Inverter

conntaxman
conntaxman Solar Expert Posts: 125 ✭✭✭✭✭
in Advanced Solar Electric Technical Forum #1
I just had to buy a small inverter because of a power outage. I only could get a 750 watt cont. out put. It did work good for my computers and LCD tv. My question. on my Boiler some may call it a furnace. But it has an oil burner on it and circulators. Its a new only 1 year old Peerless boiler. The burner motor is small compared to the old type ,and the motor only draws 1 1/2 amps. Why wouldn't that 750 watt 1500 peek inverter start that motor. Isn't one amp =100 watts? or is it the type of motor.
oh the inverter is modified sine wave.Got it at wall mart.
tks
John
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Inverter

    John;

    1.5 Amps @ 120 Volts is 180 Watts. This is very much the same as the running Watts on a refrigerator. But it is an AC induction motor, which means that when it starts it could draw more than the 750 Watts the inverter is capable of. Combine that with the fact AC induction motors don't work well on MSW and you have a "no go" situation. You can't count on the surge rating of inverters, especially not cheap ones. Very often they can't "ramp up" to the surge rating as fast as a motor tries to draw it. Instead the inverter faults.

    The motor(s) for water or air circulation will suffer the same fate. Motors are the most difficult thing to run from inverters.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #3
    Re: Inverter

    You may also have problems with the control electronics not liking the MSW waveform...

    And/or check the voltage drop right at the DC input to the inverter... It is possible that your wiring, battery connections, or even the battery bank are unable to support the minimum input voltage requirement.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • conntaxman
    conntaxman Solar Expert Posts: 125 ✭✭✭✭✭
    #4
    Re: Inverter

    Cariboocoot .tks for getting back to me so fast. That is what i though the problem was. Im looking to buy a prue sine wave about 2000 cont. watts .
    was I correct 100 watts = 1 amp? Even in the man. for that inverter it said that some motors wouldn't run. and by the way it hummed I could tell that it was the inverter.
    I want to make sure that the inverter that I buy will be able to run the Boiler and its 2 cirulators, their the small cir,not the big old B&G type.
    I look at it this way, all the things don't run at once, i know they could,but not likely, or I'll make sure that I don't go over the wattage of the inverter.
    Tks John
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #5
    Re: Inverter

    One of the power equations is:
    • Power = Volts * Amps * Power Factor
    Power factor can range from ~0.5 to ~0.95--With induction motors in the ~0.6-0.7 PF range. You can use a Kill-a-Watt meter to figure that out.

    Another issue with inverters (and generators) is that Watts is not the only design issue... Volts*Amps is too. Typically, for smaller inverters and generators, the maximum output is, for example, both 1,200 Watts and 1,200 VA. The amount of copper and iron (wiring + transformers) behave according to the VA number--and not so much to the Watts number.

    With PowerFactor ~1.0 -- Then Watts is approximately equal to Volts*Amps.

    For devices with poor Power Factor, those devices tend to draw a lot of "extra current" when operating. And with PF~0.5, then the VA is ~2x the Watt rating.

    So, say you have a motor that takes "1 amp" of 120 VAC and has a PF of 0.67
    • Watts = 1 amp * 120 volts * 0.67 = 80.4 watts
    That is, more or less (add ~15% for inverter losses) how much energy will be drawn from the battery bank.

    But, the inverter would actually need to drive:
    • Volt*Amps = 1 Amp * 120 Volts = 120 VA
    So, the inverter is, sort of, operating as if it had a 120 Watt Resistive load (in terms of wiring self heating and transformer/inductor saturation).

    And, note that a 5x starting surge would push the the VA requirements to:
    • 5x 1 Amp * 120 VAC = 600 VA
    And the surge current on the DC side would be around (for a 12 volt system running at 10.5 cutoff voltage)--Remember the DC side will draw Power (in watts) and not VA:
    • 5x 1 amp * 120 vac * 0.67 PF * 1/0.85 inverter eff * 1/10.5 volt battery bus = 45 amps DC on the 12 volt power bus
    So--That is a pretty heavy current draw during the starting surge. So, heavy and short DC power cables with good/solid connections to the Battery Terminals/common bus connections (and battery interconnects).

    I hope that sort of makes sense.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #6
    Re: Inverter

    Yes, the MSW could well take out the computer in the burner. I use a Morningstar pure sine wave SS-300 to run my Riello oil burner: http://www.riellousa.com/2_products/2_oil-burners/3a_40-series-oil-fired.asp and it has no problem starting it as long as the inverter has no other loads. Runs my circ pump as well, but there's only one and it's small. And on top of that, I'm weird. :p
    What do you have supplying power to your inverter? And do you have good, solid, bolted up connections, or are you just using "jumper cables"? Jumper cables will not do the trick for anything beyond a very light load. Learned that the hard way doing a quick test of a new inverter before permanent installation, I thought the inverter was defective. lol
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #7
    Re: Inverter

    The only reliable way to see what the inverter draws is to connect a shunt into the positive wire from the battery /ies to the inverter and as close as possible to the inverter.. this amp reading is the only true reading you will ever get of what the inverter draws.. Mesasuring the inverter output is NOT reliable.
    Every system I think should have a shunt connected as above. It will show up many problems nothing else will.
    And it serves a great secondarr purpose,but saying this will bring howls of condemnation from everybody as to why its not a good idea. Funny thing is the reasons they will give why its not a good idea is the very reasons it is a good idea. and they will contradict their own statements.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #8
    Re: Inverter

    Battery Monitors use Shunts on the Battery Bank (typically negative connection because of negative grounding)--I highly recommend a battery monitor.

    Of course, it will only give the current leaving the battery, not a specific circuit current (and charging current when applicable).

    And John P (if I recall correctly) found shunts to be very serviceable fuses too (much better than nothing it the case of a dead short--for sure).

    There is nothing more scary than a set of battery cables burning off their insulation and starting to glow red hot--And you have nothing to turn off / break the circuit... And probably 10x worse if that happens when you are not present.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • conntaxman
    conntaxman Solar Expert Posts: 125 ✭✭✭✭✭
    #9
    Re: Inverter

    Holy smokes, i just read all the responses,with ALL the number's. my head is Spinning.Thanks guys,your sure smarter then me on this stuff.Glad your here to get advice,and its a very nice bunch of guys.
    As you read,I will be getting a pure sine wave inverter.Im reading all the spec. on then.,Im trying to find the lowest current draw when there is no draw, and the efficacy,I know that sounds stupid,their will always be a draw,even a LED will draw.
    one person asked how many batteries. I now only have 2 deep cyc. 115amh. We had our power knocked out by a snow storm and that is why i had to get the cheap inverter and only 2 batteries. And it was hard to find that inverter. Everyone was buying them.
    As of now i only have 520 watts of solar panels.Soon I will have 1000watts of panels. I would like to get up to atleast 2000 watts of panels.
    My solar controller (40 AMP)could be wired parallel with another of the same type,so thats good.
    Im retired and a very tight budget.
    So next is the inverter. Then more batteries.
    Thanks for all the help,
    John
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Inverter
    conntaxman wrote: »
    I would like to get up to atleast 2000 watts of panels.

    Why?
    This is another case of "what are your loads?"
    If you don't size the system according to that you end up buying too little or too much of something, be it panels, batteries, inverter, or some combination thereof.

    So if this is for emergency power purposes, figure out what you need to run first. That would include the furnace and refrigerator which will both want that pure sine wave and have fairly hefty start surges. That would determine the inverter size. Then you need to know for how long you'll need to power them. That will determine the battery bank size. And that will determine how much panel is required to recharge them. Picking components at random gets you in to trouble, as you've already seen. :roll:

    I wonder if I could get a parrot and teach it to repeat this stuff? :p
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #11
    Re: Inverter
    Cariboocoot wrote: »

    I wonder if I could get a parrot and teach it to repeat this stuff? :p

    Not joking, some forms have a "sticky" section at the top for select posts only. Would be a perfect place for something like that info.
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  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #12
    Re: Inverter
    Cariboocoot wrote: »
    Why?

    I wonder if I could get a parrot and teach it to repeat this stuff? :p

    For one of the offroad forums I frequent I saved word files on my computer for the stuff I was always repeating. I could copy and paste in a paragraph, or a page, for the top twenty or so questions that kept getting asked.

    If you put a sticky in the "beginners corner" as a "must read before posting", it might save you ten out of a hundred reposts. I was hear a couple of months before I saw the one in the general section.

    It would probably be better to dumb your answers down a little so the math wasn't so overwhelming to a noob if you did that.
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