Page 691 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 677

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0 0 FIGURE 8.54: Drive types for
step motor: (a) unipolar drive,
(b) (b) bipolar drive.

8.6.2 Step Motor Drives
Drive controls the direction and magnitude of current in each phase, and hence controls the
direction and magnitude of torque. There are two types of drives which must be matched
to the winding type of a step motor (Figure 8.54),

1. unipolar drive,
2. bipolar drive.
Unipolar drives require the motor windings to be center tapped (Figures 8.54a, 8.51a).
By turning ON the current at one end of the winding versus the other end, the direction
of current in the winding is changed, hence the flux direction. When a phase winding is
energized, only 50% of it is used by a unipolar drive. Unipolar drives require two power
transistor switches per phase. In a unipolar center-tapped motor, each phase has three leads:
two sides and one center tap lead (Figure 8.51a).
Bipolar drives use an H-bridge (four power transistor switches) per phase. A bipolar
drive for a two-phase step motor has two H-bridges (2 × 4 = 8 transistors). The direction
of the current is changed by controlling which two pairs of the H-bridge switches are
turned ON. Although it requires twice as many power switches, it makes 100% use of the
conductors of the energized coil. A stepper motor with bipolar winding has two leads per
phase (Figure 8.51b,c). Some motors are wound such that they can be configured to be
driven by either a unipolar or bipolar drive. In this case, each winding has two identical
windings with four leads. Depending on how the leads are connected, the motor can be
made to operate with a unipolar or a bipolar type drive.
The microstepping drive was developed to solve resonance problems. In microstep-
ping, the current is not simply switched ON/OFF between phases, but gradually changed.