pwm with DC motor

pusink

When i do PWM control...i hear noise from my dc motor. the noise is become louder when the motor speed is to slow...how to reduce that...tq.·

Franklin

You could build a filter circuit that would smooth out the pulses of PWM

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- Stephen

pusink

any example circuit????

Franklin

I would have to Google something up for you but you could do the same.

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- Stephen

Peter KG6LSE

A pi circuit is a option but making one that can handle lots O Amps is no cake walk ... but a simple large Cap accross the motor should smooth things out for you too... 10- 1000 uF depending on the size of the motor...
Peter KG6LSE

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Beau Schwabe

pusink,

This may help ... www.teamdelta.com/pdf/tde_an1.pdf

For a motor, typically you don't want the filter to be part of the series circuit that's powering the motor.
Instead design your filter so that it's parallel to your motor.

In the article above, I generally agree with what it's trying to express, however I disagree to the reference where it says that noise suppression is a 'black-art". What you are doing is basically using the coil of the motor itself as part of an LC tank where the resonant frequency is determined by the inductance of the motor and the value of the capacitor(s). A tank circuit such as this becomes high impedance only at the resonant frequency, thus acting as a notch filter to attenuate any frequencies above or below this resonant frequency. Keep in mind, as the motor spins the effective inductance will change proportionally to the speed and in turn change the resonant frequency.


If your noise is a mechanical issue, the best solution that I have encountered requires that you monitor the speed of the motor and dynamically adjust your PWM base frequency to match the motor speed times the number of motor poles.

For example if your motor is running at 2000RPM and has 5 poles(<-coils), then the PWM base frequency should be a multiple of 166Hz

2000RPM / 60 = 33 RPS or 33 Hz

33Hz X 5 Poles = 166Hz

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 12/23/2009 9:02:17 PM GMT

stamptrol

A side issue with motors on PWM is that the motor normally is noisier than if running on pure DC. So, some noise is normal, varying in pitch with the effective base frequency of the PWM signal and motor speed.

Also, i assume you're using a PWM module meant for motor driving, not the PWM command from within the Stamp program.

With a PWM motor driving module, you may have an option to change the chopping frequency and perhaps minimize the mechanical sound from the motor.

Cheers,

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Tom Sisk

http://www.siskconsult.com
·

erco

You can vary, but never eliminate motor noise using PWM. A PWM signal is pulsating DC similar to a high-current audio signal and your motor is acting like an inefficient speaker. Your motor (also your geartrain and even your robot enclosure to some degree) will have several resonant frequencies, based on its physical characteristics. If your PWM signal frequency is anywhere close to a motor resonant frequency, it will be louder.

The PWM signal base frequency is constant but the duty cycle changes to effect different speeds. You can vary the base frequency to try to reduce the noise by staying away from resonance.

There is no comparable sound from a dc power signal, since there is no audio component. Lower dc voltage means lower speed, power & torque. Filtering a PWM signal with parallel caps and/or series chokes can reduce the noise by reducing the pulsating DC component, but there maybe a corresponding loss of torque & power, since you are effectively raising the steady DC level and reducing the pulsating DC component.

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·"If you build it, they will come."