompted me to try this is the fact that I found a guy on eBay that is a Xantrex dealer and he has 8 brand new, still in the box, SW+5548 inverters for sale. A single 4024, I think, in the long run will be too small. But a 5548 would handle the job with no problem. I've been wanting to upgrade to 48 volt and looked at a Radian and possibly a new XW. When I saw those 5548's on there I go curious as to whether one of those with a transformer would do the job. I'm now pretty much convinced that it would.

    The guy only wants $2500 for one those 5548's, with free shipping in the US. I contacted him and asked about the warranty and he said Schneider will honor the original warranty on them. It wouldn't cost me a dime to install one of those because I know a half dozen people who want my 4024's.

    I'm glad I tried it. I'm happy with it and reasonably convinced it's a more efficient setup than dual inverters - it just doesn't have the raw capacity. But a bigger single inverter would "fix" that.
    --
    Chris
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,746 ✭✭✭✭✭✭
    #6
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    Re: stacked vs split-phase vs transformer

    Keep in mind that the efficiency of a 5548 SW will be based on the load and so if you are now using a 2524V SW and your average loads remain the same, the 5548V may show some loss. This was hashed over pretty well in the old defunct XW forum. The same with the old single phase SW being a tad more efficient the the split phase XW. The XW split did well over an SW with a transformer/stack in systems that had longer, intermittent run time, 240 volt loads like a heat pump or well system that needed to be on most the time.

    In any case going to 48V from 24V pretty much makes these losses negligable offgrid. I would talk with Schneider before anyone told me they would honor warranty on an SW. I do not think some of the parts are available except from scavenging a used SW. Going forward an XW is very easy to replace parts on compared to an SW. The XW and it's chargers are naturally convection cooled (bottom input air and and top output) compared to an SW that will be stressed if the fan goes out. The other nice thing is the fault diagnostics and the escalating fault logic that can shut down the system in certain cases, such as lightning!

    I would really like someone else to make a complete system that is better for an offgrid residence! I like choices !
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #7
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    Re: stacked vs split-phase vs transformer

    Thanks Dave - these are all considerations, definitely. I intend to call Schneider next week and find out if, indeed, the warranty would be honored on one of those SW+ 5548 inverters, or if the warranty would consist of replacing it with a 6048 in the event it failed under warranty. With the reliability reputation of the SW+ inverters there's probably a better than 95% chance it would run 15 years without having a single issue.

    I looked in the owner's manual for my SW+4024's and they have the efficiency curves in there, vs load. What I measured on mine is amazingly pretty close to what that curve says - my measurements might be a slight bit better at 2 kW load than what the curve says it should be. They have the curve in that manual for the SW+ 5048 too, and there is very little difference between the 4024 and 5048, except that the 5048 is a little more efficient at heavier loads than the 4024.

    I too would like to see the "true" off-grid inverter. The XW's are hybrid - either grid-tie or off-grid. The SW+ never had grid-tie integrated in it (like the previous SW Series II), although it has a Grid-Tie light on it, that feature never got enabled. After trying the transformer for the other leg of the 240, I'm not totally convinced that a split-phase inverter is the answer for off-grid either because the efficiency curves assume balanced legs. You can go to maximum 75% imbalance on a XW, where with a transformer supplying the other leg, that is not an issue.

    That all being said, I believe the SW+ is one heavy duty unit based on my experience running our SW+4024's. I don't know that the convection cooling is that big of an issue because I have rarely seen the fan in our units ever start unless the chargers were putting out their max for an extended period of time. Even loaded heavily (25% over rated continuous power for several minutes) the fan don't come on unless it's 80 degrees in the utility room.
    --
    Chris
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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #8
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    Re: stacked vs split-phase vs transformer
    ChrisOlson wrote: »
    What prompted me to try this is the fact that I found a guy on eBay that is a Xantrex dealer and he has 8 brand new, still in the box, SW+5548 inverters for sale.

    I contacted Schneider Electric's Renewable Energy business and inquired about the warranty on those 5548's. I left a message on their voicemail and a fellow called me back within one hour. He said "probably not", except under special and very certain circumstances. He did not say what those circumstances are. But he asked me for the eBay listing so he could look at it himself. He said there exists a remote possibility that (providing these are actually new inverters and the serial numbers check out) that they MIGHT be under factory warranty yet. He asked for my email address, said he will investigate these units and get back to me by tomorrow afternoon with a yes or no answer.

    Then he went on to tell me that providing they would still honor the warranty the SW+5548, that in the event it would need service it would have to be sent in to Schneider's service center for repair. He said their warranty policy on the XW-series is to swap out a unit that has a warranty issue with a new one. Since that wouldn't be possible for a SW+, it would have to be sent in for any repairs or issues (if it had any).

    Overall good customer support experience (considering my question) and the fellow at Schneider was very helpful and thorough.
    --
    Chris
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