Questions about 3-phase AC motors

Discussion in 'General Electronics Chat' started by Macnerd, Jun 28, 2015.

  1. Macnerd

    Thread Starter Member

    May 22, 2014
    37
    1
    I am curious about this.

    I recently watched a program on TV about the Tesla car. It has a 3-phase AC motor. A 3-phase motor is like a 3-cylinder internal combustion engine.

    Granted the electronics to control a multi-phase motor would be complicated.

    While I watched the TV show, I wondered if it is feasible to make a 4-phase, 6-phase, 8-phase, 12-phase motor. I'm sure that it would be impractical, but is it feasible?

    Would the motor be more efficient if it has a higher frequency - 120 Hz, 240 Hz, 480 Hz?
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    A three phase motor is not like a 3 cylinder engine because, no matter how many cylinders you have, the energy is still delivered in pulses. A three phase motor has continuous, constant power as the phases overlap. More phases will not fix it because it isn't broken.

    High frequency motors need a lot less iron, but efficiency is always in the details. You can still design a lousy 400 Hz motor. :p
     
    Reloadron likes this.
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    It is not a question of phases, but poles and applied frequency for induction types, There are 3phase controllers (VFD's) that currently operate 3ph induction motors from 0 rpm to 120Hz and above.
    Also look up PM pole, ECM, BLDC and the Fischer-Paykel outrunner motor for more information. (currently used in washing machines).
    Another example is a BLDC servo that is typically 8 pole maximum, but with PID control is capable of extremely fine positioning resolution.
    Locomotives have used 3 ph induction traction motors since the 80's.
    Max.
     
    Last edited: Jun 28, 2015
    Reloadron likes this.
  4. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    The reason we use 3-phase is that it is the minimum number of phases where the sum of power of the all the phases is a constant value, the same as DC (if you sum the instantaneous power of the three phases over one cycle it equals a constant). This means the power delivered by a 3-phase motor is also constant-power with no power or torque fluctuations.

    Single-phase and 2-phase do no have this characteristic and their instantaneous power varies over a cycle.
    That's why 3-phase motors are quieter and smoother than single-phase motors.

    Certainly you can make a motor with more than three phases but it just adds complexity and wires without any other advantages.
     
  5. JWHassler

    Member

    Sep 25, 2013
    201
    33
  6. Biff383

    Member

    Jun 6, 2012
    49
    19
    I watched the video....now my head hurts. Thanks a lot. I've been working with these things for 30 years, I hope they don't change them on me now.
     
    JWHassler likes this.
Loading...