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ac induction motor speed control problem

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

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  1. Hi

    I'm experimenting with a single phase ac 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 .

    Michail
     
  2. default

    default Guest

    There's a contradiction in what you are suggesting. Pulse Width
    Modulation is generally accepted to be on-off control - the longer On
    pulse increases speed, shorter On decreases speed.

    Phase control is triggering a device (SCR or Triac) into conduction
    for part of each sine wave as a percentage of the wave.

    PWM and phase angle control are similar, but pwm doesn't synchronize
    to the power line frequency and phase angle is used for AC only.

    1. Motor overheating is normal - AC induction motors are
    synchronized to the power line frequency, they are designed to turn at
    almost the same speed as the line frequency would suggest. Two pole
    motor with 50 hertz should spin at 3,000 RPM and four pole motor (much
    more common) at 1500 RPM. (in actuality about 5% slower - called slip
    or slippage in the jargon)

    It is that or nothing for these types of motors - as you lower the
    voltage, the torque drops, slippage increases, and current goes up
    faster than the voltage goes down so they dissipate more power and get
    hotter - they are dropping out of synchronization with the line
    frequency.

    To properly control them you need to vary the frequency you supply and
    not the voltage (or with some combination of frequency and voltage).
    Frequency is important - they are happy when turning at the line
    frequency and hot when not. If you control frequency you get the full
    or nearly full torque out of them at slower speeds - and can usually
    exceed twice the normal speed with many motors.

    2. The buzzing or hum is also normal when trying to phase control
    an AC motor. The phase control itself turns on suddenly and that
    causes noise and the motor is pulling down lots of current to create
    heat and that is noisy.

    You can do what you want to do, but not the way you want to do it.

    Your choices are: get a "variable frequency drive" designed to vary
    frequency (will work with three phase and single phase motors
    providing they have no "starting winding" or "run capacitor")

    A single phase motor that drops too low in speed may re engage the
    starting winding since most use centrifugal switches, and capacitor
    run motors are close to three phase motors in design using a capacitor
    to shift the phase on half the windings.

    The other choice is to get a "universal" motor AC/DC motor with
    brushes, like you see in electric hand drills and kitchen blenders and
    vacuum cleaners as a rule. Those motors can be controlled with simple
    phase angle control with little or no problem.

    Induction motors again:

    You can often "tweak" the line voltage a bit to maximize efficiency.
    As voltage to an induction motor drops, current drops - up to a point
    (!) then starts increasing rapidly as voltage drops further. The
    "sweet spot" is just at the point where current is the lowest or a
    little below - BUT that assumes the line voltage is constant, and the
    load is constant, or your phase angle controller has some means of
    adapting to low line voltage and load by lowering (?) the firing angle
    (firing sooner).

    Really only good for things like fractional horsepower split phase AC
    motors driving fans . . . and with phase angle controls with voltage
    sensing feedback - but will save money over time so may be worth doing

    Small, "shaded pole" motors can be varied a little bit with somewhat
    less overheating by using phase control. Useful for things like
    "muffin" fans that work off the mains voltage and for lowering the
    noise level when all the cooling the fan provides, isn't really
    necessary and noise control is.
     
  3. Eric R Snow

    Eric R Snow Guest

    Greetings Michail,
    AC single phase induction motors have two windings. Generally
    speaking, one is the run winding and the other is the start winding.
    The start winding is usually only energized long enough for the motor
    to spin up to about 85% of the rated speed. These motors will have a
    centrifugal switch that turns the start winding on and off. Because
    the start winding has a lower resistance than the run winding it will,
    if left energized, cause the motor to overheat. If your motor doesn't
    have the switch then it probably uses a capacitor to shift the phase
    of one winding so that the motor will be self starting. Other motors
    are made to use an externally mounted relay that controls the power to
    the start winding. Another reason for overheating is low voltage. As
    the voltage to an AC induction motor drops it will start to slow down.
    The motor will then draw more current in a effort to speed back up.
    This happens because the motor is also a generator. And the generator
    action opposes the current supplied to the motor. As RPM drops there
    is less opposition to the current so the motor is able use more
    current. This opposition to the current is called "Back EMF". EMF
    stands for ElectroMotive Force. I learned about electric motors mostly
    from the book "Electric Motors and Control Techniques". It is
    published by TAB and the author is Irving Gottlieb. It is a very good
    book for a lay person to learn about electric motors. Here's a link:
    http://www.amazon.com/Electric-Motors-Control-Techniques-Gottlieb/dp/0070240124
    Cheers,
    Eric
     
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