AC Theory

Sorry to bother you guys again

I can’t seem to understand why an AC motor would be more efficient than a DC motor. This just seems counter intuitive. Because an AC motor, having alternating current, will produce a magnetic field to attract the stator, but all the wile the polarity constantly switches and repels the motor (for a short amount of time, but many times in one cycle) wile you want to attract it.

I also found out that the alternating current causes the electricity to flow though the skin of the wire, not though the middle (which increases resistance). And because you are switching polarity, you are accelerating the electrons constantly. When you accelerate an electric charge it gives off electro magnetic radiation. Which is an energy loss as well.

I think that this comes down to is why is AC better than DC overall. We use AC in our power lines. Because, as I have heard everywhere, there is less energy loss. But to complete my project I need to know why.

Here are some of my theories on possible reasons why AC is better (please don’t laugh, I am trying).
* Maybe, AC is better because the electrons are not moving though the entire circuit. So there would be less work done on the electrons. (question is would that affect the total energy)
* Maybe, AC is better because when the polarity changes the electrons recoil. So no energy is lost in changing polarity. But that alone would not make AC the winner. Therefore maybe if/when the electrons recoil they recharge the system.
* Or, it could be that by changing the polarity, you (some how?) change the flux field for the better?

This is all I could come up with.

Please help
Thanks for your time

Edit: grammar check

here’s an AWESOME site…

Click around, its got some other AC topics too… Like motor control…

You gave me that site already, from it I have found most of the information about speed controlers. However I cannot find a page that talks about AC efficiancy. From this site I learned how to build a transformer, how to build a variable transformer, how to use that variable transformer to make a volume controle for a spitter. ect

Have to go I’ll talk more later

Haha, oops…

I can’t answer your questions… but I do know that by having fixed commutators, you can’t advance the timing on the fields so that the mag field is lagging the electric field. Also, resistance of brushes on the armature.

AC, you just have coils and a squirrel cage on an induction motor… so you can advance the timing so that the mag field controls the speed, and it maximizes the efficiency, because its always running in its efficient mode… except when you get Way outside the frequency response of a motor, i.e. a REALLY high frequency saturates the motor field, and it can cause problems.

I have been researching for many long weeks. I used Google, eduCypedia, Wikipedia, and even (gulp) the library. I also asked people I knew, teachers, engineers, professors, and people from forums (thanks for your input by the way, if you did not give any input then this statement does not include you). No one could give me an answer, or if they did it was always related to the howl Edison vs. Westinghouse debate. Which, don’t get me wrong, is still related, however, it doesn’t answer my question. But after a long series of debates with my High School friends and instructors. I now know the solution to my problem.

So back to the topic, no matter how I thought of it, AC seemed to be less efficient than DC. And there was a very logical reason for that. Being that DC is more efficient than AC! I had been banging my head in to a wall for the last… I don’t know how long, and I had been gonging in the wrong direction (headache). Oh well, I hope this will help someone else.

Ed: spell check

Spell check is your friend, embrace it.

So, what do you think

Anyone agrea, disagrea…

OK, now that that I think about it, it’s probubly not very interesting

The real advantage of the A/C motor if it’s of the “Induction” design is that using such a motor changes the powerband to a different shape. Most permanent magnet motors work best at a very limited (narrow) high rpm, but a variable magnet (the magnet is “induced”) motor can have high efficiency even at low rpms.

So the issue is average or overall efficiency and not the ideal efficiency.

Actually some permanent magnet motors (like the pancake design) can be extremely efficient in a narrow rpm range.

The answer is just more complex than the original question allowed for. (life is like that)

Do you see where I’m going with this?

In the real world using a motor over real hills and stops and starts the AC Induction motor is better because you spend a lot of time changing speeds. It’s a little like the city verses highway mpg issue for gasoline cars. AC Induction motors are better for the city driving and DC permanent magnet motors are better for highway driving.

Things like racing vehicles will naturally prefer an AC Induction motor.

(the Tesla Roadster uses an Induction motor)

Very interesting, I did not even consider that. So an AC induction motor would have a high toque at lower rpms. And a DC motor would have higher toque at higher rmps.

What about a DC step-motor, I think it should have the same performance as an AC induction motor without wasting so much energy.

Actually I was thinking of using a DC step-motor that mimicked a tangent wave instead of a sine wave. So I would be proposing to use a 4-pole 4-(tan) phase DC (AC technically) step-motor. This seems to me that it would be a more energy efficient. But I haven’t found anyone that has done this before, so I don’t know if this would work.

Thanks for your time

Basically says that we can use a smaller guage wire at a higher voltage to minimize current losses through the resistance of long wires. The power lines normally run at 30Kvolts or so (some even up to 60Kvolts a CPS employee once told me). With such high voltage you won’t need as much current flowing, thuse eliminating unnecessary current losses. AC is easily stepped down to a lower voltage (lower voltage = high amperage since). We step down the voltage close to our home where we need the most power. DC can’t get stepped down with just a passive transformer.

In all, a higher voltages/lower amps just wins over low volts/high amps. AC wins over DC because it has magentic field properties.

Hay, thanks for the link, it looks interesting
I’m kind of busy right know so I’ll have to thoroughly explore it later

What do you think about my idea for a step motor driven by a tan wave instead of a sine wave?

I have no background in actually putting a stepper motor in use so that doesn’t make a whole lot of sense to me (even after reading your posts again). I think that it is something to be tried, but maybe on a smaller scale. I have put together my own PWM motor controller (no regen of course) and hooked up a 12v dc computer fan. With a few more fets and a few more flyback diodes I know it will power my motor, because the theory is the same.

I just wish someone would answer me back about the cheap 2.5HP “Generators” floating on ebay in another thread I started not too long ago.