B-EMF Is my understanding correct?

QUOTE (John Webb @ 22 Feb 2008, 07:45) <{POST_SNAPBACK}>Additional to 34C's comments, decoders usually power the motor by some form of pulsed voltage. Hence between each pulse of applied power there is a period when the back-EMF caused by the still-rotating motor can be sensed to give the required feed-back. There have been analogue controllers which use this principle available for many years.

Regards,
John Webb

*** I hope you didn't bet money on it Martin

Back EMF has everything to do with motor control!!

You ask what DCC manufacturers mean when they use the term back EMF. Well, they really ARE referring to regulation and control of the motor torque by regulating its action against the motors sensed back EMF.

Forget the marketing terms from brands like Digitrax such as cruise control - they do both DCC and the benefits of proper back EMF a big dis-service by just encouraging misunderstanding.

If Back EMF played no part in motor control every item that needs good speed regulation would not work - from your CD player to your computer drives to many other things.... and older things like hi-fi turntables and tape recorders would never have given good music . Back EMF as related to coils and motors is part of the law of conservation of energy and as its always a directly proportional thing. It is absolutely critical to motor control in many, many applications.

You say forget the fields BUT you simply cannot do that and have this discussion. There is NEVER a constant current, back EMF does not stop at the brushes and every revolution one part of the motor - and that is the one that generates the back EMF - is NOT in contact with a brush anyway!

Back EMF is a voltage created by the collapse of the magnetic field as each "odd" pole in a DC motor (which always have odd # of poles) is freed of brush contact.... or by field fluctuations in coreless/brushless motors. Back EMF circuitry senses this and applies an appropriately calculated VA adjustment - but it will never simply be a current adjustment, so you are a wee bit off there too!!

Look up Lenz's Law on google in relation to conservation of energy - Coincidentally its named is same as a DCC Mfr, but Mr Lenz is no relation (it was named around 1900). Iit explains it quite well in pure theory.

Back EMF is the voltage produced across a winding of a motor due to the winding turns being cut by a magnetic field while the motor is operating. This voltage is directly proportional to rotor velocity and is opposite in polarity to the applied voltage. Sometimes referred to as counter EMF

Back EMF is, as John and 34C mentioned, very easily sampled - even without a pulsed supply which all decoders and back emF capable DC controllers use, a DC motor has an odd number of poles and only two brushes in contact so for example the third motor coil / pole in a 3 pole motor is in fact regularly creating/collapsing its field and generating back EMF.

On the right instruments this makes measurements taken at the point it happens (at the brush) look like a pulse even when the supply is a relatively pure DC. So - even without the presence of a pulsed supply a little added sophistiction can take advantage of this, as although brushes are in constant contact they do not regulate current or any other factor in either direction and with a part of the motor always being a back EMF generator there is always a changing field effect generated by the motor that can be sensed by appropriate circuitry.

A sophisticated decoder generally has three separate approaches that can be adjusted - ie degree of compensation, speed or attack of compensation and rate / proportion of compensation is applied dependent on loco speed (ie as speed increases, energy used in back EMF compensation is reduced). Its generally not well explained in literature or well understood by modellers and as its sort of a 3 dimensional adjustment option it can be can be awkward to adjust if you are not methodical. Lenz attempted to simplify it with their 6 step adjustment but blew it, and their decoders are therefore good but not as good at ultimate slow running as those brands which allow a full range of adjustment on all back EMF settings.

Kind regards

Richard

QUOTE (Martin71 @ 21 Feb 2008, 15:33) <{POST_SNAPBACK}>Lets say you have a motor, ac or dc doesnt matter, and you apply 16v to run it. The B-EMF that come back throught the armature (dc) or rotor (ac) is always less that 16v say 15.8 v for the B-EMF. And I know B-EMF stops at the brushs in the motor. So how does the decoder know about this B-EMF is there is no way of it getting to the decoder?
Strictly speaking you are correct in that BEMF stops at the brushes, but BEMF is nothing more than the voltage (the EMF) generated by the motor being turned and acting like a generator. The "laws of physics" prevent the generated voltage being greater than the applied voltage that is making the motor turn. If you remove the drive voltage, then the inertia of the armature keeps the motor turning, still acting as a generator, and the generated voltage can now be measured.

If the load on the motor increases (e.g. going up an incline) then it slows down, the BEMF reduces and the current increases. The decoder senses the reduced BEMF (not the increased current) and compensate for the extra load.

Andrew