Brushless DC (and PMAC) motors - Root cause behind commutation "tick" and methods to handle it

Discussion in 'General Electronics Chat' started by andrewcutting, Jul 23, 2017.

  1. andrewcutting

    Thread Starter New Member

    Jul 23, 2017
    2
    0
    Hello, this is a general question in relation to commutation of 3 phase permanent magnet brushless motors.

    When commutating using the traditional 6 step square wave method, an acoustic "tick" is heard once for every AC cycle, eg at half the pole count of the motor. For example a 22 pole motor will create an audible tick at a regular frequency 11 times per motor rotation. The volume of the tick increases with voltage/current.

    I believe it is to do with the rapidly increasing current caused by the square wave creating a mechanical movement in the winding. Is this correct ? If not can somebody explain please the root cause for the noise ?

    What approaches can be used to minimise it's size/volume ?

    Related to this issue, I have an application whereby I am commutating using a variety of sine and space vector wave commutation patterns, and find that on some occasions the motor will commutate completely silently, but then later under the same circumstances begin to "tick" intermittently (not necessarily every AC cycle). The intermittent "ticks" occur at the same frequency as the above 6 step commutation, however the output is a simulated sine wave running at 42kHz PWM outputting 1,000 duty cycle steps per second. It would seem that the ticks are common to both commutation types, albeit with the sine drive they on occasion disappear completely which is what I'm hoping to achieve with the sine driven system.

    Any assistance on ways to reduce or eliminate the ticking on either 6 step or sinusoidal drive would be greatly appreciated.

    Thanks
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    18,249
    5,605
    I have been using BLDC motor for quite a few decades and never ever heard a 'tick'.
    And 3 phase sinusoidal you should definitely not hear it IMO.
    Although at 22 pole count it has to be a large motor such as a Fischer-Paykel variety?
    Max..
     
  3. Sensacell

    Moderator

    Jun 19, 2012
    2,344
    752
    Most of the sound comes from "magnetostriction" of the laminations, very hard to suppress.

    Making the change in magnetic flux as smooth as possible is your best option.
    Heavy varnish on the windings too.
     
  4. andrewcutting

    Thread Starter New Member

    Jul 23, 2017
    2
    0
    Thanks Max, sorry I should have mentioned that I'm dealing with quite low speeds, a speed range of 1 > 400 RPM on a 24 pole motor typically, so the individual ticks are still audible. With traditional low pole count BLDC motors running 300+ RPM minimum you wouldn't hear the individual ticks, it just becomes a buzz.

    It occurs on every motor type I've tried to varying degrees, outrunner BLDC, inrunner BLDC, 3 phase steppers, gimbal outrunner, 8 pole fan motor. The 22 pole count motor I have in front of me is a small camera gimbal style, with permanent magnets on the outer spinning rotor. On some occasions using sine output it's completely silent, other times the electrical tick returns. The same can be said of any PMAC motor type, so I'd like to get to the underlying cause, although the physics involved is probably way above my head *8)
     
Loading...