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Thread: Inverters for AC motor speed control

  1. #1
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    Default Inverters for AC motor speed control

    Hello folks,

    I don't seem to understand the explanations I hear regarding the use of
    Inverters for controlling the RPM of an AC motor. They say that lower
    RPMs are achieved without the loss of power from the motor. But, if we
    are playing around with PWM, are n't we changing the power input into
    the motor?

    ziloo
    Last edited by ziloo; July 11th, 2018 at 06:25 AM.

  2. #2

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    No. It will try to maintain the RPM you command with however much torque you allow it to exert (rather how much current you allow it to draw).

    What are you driving with what kind of motor?

  3. #3
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    Using a diac-triac combination (such as in a dimmer), we are changing the duty cycle.......that is
    the amount of power we are providing for the device. In controlling the RPM of a DC motor,
    the PWM changes the duty cycle of the power into the motor....hence controlling the speed of
    the motor. Is it any different with the Inverters?

    ziloo

  4. #4

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    I think you're thinking of this as an unloaded motor.

    AC or DC and however you control the speed and power, you can always achieve whatever torque you want at whatever RPM you want.

    If the load is low, power is low. If the load is high, power is high. The speed is separate. But if you have more load then you have available power then you can lose control of the speed.

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    That type of inverter requires 3 phas3, no?

  6. #6

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    It depends what kind of motor you are using. If you have a three phase motor, you need an inverter with three phase output. The input can be three phase, single phase, or DC. That's the beauty of inverters.

    If you have a single phase motor, you need an inverter with single phase output.

  7. #7
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    Quote Originally Posted by KC9UDX View Post
    ...AC or DC and however you control the speed and power, you can always achieve whatever torque you want at whatever RPM you want....
    Suppose I have a lathe machine with a motor rated at 1 hp. I want to do some cutting at
    300 RPM on a steel bar without losing power. If I use a dimmer for this purpose, I have to
    shorten the duty cycle by adjusting the trigger voltage on the diac that will in turn reduce
    the power I can expect from the motor.

    How does the Inverter allow operating an AC motor at slow speed?
    Where does the "Inversion" happen?

    ziloo

  8. #8
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    Is PWM the best in this case? I'd expect VFD for heavy loads might be best.

  9. #9
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    Quote Originally Posted by Chuck(G) View Post
    I'd expect VFD for heavy loads might be best.
    I don't quite understand the relationship between variable frequency and variable speed
    without the loss of power....

    ziloo

  10. #10

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    Quote Originally Posted by ziloo View Post
    How does the Inverter allow operating an AC motor at slow speed?
    Where does the "Inversion" happen?
    The AC power is simulated at a lower frequency, hence lower speed, assuming the motor is some sort of induction motor that has a slip-speed relationship with line frequency to induce a magnetic field in the rotor. With such a motor the triac-based control is a very poor idea. Be specific about what you mean by "power" as it sounds like you mean torque. The same torque at a lower speed is less mechanical power.

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