The Difference Between Unipolar and Bipolar Stepper Motors

Stepper motors are characterized as bipolar or unipolar. Bipolar stepper motors have four lead wires and require a total of eight drive transistors (i.e., two full H-bridges). Unipolar have an additional center-tap on each phase for a total of six lead wires. With the center-taps connected to a common voltage source, unipolar stepper motor drivers can be controlled with four identical “switches”, typically NPN or N-channel drive transistors (Figure 1). In conventional full-stepping mode, one motor phase is energized at a time resulting in minimum power consumption and high positional accuracy regardless of winding imbalance. Half-stepping control alternates between energizing a single phase and two phases simultaneously resulting in an eight-step sequence which provides higher resolution, lower noise levels and less susceptibility to motor resonance.

A stepper motor has the following features:

Both-direction rotation
precision angular incremental changes
capability for digital control
holding torque at zero speed
repetition of accurate motion or velocity profiles

Stepper Motor Unipolar/Bipolar 57×56mm 7.4V 1 A per Phase

This NEMA 23-size hybrid stepping motor can be used as a unipolar or bipolar stepper motor and has a 1.8° step angle (200 steps/revolution). Each phase draws 1 A at 7.4 V, allowing for a holding torque of 9 kg-cm (125 oz-in).

This high-torque hybrid stepping motor has a 1.8° step angle (200 steps/revolution). Each phase draws 1 A at 7.4 V, allowing for a holding torque of 9 kg-cm (125 oz-in). The leadshine hbs57 has six color-coded wires terminated with bare leads that allow it to be controlled by both unipolar and bipolar stepper motor drivers. When used with a unipolar stepper motor driver, all six leads are used. When used with a bipolar stepper motor driver, the center-tap yellow and white wires can be left disconnected (the red-blue pair gives access to one coil and the black-green pair gives access to the other coil). We recommend using it as a bipolar stepper motor.

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 bipolar stepper motor - circuit specialists blogcurrent 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.

Because windings are better utilized, they are more powerful than a unipolar motor of the same weight. This is due to the physical space occupied by the windings. A unipolar motor has twice the amount of wire in the same space, but only half used at any point in time, hence is 50% efficient (or approximately 70% of the torque output available). Though bipolar is more complicated to drive, the abundance of driver chip means this is much less difficult to achieve. An 8-lead stepper is wound like a unipolar stepper, but the leads are not joined to common internally to the motor. This kind of motor can be wired in several configurations.

Unipolar Stepper Motors

nipolar stepping motors have a center tap wired to the positive supply on each of two windings. The two ends of each winding are alternately grounded to reverse the direction of the magnetic field. The rotor would require proportionally more poles for higher angular resolutions. 30 degree per step motor is a common permanent magnet motor design. Control sequences in the windings spin the motor. The magnet is rotated one step at a time and the two halves of each winding are never energized at the same time.

Both uni-polar and bipolar steppers are used widely in projects. However, they have their own advantages and disadvantages from the application point of view. The advantage of a uni-polar motor is that we do not have to use a complex H bridge circuitry to control the stepper motor. Only a simple driver like ULN2003A will do the task satisfactorily. But, there is one disadvantage of uni-polar motors. The torque generated by them is quite less. This is because the current is flowing only through the half the winding. Hence they are used in low torque applications.