Page 223 - Electric Drives and Electromechanical Systems

Chapter 8 Stepper motors 219

FIG. 8.7 Current through a unipolar stepper-motor winding as a function of speed. The applied voltage is shown
by the dotted line. (A) Winding current at low speed. (B) Winding current at high speed.

equal to the average motor torque. If the sum of the motor and the load inertias is low, the
motor will stall whenever the load torque exceeds the generated torque.
Since stepper motors are designed to operate in discrete steps, at very slow speeds, the
motor will come to rest between each excitation. Due to the dynamics of stepper motors
and their loads, the single-step transient behaviour tends to be very oscillatory, and the
effects of this have to be considered in the design of an overall system, because they can
result in significant problems in poorly damped systems, as discussed in Section 3.6,the
undamped natural frequency of oscillation in a drive system was shown to be

s ffiffiffiffiffiffi !
1 K
f n ¼ (8.6)
2p I tot