AC/DC power

[Mookie]

I know we have been down this road before, but while I am working on figuring out wheel bearings in my spare time, can someone briefly explain AC vs DC traction power? (I think that is what it is called, but if not someone will correct me.)

Jen

[Anonymous]

AC is alternating current. DC is direct current. They both have advantages and dissadvantages. DC has brushes and a commutator. AC has slip rings. DC is much easier to control therefore cheaper. AC will take abuse more and is more powerfull but to control it is more difficult. Therefore more expensive because of the circuitry and computers used to control the current. There are more issues with both. The history of DC versus AC is quite interesting especially when it comes to your home.

[edblysard]

Hi Jenny,
As a side note to Dan's post, all locomotives have a alternator, which produces A/C current.
The locomotives equiped with D/C traction motors use a rectifier rack to "cut" the alternating current "in half". (it only uses one side of the the alternating current).
Your automobile has the same set up, only smaller. Your car has a alternator, which produces 12 to 15 volts A/C at 65 to 115 amps, but all of the electrical components(head lights, radio, windshield wipers) and your starting battery are 12 volt D/C.
The reason is because its easier and cheaper to produce them. Your alternator has a diode trio,(retifier rack) just like a locomotive, which changes the output to 12 volts d/c. In a D/C locomotive, it changes it to 600 volts d/c.
An A/C or D/C locomotive both have the same basic innards, the difference is in what type of traction motor they have. A traction motor is an electric motor, one per axle, inside the trucks, geared to the wheelsets. A/C traction motors have less moving parts, but are currently more expensive to purchase and maintain. D/C traction motors have been around forever, parts are cheap and plentifull, but they are prone to more breakdowns. So it a trade off, but sooner or later, I think you will see the D/C traction motor go the way of steam engines.
Stay Frosty,
Ed

[Mookie]

Ed: Inside the trucks where?

[Mookie]

Dan - the 1st line was all I knew about them. So the additional information is helpful.

Thanx

Jen

[edblysard]

Between the wheels. The inside of one of the wheels has a gear running around the inside diamiter. I will get a photo of one of the wheelsets, and a traction motor in place for you.
Its kinda hard to explain, but once you see it, it will make perfect sense.
Stay Frosty
Ed

[Mookie]

I am beginning to think those wheels are an engineering miracle - all the "stuff" that goes on in such a small place!

But it is providing some really interesting conversation at home!

Jen

[Anonymous]

Good data, Ed. Way back 30 years ago in A&P school we learned that AC which is changed to DC is then called PDC, for pulsating direct current. It uses only one side of the sine wave. The diode blocks the other side. Frankly, I think it is done with mirrors.:)

[edblysard]

Hi Dan, yeah, but they use really small mirrors...
Yup, in the auto parts "schools" we attended when I was in the business, we were taught that the same thing, it only uses one side of the sine wave. Of course, most of us had never seen a ocilliscope (sp?) so had no idea what the instructer was talking about. It wasnt untill I managed a auto repair center that I finally got a clue. I play with some small electronics, no real skill, for my models, adding that to having changed hundreds of diode trios in GM alternators over the years helps. The best explaination I ever heard was a diode was a one way valve. Am trying to remember the fancy term for the small amount of "back" current a motor generates. I know that when dynamic brakes are used, this is the force that applies, just cant come up with the term.
Maybe sleep will help.
Stay Frosty,
Ed

[Anonymous]

Hi Ed, in the aviation business that I enjoy so much and am an active part of we use oscilloscopes quite a lot so I can say that I have seen the sine wave.

[edblysard]

What? You saw it? And lived to tell the tale?
WOW!!
Not only a Train watcher, but a Wave watcher too.
Multi-tasking...what a concept.
So what is the term for the "back" current a motor makes?
Stay Frosty,
Ed

[Anonymous]

Ed, I have been out of college a few years so this is just off the top of my head. A motor has a Armature core and field windings. These are nothing more then conductors and, like other conductors, the changing magnetic field will induce a current within them. I believe this is called the "Eddy current". Best guess, hope it helps jar your memory.
TIM A

[edblysard]

Found it, my memory, that is.
Its called back EMF, back electromotive force, caused when the polarity of the armature of a permanent magnet motor is reversed, which changes the traction motors into generators, which produce current(heat) that is dissipated through the dynamic brake grid(resister). The back emf is then a retarding force on the armature, which slows the wheels. See, sleep and a memory jog do help. I should try it more often.
Stay Frosty, and thanks Tim,
Ed

[Mookie]

Wow!

Jen

[bfsfabs]

Tim, It has been a while, but I think "eddie currents" are currents induced in the laminations of a transformer core. Not usefull, a loss in the transformer.

Lowell

[Anonymous]

Sorry about that. I don't watch trains. I just like devices that transport (except boats, ships ok but not boats)and love to research them. As for the back current, I would have guessed impedance. EMF sounds correct.

[Anonymous]

Yes Sir, you are correct. Been a while for me too. Dusted off one of my old school books to confirm. Sorry for the confusion.
TIM A