I have built a PWM speed control motor for brushless DC motor.
But I found that there are some hear-able noise from the motor.
If the speed adjust to more slow, the electricity noise is more
and in lower frequency noise i hear. So, is there any methods to
solve it? My clock frequency generated are within 100Hz.
What can I do to improve such low frequency noise?
Thank you for helping.
Hello
From your original posting and your reply to some of the responses, I
think you are confusing PWM control with "commutation".
In a conventional, brushed DC motor, you can simply apply a drive
voltage/current to the motor and the armature will rotate, without you
having to worry much about what is happening inside the motor. What
is actually happening however is that, as the armature rotates, the
commutator switches between various coils in the motor to cause the
magnetic field to rotate in a manner that sustains torque on the
armature - if this didn't happen, the armature would simply align
itself with the coil that was being driven and park there.
You say that yours is a brushless DC motor, in which case *you* have
to sort out the necessary switching between windings to cause the
magnetic field to rotate. To achieve this, you need some kind of
sensor/encoder that "tells" your commutation electronics the
rotational position of the armature at any given moment and enables
the controling electronics to drive the various coils accordingly.
Depending on the rigours of your application, this control can be
anything from simple ON/OFF switching of the various coils to
progressive modulation of coil currents based on rotor angular
position and motor demand.
PWM is simply a relatively efficient means of modulating motor demand
and will usually be done at a frequency high enough for its effect not
to be audible, (and, incidently, high enough that there is relatively
small change in coil current on a cycle-by-cycle basis, i.e. coil
current can notionally be considered to be DC during any one cycle).
The nature of the commutation encoder you require will depend on the
number of windings/pole pairs in your motor and on the torque ripple
that you can tolerate at the output shaft. Depending on its
resolution, you may well be able to use the commutation encoder to
achieve speed control, in conjunction with PWM control of motor
demand.
If you need more assistance, I think you need to provide more detail
of what you are actually doing, (i.e. details of your circuit), as
well as details of what you are *trying* to do.
Hope some of this helps/makes sense
Regards
Ted
