Re: A little experiment on demagnetization

krnldmp,

I'll run that data tomorrow, all that matters is the no-load stall
speed because motors have constant torque losses. Now about the 1%
stuff:

1) A step motor winding has resistance that will develop a voltage
drop when current passes thru it.

2) The difference between an ideal step motor (superconducting coils
with zero resistance) and a real world one is this voltage
effectively subtracts from the power supply voltage.

3) This voltage drop is meaningless while the drive is current
limiting at low speeds.

4) As speed increases, motor inductive reactance ultimately begins to
limit coil current, not the drive. As speed continues to increase,
coil current becomes the inverse of frequency (speed).

5) This current will be much less than the holding current and and
the voltage drop across the coil resistance will much less as well.

6) A parallel connection halves the voltage drop compared to a half
coil connection.

Now, let's say you have a 5.5A, 2.25V unipolar motor. Based on the
20:1 max overdrive rule, you select a 45VDC power supply voltage.

Parallel connected, you would have a 1.125V advantage over a half-
winding motor. The ideal motor would then "see" 43.875V versus 42.75V
for the half coil connection. The half coil would "see" 97.4% of
parallel connected voltage, a 2.6% difference.

At 5.5A, the motor is still being current limited by the drive. Now
increase the speed to where current limiting ceases altogether (about
2.75A). Here the motor begins to deliver full power.

The parallel connection advantage now drops less than .6V and as a
percentage, (based on 45VDC), the half coil is now behind by only
1.4%.

As speed increases further, the disadvantage falls well below the
stated 1%.

Dyno tests concur empirically.

Mariss

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

> Just out of curiosity, if you still have the data, what was the top
> frequency of operation for the recent test and what was the torque

at

> that frequency for both the good motor and the bum motor.
>
> I took apart a motor and cooked the rotor to well beyong the curie
> temp and then adter reassembly dumped current in through one winding
> with a DC power supply. Hold current was a joke. Guess I'm not
> understanding how the magnets do what they so as compared to the
> switched reluctance motor without them. And... it appears that there
> is no change in permeability to speak of.
>
> I still don't understand how you are getting to within 1% of bipolar
> drive performance with unipolar drive, or did I misunderstand that
> too.
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis
> <mariss92705@y...>" <mariss92705@y...> wrote: