– 3 –
Full Coil
versus
Half Coil
w
t
t
A
H
h
t
h
b
t
t
Full Coil
Half Coil
Drive Current Setting
0.5 I
I
D.C. Supply Current
0.33 I
0.66 I
Drive Heating (low speed region)
0.15 I
2
0.6 I
2
Drive Heating (high speed region)
0.03 I
2
0.1 I
2
Coil Inductance
4L
L
Coil Resistance
2R
R
Speed/Torque Break Point
0.5f break
f break
Low Speed Torque
T
T
High Speed Torque
kVT/2f
kVT/f
Danaher Motion
•
Precision Systems Group
•
7C Raymond Avenue, Salem, NH 03079
tel:
603.893.0588
•
toll free:
800.227.1066
•
fax:
603.893.8280
•
url:
www.danaherprecision.com
Since most stepping motors are six lead devices, and the
MDM-7 drive has four output leads, two leads must be left
unconnected. The choice of which two leads to leave uncon-
nected has distinct performance impacts, as described below.
Stepping motors are normally wired as two center-tapped coils
(six leads total). Our pins #3 and 4 (similarly #5 and 6) can be
wired across the full coil, leaving the center tap disconnected,
or across a coil end and the center tap, leaving the other coil
end disconnected. These two possibilities are referred to as
“full coil” and “half coil,” respectively.
Operation in full coil mode doubles the number of winding turns
and quadruples the winding inductance (compared to half coil
operation). Accordingly, the current setting resistor (see below)
should be set to one-half the rated (nameplate) motor current
value for “full coil” operation (the nameplate value assumes
unipolar, half coil operation). Heating of the drive module and
motor will be half that experienced in half coil mode. Power
supply current requirements are also cut in half. On the down-
side, motor torque will fall off faster at higher speeds. The
graph of motor torque versus frequency can be broken into two
regions: a low speed region within which torque is constant,
and a high speed region within which torque is inversely pro-
portional to frequency. The onset of the “break” between the
two regions will be halved in full coll mode, and the torque at
any given high speed will be half that available-in half coil
mode. The chart below summarizes the differences between
full and half coil modes.
In this chart, I is the rated (nameplate) current; L is the rated
inductance; R is the rated resistance; V is the drive supply volt-
age; f is the step frequency; T is the holding torque, and k is a
motor-specific constant of proportionality. Since high speed
torque is proportional to the D.C. supply voltage, full coil opera-
tion at 50 volts will result in the identical performance as half
coil operation at 25 volts.
The basic conclusion is that for high speed performance, half
coil mode should be employed. If low to moderate speeds are
adequate, and especially if heating or supply current are of
concern, full coil operation is preferable. The pair of leads
which remain unconnected should be insulated to avoid shorts
or shocks; under some circumstances voltages of up to 120
volts can appear on these leads. There is no specific rule as to
the order of the four remaining leads and our terminals #3, #4,
#5, and #6 (excepting, of course, that pins 3 and 4 must run to a
motor coil and not to each end of two separate motor coils).
Reversing any two of the four motor leads will result in a direc-
tion reversal relative to the state of the direction line.
Accordingly, if your system “runs the wrong way,” reversing
any pair of motor leads (after carefully powering down the D.C.
supply) will reverse the system direction sense. This is consid-
erable easier than inverting the level of the direction line via
hardware. The color conventions for several stepping motor
manufacturers are shown on the next page
