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Posted by John Popelish on October 16, 2007, 1:10 am
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vorange wrote:
> 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.
Yes. 6 wires indicates two windings and a center tap on each.
> But should
> that not mean it has 4 phases. Why do they list it as a "2 phase
> stepper motor"?
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.
> 2) The motor above is listed as a "bipolar (in series) and unipolar"
> as well which is kind of confusing.
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.
> 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).
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.
> 3) Can you suggest a driver chip (IC) i can use to drive the above
> stepper motor?
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
> Would the following driver be ok :
> http://www.onsemi.com/pub/Collateral/MC3479-D.PDF
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.
> 4) Is "load voltage" the voltage rating of the stepper motor? (in this
> case 4V)
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.
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>
> http://tinyurl.com/2qr4d9
>
> I'm guessing since it has 6 wires that it has 2 coils.