Unipolar Stepper Motor vs Bipolar Stepper Motors

In previous blogs we learned the basics of stepper motors and how they function. Also discussed was the many uses at which stepper motors can be used. The next step in these articles is to take a bit more in depth look at the two different types of two-phase stepper motors. There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor, one being bipolar and the other unipolar.

Unipolar Stepper Motors
The unipolar stepper motor operates with one winding with a center tap per phase. Each section of the winding is switched on for each direction of the magnetic field. Each winding is made relatively simple with the commutation circuit, this is done since the arrangement has a magnetic pole which can be reversed without switching the direction of the current.

In most cases, given a phase, the common center tap for each winding is the following; three leads per phase and six leads for a regular two phase stepper motor. You will usually see that both these phases are often joined internally, this makes the stepper motor only have five leads. Often a stepper motor controller will be used to activate the drive transistors in the proper order. Since it is quite easy to operate these stepper motors, they are often very popular among hobbyists and are usually the cheapest way to get precise angular movements.

Bipolar Stepper Motors
With bipolar stepper motors there is only a single winding per phase. The driving circuit needs to be more complicated to reverse the magnetic pole, this is done to reverse the current in the winding. This is done with a H-bridge arrangement, however there are several driver chips that can be purchased to make this a more simple task.

Unlike the unipolar stepper motor, the bipolar stepper motor has two leads per phase, neither of which are common. Static friction effects do happen with a H-bridge with certain drive topologies, however this can be reduced with dithering the stepper motor signal at a higher frequency.

This is a brief introduction on how a stepper motor may vary between unipolar and bipolar.

Bipolar stepper motors can be a bit more difficult to operate, and the unipolar motor does feature twice the amount of wire in the same space. Different projects will require different types and settings of stepper motors. Browse the large selection to find the perfect one for your needs.

George

George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.