AC / DC motor question?

Discussion in 'General Electronics Chat' started by neorules, Apr 9, 2009.

  1. neorules

    Thread Starter Member

    Nov 30, 2007
    12
    0
    AC induction motors have a squirrel cage rotor.
    Does the magnetic field have to cross the rotor or does it run around the squirrel cage?
    I suspect it crosses the rotor due to the voltages induced in the squirrel cage.

    I am looking at rather large motors and having a rotor that is 30" in diameter is a bit heavy.

    If the field has to cross the rotor then I may go to a wound rotor.
    That way I can make the rotor appear to be a magnetic core without having a solid rotor.
    But by doing that would I have to switch from AC inverter control to DC brushless control?

    Basically the end is a ring motor more or less....
    Thanks for any advice.
    JC
     
  2. b.shahvir

    Active Member

    Jan 6, 2009
    444
    0
    Hi, :)

    I think you are unclear on the concept of induction motor. The rotor is basically made up of magnetic material and the copper rotor bars are embedded into it. The bars are then short-ckted by a shorting ring at the ends. The stator magnetic flux crosses the rotor iron as well as runs around the rotor conductors cutting them inducing current in the short-ckted. rotor bars and hence the rotor develops torque.

    A wound rotor is technically similar to a squirrel cage rotor....only difference being the windings can be opened ckted. to insert resistances in series with them for starting and speed control purposes. The wound rotor motor is used for special applications requiring high torque starting and speed control. But with the advent of variable frequency drives (VFDs) and specially designed high torque double caged squirrel cage induction motors, wound rotor motors are rarely used nowadays.

    Regards,
    Shahvir
     
  3. neorules

    Thread Starter Member

    Nov 30, 2007
    12
    0
    So a ring motor is more of a giant stepper? Since there is no path for the flux to cross the interior of the rotor?

    I guess that is what I am trying to figure out.
    Thanks JC
     
  4. b.shahvir

    Active Member

    Jan 6, 2009
    444
    0
    I repeat, rotor is basically made of iron. Being a magnetic material it has affinity for magnetic flux. The rotor is placed between the North and South rotating magnetic poles. As a result, magnetic flux will emanate from North pole.....penetrate the rotor iron....pass thru it completely and then finally enters the South pole.

    Also, at the same time this rotating flux cuts the rotor conductors embedded in the rotor iron and develops torque due to interaction between stator and rotor magnetic fields. The rotor magnetic field is the result of rotor currents induced in the short-ckted rotor conductors by the stator rotating magnetic field.
     
  5. neorules

    Thread Starter Member

    Nov 30, 2007
    12
    0
    Ok... I get it ... The motor I was looking at was a three phase DC motor not an induction motor. The motor was about 3 feet in diameter with a 2+ foot hollow center section.
    Thanks!
     
  6. b.shahvir

    Active Member

    Jan 6, 2009
    444
    0
    There is no such thing as a 3 phase DC motor....Tesla will turn in his grave! :rolleyes: You have to clarify your concepts of electrical machines first.
     
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