Stepper Motors: Unipolar/Bipolar Windings

Stepper motor windings are produced in two winding versions: Bipolar and Unipolar configuration.

Bipolar winding configuration (as in sections above)

Current flows through whole windings, in alternating direction.
Driver circuitry needs 4 transistors per winding.

JE two-winding stepper motors have 4 leadwires.

stepper motors 

 

Switching scheme of one winding:

stepper motors arrow  stepper motors 

 

Unipolar winding configuration

Each winding has a center tap.
Current flows through a half winding only, and always in same direction.
Driver circuitry needs 2 transistors per winding only.

The JE two-winding stepper motor has 6 leadwires or 5 leadwires, if the center taps are connected inside the motor.

stepper motors 

 

Switching scheme of one winding:

stepper motors

 

Unipolar motors are usually applied for small motor sizes, in combination with constant voltage driver circuitry (low cost transistor arrays available for small motors).

Applicational notes for Unipolar motors:

Driver circuitry costs Lower costs of driver circuitry, only 2 transistors per winding
Motor costs Higher costs of motor , 6 or 5 leadwires and windings need a center tap
Performance Lower holding torque TH of only 1/√2 compared to bipolar motors (same motor size, same temperature increase).
Lower torque at low and medium step frequency.
For the same TH specification - higher current needed (higher energy consumption).

Higher torque at high step frequency, due to lower ratio L/R and therefore faster current rise.