# Stepper motor question

Discussion in 'Electronic Basics' started by vorange, Oct 16, 2007.

1. ### vorangeGuest

1) I have a stepper motor in mind :

http://tinyurl.com/2qr4d9

I'm guessing since it has 6 wires that it has 2 coils. But should
that not mean it has 4 phases. Why do they list it as a "2 phase
stepper motor"?

2) The motor above is listed as a "bipolar (in series) and unipolar"
as well which is kind of confusing. Is it possible to drive it as a
bipolar motor (i.e. without using the two center taps)? Is there any
downside to driving a unipolar motor as a bipolar motor (or is this
question totally ridiculous).

3) Can you suggest a driver chip (IC) i can use to drive the above
stepper motor? Would the following driver be ok :

http://www.onsemi.com/pub/Collateral/MC3479-D.PDF

4) Is "load voltage" the voltage rating of the stepper motor? (in this
case 4V)

2. ### John PopelishGuest

Yes. 6 wires indicates two windings and a center tap on each.
2 phases means that the coils are placed in two magnetic
locations. You get 4 effective phases by driving each with
current in one of two directions.
Bipolar means that the driver circuit applies voltage in
either direction across the whole winding (the two halves of
the center tapped winding, both halves in series, carrying
the same current). Unipolar means that the driving circuit
can force current in only one direction, so you get the
reversal by applying it to only one half of the center
tapped winding at a time, both currents either toward or
away from the center tap. The effect is one each half
magnetizes the poles in one direction and the other half
winding magnetizes the poles in the other direction. The
efficiency is lower, because only half of the copper is
being used at any time.
An excellent question. It is perfectly reasonable to drive
a unipolar motor with a bipolar driver, and it will be more
efficient because the whole winding will be in use. Having
the current pass through twice as many turns also produces
about the same torque as twice the current from a unipolar
driver. The down side is that the motor will require 4
times the supply voltage to max out at the same speed,
because twice as many turns produce 4 times the inductance.
You can give up the current efficiency and recover the
full speed by driving only half of the center tapped winding
with the bipolar driver, but you will be wasting half of the
copper, so the temperature rise will be worse.
Allegromicro makes many:

http://www.allegromicro.com/en/Products/Categories/ICs/motor.asp
I would pick a dual bipolar driver with two or three times
the series current rating of the motor if you want full
torque from the motor. Perhaps the AD3992, because it is
available in a DIP package for easy prototyping.
http://www.allegromicro.com/en/Products/Part_Numbers/3992/index.asp
It is a bit wimpy, with a current rating of only 0.35 amp,
which I would not get close to, for good reliability. It
would probably turn the motor but would not get much torque
from it.
The chip is rated for up to an 18 volt motor supply, but it
puts out about 3 volts less than the supply because of the
voltage drops of the power switches. It also has no
provision for current regulation like the AD3992 has.

Current regulation allows the supply voltage to be well
above the DC rating of the motor, so that there is extra
voltage available to overcome the generated voltage as the
motor moves faster. Basically, the motor torque is
proportional to the coil current, but the voltage it takes
to produce that current is proportional to the speed, plus a
fixed amount to overcome the winding resistance.

3. ### vorangeGuest

Thank you John.

As always, you are an encyclopedia of knowledge. You really should
write a book on electronics for beginners to intermediate. I predict
it would sell like hotcakes.

Just a quick comment :
Yes I noticed that too. I had planned to put transistors on its
outputs to boost the current. But the Allegro suggestion is looking
good.

Cheers!

4. ### BRWGuest

My favorite stepper motor tutorial:

http://www.cs.uiowa.edu/~jones/step/

BRW

5. ### John PopelishGuest

(snip)

Micro stepping is used with both bipolar and unipolar
drivers, but I have seen it most often with bipolar drivers.

6. ### JamieGuest

unipolar steppers are niffy with the correct controller on it.
the CT Fields give you the option of micro stepping. That is
if you have the correct controlling electronics. what this does
is locks both fields on . You don't get as much holding torque as
you would on a single phase position but it's nice where you would
have lets say a 100 PPR stepper and emulate a 200 PPR stepper.
In some cases, this is used as a soft position index to reduce
shock movement between poles. But then again, it depends on the
controller.

7. ### JamieGuest

It works smoother with unipolar but with less torque.
the bipolar gives you more torque but is a little less
stable in the micro step. In many cases, more torque is more
favorable but does require more current to operate it while in the
split phase index.
I wrote a little code in an AVR to operate a unipolar motor
using PWM to perform soft movement of the rotor. I gotta say, it
was an interesting experience. Parameters had to be tweaked to
account for the induction sang in the coils. When it was all done
how ever, the motor it self was able to turn so smooth between
indexes, you couldn't tell it was a stepper motor.

We had a project where we needed smooth movement but be able to
lock the rotor once it stopped and index 2 notches . the locking
position didn't have to be exact! but the motion to get it there
needed to be smooth.
it was a toss up between using a stepper or a brushless DC motor.

I guess either one would work, just a matter of how you would want
to approach it.
And bipolar is more common due to it's simplicity in design.