Large audible electrical noise on under speed-controlled DC motor

[Electronic Swear]

I am designing a prototype of speed controlled DC brushless motor.
The control method is using PWM and motor can be varying in high
and low speed. I haven't used any single chip driving IC but is
using discrete component to make the PWM control.

The problem is that there are a quite big audible electrical noise
from motor when operate. There are a very low noise when in full
speed. However, when I control it into a low speed. The audible
noise is so obvious and seem not acceptable. If the speed is lower,
noise is louder.

So, what should I care in order to reduce that audible electrical
noise? Thnak you very much.

 

[Fred Bloggs]

I am designing a prototype of speed controlled DC brushless motor.
The control method is using PWM and motor can be varying in high
and low speed. I haven't used any single chip driving IC but is
using discrete component to make the PWM control.

The problem is that there are a quite big audible electrical noise
from motor when operate. There are a very low noise when in full
speed. However, when I control it into a low speed. The audible
noise is so obvious and seem not acceptable. If the speed is lower,
noise is louder.

So, what should I care in order to reduce that audible electrical
noise? Thnak you very much.

Ear muffs?

 

[John Popelish]

I am designing a prototype of speed controlled DC brushless motor.
The control method is using PWM and motor can be varying in high
and low speed. I haven't used any single chip driving IC but is
using discrete component to make the PWM control.

The problem is that there are a quite big audible electrical noise
from motor when operate. There are a very low noise when in full
speed. However, when I control it into a low speed. The audible
noise is so obvious and seem not acceptable. If the speed is lower,
noise is louder.

So, what should I care in order to reduce that audible electrical
noise? Thnak you very much.

Raise the PWM frequency. If it is already in the kHz range, I would
suspect instability in the regulation that is causing a low frequency
oscillation in the duty cycle.

 

[Electronic Swear]

I want to know how to modify the circuit in order to eliminate the
electrical audible noise....

I am afraid that is the noise from the coil or stator back emf or
other induced current to product the noise.

Ear muff....not in my expectation.
Thanks~

 

[Tony Williams]

The problem is that there are a quite big audible electrical
noise from motor when operate. There are a very low noise when
in full speed. However, when I control it into a low speed. The
audible noise is so obvious and seem not acceptable. If the
speed is lower, noise is louder.

In a PWM system, loud noise at low duty-cycles (ie, low-load,
and/or low-speed), is a classic description of operation with
a Discontinuous Current.

That is, the current is falling to zero during each OFF-period.

As well as the loud noise, operation with discontinuous current
also makes the torque erratic..... which can cause trouble for a
closed-loop speed servo.

It happens when the L/R time-constant of the motor is not large
enough, compared to the PWM OFF-period.

The solution is to increase the PWM frequency, or to use an
external (dc) inductor in series with the motor, to increase
the effective L/R.

 

[Klaus Vestergaard Kragelund]

In a PWM system, loud noise at low duty-cycles (ie, low-load,
and/or low-speed), is a classic description of operation with
a Discontinuous Current.

That is, the current is falling to zero during each OFF-period.

As well as the loud noise, operation with discontinuous current
also makes the torque erratic..... which can cause trouble for a
closed-loop speed servo.

It happens when the L/R time-constant of the motor is not large
enough, compared to the PWM OFF-period.

The solution is to increase the PWM frequency, or to use an
external (dc) inductor in series with the motor, to increase
the effective L/R.

If this is not your problem, then you might add spread spectrum modulation
to the PWM frequency to lower the noise. You can atthieve 10dB with the
correct modulation.

In practice, add white noise to your PWM reload counter or in this case the
ref. voltage

This will also lower your conducted emmision noise if you choose the values
correctly (depending upon the bandwidth of the standard you test for - 9kHz
for the Test Receiver is common)

Cheers

Klaus