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AC brush motor speed control

Discussion in 'Sensors and Actuators' started by Mark56, Sep 23, 2011.

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  1. Mark56

    Mark56

    27
    0
    Jun 15, 2011
    Hi, I'm looking for an easy method to reduce the speed of a 1300W 230V AC single phase brush motor on a power saw. I only need to reduce the speed from 7000rpm down to 5800rpm (or a little less) to use a steel cutting blade. It does not need to be fully variable but a further step down to about 4000rpm may be usefull for cutting thicker material.

    Any help would be most appreciated :)
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    A triac controller (light dimmer) should be able to do the job, at least if it's of a type meant for transformers (inductive load).
    Be careful about loading motors at reduced speeds since the built-in fan quickly becomes very ineffective and the motor can burn.
     
  3. Mark56

    Mark56

    27
    0
    Jun 15, 2011
    Thank's Resqueline, that is usefull advice. 1300W+ light dimmers do appear to be quite expensive though, €100+.

    I've been looking at motor speed controllers but don't quite understand the wattage rating. One I saw rated at 1500W (about €20) is fused at just 4 Amps which following the normal calculation (A x W = V) only gives 920W at 230V AC. This particualr unit is described as being ok for electric drills and vacumm cleaners but not for inductive loads like some lighting circuits with a ballast.

    If you have the time can you explain where my calculations may be going wrong?

    I appreciate the tip about the cooling fan. The motor will only be used in short burst to cut thin steel so wont have much load on it, even when cutting a lot of wood is has barely got warm. It's a Dewalt mitre saw and has the quietest brush motor I have ever heard, being Dewalt it will be very high quality.
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    I'm not keeping up to date with he market prices on those devices so I can't help much where to best find them.
    You're perfectly right about that wattage rating being overrated. Either it's a marketing hype or a typo, but it could still be be ok for your use.
    Fuses can usually carry twice the rated current for 20 seconds before blowing though, maybe that's where they got their number from..
    Electric drills and vacuum cleaners have brush motors so if it can run those it can run your saw. Brush motors are not very inductive.
    I'd put a thermometer probe at the cooling outlet vent to keep an eye on the temp's but I'm not sure what would be the limit. Around 80 - 100 deg. C maybe?
     
  5. Mark56

    Mark56

    27
    0
    Jun 15, 2011
    Ok, thanks for the comments. I'm purchasing the controller and will see how it goes, the hardest part is trying to find a company that will ship a steel cutting blade to Spain but that's not a problem for this forum.
     
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