Dynamic Braking

[Modelcar]

....The statement from Randy: "some one with enough patience can make a big difference in some one else's life"....So true, in all phases of life.

[cherokee woman]

I printed this thread out. Will make me think, and I will learn from it.

[Randy Stahl]

QUOTE: Originally posted by CSXrules4eva

Thankx very much guys for the responces to my question, you guys explained more than my professor did. What's funny is when I ask him for help on certain topics, he tells me that it took him 35 years to understand electricty. Then I tell him it's going to take me 50 years to understand it, and I really do think this is true given the fact that I'm having tremendous trouble trying to understand everything involving Electricty and Electronics.

Remember that when YOU are teaching a new generation of railroaders, it may not come easy to everyone but some one with enough patience can make a big difference in some one elses life.
Randy

[CShaveRR]

Don't sell yourself short, Sarah--if I'd tried to follow that first question of yours I'd have been over my head right away! AMG

[CSXrules4eva]

Thankx very much guys for the responces to my question, you guys explained more than my professor did. What's funny is when I ask him for help on certain topics, he tells me that it took him 35 years to understand electricty. Then I tell him it's going to take me 50 years to understand it, and I really do think this is true given the fact that I'm having tremendous trouble trying to understand everything involving Electricty and Electronics.

[tree68]

Super simple explanation: You know that moving a wire through a magnetic field will generate electricity. Picture a permanent magnet on the rotor, spinning inside the windings. Each time the magnetic field cuts through a winding, it generates electricity.

The permanent magnet motors used in model railroad locomotives and toys reverse the principle - the magnet is fixed and the windings turn through the magnetic fields, attracting and repelling. If you spin a motor like that, you'll generate electricity.

All of the explanations offered so far are are variations on the theme. Instead of using permanent magnets, windings are excited by various methods, creating the magnetic fields. AC, DC, phasing, etc, are simply engineering.

[chad thomas]

Great explainations guys. Makes my explaination pretty weak.

I like the comparison to marquee lights. picture a circle of them and thats the ticket.

[zardoz]

Darn dash-2 and AC technology: those flashovers of the old units were always so much fun!

[Randy Stahl]

QUOTE: Originally posted by CSXrules4eva

Ok I've got to ask a question which has puzzeled me for a long time. since we're talking about generators. I asked my Electricity 110 professor and I still don't understand it I know that this type of generator consists of a rotating magnetic feild inside three sets of winds but, how in the heck is the magnetic feild going to rotate? I can picture the armature rotating but not the field itself. Also are these windings of a choke design?// I have no clue. I aso wonder if the word phase can be given to the point at which the amature is stationed. For instance at 90 degrees or 180 degrees? /

Sarah, you ask good questions. I have been a locomotive electrician for almost 20 years and I still have to think sometimes. I remember when I was just getting started these locomotives just didn't make sense, especially the high tech dash 2 stuff. One day it seemed like something just clicked and suddenly my eyes were opened to a whole new world. I don't want to get into a long winded electrical dissertation but I think if we step back in time a bit you may be able to make some sense out of it . Lets consder the EMD D-32 main generator. This was the last D.C. machine that EMD used in locomotives. If you look at this machine you will see a bunch of brushes and a big shiney commutator. It is this commutator that makes this a D.C. generator. Sarah, every electrical machine that rotates is essentially an alternator, on the D32 gen the fields are stationary and the armature is exited by the stationary fields, the brushes then pick the current off the commutator when the rotation is at the highest voltage point on the A.C. signwave. In other words when the center of the armature coil is almost at the center of the corresponding field coil. If you move the brush holder, you will find that you can pick the current at the asending or desending part of the A.C. wave, thereby cutting your efficiency and your voltage current out put. There are positive and negative brush holders that are on opposite sides of the generator. If we take away the commutator and the brushes, we still have a very good alternator but, how do we get all that power off the rotating part? Easy, we make the rotating part the fields and pull the big power off the stationary (stator) part of the machine. We can now use low voltage slip rings to get the rotor exited, theres no danger of flashover, and we can make a smaller machine because we don't have as much heat to dissapate. The biggest reason D.C. machines went away is because they needed to make them BIGGER, they ran out of room in the locomotive carbody. Remember also that VOLTAGE LEADS CURRENT, except with A.C. locomotives in dynamics.
Randy

[Anonymous]

It's like a variable frequency drive, with an AC traction motor, it matches the speed to the frequency..

[Overmod]

Sarah, think of the resultant of the fields generated in the windings, and you'll be able to visualize the effect.


First, be careful to distinguish what happens in a generator from what happens in a motor. There's no mystery about the generator at all: you have a rotating magnetic field (and you don't care, for purposes of this discussion, how that was generated or how many poles it has; it's induced in an armature that's rotated by the external power source). As this magnetic field passes each set of windings in the generator, it will induce a current in that winding. Which will then lapse when there's no longer a B-field to excite it. All you have in the output is three separate sets of induced currents, which you can feed into three separate wires to get three-phase power.

What I think is giving you difficulty is what happens in a three-phase *motor* -- one of the great inventions of Nikola Tesla. This is where you have the "rotating field" pulling the armature around, made still more mysterious when the armature contains no electrical connections or even copper windings at all... as is the case in most modern induction motors.

The B-fields generated by each phase's windings in the induction motor, of course, just increase and decrease. But there are multiple sets of poles close to the armature -- as many as 19 or more in some designs -- so there is a small angular separation between adjacent magnetic fields. The excitation of the fields is made so that the fields 'light up' sequentially, like the chase lights on marquee signs, with the effect that the field 'seen' by the susceptible bars on the induction-motor armature seems to be rotating. Note that it does not matter whether the armature can be in perfect sync with the field modulation; indeed, there is usually a certain amount of 'slip' between the "speed" of the rotating field and the speed at which the armature turns in order to realize maximum torque.

You'll want to find a good modern textbook on motor theory and design to figure out the best configurations and materials for windings, poles, etc. -- normally, the idea is to get the maximum area of pole face as near the armature bars as possible, with the understanding that cooling, hysteresis, etc. are significant.

I'm not sure what you mean by "choke design" for the windings; they do involve coils that wrap around core material (which directs the magnetic field specifically to act at the small gap to the rotor, correcting for physical geometry etc. in the actual windings)

You could use 'phase' to refer where the armature is, relative to one set of poles, but it wouldn't make much sense to do so. The point of polyphase notation is to indicate the relationship between alternating-current signals with similar or identical waveforms that are offset in time -- the term "phase angle" does not refer to a physical angle of armature to winding, but to a mapping of the sine wave(s) back onto a circle.

Modern locomotive drives synthesize each winding phase separately, and are theoretically able to control all aspects of the excitation waveform (for example, they're not limited to sine-wave form) -- risetime, duration, and relaxation are all specifiable. I suspect you won't get a discussion of PWM or other motor-control techniques in a 100-level course, unless you ask for it, but your textbooks may have some appropriate references.

For real fun, look at the control issues of so-called double-salient motors (switched-reluctance motors being a good example). I think these will be the 'next wave' of technology for locomotive traction, and if you're interested in reliable high-power electric drives, you may be at the cutting edge of research there...

[chad thomas]

I'm not good at explaining things so bear with me.

If you had a simple 1 phase motor during the positive half of the cycle one winding (we'll call it 1) attracts one poll (we'll call it N), the other winding (we'll call it 2) attracts the other poll (we'll call it S). 180 degrees of rotation later W1 now electrically reversed attracts poll S, and W2 now attracts poll N. So it is said that the field is rotating with the armature (polls).

If this were the case and the frequency of the current was 60 Hz the armature would rotate at 60 revolutions per second (60 Hz)

If it were a 4 pole motor it would take 2 electrical cycles for the motor to make 1 revolution. Every time the voltage alternates polairity the next succesive winding attracts the poll to itself (fixed magnet).


does that help?

[CSXrules4eva]

Ok I've got to ask a question which has puzzeled me for a long time. since we're talking about generators. I asked my Electricity 110 professor and I still don't understand it I know that this type of generator consists of a rotating magnetic feild inside three sets of winds but, how in the heck is the magnetic feild going to rotate? I can picture the armature rotating but not the field itself. Also are these windings of a choke design?// I have no clue. I aso wonder if the word phase can be given to the point at which the amature is stationed. For instance at 90 degrees or 180 degrees? /

[Overmod]

Simple, if you think about it.

Whether or not an AC motor has a powered armature, it turns because the field currents are phased to 'rotate' (and carry the armature poles around as they do). If you arrange the timing of the field activation and lapse so that it impedes rotation of the armature, rather than impels it, you have effective motor braking. Note that there's no effective restriction of DB to a particular speed, as there is with DC motors, because the AC braking effect works right down to stopped-rotor torque.

Pure AC motors with powered armature coils can also produce dynamic braking as generators, if the armature is powered and the field coils excited. If the field excitation is taken from a powerline, the output will be automatically in phase (which is why there's no particular problem when generating distributed power with small engines connected to the general utility 'grid'...)

[Brhodes]

AC induction motors no not generate electricity like DC traction motors can be made to do. How does dynamic braking work on AC locomotives?