Uni vs. bipolar was Re: HobbyCNC NEW 3AMP Unipolar Chopper Driver Board!

Jess,

With unipolar you have an extra wire (or two) from your driver to
your motor... Higher connector cost... more wires to flex and break...

Unipolar drives shut down a coil by opening a switch (transistor).
There is nothing to DRIVE the current out of the coil, so it decays
relatively slowly. This works okay because as the other posted
answers have pointed out, we have a center tapped coil, and are only
using 1/2 of it at any given time. So while 1/2 is "winding" down--
pun intended<G> we're using other coils. For many uses of steppers,
by the time we get back to it the current in the coil we shut off has
decayed, and is ready to be turned on again.

Bipolar drives are more "pushy". They do not simply turn the coil
off, but reverse its polarity. To do this they use more transistors,
(4 per coil, instead of the 2 used in unipolar drives) and since the
motor voltage is always present(except during current control, but
that's another subject), the electrons are "marched", rather
than "walking" at their own pace...

Torque in a motor is a result of copper area, and amperage. For a
given amps and copper area, the torque will be identical, whether the
drive is a unipolar OR a bipolar. BUT...

Because the Bipolar "marches" the electrons through the coils, and
the unipolar "lets 'em walk"; the bipolar can switch the direction in
a given coil faster. However, the unipolar doesn't NEED to switch
direction in a coil, because in effect it uses a "different" coil...

So we're back to the difference first mentioned above; a couple less
wires from the motor to the driver. There are cases where walking or
marching is the better choice, when ALL factors are considered.

Now let's add two more truths:
The better bipolar drives ARE better than the better unipolar drives
where high speed torque is concerned. But not by nearly as much as
some would have you believe, or for the reasons often given.

The second thing is that a bipolar drive allows you to use a given
unipolar motor(6 or 8 wires) at a lower amperage with the same TORQUE
at high speeds, WHEN you are limited by the driver amperage
capability.

So you can take a 3volt, 4 amp motor and run it with a 2 amp bipolar
drive by using the whole coil. (Ignoring the center tap that you HAVE
TO USE with unipolar) This can let you use bigger motors with smaller
drives. The downside is that you won't have the same speed capability
because inductance is 4 times as high as the 1/2 coil value.
Sometimes if you have to use a certain motor and/or a certain drive,
it can be useful.

Hope this helps,

Ballendo

P.S. When I get my HobbyCNC uni-chopper board, it's gonna be
interesting to compare it to my xylotex... using the Same motors,
same volts, same amps... And THEN optomising motors for each drive
type and re-running the tests...

(Geckos can't be included in the comparison due to their added
features which make them more capable for reasons beyond the uni- vs.
bi- differences.) But the A3977 vs. the SLA7062M should be a
fair "race", as long as current and voltage are considered.

>In CAD_CAM_EDM_DRO@yahoogroups.com, <jess@p...> wrote:
>Could someone who knows explain -
>
>What are the advantages and disadvantages of unipolar and bipolar
>stepper motors. Or does anyone have a link to where this is
>explained?
>
> Jess Perez
>