Re: [CAD_CAM_EDM_DRO] Bipolar vs Unipolar vs Chopper Driver

In a message dated 30-Sep-00 06:19:42 Central Daylight Time,
jguenther@... writes:

<< What is the difference / benefits between bipolar, unipolar and chopper
drivers? I see lots of advertising for each type, what is the best type for
small machines using NEMA 23 or NEMA 34 stepper motors? >>

John: By the time you read this, someone else in the group with far more
experience and brains than I will have answered you. But here's my 2¢ worth:

The "old" way was "unipolar", where "each" winding was twice as many turns as
needed for the job, but that was then center-tapped. That way, if the
center-tap was +ve common, the magnetic polarity of the winding could be
"reversed" by connecting -ve alternately to ONE end, then the other. This
way, you needed only ONE polarity of DC-power-supply. But only HALF the
copper was actually conducting current at any instant, so, this was less
efficient in terms of space, torque-per-size motor, economy of material, etc.
Too, this method almost-always was the "L/R" type where the "slowing-down
effects" of the inductance of the winding was partially overcome by inserting
a "series resistor" between that center-tap of each of two windings (i.e.,
two resistors), which limited the maximum current to less than the max.
allowable. But, the MUCH-larger power not used at slower step-rates - more
than 3/4, usually! - was converted to heat, and totally wasted, except in the
winter in northern climes.

The "bipolar chopper drive" is "latest state-of-the-art". There are only TWO
windings per motor, i.e., "4 wires"; indeed, some refer to these motors as
"4-wire steppers". The power-supplies have a "common" and TWO polarities
out, i.e., a -ve out, a "ground" or "common, and a +ve out. The magnetic
polarity (N/S) reversal is done by reversing the electrical polarity. This
means a more complex driver-circuit, but, hey, this is 2000, wot?! Thus,
"ALL the copper" is conducting current. Stepping is done by reversing the
polarity to ONE winding, then the other, then the first one again, and so on.
Current-limiting? This is done by "chopping" the DC into "pulsed DC". By
varying the "duty-cycle" (the ratio of "current-ON" to "current OFF"), you
control the average current, and with this scheme, you can run the max. DC
voltage on up there to 40 or even 80 volts in some circuits, while the "motor
wants only 2 to six volts, depending on the motor". That much-better
overcomes the effects of the inductance of the windings in "slowing things
down".

I had built a number of small "workshop machines" with the old circuitry (L/R
unipolar), and I am now finishing-up two new CNC lathes using new NEMA-34
(KML092-??) motors by Superior Electric division of Warner/Dana, and they
have MUCH, much more torque for the same size motor. BUT, they come with a
VERY expensive driver/power-supply box, but it IS tidy and efficient! That
much more I don't have to build, as I did, before! Whew!

Jan Rowland, Troll