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BLDC motor load performance

  1. May 8, 2009 #1
    Hi,
    I am using Sensored BLDC motors for some robotic movements. I am driving my motors using motor drivers. I am using ball screw mechanism for lifting a pedal up and down. On the pedal, there will be always a very high load, placed on it.

    Now, my motor can lift up the pedal with my desired speed accurately but while going downward, the pedal just goes down freely overcoming the output speed of the motor.

    Picture1.jpg

    In summery I would like to say, when the motors are on downward rotation, they can’t hold the weight of the load and fail to conform to the desired downward velocity; instead they just let the pedal goes down freely until it reaches the lower threshold.


    If you have any questions please let me know

    thanks in advance
     
  2. jcsd
  3. May 8, 2009 #2
    If the motors were standard brushed dc motors, there are two ways they can be connected; series connected (stator in series with armature), and shunt connected (stator in parallel with armature). I believe connecting them in shunt mode, with a voltage applied to the stator and a load (or short) connected to armature, the motor will act as a brake.

    If the motor is a brushless dc motor (BLDC) with a permanent magnet rotor, then shorting the stator MAY cause the motor to act like a brake, depending on details of driver circuit. The cogging in BLDC motors may hold depending on load, but it may creep. The stator WILL act as a brake if the stator windings can be excited at a dc voltage, without commutation.
     
  4. May 9, 2009 #3
    Going down, your machine is acting as a generator. It will have retarding toque in proportion to the load seen at the electrical terminals. This means you need either a 4-quadrant drive for regenerative braking, or a dynamic brake circuit which essentially puts a resistor across the terminals, and the heating is the load. Short-circuiting the terminals makes braking torque during rotation, but it is not controllable.
     
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