Back-EMF and DCC

[Railway42]

I have noticed a lot of talk on different forums about back-EMF being turned on and off, so I thought I might try to explain that this is impossible!
Back-EMF or electromotive force in its full name is the product of current flowing through a conductor in a magnetic field that produces an opposite voltage to the applied voltage. http://en.wikipedia.org/wiki/Lenz's_Law < Link To Lenz's Law
As can be seen back-EMF cannot be switched on and off. What can be done is to use a clever circuit and measure the back-EMF opposite voltage and then apply this as feedback to the control circuit to adjust the running voltage of the motor. This both assists superb slow running and provides proportional load compensation to the motor drive. Back-EMF feedback features do vary brand by brand however back-EMF feedback operates by sensing load and it then compensates for variances by adjusting power provided to the motor under varying conditions. NO other method of control can match a quality back-EMF feedback decoder for running quality.
With back-EMF feedback enabled, locomotive decoders sense the rotational speed and current needs of the motor and automatically adjust future digital pulses to the motor to maintain a desired speed by increasing or decreasing power available to the motor very smoothly.
Back-EMF feedback can be tuned to increase or moderate its effect and even to allow it to gently turn off as speed increases. It is a VERY clever feature and highly desirable.
The first LGB MTS loco mouse and decoder 55020 made by Lenz had no back-EMF feedback. It was not until Massoth came onto the market with their decoder 55021 that back-EMF feedback became popular. Decoder 55021 back-EMF feedback is CV54, this feedback can be turned on and off but not the back-EMF.

The textbook defines:
When current is flowing through the armature conductor of dc motor, it starts rotating inside the magnetic field of stator pole. As the armature conductors move inside the magnetic field there will be an induced EMF in the armature conductor. This EMF would be due to Faraday law of electromagnetic induction. The direction of the induced EMF will be as per Fleming right hand rule and the induced EMF will oppose the applied voltage across the armature terminals. In other words in dc motor the rotating armature will generate an EMF as it is rotating under magnetic field, in opposite of the applied voltage and this generated EMF in dc motor is called back EMF E[sub:1nbez59x]b[/sub:1nbez59x].
Applied voltage V has to drive armature current I[sub:1nbez59x]a[/sub:1nbez59x] against the counter or back EMF E[sub:1nbez59x]b[/sub:1nbez59x].
Hence, armature current I[sub:1nbez59x]a[/sub:1nbez59x] = net voltage / armature resistance = [V ? E[sub:1nbez59x]b[/sub:1nbez59x] ]/ R[sub:1nbez59x]a[/sub:1nbez59x], where R[sub:1nbez59x]a[/sub:1nbez59x] is the resistance of armature circuit.

Railway42

 

[daveyb]

right,, stop that,,,, and i thaught it was hard enought trying to understand dcc,,,,

and now i have to sit in a darkened shed for half an hour to let my brain cool down....

but very informative,, if a bit physicsy towards the end,,,

nice one,,

 

[bobg]

Good explanation, even I understood! (takes some doing)

Many, many years ago when "Oi were but a lad" and we played with slot cars, a firm brought out a new hand-controller with an extra terminal which connected the motor terminals in dead short when the lever was released and the feed removed. This had the effect of slowing the motor much quicker as the generated current (induced EMF) was reapplied back through the motor (trying to make it go the other way). Made one 'ell of a difference to lap times. Now doing what you show above is a REAL clever use of the phenomena!

 

[ntpntpntp]

Yeah, it's a terminology problem with decoder manuals and some folk referring to "back-EMF" when really they mean "sensing of back-EMF" for the purposes of a negative feedback loop to create constant speed or load compensation effects.

More expensive analogue DC controllers have had such capabilities for decades (I used to love the old "CompSpeed Rambler" push-button controller with its coarse pulse-power and feedback.) Massoth may have helped make load-compensation popular for G scale but of course other DCC decoders had this feature earlier.

 

[Cliff George]

Good explanation.

Back-EMF is always present, DCC decoders can use the Back-EMF to implement load compensation. The decoders use of Back-EMF may be controlled by CV, including telling it to do nothing with the information.

 

[ntpntpntp]

By the way, I do feel that Railway42 really should acknowledge that most of the text in the first part of the explanation has been lifted/borrowed from DCC Concepts' web site. It's good stuff and should be credited (assuming DCC Concepts didn't borrow it from somewhere else!)

http://www.dccconcepts.com/index_files/DCCterminology.htm < Link To http://www.dccconcepts.co...les/DCCterminology.htm

 

[Railway42]

Yes I did use that part as I thought it was a reasonable explanation but it did say back-EMF. And this not what controls the motor it is the feedback of the oppose voltage back-EMF that controls the motor and to do this you have to stop driving the motor this is done in DCC between the pulses. When the motor speeds up it produces more back-EMF. (Going downhill) when the motor slows down there is less back-EMF. (Going uphill) and when stopped or stalled no back-EMF. So as I said it is the feedback that is the clever part. If you don?t have the feedback the motor still produces back-EMF and if uncontrolled can and will burnout a motor if stalled for sum time. Also in a lot of circuits they are protected from the back-EMF by a diode in reveres of the voltage.
I apologize if I offended anyone.
Railway42

 

[whatlep]

Excellent summary. Thank you.

 

[alec dawe]

WHY would anyone be offended??