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ac incution motor speed control problem ..

Discussion in 'Electronic Design' started by mike theodore, Jun 6, 2007.

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  1. Hi

    I'm experimenting with a single phase 220V ac induction motor.
    I want to regulate speed and for this i'm using a triac together with
    an optocoupler to drive it.
    I'm using PWM to control firing angle and hence the speed of the motor .
    I see two strange things as speed is reduced
    1. Motor overheats. Is this normal ?
    2. I hear a buzzing sound coming from the motor .From what is it?

    I want to mention that the motor has a primary and a secondary winding
    The primary has the triac circuit and the seconadry is connected directly to
    mains throug a capacitor.
    Any help would be greatly appreciated .

  2. I'm afraid it is.
    Vibration caused by the rapidly changing magnetic forces
    between the rotor and stator.
    Induction motors run efficiently only near synchronous
    speed. The applied electrical energy produces a magnetic
    wave that rotates around the stator, while both magnetizing
    and dragging around the rotor.

    The only efficient way to slow an induction motor is to
    lower both the applied AC frequency and the voltage. Just
    chopping parts of each half cycle out of the applied
    waveform does not change the applied frequency, though it
    does lower the effective voltage of the fundamental, while
    adding all those noisy harmonics.

    There are low cost drives made to produce variable speed
    operation for 3 phase induction motors.

  3. John,
    Thanks for the immediate answer

    My questions are .

    Why is it overhated since by chopping the cycle i insert less energy ?

    If i use another method it will not be overhated ?

    What is the best method to make speed control with single phase
    ac induction motors without these problems ?

    Should i use a rectifier and PWM with IGBT for DUTY cycle ?

    Is there a low cost solution to make single phase motor speed control?

    If i use PWM with "full cycle stealing" so some 50 hz cycles are ommited
    will i get better results ?

    The motor power is 250 Watts.
  4. duty cycle I Imagine it cosnumes less energy than full cycle
    operation .

    The induction motor is essentially a transformer with the
    secondary shorted but rotating within the primary. The
    rotation keeps the short circuit from looking like a big
    load. When you slow the motor by lowering the average
    voltage by any means, the short circuit in the secondary
    (rotor) shows up. This creates heat just like shorting the
    secondary of any transformer would.

    If the single phase motor normally runs with a capacitor,
    then you can use one of the three phase variable frequency,
    variable voltage drives, with a transformer, to convert the
    three phase output to the two phases the motor would get
    from the direct line connection and the series capacitor
    connection to the second winding. And it will run
    efficiently ad variable frequency. Any voltage control
    method (at fixed frequency) will produce a lot of heat.
    and some on ) instead of phase chopping will I get better
    results ?

    If you pass one half cycle, and then remove the next two
    half cycles, you effectively reduce the frequency by a
    factor of three and the motor will run slower. But the high
    harmonic content will produce a lot of vibration.
    The link I gave you has drives for sale that are lower in
    cost than the parts cost to make your own.
  5. MooseFET

    MooseFET Guest

    Just the sudden magnetic field changes in the motor field windings can
    also be causing noise. There can be magnetostriction in the iron and
    a bit of jiggle in the windings. A transformer will make noise if
    connected to a light dimmer.

    The motor is likely not good at radiating vibrations at 60Hz into the
    air. This will make th eviration at higher frequency seem bigger
    because the higher frequencies do get radiated well.
  6. Steve Sousa

    Steve Sousa Guest

    A diagram please?

    Thank you

    Steve Sousa
  7. Jamie

    Jamie Guest

    that really isn't the proper way to do that how ever, induction motors
    have been mode to deal with heating stators with what looks like an open
    frame chassis to allow for air circulation around the stator. These type
    of motors are normally referred to as tension/stall motors where they can
    be used as pull and hold drive motors.

    If you want to vary the speed, then you should be varying the frequency
    going to the motor.
    A Vector type feed back system (encoder on the shaft) can precisely
    get proper frequency at lowest voltage possible for more energy
    efficiency.. this also allows for the motor to run as slower speeds with
    torque with out losing it. This is because the frequency being generated
    matches the rotation speed desired with the number of poles in the
    factor. from there, all that has to be done is selecting only the amount
    of voltage required to the stator to achieve this rotation speed at
    matching frequency.
    The rotation RMP is 95% of frequency *NumberOfPoles, this is
    considered to be in Asynchronous. You can not achieve 100% rotation speed
    with the supply frequency with this type of motor..
    That's where Synchronous motors come in

    something to think about.

  8. Damn they are cheap. About 18 months ago i blew about a $1000 to
    design and build a half dozen of those for my father. I feel robbed.
  9. MooseFET

    MooseFET Guest

    On Jun 6, 8:06 pm, Jamie

    Make that (2 * Frequency) / NumberOfPoles
  10. The exact design depends on the winding ratios in the motor.
    Capacitor run motors often see more voltage across the
    capacitor driven winding than the full line voltage applied
    to the main winding, because of resonant boost effect. But
    you have to run the motor unloaded, and with the removed
    (starting the motor with a spin) and measure the voltage
    across that winding, to know what the optimum relative
    winding voltages are.

    The relative phase of the main and capacitor windings is
    generally 90 degrees (but that can be measured with the
    unloaded test, also). So the trick to run them with a 3
    phase drive is to extract the two required voltage ratios at
    the required 90 degree phase shift from the 3 120 degree
    shifted outputs from the drive. Here is a phasor diagram
    (where the length of the windings represent voltage and the
    orientation represents relative phase shift) of how to use
    an auto transformer to get 2 phases from 3.

    Cs represent capacitor driven winding, Ms represents main
    winding, Ws represent autotransformer winding:

    * @ phase A
    * C
    * C
    * C
    * C
    * C
    * phase B +MMMMMMMM\
    * @ | >@ phase C

    When driven from a variable speed drive, the capacitor is
    not needed, since the drive provides the 90 degree phase
    shift between the two windings.

    There are lots of other possible ways to derive arbitrary
    ratios voltages with a relative phase difference of 90
    degrees from a 3 phase source. This one is just the easiest
    for me to draw with characters. Doodle a little and see
    what you can come up with.
  11. Jamie

    Jamie Guest

    Yup.. Thanks for the over sight, i must of been tired when i did that one./

    So that only means that some do read my post.
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