Stacking Inverters

[Windmill1]

Hello
I have a Aims 1500 watt pure sinewave inverter that I run a refrigerator and two garage lights on. I was thinking of getting another inverter just like this one and stacking them. Is stacking inverters  a good idea, or is this something that does not work well? Also how do you wire them up ? Any info would be very helpful.
Thanks

[Frank S]

I have an Aims 1500 PSW and an AT&T 1800 PSW both look identical inside and out neither specifically states that they are stack-able one reason is neither have a DC output I also have a Xantrex 3000 MSW . in stacking you would connect one inverter as a master and one as a slave the master would connect to the bank whie the slave would connect to the DC out or this is how I understand it
 But I could be wrong on how to connect so look at the wireing diagram of a stackable unit to be sure.
 I think the connection is to match the frequency circuit so when the AC leads are paralleled together you don't let out the magic smoke. this also allows for 1 inverter to be in standby mode drawing minimal current until the other reaches 75% load then the second inverter comes online .
 another way and this is a way I have done it with mine although I may have been tempting the smoke signal was to wire both inverters into the bank then tied the neutrals together to get 240 split phase. I only tried this once and doubt if everything was according to Hoyle but  the setup did start a 1hp 240v automotive lift pump.
 Since the 1800w AT&t will run most things in my RV by itself I don't use the other 2 inverters except for playing around when I am bored which is hardly ever.
 My recommendations would not to try and stack the Aims units but if you need more output than 1 can do then split a circuit and dedicate one for one use and one for the other only switching it on when needed
 Someone like Sparweb or Flux or possibly Chris O may be able to tell you for sure.

[Flux]

Unless the inverters state that they can be stacked then they can't.

You can't connect independent individual inverters in parallel. They must share a common oscillator or be locked in frequency and phase and they will also need current sharing circuitry.

Some are designed for this ( all the high end units are) and they have linking ports and cables.

Those designed for stand alone use will fry instantly if paralleled. I don't know the N American market but this sounds like a standard run of the mill stand alone unit, I seriously doubt that it has this facility.

Flux

[SparWeb]

It's a mobile inverter.
It has no ground reference, and no way to synchronize its clock frequency.
Not only can it not be interfaced with a second inverter unit, it cannot have any interface with the grid AC, not even in the way a UPS/standby unit can.

The trick Frank says he tried is one that I wouldn't do, not without instructions from the manufacturer.  Once the two inverters were paralleled, there was nothing keeping their 60 Hz frequency in sync or relative loading balanced.  This might not have caused them to blow up immediately, but that poor motor...  I'm sure it was a moment of force majeure that made him do it.

[Frank S]

Sparweb  I did it just for giggles & grins LOL
 I have  the to inverters connected in MY RV one powering 1 line and the other powering the other line since the only thing in the RV that uses 240 split phase is the cook top and it only get switched on with inverters off and generator running, all of the  other loads are completely separate.
 I checked the voltage across the lines it showed 240, 120, 240 real erratic  which I figured would be conducive of random HZ mismatch so as I connected the motor with 2 momentary contact switches held 1 in the on position then swatted at the other one a couple of times the motor spun up so I disconnected the mess and left things alone thinking I might be able to figure out how to cut wire in some kind of a frequency matching oscillator in someday if possible, and if I retire the inverters from needed use 

[Windmill1]

Thanks
All the info I have on this inverter does not say that it is stackable, so I guess that its not.

[SparWeb]

Ok Windmill1, play safe!

Frank I doubt you can just "plug in" something to keep the inverters in sync.
If you were very very clever, you would know where on the inverters' boards you could pair up the oscillators to keep them synchronized, however you and I both know, neither of us is smart enough to do that without turning our hair all curly and black!

[Frank S]

Sparweb I'm fairly clever when it comes to things that I can see or touch. I can not see electricity nor can I touch it  without curling my hair the turning black part might be accepted since mine is mostly gray LOL
 I can however lay my hands on the spark wires of a  running model "A" ford and stall it,  been there done that hurt like the dickens.
 About stacking inverters, I'm pretty sure this is why the ones that can be safely stacked to get 240 split phase have only 1 directly connected to the batteries and the other wired in as a slave. with some jumper between them to comunicate with each other.
 Thinking about this though I do have a pair of identical 1000w UPS units tossed in a corner somewhere they  are older units and I don't even know the brand except that they weigh 40 lbs each and have numerous connecting ports I might drag them out someday and see what good they might be

 But for Windmill1, Iwould never recommend doing doing what I tried with my inverters.  Nor would I even try to parallel them.

[joestue]

if there is voltage isolation you can connect in series, but why would you want to? if they are modified sinewave the 240 volt out is going to be erratic as hell, if sine wave then you get a smooth decay from 240-0-240 at the difference in frequency.

Frank, various rack mount pure sine wave UPS systems lose sync with the grid (their clock is not tuned with the local mains freq, it is locked at 60+/- .xxx.
but they have a pll that re-synchronizes the frequency before switching the relays over to mains feed and shutting down the inverter.
at least one person has found out how to activate that PLL without switching back to mains. .. this could be used to run them in series, but not in parallel.
so there "is" hope out there lol.. might be easier and more reliable to parallel two identical transformers at the 24/48 volt side, solder in twice as many mosfets, and change the resistor on the current sense transformer.
as i recall one method was to just plug the inverter into something like 2/3rds nominal line voltage. it won't switch over, because the line is too low, but it will synchronize.

[Frank S]

jostrue; thinking about what you just said, and after digging these things out today to see just what they were.
 the 2 units I have are APC  smart 1000Xl . tye are older units no USB but have a serial and RS 232 port. they are SW not MSW .
  After reading your post here is what I started thinking about
1 wire both units to a bank
2 get a transformer 120v in 80 to 90v out,  plug the transformer into one of the out put receptacles, wire the 120v input of the 2nd unit to the output of the transformer the voltage should be low enough to prevent it from switching as it would see a low voltage input fault.
I'm thinking that if I were to tie the output  neutrals together the  lines would give 240v  The only problem is I don't know if they will be or would stay in frequency sync this way. If not then I would have to figure out what would need to be added if there is anything that would hold them in sync.
 If I let out the smoke nothing is lost as I don't even have $5.00 tied up in them.
 But it is not anything that I am going to do just now because I have other irons to melt

[joestue]

honestly i wouldn't bother with this mess...
what is the point?

solder in more mosfets to the H bridge, connect the other transformer up as normal, run the secondary in series.. i just realized.. there is likely no primary (low side) current sensing, so it won't even know...

I'm thinking that if I were to tie the output  neutrals together the  lines would give 240v  The only problem is I don't know if they will be or would stay in frequency sync this way.

well that is exactly what it should do, however another mod i read on a forum from several years back involved some soldering, so there's no telling exactly what's going to happen, wouldn't take long to figure it out.. hook a lightbulb up in series with the mains and the UPS and disconnect and see if the frequency drifts.. plug it into a 80v source and if it synchronizes then you're good.

[Frank S]

honestly i wouldn't bother with this mess...
what is the point?

 There's really no point to it, its just something that I might want to learn how to do

I'm thinking that if I were to tie the output  neutrals together the  lines would give 240v  The only problem is I don't know if they will be or would stay in frequency sync this way.

[well that is exactly what it should do, however another mod i read on a forum from several years back involved some soldering, so there's no telling exactly what's going to happen, wouldn't take long to figure it out.. hook a lightbulb up in series with the mains and the UPS and disconnect and see if the frequency drifts.. plug it into a 80v source and if it synchronizes then you're good.
[/quote]

 No mains to hook up to  just a generator or batteries  can call the generator the mains I guess