Unipolar vs. Bipolar Stepper Motor

The terms UNIPOLAR and BIPOLAR, refers to the configuration of the coil winding on each stator within a stepper motor, and the type of magnetic field it produces. However, the coil configuration is the most important difference between the two. In a bipolar stepper motor, each stator coil winding has two terminals, however in a unipolar stepper motor, each stator coil winding has three terminals because of a centre tap.

A stepper motor can have many stators, and each stator has a dedicated coil winding. The magnetic field on each stator has to change in order for the stator shaft to rotate. The polarity changes when the voltage polarities to each coil changes. This design feature is the same in both types of stepper motor; however, the major difference between unipolar and bipolar stepper motors is with the coil winding, and the switching logic they require.

With the bipolar coil scheme, each coil requires two changeover switches to reverse the voltage polarity to the coil.

For simplicity and clarity, this diagram shows the operation of just one stator and its coil. Usually, a step motor can have many stators and coils depending upon the model and step resolution requirements.

With unipolar coil scheme, the coil has a centre tap, which could remain fixed to the positive rail, and then we only have to switch the negative supply to either ends of the coil. With a centre tap, the advantage is that we can have a unidirectional flow of current, however as you can see by the arrows, it is only through half of the coil at any one time. A major advantage of this scheme is that you only need one changeover switch.

In practice, manufacturers often do not provide the centre tap of the unipolar coil, and instead they bring out four wires of the coil, which gives the engineer more flexibility.

Since changeover switches represent semiconductor power transistors, the driver IC for the bipolar stepper motor is usually more expensive, as each coil requires double the amount of transistors per coil when compared to the unipolar stepper motor coil.

As you can see from the diagrams, the unipolar coil scheme has a much simpler switching logic, which engineers usually implement using a flip-flop circuit to drive a pair of transistors. The complementary outputs of a D-type flip-flop, or a JK flip-flop, are ideal for this application.

There is also a major difference in torque between the two types of motors. Usually, bipolar motors have more torque compared to the unipolar motors. This is because in a unipolar motor, the stator coil has a centre tap, which results in only half of the coil generating a magnetic field at any one time. Fewer coil turns means less magnetic field and less torque. In comparison with a bipolar motor, we are using the complete coil, and therefore we get more torque.

Therefore, the unipolar motor is usually cheaper and easier to implement, but it is not as powerful, compared to the bipolar stepper motor.