Re: [CAD_CAM_EDM_DRO] Re: Running Unipolar steppers with a Bi-polar driver?

jeffalanp wrote:

>Hello Peter,
> When you run stepper motors in bipolar mode you will be driving
>twice the amount of copper (compared to unipolar). This turns into
>TWO times the resistance, and FOUR times the inductance the drive has
>to overcome, thus the time to fully energize the coils will DOUBLE.
>

Yes, it seems so, but it isn't. The difference is that bipolar drives
actively apply voltage to
reverse the current in the winding, while unipolar drives just let it
dissipate passively through
'catch' diodes. It turns out that, even using the full winding, a
bipolar drive is almost always
capable of faster rotation than a unipolar. Using only half the
winding, which is what the unipolar
drive always uses, ends up with the same inductance, but provides far
superior performance.

>
>As you start getting faster and faster step rates, your drive doesn't
>have time to fully charge/discharge the current in the coils to the
>proper level. Thus at HIGHER step rates bipolar can end up having
>less torque than unipolar (given equivalent voltages for both
>drives).
>

What you don't understand, here, is that the bipolar drive effectively
has twice as much
applied voltage as the unipolar, since it actually reverses the voltage
applied to the winding.

> The good news is that the holding torque, or torque at slower step
>rates will be about 1.414 times MORE than a unipolar drive. This is
>where most of the power is really needed; when cutting metal at the
>slower, non-rapid feed rates.
>
>

The holding torque is about the same for most bipolar drives. I think
some microstepping
drives accomplish the 1.414 (square root of 2) torque improvement by
providing partial
current to both windings when between full-step positions.

Jon