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Protection for a permanent magnet dc motor

Discussion in 'Electrical Engineering' started by Palindrome, Jan 20, 2007.

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  1. Palindrome

    Palindrome Guest


    I'm after suggestions for protecting a DC permanent magnet motor.

    The motor is used to drive a hydraulic swash plate pump for typically 30
    min -1 hour at a time. Mostly the motor is on light load when running,
    but it is intermittently loaded to higher than its continuous rating,
    for 5 mins or so.

    The failure mode is overheat of the rotor winding - but it isn't going
    to be easy to sense the temperature of the winding directly.

    I've been thinking of:

    1) putting a small value resistor in series with it and bonding a
    temperature sensor to that - tripping the supply if it gets too hot.

    2) putting a temperature sensor in the airflow from the motor's cooling fan.

    3) sensing the motor current and feeding that into an integrator and
    setting a trip level on the integrator output.

    Any thoughts? The motor only has a fuse* at the moment, plus a nose to
    sense that it is "smelling a bit hot"..

    * A fuse with the correct i2t curve would do nicely, perhaps. The
    present one, fitted by the system manufacturer, is a quickblow one that
    doesn't even blow if the motor stalls..
  2. John G

    John G Guest

    You did not tell us how big this motor is.

    A properly rated magnetic/thermal breaker should do what you want I

    A temperature sensor in the air flow will most likely fail to react if
    the motor stalls.

    Surely a quickblow fuse must be too big if it does not blow on startup
    and a closer rated slowblo fuse would seem more appropriate.
  3. A self-heating thermister can solve that -- no airflow and it will
    heat up anyway.
  4. This is the basic principle behind motor overload protection. The
    resistor needs to be sized (both in terms of resistance, thermal mass
    and dissipation) to mimic the motor's thermal time constant. The temp
    sensor is usually as simple as a bimetal switch that opens at a
    calculated temp. and de-energizes the control circuit.
    Fuses and circuit breakers are usually sized to protect against faults
    in the branch circuit or motor winding. There may be a slow-blow fuse
    suitable to what you need, but that will require some knowledge of the
    motor's maximum time-current characteristics.
  5. Bud--

    Bud-- Guest

    In the US, AC motors typically use an overload unit. The mechanical
    units have thermal heaters wired in series with the motor and sized to
    the motor current. They cause a switch to open when they get hot enough.
    I presume the UK has equivalent units which you have probably used. One
    advantage is overload units are built to match the thermal
    characteristics of a motor. Fuses, circuit breakers and series resistors
    probably would not be a good match. Overload units also have many heater
    sizes to match the motor current - more variety than fuses.
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