Brushless motor hard to turn with servo cable connected

Discussion in '3 Axis Gimbal' started by Peer Gopfrich, Oct 29, 2013.

  1. Peer Gopfrich

    Peer Gopfrich New Member

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    I recently bought three GBM8108 motors. Two of them work flawlessly, the third one has an odd problem: Whenever I connect it to my Alexmos controller board, the motor becomes very hard to turn, i.e. there is a lot of resistance. Both when the board is powered on and when its without power.

    Performance-wise, this makes the motor very slow to respond. I'm trying to use it on the pitch axis, where it stutters and struggles to follow my moves. Every movement is delayed and slower than it should be, as the motor fights against some internal resistance.

    As soon as I disconnect the cable from the board (leaving the other end plugged into the motor itself), the resistance goes back to normal, and I can easily turn it.

    I've tried hooking the motor up to all the other connectors on the board (X, Y, Z axis) and nothing changes. It also makes no difference whether the board is powered or not. I then used different cables from my other motors. Same result.

    Any idea what's going on here? I guess the magnetic resistance of the motor increases when something is connected to the servo cable, but I don't know enough about the technology to really make more than an uneducated guess.
     
    Brad Meier likes this.
  2. Tim Joy

    Tim Joy Active Member

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    If you have a multi-meter, check the impedance between the motor lead pairs (1-2, 2-3,1-3). They should all be the same. If not, then the motor is wound wrong or there's a short somewhere.
     
  3. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Apextwin:

    Welcome to the forum.

    If it's not too much trouble, would you be kind enough to change your user name to your real first name and last name, please? The reasons for this (and how to do it) are explained here: http://forum.freeflysystems.com/index.php?threads/real-names.497/

    If you connect the wires of a brushless motor it turns into a generator that then produces current to oppose the motion you're trying to induce, so Tim is correct.

    Thanks
    Andy
     
  4. Peer Gopfrich

    Peer Gopfrich New Member

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    Thanks for the reply, Jim. Unfortunately I don't have access to a multi-meter, so I can't check the impedance. But RCtimer has agreed to let me return the motor and get it replaced. They'll let me know what kind of defect it has. I'll update this thread once I hear back from them.

    And sorry about the username snafu, Andy. I changed it to the correct format.
     
  5. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Not a problem, Peer!
    Do let us know if the replacement motor has the same symptoms or not....

    Andy.
     

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