Re: Unipolar as bipolar.

Mariss,

Then your white paper was only talking about "series" connection of BOTH
coils being good for 1/2 power? There must still be some drawbacks.
Nothing's for free! You still have twice inductance, compared to being
allowed to parallel connect the coils. So I suspect there is still
considerable power drop-off at higher speeds? The motor would be
limited by it's inductive reactance earlier then the parallel
connection. Still worthwhile.

Alan


Mariss Freimanis wrote:

>
> Alan,
>
> It is counter-intuitive, but here's the reason:
>
> A step motor reaches its rated power at the corner frequency. The
> corner frequency is where the motor's inductive reactance begins to
> limit current and not the drive.
>
> Beyond that speed, inductive current (I = V / 2 pi f L) drops rapidly
> with speed and I squared R losses drop even more rapidly.
>
> The main effect of using only 50% of the copper is the winding
> resistance is twice as high. This "extra" resistance develops a
> voltage drop that subtracts from the voltage the motor "sees".
>
> This voltage drop becomes insignificant (<3%) because of the
> diminishing inductive current at higher speeds.
>
> Since motor output power is proportional to supply voltage, this 3%
> voltage drop results in a 3% power drop.
>
> At low speeds there is no performance penalty at all because torque
> is proportional to current, and it is the drive that sets the current.
>
> Dyno tests bear this out.
>
> Example: a 3V, 4A unipolar motor would be the same as a 1.5V, 4A
> parallel wired 8-wire motor. The difference is 1.5V. If this motor
> were run at 60VDC, the first motor would see 57V (60-3) while the 2nd
> motor would see 58.5V (60-1.5). The ratio would be 57/58.5 or 97.4%
> the power of a parallel wired motor.
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@egroups.com, Alan Marconett KM6VV <KM6VV@a...>
> wrote:
> > Thanks Mariss,
> >
> > I wasn't aware that only using 1/2 the winding would be that close
> to a
> > parallel connection. The old Slo-Syn tables I have don't look that
> > good. More like 20%, as I recall. Still, you waste 1/2 the
> windings!
> >
> > Alan