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I have purchased three 750-watt power inverters in the last three weeks. They have all failed to work on the second or third use. The first two were the same brand. I replaced an old one that ran for several years. I am using it to run a 4.7 amp 120-volt vacuum cleaner motor that pumps up my air dock. I have it connected directly with marine cable to one of the boat batteries. The battery outputs about 12.8 volts DC. The fuses don?t blow, just No Output. I am using a three-wire 120-volt extension cord and am, wired neutral, hot and ground to the AC motor vial an on/off switch. The motor works fine when I run house power with an extension cord from the dock next to me. Could I be, just unlucky with new inverters and need to buy a forth one?

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  • hboyle362 Jul 22, 2010

    the first two inverters were 750 watts from O'Reilly auto parts

  • hboyle362 Jul 22, 2010

    Should I put a shut-off switch on the DC input?

  • hboyle362 Jul 23, 2010

    Yes this was helpfull.

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The problem may be that the inverter is undersized for the load. Is the 4.7 Amp rating the motor peak draw or the nominal one. What I mean is that it takes many times more power to start start the motor than when the motor is running. Typically it is 3 to 7 times more amps to start the motor than when it is running. It could be that starting the vacumm cleaner is stressing the inverter and it is causing it to fail prematurely. If you can find out the locked rotor current draw of the motor, the that would be the mininum size of the inverter.

Also, the type of inverter will make a difference on how the motor operates. A modified sine wave will make the motor run hotter than a pure sine wave inverter. Hope this helps.

Posted on Jul 22, 2010

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  • Stephen Lam Jul 22, 2010

    I don't think a switch on the input would do anything to keep the inverter from damage. I think the inverter that you are using are undersized and the startup current of the motors are causing them to fail. I would recommend that you get inverters with at least 4000 watt peak capacity if you want any kind of life from it. Hope this helps.

  • hboyle362 Jul 23, 2010


    Thanks, The manufacture of the Air Docks recommends a 750w Black & Decker. Do you think, as the motor ages it draws more current? Should an inverter fail because of an over load? I think I will try a 1000W

  • Stephen Lam Jul 23, 2010

    Somethings not making sense to me. You are saying that you are using a "4.7 amp 120-volt vacuum cleaner motor that pumps up my air dock" and the manufacturer is recomending a 750 watt inverter. If you multiply the current with the voltage you get watts. In this case it is 4.7 X 120 = 564 watts. However this is the running power, not the startup or surge power. For an AC motor, it can be anywhere from 3 to 7 times the running power. If we assume the worst case of 7, then the surge power is 3948 watts. I don't understand how the manufacturer could recommend a 750 watt invertor.



    Typically inverters have two ratings, a continuous and a surge power rating. Typically the surge power rating is twice the continous. Since you were using 750 watt (continuous power) inverters, even with the best case surge of 3 times the continuous power, 1692 watts, would be exceeding the surge power of the the inverter which would be 1500 watts.



    I tried to call Air Docks this morning, but only got a message. I would like to speak to someone there before giving you a recomendation on what should be the size of the inverter. Clearly it is not 750 watts, since 3 of them has failed you in a short period of time.



    Just to let you know, I do have a degree in electrical engineering and have knowledge with motor start up currents. I will try get a hold of the manufacturer to get back to you by this afternoon. In the meanwhile, if you need to get an inverter, for your air dock, I would recommend that you get at least one that has a surge capacity of 4000 watts to be on the safe side.

  • hboyle362 Jul 23, 2010

    Hi This is the motor that air dock uses

    http://www.blowermotorsplus.com/ProductD...



    I did have a used 700w inverter from Harbor Freight for years. Air Dock recommends an 800w from Block & Decker. Thanks

  • Stephen Lam Jul 23, 2010

    Thanks for the info. I couldn't get the start current draw for this blower. However the web site show the max Amps as 5.7 so the continuous wattage is 684 watts. A 750 watt inverter is just marginally adequate to supply the continous power for the blower. As the bladders that you are trying to fill become full in order to lift your boat out of the water, that will increase the load to the motor, and thus overload your inverter, shortening out it life quickly.



    A larger inverter will give you more margin than a smaller one. Also a larger inverter will have better cooling capabitity as well. Typically an inverter are 90% efficient, so 10% of the power consumed by the inverter will be wasted as heat. So for a 700 watt load, will cause the inverter to generate 70 watts of heat. Your inverter is generating heat equivalent of a 75 watt incandescent light bulb when your blower is running.



    Plus a 2000 watt inverter is only twice as much as a 750watt inverter. Checking Amazon.com, a B&D 750 watt is $69, while a B&D 2000 watt is $139. You get nearly 3 times the power with at twice the price. You already spend that much on inverters. Get the larger inverter and spend more time on your boat instead of at the hardware store.

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Hello,
All you need is to reduce the load that is placed on the inverter, if still decide to use the same numbers of load , you can go for the fourth inverter but of higher of higher wattage.
With this hope this will helpout......

Posted on Jul 22, 2010

  • Sanyade Akinwale
    Sanyade Akinwale Jul 22, 2010

    Hello,
    Just follow the solution provided initially.
    Good luck.....


  • hboyle362 Jul 22, 2010

    I had a 700 watt in the boat for years with the same blower motor. I may try a 1000 watt from Black & Decker. Thanks

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  • hboyle362 Jul 22, 2010

    That the most recent one to fail. Thanks

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