How Does a Decoder Affect Motor Noise?

[gregc]

it looks like Digitrax adds a CV to adjust the PWM frequency (see article).

Since things tend to vibrate are certain frequencies, I'm not sure if they intend to adjust it above the human audible range or simply change it to something other than the natural frequency of the motor that it might vibrate at.

[richg1998]

This is from the link I posted not to long ago. Notice the difference in frequency.

Older decoders used a motor drive (PWM), pulse width modulation frequencies of about 30 to 200 Hz (cycles per second). PWM takes a constant like 12-18 v dc and drops to zero volts quickly and back to the higher voltage to get effective voltages between 0 and 18V. These frequencies were low enough to cause a hum or buzz in the locomotive. Most decoders today use a high frequency drive of 15kHz to as high as 43kHz. These frequencies are above the range of hearing and that is why they are called silent. This type of drive comes with many different names, depending on the manufacturer. One of the problems with the high frequency drive is slow starts. With these high frequency drives the motor tends to act like it is on dc power and is subject to stiction, where the motor does not start smoothly. The combination of back-EMF and the PID motor control can over come this problem.

[DigitalGriffin]

Much older decoders used to send out pulses with a frequency of I think ~14KHz.  This is well within our hearing range.  This created a buzzing at low speeds.

All newer decoders I believe send out a much higher frequency pulse which is well beyond our hearing range.

I don't think any low frequency pulse decoders are still being made.   I could be wrong about this....Maybe the bachmann DCC onboard decoders still use the low frequency pulse.  

[richg1998]

Explanation in the link.

http://tonystrains.com/dcc-motor-control-with-back-emf-and-p-i-d/

Rich

[mlehman]

DCC is a square waveform. Originally, this was just fed to the motor once rectified to DC. In some sense, the result was a lot like the old DC pulse power, which made similar chattering noises. Depending on the motor design, this can be noticeable or not so much. What the various "silent drive" features do is process the signal to eliminate the "chatter." With sound, this is more important now, as you don't want this very unprototypical noise interfering with the decoder's ability to create realistic sounds from the loco.

[KisNap]

Hi Everyone,

               While looking at decoders I notice that they may have features suce as "SuperSonic motor drive for silent operation."  How does the decoder affect the noise of the motor?  Isn't a motor a motor regardless of what's driving it?  Any input would be appreciated.  Thanks.