The car analogy only goes so far.
I did some work the effect of DCC pulse stretching back in 2004 or so, including making some measurements. I suspect not much has changed.
A motor under PWM from a DCC decoder is getting alternating positive and zero current. At zero speed, it¡¯s all zero current; at top speed, it¡¯s all positive current. In between, the pulses generate alternating positive and zero torque. (Except for some very old decoders, e.g. for decoders from after about 2002) this happens so fast that the motor rotational speed doesn¡¯t significantly change between those two. The mechanical inertia completely averages it out.
A motor that¡¯s using DCC pulse stretching is getting alternating positive and negative current. At zero speed, it¡¯s positive and negative alternating at 50% time. At top speed, it¡¯s almost al of one or other other. The DCC bit pattern isn¡¯t slower, so there can be a small build-up of rotational speed with DCC pulse, but it¡¯s not very much; in the 2004 measurements I saw motions of 0.002-0.003¡± at the outer diameter of some HO motors from Atlas and Athearn, nothing that would hurt anything or even be audible.
So what¡¯s the difference?
1) Note that the DCC pulse stretching has a _thermal_, not mechanical, load on the motor: It¡¯s always generating heat as if it was at full power. Maybe this is OK, maybe not. That depends on the size of motor, ability to dump heat, etc. I¡¯ve seen G scale locomotives with motor fans that might be in trouble because they¡¯re at full current but their fans aren¡¯t turning.
2) To add to (1), the DCC pulse-stretching current can be significantly higher at _zero_ speed because the motor isn¡¯t turning, hence not generating back EMF. (Not a problem for full speed running under either DC or DCC pulse stretching). Normally, as a motor runs faster, it¡¯s generator-effect generates voltage that cancels part of the drive voltage, which in turn reduces the current. People with Tortoise stall motors and LEDs in series circuit actually use this: While the Tortoise is moving, the motor is turning fast enough to reduce the current and LED is off/dim; once it stalls, the current goes up and the LED brightens. Many small, high-speed motors rely on this effect strongly, which might be why there¡¯s lore about ¡°small coreless motors¡± have trouble with DCC pulse stretching: They¡¯re at full stall current all that time.
One of the four motors I was experimenting with (not in locomotives at the time) burned out; one would run too hot to touch, and two were just hot.
3) There¡¯s an increase in magnetic stress on the windings, because the change in current is a factor of 2 more (from +V to -V instead of +V to 0). The energy goes as the current squared, so this is a bigger issue in the coreless motor case, where there¡¯s extra current.
It¡¯s that last point that causes the whine that some people can hear. (Even back then, I couldn¡¯t, but my kids could) The stress on the windings, or perhaps the stress on the core, can cause enough motion to be audible. The difference between this and the torque effect is detectible: The back-and-forth of the torque is at the drive frequency with only odd harmonics, but the magnetic stress sound is at twice that with lots of even harmonics
The test motor that burned out had a significant magnetic vibration; it¡¯s coil might have been wound just a bit loose, and resulting mechanical motion dumped heat into it until the insulation melted. The one that ran hot also had a radial vibration (that¡¯s the one my kids could hear; they weren¡¯t present before the 1st one failed) of about 0.008¡±, which is a lot for such a small motor. The other two had no detectable vibration or sound.
It¡¯s possible that older, heavier, iron core motors would be fine with all this. It¡¯s certain that some others were not.
The reason I was looking at this back then was to understand its effect on track occupancy detection, which is a _nightmare_. An Athearn locomotive moving at about 1/3 speed threw enough HF (harmonic) current into my layout of the time to trigger about half a dozen occupancy detectors in other sections. (Decoders do a better job of keeping the very high frequencies off the rails). Basically, it became a little radio modulator. My wiring wasn¡¯t perfect, but it wasn¡¯t bad, and I wasn¡¯t going to redo it.
Bottom line: DCC pulse-stretching was just another of the many Bad Ideas that tossed the NMRA into the pot when DCC was young. Mostly, we¡¯ve gotten past them now.
Bob
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Bob Jacobsen
rgj1927@...
Bob Jacobsen