I've been going through some old MR articles on throttle design. Regarding the use of pure Pulse Width Modulation (PWM) DC for locomotive power they all advise to avoid it because, 1. the pulses cause large current spikes that cause motor over heating, 2. the current spikes will cause excessive wear to the motor brushes, 3. the pulsed power causes excessive mechanical wear. I've been using a Snail controller, which is pure PWM, for a number of years now alongside a Marklin DC throttle. I've seen none of the issues mentioned above. I was wondering if anyone here has experience these problems in Z? Or perhaps it's just an issue with larger scales? Or the motors of 40 years ago? Or maybe it's just a model railroading urban legend? Mark
Not from 40 years ago, maybe just 10, using the most advanced (coreless) motors (as well as the antiquated open frame DC motors in the F7)... MTL says don't do dat. But its your railroad, enjoy!
Looking at sumgai's post, I should add that my N Scale is all pre-2005, and mostly from the '80s and '90s. No DCC.
Just like food, alcohol and California's Prop 65 Cancer warning. Folks, don't worry about. Do honestly think that all the DCC manufacturers are selling decoders that use PWM for YEARS would either be out of business from all the failures or have manufacturers barring them from selling to their products. Please, stop worrying about it. The issue is when motors stall, creating higher power (due to the typically higher voltage used on many DCC supplies0.
Thanks all for the response. I guess in this case since there were no posts of locos bursting into flames or melted shells, that nobody has problems with PWM. The potential for overheating does exist using a pulsed power source, but I haven't seen it and apparently nobody else. I was aware of the MTL warning: it goes all the way back to the first units sold under Kadee. Perhaps the warning comes from Mr. "K" or Mr. "D" reading the articles I found. Mark
Do they warn against PWM, or Pulse power? The attached sheet warns against Pulse Power, not PWM. Pulse Power and PWM are NOT the same thing. Pulse power adds pulses to the DC, usually by allowing some of the AC waveform to pass through to the DC. Pulse power is usually a low 60 hz and tends to have very high peak voltages, two things that contribute to motor heating. Pulse Width Modulation (PWM) usually uses a much higher frequency and the peak voltage is much lower.
Neither pulse nor PWM will hurt anything as long as you...you know...don't do things like watch your locomotive stall and then turn the throttle all the way up, trying to get it to go. I think a lot of damage is done by people who don't understand electrical principles and then the blame is placed upon the technology. Doug
I went back and reread the articles: MR Mar 69 and Apr 88. While they don't use the term PWM, the pulse they describe: DC only rail-to-rail swing, fixed switching frequency (about 50-100 Hz) and variable pulse width, seems like PWM to me. The proposal from these articles that interested me was the idea that pulsed power was bad, except for acceleration and deceleration, when it's good. The proposal of the articles is a throttle that produces a basement DC voltage, increasing with throttle superimposed with a rail-to-rail PWM decaying pulse chain that is controlled by the changes in throttle. Mark
