Re: Unipolar/Bipolar Parallel/Series?!?!?

> When Mariss wrote this, surely he was experiencing a taste of the
> grape

Jan,

Regarding the grape, I myself prefer a 1968 Louis Maritini Cabernet
Sauvignon. Hard to get now but worth the effort if it hasn't turned.
I have two cases myself that I guard as if it were gold.

My enjoyment of it however does not adversely affect my understanding
of the basic physics underlaying the operation of step motors.

I had a post on this list called "What is this unipolar, bipolar,
series stuff anyway?" or some such name. I wiil take the time to find
it and repost it.

All this stuff about step motor connections seems complicated unless
you understand a couple of very basic principles which make things
very simple.

(1) Step motors work using only 2 coils. The sequence in which they
are energized is the same as drumming two fingers on a desk; no
fingers down, one down, both down, first finger up and so on.

(2) Holding torque is equal to ampere-turns. This means torque is
proportional to the current passing thru a winding multiplied by the
number of turns of wire it passes thru.

(3) Iron saturates. This means The more current that passes thru a
winding, the more iron magnetic di-poles line up with the generated
field until all are lined up. At that point increasing current cannot
increase torque because all di-poles are lined up. There are none
left, so the iron is saturated.

(4) Motor output power is proportional to the power supply voltage
divided by the square root of the motor winding inductance.

Here are the rules:

4-wire motor. No problem. Use the rated current and you will be fine.
No options here.

6-wire motor (unipolar). This is the same as the 4-wire except the
windings are center-tapped and the center taps are brought out.

You have two choices here; half-winding or full winding. You use half-
winding if yours is a high speed application or full winding if it is
a low speed one. Use the unipolar current rating for half-winding and
1/2 that for full winding.

8-wire motor. The same as a 4 wire motor except the windings are
center-tapped and brought out as seperate wires for the center tap
instead of being internally connected.

The performance in parallel is only 2% better in parallel compared to
half-winding connection on a 6-wire motor. The series connection is
exactly identical to full winding performance on a 6-wire motor.

Of all the motor types I prefer the 6-wire, or unipolar motors. You
can't screw up the winding phasing like you can with 8-wire motors
(buck vs. boost) and the performance penalty of 2% is essentialy
meaningless. You simply pick 4 out of the 6 wires available and it
will work.

Jan, getting back to your 5A motor being run at 3.5A in series. This
is a common mistake if you use a microstepping drive. 3.5A is the
square root of 2 divided by 2 times 5. This normalizes heat
dissipation in a winding but it does not address the 1.414 (square
root of 2) increase in ampere-turns. This would certainly saturate
the iron in the motor and cause "lumpy" behavior in microstep
placement.

The magnetic flux path is not shared between the windings; that means
one winding has no knowledge of what the other winding is doing. If
you do not believe this, try to use a step motor as a transformer.
You will see there is no common flux path.

Anyway, it is my daughter's 17th birthday and I have to get back to
the people here. We are in fact enjoying one of our '68s and I must
say it is the elixir of the gods.

Mariss
Geckodrive Inc.