Re: [CAD_CAM_EDM_DRO] Xylotex or Gecko step driver for my Mill/Drill?

On Sunday 16 May 2004 12:08 pm, Jon Elson wrote:

> Roy J. Tellason wrote:
> >On Saturday 15 May 2004 11:05 pm, Jon Elson wrote:
> >>>but I ruled that one out because it's a unipolar only board.
> >>
> >>Good move, as most unipolar drives are very poor performers at higher
> >>speeds
> >
> >Why is that?
> >
> >Would it have something to do with a bipolar setup energizing coils in the
> >reverse direction rather than just turning them off?

> Yup, that is basically the problem. With the typical unipolar drive,
> the power source is connected to the center tap, and transistors ground one
> end or the other of the winding. When you ground one end, there is still
> circulating current in the other half, fighting the field you are trying to
> build in the newly powered winding. This energy has to go somewhere, and
> it ends up being burned off in some diode-resistor arrangement. It is a
> hard topology to harvest this energy constructively. If there is no
> resistor, the current in the shut-off winding dies very slowly. If there
> is a resistor, then it gets quite hot at high step rates.
>
> With a bipolar drive, the power source is applied across the whole winding,
> and is reversed to reverse the field. The field already in the winding
> opposes the power source at the moment the transistors are switched,
> and the reversed voltage across the winding collapses the existing field
> and quickly builds it again in the opposite direction. The energy mostly
> ends up back in the power supply, so there are no big resistors to burn
> off the left-over field.
>
> There are ways around this, but they are generally more complicated than
> the bipolar drive scheme. The only advantage to unipolar is it saves a
> 4 transistors per motor.

Thanks for the clear explanation there. Now, I have another question: Is
it feasible to use a motor with six wires (that an ohmmeter shows me as being
wired for unipolar operation) as a bipolar motor by simply ignoring the
center taps? I'm wondering about the phasing of the two half coils there...