How to find out whether I have a unipolar or a bipolar stepper motor?

You have a third type of motor: A "Universal" motor.

This is a motor can can be configured as either a unipolar or a bipolar motor.

General rules:

• 4 Wires: Bipolar only
• 5 Wires: Unipolar only
• 6 Wires: Universal
• 8 Wires: Universal

A unipolar only motor has the center of both windings connected together internally. This precludes the use of the motor in a bipolar system.

With a motor with six leads, if you connect the leads from the center of each winding together, you get a unipolar motor. If you leave the center connections unconnected, and drive each winding using only the connections to the winding ends, you are using it in bipolar mode.

If your stepper has six wires, it can be used as a unipolar motor because you have 2*3 wires, where one of the three wires (each) is a center tap and the other two wires are one or another end of the same winding started at this center tap. Please see the appropriate section of the wikipedia article about steppers.

Bipolar motors have four wires; two for each winding. To reverse the poarity of one winding, you electronically switch it the other way round instead of switching on one end or the other of a center-tapped winding.

There are also motors with eight wires. These have four windings that can be connected in series like a center-tapped, bipolar motor or in parallel like a unipolar motor.

Here is a link to a well-written example of a datasheet. It details how you can configure a stepping motor in a biploar or unipolar way. You can figure out how pre-made unipolar or bipolar motors with six, five or four wires are really just cost- or space-cutting "wire-savers" compared to the flexible eight-wire types.

Hint: Use an ohm-meter to find out which three wires belong together. To see how the polarity is configured (What's the center tap? Which end is which?), connect an oscilloscope with at least two channels and turn the motor by hand.