How exactly does a UPS work?

Discussion in 'Power Electronics' started by Antonyc, Aug 14, 2017.

  1. Antonyc

    Thread Starter New Member

    May 17, 2016
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    I have an application for something similar to a UPS, but I need to know how they work.

    I know that it has a battery, and the battery gets charged by stepped down, rectified mains.
    It has an inverter within it.
    When there is no mains power, the inverter powers the load.

    But what exactly happens?
    Does mains actually feed through to the output whilst charging the battery at the same time?
    Does mains charge the battery and at the same time, the inverter is always on, powering the load?

    How does it work?

    The reason why I'm asking is because I've got a system whereby I have two mains connections, and a system can switch between the two. I want to know how the UPS can 'switch over' so ridiculously fast! (If it actually does switch over).

    Thanks, Antony...
     
  2. Reloadron

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    Jan 15, 2015
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    Uninterruptible Power Supplies (UPS)

    You may want to read the above link and understand that uninterruptible power supplies come in several design types and how they respond can differ.
    Once you understand the different types of UPS you will understand the answers to your questions could be yes and no. Additionally a UPS can have different types of AC waveform output, such as MSW (Modified Sine Wave), and TSW (True Sine Wave) to name just a few, there are more output types.

    Different Types of UPS Systems

    Is another good read on the subject of UPS systems. Keep in mind we choose a UPS system based on out individual needs and intended application. There is no one size fits all.

    Ron
     
  3. KeepItSimpleStupid

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    Mar 4, 2014
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    An always on or dual conversion may be what you need. Some UPS's are not generator friendly.
     
  4. Reloadron

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    Good point. I use several older APC 1500 units and if power fails while they work fine within about 10 seconds my generator comes online but the older UPS units constantly beep switching back and forth between mains and UPS power. Mains voltage is a very constant 123 VAC at 60 Hz while the generator varies between 119 VAC and 125 VAC at about 59 to 61 Hz. The older UPS units don't like that so while they do work they are less than ideal on generator power. My wife's workstation does fine running on a newer UPS which actually has a "Generator" setting.

    On a side note with 60 Hz mains the switch over or change over time is about 1/2 cycle of mains or about 8.3 milli-second, for a 50 Hz system it's about 10 milli-seconds, again 1/2 cycle of mains frequency. This will always go back to choosing a UPS based on your application. A simple phone call to a manufacturer like APC and talking to one of their applications engineers is a big help if one does not know how to choose the best UPS for their needs.

    Ron
     
  5. Antonyc

    Thread Starter New Member

    May 17, 2016
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    Hey! Thanks everyone.

    At the moment, I have an offgrid solar power system and mains power, with a pretty simple setup. I have a contactor with a mechanical interlock which simply connects either one or the other to the load. The problem is that when it switches to/from, there is ofcourse a switching delay. The delay is too much for some devices.

    I'm considering a good way of getting around this. I know that those APC units somehow switch at a stupidly fast rate, and I want to know the secret!

    My PC is the main sufferer of this problem, and I guess I could parallel in a small 12v battery into the PSU, but it would be better to tackle the problem from the root cause, which is the contactor.

    :)
     
  6. Reloadron

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    Jan 15, 2015
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    Oh yeah, I see the picture. Big contactors depending on the contact voltage and current ratings can take time to switch. I have measured a few large units at 20 milli-seconds or more. There are UPS units which actually supply inverter power 24/7. The inverter driven by rectified AC mains power switching to battery power on demand. While I know the typical switching or transfer time is normally within 1 cycle or 1/2 cycle of mains power I am not sure exactly how the units transfer. I guess one could find a schematic and with some time and effort figure it out.

    I can see where switching between mains grid and your inverter can be a problem. I would just consider a basic UPS for PC use and in the event of grid and solar panel failure allowing enough time for a normal shutdown of anything you have running.

    Ron
     
  7. Antonyc

    Thread Starter New Member

    May 17, 2016
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    I've had an idea. If I was to put a UPS on the load side of the contactor, not on the PC, but on the contactor, that would mean that everything on the load side of the contactor would be protected.I just need the UPS to be one which lets mains pass through it whilst only using battery in blackouts. This is because solar will literally switch over around 2-4 times a day, totaling probably around 10ms of switch time! Antony...
     
  8. Reloadron

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    Jan 15, 2015
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    Yeah, that would work just fine as long as the UPS was capable of supporting the entire load side. On average I can run our entire household on about 4.0 KW less the electric clothes drier. As long as you have a UPS capable of supporting the entire load side at any given time it should work fine. Also make sure everything on the load side will work on the output waveform the UPS provides.

    Ron

    Ron
     
  9. Antonyc

    Thread Starter New Member

    May 17, 2016
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    Cool cool. I went through the entire house ripping out inefficient items. All items are now A++ rated. I've replaced the electric kettle with a gas one as our hob is gas. I'm running the shower off the boiler too. I estimate that I can power the house off around a 1KW inverter. I've actually got a 2.5KW inverter though, just incase we have the TV on (100w), the washing machine (120w), the microwave (800w), my computer (120w), a hair drier (800w) and erm, another 500w worth of stuff all on at the same time. :-D

    Is it possible to use a some sort of fast switching relay instead of the contactor with mechanical interlock? What do you use?
     
  10. Reloadron

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    My situation is different. I have several 1.5 KW (True Power about 1.0 KW) and things like the cable phone line, Cable set top box, my and her computers (all computer related stuff) and a few lamps. When we lose power off the grid my generator automatically starts and runs on natural gas. Everything is auto transfer and within about ten seconds we get transfer. The generator is 18 KW which is major overkill but we got a good price.

    Years ago we were home care for my wife's mother. As her health deteriorated more and more home health care equipment came into the house. We reached a point where we were good for about 20 min on batteries before we would be in trouble. That was when initially got a 4.0 KW (5.0 KW Peak) gasoline powered generator and I set up a manual transfer pony station but it was a nightmare to setup and if I was not home I doubted my wife could handle everything. Hell, I was rotating 3 each 5 gallon cans of gasoline every few months. That was when I decided on a whole house automatic transfer natural gas fired generator. It runs every Sunday morning for exercise for 30 min. My wife's mom passed years ago but that generator has come in handy, especially in dead of winter. Anyway, my UPS units only need to run several seconds before the generator is online and when grid power returns it waits, then transfers and then runs something like 10 more min for a cool down. More than once we have supported neighbors freezers with extension cords through the yards. :) I always have a few hundred feet of AWG 12 extensions.

    Your situation is obviously much different. They use a mechanical interlock for good reason and yes, the mechanics do slow down the switching action. I really don't know what is out there for fast switching contactors or relays which would improve what you have. Heck, on power loss our cable TV box takes a good 10 min to reboot which is why I even have that on a UPS. Your UPS needs would be similar as the UPS need only maintain momentarily during your grid to solar and back intervals.

    Ron
     
  11. Antonyc

    Thread Starter New Member

    May 17, 2016
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    Thanks, yes, very different! But there are some similarities. Basically, what I need is something fast switching and efficient.
     
  12. Reloadron

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    Then I would run with a good UPS on the load side. That should take care of thongs and insure constant power.

    Ron
     
  13. Antonyc

    Thread Starter New Member

    May 17, 2016
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    Thanks Ron,

    The whole thing is confusing me. This issue is this:

    There is a switching delay from mains to solar or from solar to mains.

    I could get around this by adding a UPS, like we have said, which is effectively a battery charger, a battery and an inverter on one, and this would apparently be fast and eliminate the delay.

    The UPS I'd use would need a battery pack, a 24v battery pack. Well, I'd use the system battery pack. The UPS charger wouldn't be needed as the system batteries are always topped up, as it's solar. Effectively, all I'd be using from the UPS is the inverter and the internal switch.

    However, the actual solar power system itself is also a charger, a battery bank and an inverter, so it feels really stupid to do this!

    Another thing, I don't think it will work, because the input of the UPS would be the output of the system, but the output of the UPS would also be the output of the system. So as soon as the inverter detects an outage, it starts inverting and outputting, but as soon as it outputs, it's input would read power in and shut off the inverter! That sounds incredibly dodgy!!! Haha!
     
  14. Reloadron

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    Yes, and that is the main part of your problem in that the switching delay is too long in that systems like your PC will reboot.

    Most goos UPS Units allow for an external battery pack. This where I suggested earlier you talk with a UPS Applications Engineer from any of the major UPS manufacturers. These guys can set you up and explain exactly what you want and why. Additionally I would be talking to the company which made your existing system and question why grid to solar and solar to grid transfer has such a long delay? They may have a solution to the problem.

    The UPS is only going to work when the input sees no signal, a lack of mains be it from solar or the grid. The UPS would be in series with the load on the load side. Again, there are different UPS designs for different UPS applications. Your best solution will likely come from an applications engineer of a UPS manufacturer.

    So in conclusion I would be calling the guys who built the solar power transfer system and question the change over time delay. Should a UPS be viable I would be calling a prominent UPS manufacturer, for example APC (American Power Conversions).

    Ron
     
  15. Antonyc

    Thread Starter New Member

    May 17, 2016
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    I designed, build and programmed the power station myself. :-D
     
  16. Reloadron

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    With that in mind what was the data sheet for the transfer relay? Generally they call out the time and things like pull in and drop out times and coil voltages. Typically you want to consider a time less than or equal to 1/2 of mains frequency time. Longer times and things you are supporting can drop out, such as PCs.

    Ron
     
  17. falade47

    Member

    Jan 24, 2017
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    What if u imply a solid state switching device like SCR
     
  18. Reloadron

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    The original poster is switching between Grip Power and Solar Inverter Power to a Load. So what he has is along the lines of a DPST or DPST switch (Relay) I don't know of any simple off the shelf SSR that will do that, do you? I also don't know the switching voltage and configuration, for example in my residence in the US you would be switching between 240 VAC 60 Hz. Split Phase so you are switching two mains lines. I don't know what the oroginal poster is switching.

    Ron
     
  19. Antonyc

    Thread Starter New Member

    May 17, 2016
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    DPDT, I'm switching mains L/N OR both solar L/N TO load L/N. Load is 250v 50Hz, single phase.
     
  20. falade47

    Member

    Jan 24, 2017
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    Sp
    Spdt relay would work for that..but that delay still occurs in powering up the contactor
     
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