Rewirering 5-wire unipolar stepper motor to 4-wire bipolar

I had couple of unipolar stepper motors made by TEAC, and SANYO.

They were probably removed from old hard drives long time ago, and were lying in my parts drawer. I was considering buying 4-wire motors for my project, but I spotted this blog post, and decided to give it a try. Post gives 6-wire unipolar motor as an example, but rule presented can be applied to my motors as well.

Unipolar stepper motors come in at least 2 variations:

• 5-wire - center taps of both windings are soldered together internally
• 6-wire - center taps are available on the connector

They are easy to drive - put supply voltage to center tap, and pull side taps to ground sequentially. Problem is that most readily available stepper motor drivers with microstepping capabilities (like A4988) drive only bipolar motors. Bipolar motors also provide better torque (however in case of rewired unipolar motor you need to provide twice as much voltage that it was rated due to higher serial resistance, and inductance).
6-wire motor gives easy access to side taps. 5-wire requires some fiddly soldering skills, and motor disassembly.

It's good to take ohm-meter and test windings before doing any soldering work. This will confirm type of motor, and winding condition.
In my case all windings showed around 75ohm.

Getting inside TEAC motors is very easy since there are 4 bolts holding back cover. Most often case is held with welded or pressed joints (like SANYO one).

One end of all windings is soldered to center tap pad (uppermost left pad).
Wires should be unsoldered, and separated.

There are 2 ways winding pairs can be soldered together. Unfortunately there is no way of telling witch is good. In my case first try was incorrect. Motor will oscillate instead of rotating when soldered incorrectly.

After soldering wires were secured with insulation tape.

Then everything was assembled, and here is my test setup:

• MSP430G2 Launchpad board
• Two A4988 motor drivers configured for 1/2 step resolution
• External power supply for motors.

I can't exactly tell, but I think that motors were originally designed for 12V operation. My power supply is limited to 15V, so I couldn't actually push them to the limits. I'm going to make next post about testing torque of these motors in various stepping modes, and speeds.

Video of motor operation:

Just for curiosity, here is disassembled view of SANYO motor that was actually destroyed in process.

Main fault is cracked rotor magnet. There is a lot of cost savings visible in construction. Stator coils are just wound around in two layers, and special tabs arrangement shapes magnetic field. Rotor is casted of magnetic material similar to fridge magnets, but much harder, and fragile. Center tap is connected to 5 windings 35ohm each.

Software started from TI stepper motor driver demo that can be downloaded here. Code for this project can be cloned from github.
Code can be imported to TI CCS workspace (tested with version 5.3.0).

Update (2015/12/07):

Pictures of motor resoldering are not clear enough, and I decided to add schematics below to help understanding.

Left schematic shows motor before conversion - there are 4 windings, that all have one end soldered together with others, and going out as single wire (connector may not be drew exactly to the motor specification, pin 1 corresponds to uppermost wire in the pictures). All four winding ends labeld 2, 3, 6, 7, are spliced together, and soldered to wire 1.
Right picture shows motor after conversion. So wire splice is unsoldered, and untangled. I identify phase windings by trial and error, and solder each pair together (2-3, and 6-7) effectively making 2 windings.
Let me know if this explanations is better.