28
CENTENT CN0162 MICROSTEP DRIVE
MOTOR LOAD
Generally speaking, motor load is the
single most significant contributor to
microstep positioning error. A step motor
only generates torque when a rotor error
angle exists. The relationship between
rotor displacement angle and restoring
torque for a typical motor is shown in
Figure 10.
The function that relates error angle to
torque is approximately sinusoidal. An
error angle equal to one microstep occurs
when motor load equals the holding
torque divided by the microstep
resolution. If the motor load is transient or
due to acceleration, the rotor error will
decrease to a residual level upon removal
of that transient.
-3
0
0
T
O
R
QU
E
(O
Z
-I
N
)
+
300
-1.8°
0
+1.8°
MECHANICAL ANGLE
Figure 10: Torque vs. rotor angle
TORQUE AND POWER
Step motor performance curves exhibit two distinct regions with respect to speed. In
Region 1 (Figure 11, page 29), motor torque is constant with speed while motor shaft
power is proportional to speed. In Region 2, motor torque decreases as the inverse of the
speed while motor shaft power remains constant.
The current set resistor determines motor torque in Region 1. Motor torque is held
constant by controlling the magnitude of the motor phase current. The step rate in
Region 1 is low enough to permit motor phase current to reach the programmed value.
