Page 227 - Electric Drives and Electromechanical Systems

Chapter 8 Stepper motors 223

FIG. 8.10 Static-rotor rotor-position characteristics when: (A) one phase is excited and (B) two phases are excited.
If both curves are combined the step angle is reduced from 18 to 9 degrees.

If, on the other hand, two phase-windings are excited, the resultant torque sum-
mation will produce two new equilibrium points, BA and CB , Fig. 8.10B, which are
0
0
midway between the single-winding equilibrium points. Therefore, if the windings of
the variable reluctance stepper motor considered earlier in this chapter are excited in
thesequence A, BA, B, CB,C,AC, A., each excitation change will result in a movement
of half its normal step. This approach to stepper-motor control is termed half-stepping.
It should also be noted that the peak-torque resultant for multiphase energisation is
greater than that which occurs when a single phase is used. This is of particular
importance when the number of stacks is greater than three. It is normal practice to
energise three or four phases in any one time in the control of a seven-stack stepper
motor. Half-stepping operations can be applied to hybrid stepper motors, but due to
the bipolar nature of these motors’ drives the power capacity of the drive system has to
be increased. For a forty per cent increase in the torque, the power supply has to be
increased by one hundred per cent. As the control of a stepper motor are purely digital,
the complete translator and control system can be realised through the use of a small
microprocessor (Kenjo, 1990). The low cost of computing power has led to the
implementation of closed loop control of the stepper to optimise its performance (Le
et al., 2017), in part this type of work is based on extensive simulation of the motor,
control algorithms and load as discussed further in Chapter 10.