Re: [CAD_CAM_EDM_DRO] bipolar or unipolar

One of us asked for an explanation of "BIPOLAR" vs. "UNIPOLAR" steppers and
drives, and I said I'd write one, but now, my e-mail "filing-cabinet" on AOL
is GONE! [never mind!]

SO... Whoever asked:

A stepper motor has a solid PERMANENT-MAGNET armature with NO electrical
connections, as a "universal motor" (the kind in a hand-drill or router) has.
Thus, the COIL WINDINGS are all in the "stator", the part AROUND the
armature which does not move.

The armatures of steppers have "teeth" kinda like a pinion-gear, with several
changes in permanent-magnetic polarity as you go around the surface of the
armature. The inside of the stator has like teeth, but not the same number,
so that the armature "finds" a position it likes, and will STAY there until
some magnet-change takes place, as the current through the stator-windings
change.

Normally, "electrically speaking", there are TWO "phases" in a stepper (some
have five, but I won't go there!). To make a STEP, the magnetic polarity of
the "teeth" in the stator must change, this then causing the armature to
"jump" to the next preferred position. If the SECOND winding changes
polarity next, then ANOTHER step takes place, and so on.

The BIPOLAR is simple: There are simply two windings (schematically
speaking, that is), each with a beginning and an end, or "two wires, each".
So, we have a total of FOUR wires from a BIPOLAR stepper. So, the driver
feeds DC through both windings, and for ONE step, it REVERSES the polarity
through ONE winding. For the NEXT step, it reverses polarity through the
SECOND winding. Then the first winding again, and so on.

The UNIPOLAR is a bit harder to explain in text without schematic drawings,
but I will try: There are two windings, but these are CENTER-TAPPED. Thus,
THREE wires per winding, a total of SIX wires per motor. Both center-taps
are normally "positive common" (both +DC). If ONE end of each winding is
"grounded" (connected to -ve), the armature will find that "preferred" static
position. Note, the OTHER end of each winding is NOT connected to -ve (or
anything) at this instant! Now: Disconnect ONE of those winding-ends, and
connect the OTHER end to -ve. A step takes place. And, if you do the same
with the OTHER winding, the second step takes place. Note, here, the driver
is connecting two of the "four ends" ONLY to negative, the +ve (center-tap)
being ALWAYS connected. Thus, we call this "UNIpolar". Electrically, this
kind of driver is easier to make, but the motors have "half the copper unused
at any instant". The bipolar driver uses TWICE the number of
power-transistors and the "logic circuitry" that switches these eight is a
bit more complex than it need be in a unipolar drive, but ALL the "copper in
the motor" is conducting current at ANY time. Thus, the bipolar system is
more efficient. A bipolar stepper of a given size has MORE torque for the
SAME "wattage" as the unipolar. But there's more!

For the old unipolar, large "ballast resistors" were used in series with the
center-tap +ve common (electrical reasons for this are beyond the scope,
here!), and these wasted MUCH more power than the motor actually used. It
was a "simplification" of a problem. But with (nearly-?) all bipolar drives,
the output-current is "chopped" at a supersonic rate, and the "duty-cycle" of
the chopped DC is regulated to maintain the AVERAGE DC-CURRENT for the motor
being driven. Thus, there is NO wasted energy in big power-resistors. For
unipolar "L/R drive steppers", if you had a 24 VDC supply powering a unipolar
stepper wanting, say, 3 volts @ 3 amps per phase, then you had a total of 6
amps, 144 watts of DC per motor, with 144-18 = 126 watts being "wasted" by
EACH ballast resistor! But for the bipolar, you'd have two windings drawing
an "average" 3 amps each with an effective 3 VDC per winding, or 18 watts per
total per motor. MUCH more efficient, AND there's be MORE torque output
each, since ALL the copper in the windings is used at any time!

HOPE I got it all correct! Wot? Jan Rowland


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