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

Ballendo,
Below you make mention "When I get my HobbyCNC uni-chopper
board..." Can you tell me what name you used to order it?

Dave Rigotti
www.hobbycnc.com



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "ballendo" <ballendo@y...>
wrote:

> 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
> >