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UPS system

Discussion in 'Mathematics and Physics' started by PG1995, Feb 21, 2015.

  1. PG1995

    PG1995 Active Member

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    Hi

    These are the specifications of UPS system being used:

    DC input 12V
    AC output 220V, 50Hz
    10A/20A charging current
    Power factor 0.6
    Power 1 kVA
    Modified sine wave
    The battery being used is 12V, 175Ah.

    Q1:
    If I go with mathematical calculation then I would say that the UPS could only supply 600W because power factor=real power/apparent power. I am still confused. Why does the UPS system have such a low power factor for itself? I had that it's a load which dictates power factor and not the supply (which in our case is UPS system). Even generators tend to have unity power factor.

    Note to self: The inverter used in the UPS system could only produce 600W.

    Q2:
    Although at the moment only one battery is being used, I think that another 12V battery could be connected in parallel to the present battery. But connecting a second battery would take the system a little longer to get fully charged.

    i: Do I have it correct?
    ii: Should the other battery also be of 175Ah because the one being used in 12V, 175Ah?

    Please help me with the queries. Thank you.

    Regards
    PG
     
    Last edited: Feb 21, 2015
  2. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    It might be the Chinglish translation:

    Power factor 0.6
    Power 1 kVA

    1 KVA is indeed 1000 W resistive. 0.6 pf is IMPOSSIBLE to apply to the input unless it's misinterpreted as 60% efficient. You would get the same result. A 0.6 pf is "60% efficient"

    Another way to interpret it is a minimum 0.6 pf, but somehow I think it's supposed to be efficiency

    So I think things got lost in the translation.
     
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  3. PG1995

    PG1995 Active Member

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    Thank you, KISS.

    Yes, it's really a Chinese made UPS system. I believe that someone later tried to translate the Chinese booklet into English.

    Did you mean to say "to the output"?

    The figure of "60% efficient" is way too low for a UPS system, don't you think?

    I think that the best way to confirm this is to conduct an experiment on the system itself but at the moment I don't have the required equipment.

    I'd tend to agree with this. Thanks a lot.

    Regards
    PG
     
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Here http://www.cyberpowersystems.com/pr...?selectedTabId=specifications&imageI=#tab-box is a UPS with a higher VA rating than the Watts rating.

    So what does that mean? Probably that it can supply the Watts continuously, but can briefly say start a motor. In-rush has to be covered with the VA rating.
     
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  6. PG1995

    PG1995 Active Member

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    Thanks, KISS.

    I still don't get it. If you have a resistive load then VA and watts should be equal. For instance, you can say that the system is 1 kVA and the watts extracted out of these '1 kVA' depends upon the power factor of connected load. Thanks.
     
  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    OK. "me too", but I found the answer. http://www.power-solutions.com/watts-va

    This is starting to sound like th peak/average power stuff that still happens with audio.
     
  8. PG1995

    PG1995 Active Member

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    Thank you.

    Q1:
    This is my understanding. A 1 kVA UPS can supply 1000W for some time but not continuously. It can only supply 600W continuously. An reactive load with rating 1 kVA can be connected to the UPS assuming its power factor is not greater than 0.6. Do I make sense?

    Q2:
    I'm going to assume that what I have said above is correct. A 1 kVA load is going to demand the same amount of current as a 1000W load then why is so that one can use reactive load of 1 kVA for continuous period of time and not one of 1000W? Thank you.

    Regards
    PG
     
  9. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Q1: That's what it looks like. I don't agree with it. Why rate higher output UPS's correctly?

    Q2: Larger UPS's are closer. Here http://www.apc.com/products/resourc...cfm?base_sku=surt10000rmxli&tab=documentation is a 10,000 KVA, 8000 W system. The numbers are closer.

    Maybe I could sum it up in another mentality. Friends of mine owned an electronics store and one guy came in and basically asked one question for all of the products: "How many Watts is that?" VA is a bigger number, so more "bang for the buck"?

    Here http://www.xantrex.com/documents/Power-Inverters/PROsine/DS20120912_PROsine1000-1800.pdf is a datasheet of a company I respect, but I think there surge rating should be in VA and not Watts.

    The word "surge" in their datasheet gives them more respect, but I THINK the units should be VA.
     
    Last edited: Feb 22, 2015
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  10. PG1995

    PG1995 Active Member

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    Thank you, KISS.

    I was reading this PDF and the following are excerpts:

    "Manufacturers of smaller desktop UPS often only include VA specifications for their UPS products. When the Watt rating of a UPS is not furnished, it can be very difficult to determine if the UPS is capable of supplying a specific load. Lower cost UPS products often have a Watt rating of 50% of the nameplate VA rating. This can cause confusion, for example a 1000 VA UPS that will not run a 600 W load (the Watt load rating is either not provided or in the fine print and in reality is only 500 W)"

    "Surge factor relates to the momentary overload capacity of the UPS and is a measure of the ability of the UPS to start-up loads which temporarily require extra power when they start-up. Examples of loads which have a high surge factor requirement include motors, compressors, and to a lesser extent disk drives. "
     
    Last edited: Feb 23, 2015
  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I think it's called marketing (by deception?)
     
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  12. PG1995

    PG1995 Active Member

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    Hi

    I believe that from the discussion above we can simply conclude that when buying a UPS system one should go for the watt rating. Moreover, also check VA rating of your load. The VA rating of your load shouldn't exceed maximum VA rating of your UPS system. Also, the watt rating of your load shouldn't go above the maximum watt rating of your UPS system (watt rating of your load = VA rating of your load * pF of your load).


    Now let's see how much time you can get with a given battery. For the calculation, we will use 12V, 175Ah lead-acid wet battery.

    A flooded lead-acid battery shouldn't be discharge below 50% of charge capacity otherwise its life span is shortened.

    50% discharge = 175Ah/2 = 87.5Ah
    (12V*87.5Ah)/1000 = 1 kWh

    Assuming that the UPS system is 80% efficient then the available capacity is 0.8 kWh.

    It means that a UPS system using 12V, 175Ah lead-acid battery would give us 0.8 kWh. If your total load is rated 400W then it will give 2 hours and so on. Please correct me if I'm wrong. Thank you.

    Regards
    PG

    Helpful links:
    http://forum.allaboutcircuits.com/threads/which-ups-to-buy.63962/
    http://www.solarstik.com/sites/default/files/content/state-of-charge.jpg (the table gives open circuit voltage and it is usually assumed that the battery is sitting unused for four or more hours; closed circuit is always lower for the give state of charge)
     
    Last edited: Feb 23, 2015
  13. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,


    It sounds like the 'charger' part of the system has a power factor of 0.6 not the UPS itself. With a battery this big, the charger power factor would be a concern.
    If it charges from a 220vac line, then the load it presents to the line when charging has a 0.6 power factor.
    It's a little hard to tell for sure, but that's what it sounds like.

    The 'output' is rated at 1kVA, and that alone tells you what you can connect to the output . That means you can connect a load that draws current times volts out equal to 1000 VA. If the output was 100vac for example then the max load current would be 10 amps, regardless of whether or not the load was reactive or purely resistive or a combination of the two. So for this inverter you could connect a 220v heater that dissipates 1000 watts if it is purely resistive. If it is not purely resistive but partly reactive, then you can connect a load that draws a max current of 1000/220=4.54 amps approximately.
    Since the max resistive current is the same as the max reactive current, another way to rate the output (since the voltage is fixed at 220) is to say that the max output current is 4.54 amps AC.

    So it sounds like one rating is for the CHARGER INPUT, and the other is for the INVERTER OUTPUT.

    Usually converters also have a 'continuous' rating, where the continuous rating might actually be half of the peak rating. This is usually specified in the docs. We'd have to find out if this 1000VA is peak or continuous.
     
    Last edited: Feb 23, 2015
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  14. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    In principle, this should be true 1000 W = 1000 VA, but it's not. With UPS specs, Watts seem to mean continuous and VA seems to mean surge.
    Watts seems to always be less than VA. I invite you to disprove my theory.
     
  15. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi there KISS,

    Thanks for the invitation :)

    Well, first two inverters i look up on the web gives two ratings for each one. One is a continuous rating, the other is a peak rating. That's what i said before.

    I cant argue that they all do this, but what you are suggesting is that they use Watts to show the continuous rating and VA to show the peak rating. If they do that then they dont know how to rate an inverter properly.
    We could look for more inverters and see what gives here.

    Then there is the global rule of power supplies we used to call the "Fool Load". The word 'Fool' is a play on words which would mean 'Full' as in "Full Load", but with "Fool" replacing the word "Full". The idea is that anyone that uses an inverter at the full load rating is a fool :)
    We joked about this for years, but unfortunately sometimes it is all too true.

    Check out some more inverter ratings on the web and see what you can find. I did a quick search on the web and on Amazon. I quickly found one which was an Energizer "500 watt" unit, spec'd as: "500 watt continuous, 1000 watt peak". Another made by Wagan, "1500 watt continuous, 3500 watt surge".
     
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  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Totally agree 100%. What it probably does is allow the consumer to guess correctly most of the time.

    The literature research done by me an PG in this thread supports the VA and Watts specifications. As I said earlier. Xantrax is manufacturer I would trust. They show Watts and Watts surge which is more like Watts and VA surge. Probably turned to Watts to be consumer friendly. I'd like to contact Xantrax.
     
    Last edited: Feb 24, 2015
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  17. tvtech

    tvtech Well-Known Member Most Helpful Member

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  18. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  19. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Ok no problem. Just provide me with a link to an inverter for sale on the web that uses VA for surge or peak and watts for continuous and i'd be happy to take a look.
     
  20. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    These http://www.theinverterstore.com/tips.html guys make it fun. They kinda say select a continuous rating to match the "surge rating" of your load. Basically good advice and urge, is less than 1 second typical One that I looked at on the site says 30 sec surge.

    For a moment, lets size an industrial generator http://www.kohlerpower.com/onlinecatalog/pdf/g4170.pdf or where the "professionals" know stuff.
    note that for single phase the KW and KVA ratings are equal. For three phase, however, they are not (ON PURPOSE). Three phase units AUTOMAICALLY assume a pf of 0.8, thus the KVA rating is about 20% higher than the Watt rating.

    Furthermore, there is a mention of a TB that suggest these ratings can be exceeded for short times, but we are not talking seconds here.
     
  21. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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