Most stepper motors have a full step angle of 1.8 degrees, half step angle of 0.9 degrees. A unipolar permanent magnet motor is identical to the bipolar motor described above except that bifilar windings are used to reverse the stator flux, rather than bidirectional drive. This motor is driven in exactly the same way as a bipolar motor except that the bridge drivers are replaced by simple unipolar stages - four darlingtons or a quad darlington array can be used. Unipolar motors have twice as many windings. Unipolar motors give less torque for a given motor size because the windings are made with thinner wire.
Unipolar motors are attractive to designers because they simplified the driver stage.
All permanent magnet motors suffer from the counter EMF generated by the rotor, which limits the rotation speed. When very high slew speeds are necessary a variable reluctance motor is used. Experience has shown that 3-4 revolutions per second is possible with simple conventional driver circuits. A variable reluctance motor has a non-magnetized soft iron rotor with fewer poles than the stator. Unipolar drive is used and the motor is stepped by energizing stator pole pairs to align the rotor with the pole pieces of the energized winding.