What does the switching in a UPS?

Hello Stefan,

It depends on what your load is. An incandescent bulb may be harmless. But imagine a motor: It will continue to spin. Since your inverter's phase is most likely not in sync with the power grid you might, within

2msec, apply an AC voltage that can be up to 180 degrees out of phase with what it saw just 2msec earlier. Kaboom. Maybe.

Then there are capacitors. Every larger motor has one, some fluorescents do, and so on. So let's say the whole collection of capacitors hanging on the circuit happens to be at -160V and 2msec later you apply +160V. The output devices in your inverter might express their unhappiness with a dazzling pyrotechnic show.

Well, a good UPS tracks the phase of the line. When the line goes it can take over at the correct phase angle. I have also seen some not so good UPS that were more or less slapped together. Mostly the remains thereof. So be careful.

Same to you.

Regards, Joerg

If not else, I am learning more on how UPS function :)

So my idea of using a "cheap" inverter and making a transfer switch that's fast enough turning it into a UPS doesn't seem to fly... If I have to check the phase and put them in sync.. I can't change the phase of the mains comming in, so if I want to control the phase of the inverter, I would have to hook into it somehow to control its oscilator. Looks like a big step further then what I wnated to do in the first place with some "universal inverter to in-line UPS adapter"

What about delaying the switch-over 10 ms. This would make that the power comes back after half a sine wave. I figure what ever value it comes back with at that time, it will not have the big "spike". How would this deal with power supplies (computers is what we are switching here).

Stefan

Hello Stefan, I worked on some old UPSs years ago. Maitec was the brand.

Dirty mains in ---> DC and batt --->AC ----> Clean AC out

While the dirty mains was present, the clean AC out was synchronised to the dirty mains. If the dirty mains dropped off then the batteries took over powering the UPS which still produced clean AC out. The UPS output frequency was xtal controlled.

When the dirty mains came back on the UPS would synchronise its output to that of the dirty mains. I think this took a few seconds for the xtal oscillator to be pulled. I can't remember exactly.

If the UPS was over loaded a fast change over relay would switch the UPSs output connector from clean UPS output to that of the dirty mains until the overload was removed then the relay switched back to clean UPS output again. Since both the dirty mains and UPS output were in phase with each other the tiny break in the supply for a couple of milliseconds was not noticed if powering light globes or computers.

Regards, John Crighton Sydney

I had a pump load wired to a DPDT BREAK BEFORE MAKE relay with one set of contacts contacts powered by the power line and the other by a free running modifed sine wave inverter. When I tested the system by switching the relay (I don't remember now which way it was going line to inverter or inverter to line) the inverter blew up!!!!!! I was not happy :-(

Either one of two things happened....

1) The load was a motor that kept running on interia during the switch over and was therefore out of phase with the inverter

or

2) the relay contacts arced enough to bridge the gap and momentarily connected the inverter directly to the line.

I never figured out which problem it was and abandonded the plan using a real UPS instead. I think I would have had to use two relays and lots of time delays to make it work. (I didn't care if the pump went off for a few seconds during the swittch)

Seamless switching is not easy. I think you do have to keep the inverter running all the time and keep it in phase with the power line so it is ready to take up the load at any time should the line go off.....

and when the power returns, you have to slowly slew the inverter phase to be back in phase with the line before you switch the load back over to the line....

it is a very interesting problem....the solutions are MUCH easier if you can afford to let the power go off at the load for a second or so, which won't work too well for a computer....

let us know what you figure out...

Mark

I've seen a mongo inverter that had a sync input/output, so that they could be sync'd together for higher power. I could see squaring up the line, and using that for the sync signal, but you'd have to have something that could keep up the phase during the outage, like a slow PLL or something, then just gate the drive to the driver transistors.

Cheers! Rich

Well, as an educational project, it sounds dandy, albeit I'd schedule about 6 months to get the thing done. :-)

If the goal is to power computers uninterruptably, I'd look into computer power supplies that can run right off the batteries (and their float charger, of course. :-) )

Have Fun! Rich

And find a deep-cycle battery that's happy with a continuous float charge. :-)

Cheers! Rich

I just took apart a APC 2200 UPS.

I believe that the relays control the source. These are "on board relays" that solder right into the board. Rated for 20 amps.

They use an array of mosfets to do switching. They have transformers that serve as both down and up transformers, based on relays, I think.

i

check

Hello Stefan, why not keep things simple. You say you already have an inverter. Let your inverter power your computer equipment continuously. Don't overload the inverter, run it conservatively and forget about change over relays. Regards, John Crighton Sydney

I appreciate the comments.

As mentioned I see this as a chalenge and an educational project more than a way to keep my computer going. What about this one,... When the main power goes off, I sense this in about 1ms. I then wait for the next zero-crossing from the inverter in max 10ms (likely way less as inverters have more like modified block waves that are zero 1/3 of the time.)so in 10-11ms max I swith my relay. When the power comes back, I switch the inverter off on a zero-crossing and wait for the mains zero crossing to switch back to mains.

- Problem 1 would be, a relay is just to slow, I can't afford to add another

20ms of relay switching to my delay, and that would make me loose the zero-crossing by about 200% :) Go to Solid state?

- Problem 2 could be, zero-crosings appear from neg-to-pos and from pos-neg. If I catch a neg-to-pos from the inverter and then swith to a pos-to-neg from the mains, meaning getting 2 negative sine halves (or 2 positive sine halves) to follow each other, what will be the result?

Unfortunately, this is something my old scope will never show me as it doesn't have memory or storage. Looks like I will need to upgrade to find out what's really going on.

Any thoughts on this?

Thanks StefanV

Hello Rich,

It's no big deal. You have to keep running the 60Hz as accurately as possible. Cumulative error is important. Then when the line comes back a good UPS begins to adjust its phase towards what the line has while monitoring the line for how solid the restored power is.

Sync is a tad more complicated that just squaring up the line voltage.

Regards, Joerg

Hello Stefan,

Depending on what's connected, anything between 'nothing' and 'Kaboom'. A motor or capacitive load might fry the inverter if it tries to pull it into the wrong phase polarity.

Oh, I scoped out that kind of stuff with my old 8MHz Hameg. 3" tube, no trigger, just a 'synchronizer'. Back in those days I built dimmer type circuits for inductive loads and you had to really know what was going on or the mounting box would become one hellacious black hole.

Guess what, 30 years later that Hameg is still working fine. I just can't part with it.

Regards, Joerg

Irrelevant. If you design the charger appropriately, the normal voltage will not differ from the other case.