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Question about VFD (variable frequency drive)

Discussion in 'Misc Electronics' started by [email protected], Jan 18, 2013.

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  1. Guest

    Hi,

    I read an article intruducing the principle of VFD at:
    http://www05.abb.com/global/scot/sc...52573070053da9e/$file/lvd_eotn01u_en_reva.pdf

    When I read it, I do not understand the line:

    "As long as this ratio stays in proportion, the motor will develop rated torque."

    It means that we only change the frequency to drive the motor to change thetorque?

    It seems there is also a method to change the AC voltage to drive the AC motor (I am not sure that there is the precise statement, but I think theoretically it can change the on/off time to change RMS voltage at the motor). That VFD introduction does not include voltage variation?

    Thanks,



    ...........
    Fig. 5 shows
    the torque-developing characteristic of every motor:
    the Volts per Hertz ratio (V/Hz). We change this
    ratio to change motor torque. An induction motor
    connected to a 460V, 60 Hz source has a ratio of 7.67.
    As long as this ratio stays in proportion, the motor will
    develop rated torque. A drive provides many different
    frequency outputs. At any given frequency output of
    the drive, you get a new torque curve.
     
  2. Jamie

    Jamie Guest

    Normal mode for a basic VFD is volts to hz... Which means by the time
    the drive reaches max output, it is generating max voltage and frequency
    the motor is designated for.

    At minimum output (zero speed), it will generate no voltage and no hz..

    At 50% output, it is generating half the motor rated voltage and half
    the motor base frequency..

    It is a proportional operation and since this is true, the torque
    capabilities for the motor equal.. In other words, we can generate max
    current for that motor at any speed except 0 of course.

    Lets say you have a motor that handles 10 amps max, if you were to
    lower the frequency to 10 hz for example, you also need to lower the
    voltage to the same proportion. Reason for this is induction of the
    motor coils. Lower frequencies will generate higher currents and currents
    are what drives the machine (magnetically). If you don't lower the
    voltage to match the ratio of frequency, you'll exceed the motor's current
    rating. Of course, higher Hz requires higher voltages to maintain that
    max current due induction.

    If you wanted to flux the motor with constant current then look at
    operations like vector control. Vector systems attempt to maintain a
    near constant current in the motor, even when it's not spinning (0 hz).

    Basically, it would appear like having pure DC current being applied
    to the coils, which basically it is. This will give you a stiff holding
    drive shaft. THe control electronics will modulate this constant current
    (Mag current) to form the AC frequency and voltage required to move the
    shaft. It's almost like having a servo motor as a results in effective
    output.

    When things are operating smoothly in vector mode the motor current
    should be reading the same through out the power band. If the current
    starts to increase in vector mode it means that you don't have the
    mag current up to the 50% mark or, you have exceeded the motor and
    drive's ability and most likely will respond to things like speed and
    over voltage errors.

    Jamie.
     
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