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Ac Motors

  1. Mar 19, 2004 #1
    main features of dc motor (input dc)
    Two permanent magnets on opposite sides of the motor, with opposite poles facing each other. The pole faces are curved to fit around the armature.

    The armature consists of a cylinder of laminated iron mounted on an axle. Often there are longitudinal grooves into which the coils are wound.

    rotor ---> There may be only one, in a very simple motor, or several coils, usually of several turns of insulated wire, wound onto the armature. The ends of the coils are connected to bars on the commutator.

    The commutator is a broad ring of metal mounted on the axle at one end of the armature, and cut into an even number of separate bars (two in a simple motor). Each opposite pair of bars is connected to one coil.


    dc motors uses a split ring communtator and dc input

    so is dc generator (genertor which reverse the funtion of motor)(with dc output)

    in addition genertor can also use slip ring instead of spilt ring communtator to get ac output

    but it seems not right for a motor if change split ring communtator to slip ring (full ring)
    it would not work as a ac motor (if input is ac). is this true?
  2. jcsd
  3. Mar 19, 2004 #2
    You seem to be talking about an alternator here. This will not function as a motor in any circumstance. It takes DC imput, yes, but it must be mechanically rotated by a belt from a separate engine. These are found in cars and portable generators. I am not aware of a DC motor that is configured to also deliver AC output.

    The kind of DC motor you described with permanent field magnets would not work as an AC motor. I believe you might get it started if you mechanically spun it into operation but it would stall as soon as you put a load on it.
  4. Mar 19, 2004 #3
    oh thx btw

    The type of electricity transmitted over long distances is predominantly AC, since AC can be changed easily to high voltages and correspondingly low currents by the use of a step-up transformer. With advances in solid state technology it is becoming easier to step DC voltages up and down, and DC is increasingly being used for long distance power transmission.

    i think the bold is not correct! is it?
  5. Mar 19, 2004 #4
    I do not know if advances in solid state technology have made it easier to transmit DC over long distances. I haven't come across any information to this effect.
  6. Mar 19, 2004 #5
    hey thx anyway i got it:

    But DC has the advantage of not causing losses through electromagnetic radiation or magnetic induction. With solid-state switching it is now relatively simple to change between DC and AC at high or low voltages. High voltage DC transmission is now practicable. Scientists are striving to develop super-conducting wires for power transmission. If they do, DC could become the preferred current for long distance transmission. There is already a 500 kV DC submarine transmission line carrying 2800 MW over 50 km between the two islands of Shikoku and Kansai in Japan.
  7. Mar 19, 2004 #6
    This is amazing. What material is used in the line?
  8. Mar 19, 2004 #7
    hehe ask the expert or search, thats all the info i got.
  9. Mar 19, 2004 #8
    I just did a google and I see what this is about. These solid state transformers are promising to make it as easy to transmit DC at high voltages and then step it down to where the consumer can use it as easily as the transformer now does this with AC.

    The DC system as started by Edison was limited to the voltages that would be used by the consumer since there was no way to step it down. The result was he couldn't transmit it very far. AC which could be generated at very high, far traveling voltages, and automatically stepped down in a transformer, won the day as the preferred system. These solid state DC transformers may change that.

    This is quite interesting.
  10. Mar 19, 2004 #9


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    Any links that explain the mechanism here as being anything new and different guys? I searched on google and all I got was regular transformers...

    A switching power supply is nothing new and whether the switching is used buck-boost or whatnot, fundamentally the DC is being chopped into AC, transformed, and then rectified back down to DC again.

    I did find this link, but it provides little insight into manipulating DC using SS devices.

  11. Mar 19, 2004 #10
    cool but dc havnt got that much advatage over ac wats the point to use dc than?
  12. Mar 19, 2004 #11
    At the bottom of that page is this link:

    Product Information [Power Transmission Lines]Furukawa Electric Co.,Ltd.
    Address:http://www.furukawa.co.jp/english/product/soden1_e.htm [Broken]

    If you scroll down to "Worlds largest-scale DC power transmission line" you'll find the best I could find.

    If you google "Shikoku Hansai DC transmission" there are alot of PDF sites that I didn't look at where you might find something more detailed.

    My impression is that they are generating DC at very high voltage, transmitting it as DC, and both transforming it down and inverting it to AC at the recieving end. I don't think there's anything new in particular just that its the first large scale set up to do it this way. One advantage would be that one wire instead of two over 50km would save alot of wire, I suppose. And it claims there are no losses through electromagnetic radiation and magnetic induction, for what that's worth.
    Last edited by a moderator: May 1, 2017
  13. Mar 19, 2004 #12


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    Perhaps the solid state circiutry used to perform the DC step up/step down is smaller and cheaper to manufacture then the huge transformers required by AC.
  14. Mar 19, 2004 #13
    That's a definite possibility.
  15. Mar 19, 2004 #14


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    Well, for a given power range a swiching PS can use a much higher switching frequency like 100KHz instead of a fixed 60Hz and get away with a much smaller transformer (compare a car audio amplifier transformer versus a home audio amplifier transformer) but there's still a few transistors and diodes and a transformer directly in the current path, each with respective losses. For a low duty-cycle application, and I guess a home might fit that description, the lower efficiency at actual tranmission of peak power could be offset by the substantially higher efficiency during low power modes.

    Whether or not the upfront investment and power savings during those low power times are cost effective, well, I guess that's why its called engineering with that many compromises. :smile: I think the oil filled transformer has no reason to fear extinction given the investment needed to swtich everyone over.

    Personally, this seems comparable to the instant hot water heater figures and arguements. Most homes have a low duty-cycle of hot water useage (hot water used < 1hr per day) yet maintain a 30-60 gallon tank at full temperature 24/7. But from an economics only standpoint (forget the 'green' aspect) it takes a lot of utility bills savings over time to offset the intial cost of a instant hot water heater setup over the old tank system.


    P.S. For the typical 'wall-wart' transformer to charge cell phones or run small equipment around the house, a SS and IC based solution makes tons of sense. But we're talking <10W there, pretty different economy of scale.
  16. Mar 20, 2004 #15
    by the way if you put a certain amout of load on a 1. dc motor 2. Ac induction motor 3. Three phase synchronous motors

    what would happen to each?

    in speed, and effect --->( stops or keep going)


    somehow in my memory that

    Three phase synchronous motors or induction motor will stop if there is a extra load becasue it is not in constant speed anymore.
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