Slowing down a strimmer motor

Discussion in 'The Projects Forum' started by arose, Apr 6, 2010.

  1. arose

    Thread Starter New Member

    Apr 6, 2010
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    I have a motor from a strimmer that I am trying to slow down. Will a simple potentiometer do the trick or do I need something a little more sophisticated or complicated?! Not sure what the rpm is, I litterally took it out of a wrecked strimmer and I want to use it for something quite slow.

    Any help really appreciated!!
    Thanks!
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Electric grass trimmer?

    Ken
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    If it operates on 120 V AC or DC [ has brushes ], try operating on 12 V DC. More information will probably bring more answers.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You would be better off to use V-belts and pulleys or a transmission to reduce the speed.

    If you try to use PWM or reduce the voltage to run it slowly, you will lose a great deal of the motor's torque.
     
  5. arose

    Thread Starter New Member

    Apr 6, 2010
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    Yes it is an electric grass strimmer.
    It runs on 240V. The problem is I dont have a lot of space to put in pulleys etc, as it all has to fit in under a bench.
    It doesn't have to have much force when it is slowed down, just enough to turn a small load of about 5-10 lbs.

    I have though about PWM, but can't seem to find a kit in Ireland that I can get quickly.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Then you need to find another motor that is better suited for your task.

    String trimmers don't have much torque to begin with, even when running full speed.

    A gear-reduction transmission or V-belt arrangement would multiply the available torque while reducing the speed. However, simply using PWM or current limiting would reduce the amount of torque available to practically nothing; you'd be lucky if it had enough torque to turn over an egg.
     
    arose likes this.
  7. arose

    Thread Starter New Member

    Apr 6, 2010
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    Thanks! I'll have a look around
     
  8. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    A simple thyristor based 'drill speed controller' would be perfect for the job.

    These are not much different to a light dimmer, but allow a good speed range whilst retaining the motor torque.

    Examples:
    http://www.quasarelectronics.com/kit-files/smart-kit/1074.pdf
    http://www.southgatearc.org/techtips/drillspeed.htm

    They are quite minimal circuits, but as they work on mains you must be extremely careful with contruction and insulation.

    Unless you have experience building electronics it's probably easier and safer to buy a ready built unit on ebay etc - eg.
    http://cgi.ebay.co.uk/POWER-CONTROL...ial_Automation_Control_ET?hash=item2eac3351fa
     
    arose likes this.
  9. arose

    Thread Starter New Member

    Apr 6, 2010
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    Thanks Robert, that looks great!!
     
  10. DickCappels

    Moderator

    Aug 21, 2008
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    Here is a modification I made to a cheap Chinese-made string trimmer, running on 240 VAC. After smoking a number of snubbers, I changed to an ST Micro "snubberless" triac. Not noted on the schematic is that the Diac breaks down at 32 volts. The Triac has about 8 square cm of aluminum heatsinking inside the handle of the trimmer.

    You will probably have to trim the resistor values to meet your trimmer and specific needs.

    i am very happy with this design, becuase on all but the highest speed, the plastic "string" seems to last forever.

    [​IMG][​IMG]Schematic:
    [​IMG]http://yfrog.com/4patrimj


    Photograph:
    http://yfrog.com/5matrimj (Note the red buttons which are the switches).

    [​IMG]
    [​IMG]




    [​IMG]
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Dick,
    Somehow your links didn't come through. Trying a different tactic...

    [​IMG]

    Schematic:
    [​IMG]

    Photograph:
    [​IMG]

    Seems that you were trying to post the page link as an image, not the images themselves. Not sure that I got them all.
     
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