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Author Topic: Thread about Pulse Width Control Deleted?  (Read 11035 times)
« Reply #15 on: February 19, 2011, 09:52:23 PM »

Decoder signal to the motor.
Zero throttle. Zero volts.

Forward, half throttle.

Reverse, minimum throttle.

DCC signal as applied to a DC motor when using stretched zero bit option. This causes the motor to rotate fwd and rev in very small amounts.


« Last Edit: February 19, 2011, 09:53:54 PM by richg » Logged
« Reply #16 on: February 19, 2011, 10:07:29 PM »

Download and study the photos. A picture is worth a least a thousand words.
A meter connected to the motor will read DC voltage when the motor is connected to a decoder. The 12 volt pulse gets wider for more speed. Even at speed step one on 128 mode, there is a 12 volt pulse but very narrow pulse width.

With a DC motor connected to the DCC signal/track voltage, you will see AC volts because of equal positive and negative signal transitions. As the positive or negative zero bit gets wider, you will start to see DC voltage on the motor but it will buzz because of the AC.

Some motors will buzz with older lower frequency decoders. Newer decoders now operate at a higher frequency. I have the frequency number somewhere.
My older SoundTraxx LC decoders could cause buzzing in some locos but the Tsunami do not, at least in my locos.

Jim Banner

Enjoying electric model railroading since 1950.

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« Reply #17 on: February 19, 2011, 10:07:52 PM »

If the following discussion is too technical and boring, please skip on down to the last paragraph.

Pulse Width Control, even though unipolar, is still a mixture of dc and ac.  The lower the throttle setting, the higher the frequencies of the ac component.  The higher the frequency, the better the motor inductance rejects it.  So the current flowing through the motor at low throttle settings is mostly dc.  As you advance the throttle, the ac component contains more lower frequencies.  At some point, these will try to rotate the motor's armature (not its stator.)  But unlike DCC, the current will rotate it in one direction only.  Cogging action, the magnetic attraction between the armature sections and the motor's field magnets, will pull the armature back again.  Past some critical throttle setting, the pulses will be wide enough to advance the armature in spite of the cogging action and the motor will begin to run.  Just below this point is the maximum current that can be run through the motor without it running.  If you compare this with the amount of pure, filtered dc that may be put through the same motor just before it starts to run, you will find that the motor consumes more power with the pure dc than it does with the PWC ac + dc.  And since in neither case is the motor dissipating heat by turning, it will heat up more with pure dc if left barely not running.

There is one exception.  That concerns coreless motors which have no iron in their armatures.  With no iron, the inductance of the armature is much lower.  With lower inductance, the motor is less able to reject the ac component of the PWC.  So the total ac + dc current that PWC can push through a coreless armature is higher.  Add in the reduced cooling of the armature windings when there is no iron to conduct the heat away and you find that coreless motors are not good candidates for running on PWC.  The only model locomotive with a coreless motor that I am aware of was one that LGB put out.  It started smoothly on dc because coreless motor have no cogging action but there were reports of it being damaged by PWC and by DCC.

Bottom line, we use PWC to start the motor turning at lower speeds than is possible with straight dc.  So if we apply straight dc, we can apply more of it before the motor starts than we can by applying pulses.  More power = more heat in the stopped motor.  


Growing older is mandatory but growing up is optional.

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« Reply #18 on: February 19, 2011, 11:28:44 PM »

I'M going to read these post a few more times and get it clear in my head . As I have a few more questions I need to ask . Sorry Jim I meant to say armature .

I'm running G scale DC locomotives , with no TE

thanks again Jim ,Rich , Hunt and AC

Regards: mark fuller  MF5117 RR

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« Reply #19 on: February 19, 2011, 11:53:40 PM »

That's what I was told Hunt but the way it was explained to me was that . OK I have 3 USA Locomotives . The Aristo-Craft Locomotives I have say use PWC ,they are DC Locomotives though, not DCC ready . I took The USA Trains Locomotive One of them to get sound installed in it . I had already bought the ARC 10 amp controller ,due to I had caboose's with lights and smoke one of the ArC Locomotives had sound and smoke, lights already . the Manuel said recommended PWC controller . So I bought the higher amped controller for the power to run smoke ,lights another engine etc. " consist" . My Bachmann Loco I run in the house .

The tech at the Hobby shop said I shouldn't use PWC on the USA or Bachmann Locomotive .As It wasn't good for the sound boards of the Locomotives . If they sit at an Idle the armature would sit and "as Jim said " cog back and forth . And eventually burn up the motors and even the boards . So I was thinking well hmmm guess I'll run one manufacturer's train with their controller and the other with theirs . So that's kinda where I'm at .
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