• A stepper motor with 5 wires is almost certainly 4-phase unipolar.

• A stepper motor with 6 wires is probably also 4-phase unipolar, but with two Common Power wires. They may both be the same colour.

• A stepper motor with only 4 wires is most likely bipolar. Information on that type is available elsewhere.

There are two stages to sorting out which wire is which in a 5- or 6-wire unipolar stepper motor:

1. Isolate the Common Power wire(s) by using an ohmmeter to check the resistances between pairs of wires. The Common Power wire will be the one with only half as much resistance between it and all the others.
   

   This is because the Common Power wire only has one coil between it and each other wire, whereas each of the other wires have two coils between them. Hence half the resistance.

2. Identify the wires to the coils by supplying a voltage on the Common Power wire(s) and keeping one of the other wires grounded while grounding each of the remaining three wires in turn and observing the results.

   Select one wire and ground it Assume it's connected to coil 4
   Keeping it grounded, ground each of the other three wires one by one
   Grounding one wire should make the rotor turn a little clockwise That'll be the wire connected to Coil 3
   Grounding one wire should make the rotor turn a little anticlockwise That'll be the wire connected to Coil 1
   Grounding one wire should do nothing That'll be the wire connected to Coil 2

   Note:
   The numbering of the coils as above is purely arbitrary - it's the order they are in that's important.

Examples

• Generic half-height 5.25" floppy disk drive

• TEAC half-height 5.25" floppy disk drive

• Apple II full-height 5.25" floppy disk drive

Stepper Motors Index

last updated: 10-Sep-98 Ian Harries <ih@doc.ic.ac.uk>