Series Bipolar and parrallel bipolar?


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    Default Series Bipolar and parrallel bipolar?

    Bipolar chopper drivers - tradeoffs of various connection options

    Two phase stepper motors are supplied in four main wiring
    formats: 4, 6, 8 and 5 wires. The first 3 are usable with
    bipolar drivers, and we will discuss each in turn.

    A (two phase) bipolar stepper driver has 2 pairs of bi-directional
    power circuits (I'll call them phase A and phase B). Therefore,
    phase A has a pair of terminals, and phase B has another pair of
    terminals, for a total of 4 motor connections.

    A 4 wire stepper motor is manufactured specifically for
    bipolar driver applications, and is the simplest to explain
    and hook up.

    Each coil (of two) is rated (we call this the "sticker rating")
    for a maximum amperage, and a voltage (when continuously applied,
    gives rise to the stated amperage). If the voltage rating is not
    given, it can be calculated by measuring the phase resistance with
    an ohm-meter an multiplying by the amperage(I) rating (since V=IR).

    More current gives rise to more torque, up to the maximum sticker
    amperage rating. Currents above the amperage rating will cause the
    motor to overheat and fail.

    However, voltages above the sticker voltage ARE allowed, if they
    they are applied through a duty cycle ("chopped" or "chopper driver").
    Due to the inductance of the wires in the motor coil, there is a time
    delay between the application of the voltage and the onset of current.

    Therefore it is easy to see that applying a higher voltage with a
    chopper driver (5 to 25 times the sticker rated voltage) results in a
    faster current rise (and thus faster accelerations and motor speeds).

    Getting back to a 4 wire stepper motor, the current & voltage
    "sticker" rating may be used to select the appropriate power supply
    voltage (to power the driver, to drive the stepper).

    For example, if the current (I) rating was 6.4A and the phase
    resistance (R) was 0.5 ohms, and the inductance was 2.5mH(milliHenrys)
    then the phase voltage could be calculated to be V=IR or 3.2V = 6.4A *
    0.5 Ohm.

    Using a Gecko G201 (7A max drive, 80V maximum) would be a great fit
    for this motor, as an 80V (power supply) divided by the phase voltage
    3.2V
    gives a 80/3.2 = 25 ratio. This is optimimal in our target range
    of 5-25X sticker voltage. The driver would be configured to trim
    maximum current output back to 6.4 Amp.

    Six wire motors have 3 wires per phase, each coil having a center
    tap. You have two choices (use 1/2 coil (called unipolar), or
    use the full coil (called bipolar series).

    Eight wire motors have 2 pair wires per phase, in which case
    one has 3 options (use just one coil of the two (unipolar), put both
    coils in series (bipolar series), or put both coils in parallel
    (bipolar parallel).

    The sticker rating on these usually assumes to be rating each
    half coil (for unipolar driver applications). Since we are
    concerned with bipolar drivers, so we can choose to use only
    half coil (of each of the 2 separate coils), in which case the
    sticker rating would apply.

    However, if we choose to utilize the entire coil (called a bipolar
    series hookup, or bipolar parallel hookup) we have to apply some
    factors
    (which are multiplied by the sticker ratings). I will give a table of
    these factors:


    unipolar bipolar series bipolar
    parallel
    current factor * 1.0 * 0.7 * 1.4
    phase resistance * 1.0 * 2.0 * 0.5
    phase inductance * 1.0 * 4.0 * 1.0



    For example, if the above 6.4A 0.5V steppers were 8 wire, then using
    the table above, these would be ratings of the various hookups:

    unipolar bipolar series bipolar
    parallel
    current factor 6.4 * 1.0 = 6.4 * 0.7 = 4.48 * 1.4 =
    8.96
    phase resistance .50 * 1.0 = 0.50 * 2.0 = 1.0 * 0.5 =
    .25
    phase inductance 2.5mH* 1.0 = 2.5 * 4.0 = 10.0 * 1.0 = 2.5


    comments:
    The unipolar setup looks good at 6.4A and 80V with a gecko. However
    only using 1 coil (of two) per phase will give us less torque than
    the sticker torque. Note the small inductance which will give high
    maximum speed.

    The bipolar parallel doesn't look good, we will only get 7.0/8.96 of the
    rated torque (because of the 7 amp maximum/phase of the Gecko), but
    note the 43.75V power supply limit (because 43.75V/(7A*.25ohm) = 25).

    The bipolar series looks best (4.48A the 17.85 ratio given by
    (17.85 = 80V/4.48V), but note the increased 10mH inductance limiting
    the uppermost speed achievable.


    You might read through the links below as well;

    http://www.cs.uiowa.edu/~jones/step/index.html
    http://209.41.165.153/stepper/Tutorials/UniTutor.htm

    Similar Threads:
    Last edited by cncadmin; 03-02-2005 at 01:06 PM.


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Series Bipolar and parrallel bipolar?

Series Bipolar and parrallel bipolar?