What Is A Stepper Motor? (continued from page 5)

Stepper Motors: Multiphase Motors

A less common class of permanent magnet stepping motor is wired with all windings of the motor in a cyclic series, with one tap between each pair of windings in the cycle. The most common designs in this category use 3-phase and 5-phase wiring. The control requires 1/2 of an H-bridge for each motor terminal, but these motors can provide more torque from a given package size because all or all but one of the motor windings are energized at every point in the drive cycle. Some 5-phase motors have high resolutions on the order of 0.72 degrees per step (500 steps per revolution).

With a 5-phase motor, there are 10 steps per repeat in the stepping cycle, as shown below:

Terminal 1 +++-----+++++-----++
Terminal 2 --+++++-----+++++---
Terminal 3 +-----+++++-----++++
Terminal 4 +++++-----+++++-----
Terminal 5 ----+++++-----+++++-
time --->

Here, as in the bipolar case, each terminal is shown as being either connected to the positive or negative bus of the motor power system. Note that, at each step, only one terminal changes polarity. This change removes the power from one winding attached to that terminal (because both terminals of the winding in question are of the same polarity) and applies power to one winding that was previously idle. Given the motor geometry suggested by Figure 1.5, this control sequence will drive the motor through two revolutions.
To distinguish a 5-phase motor from other motors with 5 leads, note that, if the resistance between two consecutive terminals of the 5-phase motor is R, the resistance between non-consecutive terminals will be 1.5R.

Note that some 5-phase motors have 5 separate motor windings, with a total of 10 leads. These can be connected in the star configuration shown above, using 5 half-bridge driver circuits, or each winding can be driven by its own full-bridge. While the theoretical component count of half-bridge drivers is lower, the availability of integrated full-bridge chips may make the latter approach preferable.

article contributed
by Douglas W. Jones