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DCC Track Power 3

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Bundesbahn View Drop Down
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    Posted: 07 May 2013 at 9:55am
Please, read topics DCC Track Power 1 and 2 first.

Once the decoder receives the ~ 10kHz encoded AC signal it will do the following. Here I am utilizing text provided by my Buddy BR42.
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The decoder rectifies this signal by sending it through a bridge rectifier circuit to get a smooth DC power supply. The decoder then uses the data and the rectified voltage to produce a pulsating DC voltage for the motor.
The more DC pulses the decoder provides, the faster the locomotive will go. The decoder does not supply the motor with straight DC at changing voltage because this would make the decoder too hot. Moreover, the DC motor voltage is about 2V less than the original AC track voltage due to the voltage drop in the electronics contained on the decoder.
With a 16V track voltage, you get about 14V applied to the motor, at the fastest speed setting. If the pulse times are so that the voltage is off 50% of the time, you get about 7V effective, at 30% about 10V effective.
Most standard decoders provide something like 100 - 200 pulses per second, which may cause some rattle and vibration at low speed. This also kills coreless motors, such as Faulhabers (can motors e.g.), by overheating them. Silent running decoders provide about 15,000 pulses per second, thus preventing overheating of coreless motors.
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Older, pre-digital era, Maerklin locomotives ran on AC, something that cannot happen in the DCC world. These old motors need to be modified or replaced. This is a different chapter in the book, altogether.

Obviously, the decoder will also handle all other functions, such as Lights off/on, the whistles and bells, steam puffing etc etc, depending on the features provided by a given decoder. Those signals are generated by the control station, at your command, and then interpreted and put into action by the decoder.

With all these complex processes going on, you may understand that it is highly adviseable to keep your tracks clean to ensure smooth, uninterrupted operation.

This 3-part presentation is a BR42 and ChooChoo co-production. We hope this will answer some questions you were always afraid to ask.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BR42 Quote  Post ReplyReply Direct Link To This Post Posted: 07 May 2013 at 1:32pm
Hello Everyone:

Here is a little bit more info about AC-motors.  There are two ways to run them with a decoder. The easiest is to replace the electromagnet which produces the magnetic field by a permanent magnet, thus turning the motor into a DC-motor.  The other is to use a decoder which, in addition to the power for the rotor, will also provides constant power for the electromagnet.  The decoder will switch the polarity of the current through the electromagnet (by using the diferent windings of the fieldmagnet) instead of the polarity of the current through the rotor.  Basically, the decoder has an auxiliary output for the field magnet.  This option is used for motors for which no permanent magnets are available.

Ulrich
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RandyRail Quote  Post ReplyReply Direct Link To This Post Posted: 10 Jul 2013 at 12:53pm
Ulrich,

  You do not have to have a magnet on a DC motor. You can run a AC motor with a commentator and field coil on DC, by putting a full wave rectifier attached only to the field coil only. Then the motor will run fine even respond to a reversal of current to change direction. I have seen and operated Lionel locomotives on DC wired that way. (on an old O scale outside 3rd rail railroad)  
    I had MTH O scale locomotives with CAN motors in them. the decoders in them even ran the engines on regular AC variable voltage controls. 


Randy Knaub
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tristan Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 2013 at 1:23pm
Randy,
 
Yes, on an AC motor with a split field all you need is two diodes to run it on DC, but is it the best solution?
 
The reality is that the only advantage a wound field motor has over a permanent magnet motor is that it coasts for a greater distance due to the loss of the field when power is removed.
 
A series connected wound field motor does not allow back-emf motor regulation and is generally less powerful than a permanent magnet motor. I have had good experience playing both ways with older Marklin motors.
 
At low speeds the field is weak, and ultimately the loco draws a higher current. I believe that Uhlenbrock once offered a decoder that would separately excite the field and allow back EMF control.
 
The "Hamo" (and later Trix) versions of traditional AC locos from Marklin use a permanent magnet pole piece that is relatively weak, probably to give similar performance to the wound field motors, as all things being equal, the stronger the field, the slower the motor will run.
 
Tristan
Certified equal opportunity train nut: All countries and all gauges.
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