Re: [CAD_CAM_EDM_DRO] current limiting resistors??????????

RC: I assume you are talking about the "current-limit resistors" used in
series with the "positive common center-tap" of each CT'ed winding of
"six-wire" unipolar stepper motors? The reactance of the motor-winding
increases as the pulse-rate ("steps/second") increases to the motor. So, if
you have a power-supply voltage say ten times the "normal, steady-state, DC"
coil-voltage, you have to somehow REDUCE it so you don't smoke the motor.
So, you insert this big, energy-wasteful series I-limt R. Calculate R so
that the "DC voltage" is the max. for which the motor is specified. NOW, as
you "ramp up" the stepping-rate, the reactance increases, but the
VOLTAGE-available is ten times as high as the rated winding-voltage, so the
voltage-drop begins to "share" between the increasing coil-reactance and the
I-limit R, keeping the current more-nearly "what it should be" than it could
possibly be, were the DC voltage LOW, with NO I-limit R. An old-fashioned
compromise which "kinda works", particularly for summa us po' ol' boys!

HOWEVER! The new "[bipolar!] chopper-drive", also using a higher-than
steady-DC voltage, "chops" the DC into pulses that total a "percent" of
constant-on-time, so that the AVERAGE DC-current is the max. rating of the
coil. BUT, the voltage is WAY above the coil's "max. DC rating". This way,
when reactance increases when pulse-rate goes up, the "duty-cycle" % of
time-ON increases to maintain that average DC. NO wasted power in I-limit
R's in series. MUCH more efficient!

Hey! This is a very difficult concept for an ignernt old troll to convey,
and Mariss is the genius EE "in here", and I am SURE he can explain it much
better! Wot, Mariss?????

HOPE I have been, however, of SOME use! Jan Rowland, Ignernt old
troll