Re: Hobby CNC NEW 3AMP Unipolar Chopper Driver Board!

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Roy J. Tellason"
<rtellason@b...> wrote:

> On Saturday 08 May 2004 02:06 pm, jess@p... wrote:
> > Could someone who knows explain -
> >
> > What are the advantages and disadvantages of unipolar and bipolar

stepper

> > motors.
>
> Here's my understanding of things...
>
> Assuming you have a motor that can be wired either way (4 wires,

or 8 coming

> out of it), a unipolar setup will have a *much* simpler drive

circuit, as

> far as needing fewer power semiconductors to make it work, while a

bipolar

> circuit will give you more torque with any given power supply

voltage, but

> requires more complex circuitry.

Unipolar drivers require a center tap connection for the + voltage
and transistors/FETS switch each leg of the CT coil to ground. 4
lead motors cannot be used. 6 lead motors have a integral centertap
while 8 lead motors can be wired with two coils in series and the
common point becomes the CT. Since only half of the winding is being
used at anytime, the unipolar configuration tends to yield lower
speeds.

Unipolar drivers normally have 4 power semi's while bipolar's have
two H bridge configurations for each coil (8 devices). Since the
transistors on a unipolar are all referenced to ground the drive
circuitry is simplier than a bipolar bridge which has to have two
high side devices. Turning on an NPN transistor or N-Channel FET
requires more voltage than is on the emitter. That means if
your "head" voltage is 35VDC you need to find a way to generate more
than that to turn on the high side. That takes added parts.

A lot of CNC equipment is running on unipolar drivers but for maximum
speed and torque bipolar is the choice.