Stepping Motor Driving: Bipolar Connections and Unipolar Connections

From this article, on, driving circuits and driving methods for two-phase bipolar stepping motors and for two-phase unipolar stepping motors are explained. We begin with an explanation of bipolar connections and unipolar connections.

Stepping Motor Driving: Bipolar Connections and Unipolar Connections

Stepping motors include types with bipolar connections and types with unipolar connections; each type has its advantages and disadvantages. The features of each type should be understood so that each can be used in applications accordingly.

■Bipolar connections

Bipolar connections are as shown in the above diagram; a driving method (bipolar driving) is employed in which current is passed in both directions through for one winding. The motor structure is simple, and there are few terminals, but the polarity of each terminal must be controlled, and so the driving circuit is complex. However, the efficiency of use of windings is good, and precise control is possible, so that high output torques can be obtained. Further, the back emf occurring in the coils can be reduced, so that motor drivers with lower withstand voltage ratings can be used.

■Unipolar connections

Unipolar connections have center taps, as shown in the above diagram; a driving method (unipolar driving) is employed in which current is always passed in a constant direction for one winding. The motor structure is somewhat complicated, but only on/off control of the current is needed, so that the driving circuit is simple. However, the efficiency of use of the windings is correspondingly worse, and only about half or so of the output torque is obtained compared with bipolar connections. Moreover, when turning current on and off, a high back emf occurs in the coils, and so a motor driver with a high withstand voltage rating is required.