Identifying the leads of Unipolar stepper motor

Unipolar stepper motors have four coils and either five, six or eight wires. No matter how many wires, unipolar stepper motors are interfaced in the same way. Five wires will go to the controller circuit. Four wires will be excited by pulses from the control circuit and the remaining wire(common) is given to the power supply. To identify the wires, you need a resistance meter.

First, identify the common lead. For the six and eight wire versions, some wires are twisted together to form the common lead and reduce the lead count to five. The common lead is connected to the positive of your battery or power supply.

• Eight wire motors: Find the four pair of coils. These will be the wire pairs that have the coil resistance between them. There will be infinite resistance between pairs. One wire from each pair will be all joined together to form the common lead. Without a wiring diagram, you will have to do a lot of experimentation to determine which lead of the pair is for the common.

• Six wire motors: Find the two wires whose resistance to two other wires reads the lowest of the measured resistances between wire pairs. Join these two wires together to form the common.

• Five wire motors:Measure the resistance between all pairs of wires. One wire will read about one half the resistance to all other wires when compared to the resistance between other pairs. This wire is the common wire. No wires are joined in this case.

Now, proceed to identify individual coils in order of sequence.

• Connect the common lead to the positive of your battery or power supply.
• Connect any one of the other four leads to ground. This will be coil 4.
• With coil4 still grounded, connect another lead to ground. If the shaft does not move, you have coil2. If the shaft rotates clockwise, you have coil3. If the shaft rotates counter-clockwise, you have coil1.
• Repeat until you have identified all four coils.