Inverter questions

experimenterexperimenter Registered Users Posts: 20 ✭✭
Looks like I screwed up and purchased the wrong inverter for my needs. I have a non-grid-tied (ie standalone) system and wanted to run 12V DC from a battery bank to an inverter, then to a standard circuit breaker panel. But I'm realizing the inverter outputs 120V, not 240V. I have mostly 120V circuits to put into the service breaker panel, but I was hoping for a couple 240V circuits as well.

So, I'm looking for ideas to get me out of this mess. I can think of these options:

1. Sell the 120V inverter and get an inverter that outputs 240V (this will be expensive, and I'm not sure where to sell the 120V inverter to realize best price).
2. Wire up the 120V inverter and feed only 120V circuits. No extra cash outlay, but it doesn't give me all the functionality I wanted.
3. Get a transformer to step up the voltage to 240V (not sure if this will actually work -- will the connections be phased properly? -- and I believe I'll lose half my amperage)
4. <insert your great idea here>

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter questions

    Finding a 12 Volt inverter that puts out 240 VAC will be difficult.

    How heavy are these 240 VAC loads, and how large is the 12 Volt inverter? Hopefully the inverter is not large because high Watts (over 2kW) and 12 VDC don't go together well. If it's not too much you can use an autotransformer to get your 240 VAC: http://www.solar-electric.com/x-240.html
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Inverter questions

    maybe you should explain this to the place you bought it from to see if they can accommodate you with the proper inverter for your needs and take back the one you have.
  • PhotowhitPhotowhit Solar Expert Posts: 5,592 ✭✭✭✭✭
    Re: Inverter questions

    What are you trying to power? weren't you doing a 3 panel 600 watt array?

    I think people here have done the transformer, and some have said it actually is more effiecent than using a built in 120/240 inverter. I don't think there are any 12 volt inverters that have 120/240 output, so that is likely your only option.

    240V is usually reserved to motors and big heating, I think you were originally talking about heating pads? What wattage are they?
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Inverter questions

    Adding a stepup transformer is probably your easiest option. Starting something like a 240V well pump may well be hard to handle though. The transformer needs to be sized for the surge load of the pump, not just the continuous load.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Inverter questions
    Photowhit wrote: »
    What are you trying to power? weren't you doing a 3 panel 600 watt array?
    I think people here have done the transformer, and some have said it actually is more effiecent than using a built in 120/240 inverter. I don't think there are any 12 volt inverters that have 120/240 output, so that is likely your only option.

    I am sure that the transformer is more efficient than running two synchronized 120 volt inverters, and probably more efficient than running a higher wattage 120/240 volt inverter because you cannot get a lower wattage in that style. I do not recall seeing a similar comparison between an X watt 120 inverter with transformer and an X watt 120/240 inverter.
    SMA SB 3000, old BP panels.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter questions
    inetdog wrote: »
    I am sure that the transformer is more efficient than running two synchronized 120 volt inverters, and probably more efficient than running a higher wattage 120/240 volt inverter because you cannot get a lower wattage in that style. I do not recall seeing a similar comparison between an X watt 120 inverter with transformer and an X watt 120/240 inverter.

    They'd have to be the same brand too.
    So the closest we can get is comparing the Outback 120 VAC 60 Hz models to the Outback 230 VAC 50 Hz models. Change the 'clock', different output transformer. Guess what? Same power consumption and conversion efficiency.
    I'd say that indicates inetdog is right again. :D
  • experimenterexperimenter Registered Users Posts: 20 ✭✭
    Re: Inverter questions

    The heating pads are currently running (and have been for 15 years) on a 30 amp circuit. I don't believe there is any "startup surge" with them -- they are kindof binary in that they are either on and drawing full power or off.

    The inverter I have is 3kW continuous. I suppose if I could find a 48V to 240V inverter I could wire the battery bank (four 104 amphour 12V batteries) in series to give 48V to a different inverter.
    Photowhit wrote: »

    I think people here have done the transformer, and some have said it actually is more effiecent than using a built in 120/240 inverter. I don't think there are any 12 volt inverters that have 120/240 output, so that is likely your only option.

    240V is usually reserved to motors and big heating, I think you were originally talking about heating pads? What wattage are they?
  • experimenterexperimenter Registered Users Posts: 20 ✭✭
    Re: Inverter questions
    If it's not too much you can use an autotransformer to get your 240 VAC: http://www.solar-electric.com/x-240.html

    That autotransformer looks like something to try. Does it output 240V on two 120 volt legs with a neutral between like the circuit breaker panel expects, in proper phase? (In other words, are there 2 black wires, a white neutral, and a ground coming off of it?) If not, I'm not sure how I would feed the autotransformer output to the circuit breaker panel.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter questions

    There are definitely both 24 Volt and 48 Volt 240 VAC inverters and they are a much better choice if you need 3kW+.
    Look at the Magnum inverter line: http://www.solar-electric.com/maensiwainac.html
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter questions
    That autotransformer looks like something to try. Does it output 240V on two 120 volt legs with a neutral between like the circuit breaker panel expects, in proper phase? (In other words, are there 2 black wires, a white neutral, and a ground coming off of it?) If not, I'm not sure how I would feed the autotransformer output to the circuit breaker panel.

    Yes: it creates a "mirror image" of the output waveform in series with it.
    I don't have a wiring diagram handy, but there are four wires: black, red, and two whites. The two whites are tied together and form the neutral and half the input for 120 VAC. the other half goes to the black. 240 VAC is then available between the black and red, with two legs of 120 between the whites and black or red.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Inverter questions
    Yes: it creates a "mirror image" of the output waveform in series with it.
    I don't have a wiring diagram handy, but there are four wires: black, red, and two whites. The two whites are tied together and form the neutral and half the input for 120 VAC. the other half goes to the black. 240 VAC is then available between the black and red, with two legs of 120 between the whites and black or red.

    The thing that makes it an autotransformer rather than an isolation transformer is that both the neutral and the hot input leads also form one side of the output circuit.
    A true autotransformer could have just one double-sized neutral instead of two wires which must be connected together. The results are the same either way.

    If you have the choice, you get better voltage regulation and better efficiency if you connect most of your 120 volt loads to the side which forms the transformer input. If your 240 volt panel is wired with 120 volt loads on each phase, choose the phase with the heaviest loads to connect that leg of the output to.
    SMA SB 3000, old BP panels.
  • PhotowhitPhotowhit Solar Expert Posts: 5,592 ✭✭✭✭✭
    Re: Inverter questions
    The heating pads are currently running (and have been for 15 years) on a 30 amp circuit....

    If they are on a 30 amp circuit, and they have no surge, we can assume they draw more than 20 amps, 20 amp x 120 volts would be a 2400watt load, this would challange any 12 volt inverter, with 93% conversion that's close to 2600 watts/ 12 volt = 216 Amp draw from a 12 volt battery bank... It would take a pretty massive battery bank to tolerate that large a draw for any time at a C20 discharge (or discharging the battery past dead in 20 hours) you would want a 4320 amp battery bank...

    Just big ugly numbers...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • experimenterexperimenter Registered Users Posts: 20 ✭✭
    Re: Inverter questions

    Thanks to all the responders for the info, I am going to try the autotransformer.

    Regarding the quote below, this is an area I am still somewhat confused about. I know I'm not going to magically get enough power with my system to power these energy-hog panels for any length of time, but my main purpose here is to learn how this works so I still have questions. You say "2600 watts/ 12 volt = 216 Amp draw from a 12 volt battery bank" -- I have 416 amp/hours of battery, half of which (to stay above 50%) is 208 amp hours. 216 amp draw actually is pretty close to this capacity, and I doubt the heaters have to work for a full hour to warm the place up -- 10-15 minutes is more likely. I could also add another 104 amp/hour battery or two.

    Photowhit wrote: »
    If they are on a 30 amp circuit, and they have no surge, we can assume they draw more than 20 amps, 20 amp x 120 volts would be a 2400watt load, this would challange any 12 volt inverter, with 93% conversion that's close to 2600 watts/ 12 volt = 216 Amp draw from a 12 volt battery bank... It would take a pretty massive battery bank to tolerate that large a draw for any time at a C20 discharge (or discharging the battery past dead in 20 hours) you would want a 4320 amp battery bank...

    Just big ugly numbers...
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter questions
    Thanks to all the responders for the info, I am going to try the autotransformer.

    Regarding the quote below, this is an area I am still somewhat confused about. I know I'm not going to magically get enough power with my system to power these energy-hog panels for any length of time, but my main purpose here is to learn how this works so I still have questions. You say "2600 watts/ 12 volt = 216 Amp draw from a 12 volt battery bank" -- I have 416 amp/hours of battery, half of which (to stay above 50%) is 208 amp hours. 216 amp draw actually is pretty close to this capacity, and I doubt the heaters have to work for a full hour to warm the place up -- 10-15 minutes is more likely. I could also add another 104 amp/hour battery or two.

    It's worse than that: the heavier a battery is discharged the less actual capacity it has.
    When you read that 416 Amp hour rating that's at the "20 hour" rate; drawing about 20 Amps (might have done the math wrong there). Your heaters will draw approximately 10X that current, with a resulting drop in capacity. In reality that heater draw could 'flatten' those batteries in a matter of a few minutes.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Inverter questions

    Ideas of "electric heat" powered by an off grid solar/battery systems, go together like a dog wearing false teeth.
    But if you're extremely rich, you could always purchase what you want. Thing is, you wouldn't be rich after you paid the bill :cry:
  • PhotowhitPhotowhit Solar Expert Posts: 5,592 ✭✭✭✭✭
    Re: Inverter questions

    It's just part of the learning curve in solar, this has to do with the size battery at a C20 rate that I refered to in my last post.

    I think Bill had done some battery capacity at different discharg rates, I have a fork lift battery that has a 800 Amp hour capacity at C20(a charge/discharge rate usually used in solar application), but only a 500 Amp hour rate at C6(a capacity normally used for forklift batteries).

    It is not uncommon for people using 12 volt systems hitting the inverters low voltage cutoff when trying to draw large amounts of current. If you check amazon reviews for xantrex mx1800 or prowatt 2000 (I think I got these correct) you'll see people complaining about them being over rated since they couldn't get them to run their A/C or some other high draw appliance. I think your using one or the other of these.

    You might even look for a review done here that showed the inverter performed well with correct wiring and battery bank. I in the reviews at amazon, you'll find some statements that might help you understand what your up against, like;

    "...a test using a heavier load than what I need: When I start with fully-charged batteries and plug a 1500-watt electric heater into the inverter, my battery/wiring system will only supply 11.4 volts to the inverter power input terminals. After about 15 minutes, the voltage drops down to the 11-volt level and the inverter beeps its "low battery" alarm. After a little while longer, the voltage drops down to 10.5 volts and the inverter shuts itself off.

    This is important: when the "low-battery" alarm beeped, and later when the inverter shut itself off, THE BATTERIES WERE NOT ACTUALLY LOW and THERE WAS NOTHING WRONG WITH THE INVERTER. The problem is that my battery/wiring system is not sufficient to support the heavier, 1500-watt load for very long. The specific gravity of the battery acid did not indicate a low charge in the batteries. The inverter was not defective."


    Link to amazon reviews.

    Hope this helps your understanding.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Inverter questions
    Photowhit wrote: »
    It's just part of the learning curve in solar, this has to do with the size battery at a C20 rate that I referred to in my last post.
    "...a test using a heavier load than what I need: When I start with fully-charged batteries and plug a 1500-watt electric heater into the inverter, my battery/wiring system will only supply 11.4 volts to the inverter power input terminals. After about 15 minutes, the voltage drops down to the 11-volt level and the inverter beeps its "low battery" alarm. After a little while longer, the voltage drops down to 10.5 volts and the inverter shuts itself off.

    This is important: when the "low-battery" alarm beeped, and later when the inverter shut itself off, THE BATTERIES WERE NOT ACTUALLY LOW and THERE WAS NOTHING WRONG WITH THE INVERTER. The problem is that my battery/wiring system is not sufficient to support the heavier, 1500-watt load for very long. The specific gravity of the battery acid did not indicate a low charge in the batteries. The inverter was not defective."

    Very well said! We sometimes (especially when we're new to solar) tend to think of the power stored in our batteries, as if it were fuel in a vehicle's tank. What went in, should come out. But there's a whole lot more to the way a battery system works than that.
Sign In or Register to comment.