Square wave on AC motor

Discussion in 'General Electronics Chat' started by Voltaire, Aug 3, 2010.

  1. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    Hi
    A small AC pump motor needs to run on rectified power. Pure sine is more expensive and my client may want to get a square wave rectifier. What effect will this have on the motor?
    Thanks
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    The motor will smooth out the square wave to some extent but you're going to end up with a lot of harmonics and voltage spikes some of which the motor may not like at all. The addition of a properly sized PFC capacitor would help but I'd still be worried about the peak voltage of the resultant spikes as the field tries to build and collapse from being driven by a square wave.
     
  3. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Is there any reason a DC motor could not be used? A small DC motor and conversion is likely to be simpler than the inverter in the long run.
     
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    typically resulting in injury to the first few turns of the stator windings. 'Inverter' duty motors are a bit more robust in that area.
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I'm not so certain even an inverter duty motor could take it. Graanted, their insulation breakdown rating is higher but not by a lot since VFDs simulate a sine wave and smooth it to some extent.

    Places like Harbor Freight Tools sell DC:AC inverters at fairly inexpensive prices nowadays. They output a modified sine wave that most any motor could handle, you'll just need to start with 12V.

    How small of a pump or, rather, how big of a motor in HP or voltage and current ratings?
     
  6. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    i agree. Service factor and loading will impact it's ability to handle such a source.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    You have not given us the basics on the motor (frequency, voltage, volt-amps) and you have not given us the DC power that is available.

    I don't know how that you thought you could get a decent reply without such basic information. However, you did. Weigh each with a pound of salt.

    Then give us specifics about your motor, and your current that is available.
     
  8. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    Ok Wookie :p
    My question was general and if someone had said it's going to blow up then I would not have pursued it further. But since you insist...
    The motor specs are: 0.37kW (0.5HP), 220VAC, 50Hz, induction, cap
    The DC voltage has not been confirmed but is either 12V or 24V (green fields)
    1. the pump is out in the paddock and too far for DC (100 metres, NOT meters)
    2. it has already been procured and installed

    How about Marshall's 'correction package'?

    PS does anybody have a quick and dirty circuit diagram for an aircraft fuel safety backup system?:eek:
    Just kidding :)
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
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    1) Can the motor be wired as 110/120V?

    2) Would a slight speed increase by running it at 60 Hz (most 50 Hz motors will) be a problem?

    If so and you've got 12V the inexpensive inverter makes more sense than anything, however 24V input inverters can be found almost as easily, they're just less common thus more expensive. You can also find them in 50 Hz output which is probably going to be more common to you anyway since you apparently reside in a 50 Hz country region.

    Check with some local farm supply stores, sounds like the situationis solvable for a reasonable cost.
     
  10. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    Hi Marshall
    1. No. The client supplies the power and my price is fixed and I'm about to invoice
    2. Ditto. No can do
    So, the customer must install a sine wave inverter and that's that.
     
  11. whitehorse

    New Member

    Aug 5, 2010
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    i'm new to this site but wouldnt a wheatstone bridge rectifier circuit similar to a car alternator do the trick
     
  12. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    As far as I know it does measurement of some kind. How does it work here?
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    The key word is BRIDGE. A Wheatstone BRIDGE is indeed used as a measurement tool. A BRIDGE Rectifier is used to rectify an AC voltage. I'm sure that Whitehorse intended the Bridge Rectifier section in his suggestion. As for the original problem..... My read is that there is a DC source and it needs to be converted to run a motor designed to run on AC. Running DC into a bridge rectifier will not make AC come out.
     
  14. Voltaire

    Thread Starter New Member

    Aug 3, 2010
    25
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    Thanks Bill. You read it right: DC --> AC for AC motor
     
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