Why Bipolar?

If one is going to run a stepper half-coil as has often been
recommended here in order to control inductance, what is the advantage
of having a bipolar driver compared to unipolar?

The disadvantage of course is that an L298 dual H-bridge is limited to
2-3 amps, wheras a IRF540 N-FET which is far easier to use in a
unipolar drive which needs only low-side drivers can do 60 amps...

This mostly applies to medium-large machines, but the question came up
as I was trying to figure out why the light machines sherline lathe I
finally rewired for the PC/EMC tops out at just over 6 IPM (yeah, six,
not sixty) turns out it is unipolar, but more importantly the motors
have about 20 ohms of resistance per half coil! That 30v L/R drive
doesn't stand a chance of putting any current through them at speed.
Of course being a lathe, and being small, it doesn't need to go THAT
fast anyway.

Chris


--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
http://www.mdc.net/~stratton
NEW PHONE NUMBER: (617) 628-1062 home, 253-2606 MIT