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Breaking of Universal Motor

Discussion in 'Electrical Engineering' started by Rasmus Solmer Eriksen, Sep 18, 2008.

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  1. Hi Everyone



    I am working on a project where I am utilizing a motor form a normal power
    drill to drive a lift up and down I choose the drill motor because it comes
    complete with gearbox, is reversible and cheap.



    However, I now experience too much free run after I have released the
    up/down button. Is there a good way to break such a standard "Universal
    motor"?



    Thank you

    Rasmus S. Eriksen

    Copenhagen, DK
     
  2. Palindrome

    Palindrome Guest

    I think that word that you need to search on is "brake" rather than
    "break".

    However, have you tried shorting the motor out - rather than leaving it
    open circuit, in the "off" position?

    Essentially, you need to absorb the kinetic energy by dumping it into
    something. There is a wide range of small electromagnetic brakes available -

    Try http://uk.rs-online.com/web/ and search on "electromagnetic brake",
    for examples.

    IIUC, they have a branch in Copenhagen..
     
  3. You mean "brake", as in slow down, not "break" as in damage.

    The problem I can see with a series universal motor is that it will make
    a lousy generator. Can you find a drill that uses a permanent magnet DC
    motor instead? Portable (battery powered) drills all seem to use PMDC
    motors, and I've seen AC line-powered routers with PMDC motors, so
    perhaps there are line-powered drills with such motors too.

    The advantage of a permanent magnet motor is that it makes an excellent
    generator, and if you connect the motor terminals to a low-valued
    resistor you get a nice braking torque that's proportional to current.
    If the load is a resistor, current and torque should be proportional to
    shaft speed.

    Dave
     
  4. Yes, I am looking for a electrical solution. The problem with utilizing
    DC-breaking is the non-existence of a magnetic stator field when the motor
    is not energized. Somehow I need to energize the stator coil and
    short-circuit the rotor coil to achieve braking.



    I have seen such a system working on a Bosh power drill but have been unable
    to reverse-engineer the solution.




    /Rasmus S. Eriksen
     
  5. daestrom

    daestrom Guest

    That's referred to by some as 'plugging' a motor. It used to be quite
    common in a lot of controllers for conveyors and that ilk. The trick is
    cutting off the power just as the shaft reaches a dead stop. Some old
    systems used simple switches that were driven by the shaft through a
    friction clutch. As soon as the shaft stopped, a spring would open the
    switch and kill the plugging circuit.

    daestrom
     
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