BLDC motor encoder giving wrong pulses when motor is loaded

Discussion in 'The Projects Forum' started by student_1_2014, Jan 2, 2014.

  1. student_1_2014

    Thread Starter New Member

    Jan 2, 2014
    I am working on guided navigation and the motor used in my project is BLDC motor (24V, 300rpm) with Quadrature encoders.
    Now i am experiencing some problem with the encoder data. It is giving proper pulses when the motor is not loaded and also when the motor is manually roated. But it is producing wrong pulses when the motor is being loaded.

    When the motor is actually rotating in the clockwise direction the data from the encoders are showing that it is rotating in the counterclockwise direction.
    Also the speed calculated is much higher than the actual value. This happens only when the motor is being loaded, otherwise the calculated data is proper.

    What can be the possible reason for this behavior and how can it be rectified?
  2. wangjian

    New Member

    Jan 3, 2014
    Wide variety of brushless DC motors for different types of applications. Product offering includes Instrument Grade, Automation Grade, Slotless Brushless, and High Speed Slotless Brushless DC motors.
  3. MaxHeadRoom


    Jul 18, 2013
    Is this a industrial servo type motor or model aero motor?
    If the former there are different types or outputs, from single ended, open collector to RS485 differential 24vdc - 5vTTL, you need to specify the type you have as you may have the wrong interface, there are also quadrature sine wave types that have very a low output signal level.
    Modern BLDC motors also have the 3 commutation pulses derived from tracks on the encoder.
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
    What are you using to determine if the pulses are "correct" or not? Is your encoder cable shielded? Do you have it grounded properly?

    My half baked theory as of now, is that when the motor is loaded, pulling more amps, it creates more EMI (more noise on the line) that is confusing whatever instrument or drive you are using to determine "correct" rotation. unloaded, it creates less EMI, and the mystery go-no-go device is able to differentiate noise from actual pulses.
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Strantor's hypothesis sounds good to me too. :)

    One other possibility is that the BLDC driver when loaded at 300 RPM is producing current pulses in the windings which is causing rotational resonance (as occurs in stepper motors).

    That shaft resonance has components at a much higher speed than the 300 RPM rotation, and might be pushing the encoder decoder past its sampling top speed limit so it gets a wrong count (or counts backward).
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
    how about the shaft moving under load? in and out of the motor? might cause misalignment of the encoder.