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Direction of AC motor

Discussion in 'Troubleshooting and Repair' started by petercj, Apr 16, 2012.

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  1. petercj

    petercj

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    Apr 16, 2012
    I have just replaced the carbon brushes in a Black and Decker GX300 lawnmower - no problem with that, and the machine is running fine.

    However, while replacing the brushes I noticed that the blade was sharp and as new on the cutting edge, but showed signs of wear and tear on the back edge.

    A quick check showed that the motor is running in the wrong direction.

    Apart from the blade cutting with the blunt back edge, the air scoop disc that creates the hover effect is also running backwards.

    I inherited this hover mower from my mother - it's over 30 years old - and it has given very reliable service throughout, although she did pay to have a new motor fitted around 20 years ago.

    What I suspect may have happened is that whoever fitted the motor muddle up the Live and Neutral connections when reconnecting the motor.

    Both terminals are exactly the same, and the wires are long enough to reach either terminal.

    I have stopped short of switching the wires around just in case I'm wrong and it causes damage to the motor.

    But I can't think why else the motor would be spinning in the wrong direction.

    The only other possible explanation I can think of is that B&D made a motor that would fit but ran in the opposite direction to the original - seem very unlikely to me though.

    Many thanks for any help with this issue.

    I really don't want to replace this mower, as it's been a fantastic workhorse and is a credit to B&D - if only they'd marked the Live terminal during the design stage!

    ATB

    Peter
     
    Last edited: Apr 16, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,226
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    Nov 17, 2011
    Welcome Peter.
    From what I know about AC motors, the motor itself cannot distinguish between life and neutral. Thats like your kitchen mixer running backwards when you turn the plug by 180 °. Very unlikely :)

    A single phase AC motor needs some additional circuitry so that a rotating magnetic field with a defined direction is generated (this is different for e.g. 3-phase motors). It is described here http://en.wikipedia.org/wiki/AC_motor under "split phase induction motor.
    What may have happened is that the capacitor is defect or a wire leading to the capacitor is loose or broken. You may have a look at the capacitor (part 60, http://www.toolsandpartsdirect.co.uk/Black--Decker-GX300-Type-1-Hover-Mower-Spare-Parts__p-2629.aspx).
    Notice anything unusual?


    Harald
     
  3. duke37

    duke37

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    Jan 9, 2011
    If the motor has brushes, then it will be a series connected 'universal' motor.
    To reverse rotation, you will need to change over the brush connections OR the field connections.
    You could try it out with a light bulb in series with the supply to check that nothing is untoward.
    Are you sure that it is running in the wrong direction and that the blade has not been fitted upside down?
     
  4. duke37

    duke37

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    Jan 9, 2011
    If the blade has a slight twist at the cutting end, it should run in the direction that lifts the grass from the ground.
     
  5. petercj

    petercj

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    Apr 16, 2012
    Thank you for your reply Harald.

    As suggested, I have taken a look at the capacitor, but it shows no visible signs of being a problem - wires in good condition and well attached, etc.

    Does it make any difference as to the flow of current through the capacitor?

    If so, could it be that having the Live and Neutral around the wrong way would alter the effect the capacitor has on the motor?

    If I try switching the L and N around, do I risk damaging the motor?

    Thank you for your help with this - it is much appreciated.

    Peter
     
  6. petercj

    petercj

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    Apr 16, 2012
    Hi duke 37 - thanks for you reply.

    The blade doesn't have a twist, but it has a throw upwards.

    \________/ ... no where near as acute as the angles shown, but that is the general profile.

    If the blade were turned the other way up, it would tend to dig into the ground on uneven surface, tufts, etc..

    The other issue is the impeller that generates the hover effect - I'm assuming the ducts are supposed to face in the direction of rotation so as to scoop the air and so create a cushion. At the moment they are facing away from the direction of rotation. The only issue I've ever had with this mower is that it doesn't hover very well. The action is more one of pushing the case over the grass in sledge-like manner. There is a slight hover effect, but it doesn't lift the mower as it should. If the impeller is spinning in the wrong direction, that would explain the problem. Over the years I've just assumed it was a design fault.

    Peter
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
    2,779
    Jan 21, 2010
    I have to say, your mower sucks!

    Is it possible that the plate or whatever that the blades are mounted on has been removed and replaced upsidedown?
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    10,226
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    Nov 17, 2011
    Changing live and neutral will probabply have no effect at all on the motor. At least it will not damage the motor.

    Direction of rotation of the motor is defined by the direction of rotation of the magentic field within the motor. The capacitor generates a phase shifted current through a second coil in the motor such that the magentic field has a defined rotation. If the wiring looks o.k., other possible defects you can loook for are:
    1) capacitor defect (remove, measure capacitance, compare with nominal value)
    2) id the motor is of the capacitor start type (see Wikipedia link) the starter switch could be defect. Measure with Ohmmeter (disconnect power first)
    3) the motor itself could be defect (e.g. broken wire in one of the coils)

    Harald
     
  9. duke37

    duke37

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    Jan 9, 2011
    The motor has brushes so I do not think that it is of the induction type with capacitor start or start/run. Any capacitor is more likely to for interference suppression and will have no effect on the motor behavior. Is it connected across the supply?

    Motor capacitors will have values of uF, suppression capacitors much less than this.
     
  10. petercj

    petercj

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    Apr 16, 2012
    Thanks to all for your replies.

    I found a handbook on line for this model, and it seems that the the blade should be fitted the other way up. Which I can see causing problems because it will dig into any uneven ground of tufts. But there it is.

    The impeller can only be fitted one way around, so it seems that the poor hover effect is down to bad design.

    Presumably, the impeller is supposed to draw air down from the top of the machine and blow it out at the bottom to created a cushion - unfortunately it looks like one of those theoretical designs that doesn't work too well in practice.

    For one thing, the ports where the air is supposed to be taken in gets covered in grass, which blocks the intake and flow.

    To my surprise, replacement filters are still available, so I might try fitting a new filter - I have made up filters from fine plastic mesh, but they are probably not as good as the original.

    I don't have huge lawns, and this mower has given good service for the past 20 years, so I will carry on with it, design flaws and all.

    Thanks again for your input.

    Peter
     
  11. petercj

    petercj

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    Apr 16, 2012

    Unfortunately, it doesn't suck enough! :)
     
  12. duke37

    duke37

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    Jan 9, 2011
    Fans are made to various designs.
    Forward inclined vanes give high volume but low pressure.
    Backward inclined vanes give low volume but high pressure.
    B&D will have set the vane angle to the optimum.
     
  13. petercj

    petercj

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    Apr 16, 2012
    I think the problem is that the impeller is just not big enough to create enough of a cushion to lift what is quite a heavy machine.

    Hover mower design has probably come a long way since this old faithful was built.
     
  14. john monks

    john monks

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    Mar 9, 2012
    You have brushes. That tells be that you have a parallel wound motor. That means that the brushes are connected in parallel with the field winding. If that is the case you must reverse the wires going to the field winding or the brushes. This will reverse the direction of the motor.
     
  15. duke37

    duke37

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    Jan 9, 2011
    The motor will be series connected NOT parallel connected. This type of motor is called a 'universal' motor since it will run on AC or DC. This is how electric drills and hair dryers are made.
     
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