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Bipolar Drives and Unipolar Drives for Stepper Motors: A Comparison

September 9, 2021

Bipolar Drives and Unipolar Drives for Stepper Motors: A Comparison

 A stepper motor is a type of brushless DC motor composed of connected coils — called “phases.” These electromechanical devices are generally driven in open loop without a feedback sensor, with current applied on the phases without knowing the rotor position. The rotor moves into alignment by means of the stator’s magnetic flux, generated by the current flowing in the phases. At each pulse, the current can be supplied to the next phase, allowing incremental rotational movements, or steps. There are two methods to supply current in the coil: bipolar and unipolar. This blog post will compare bipolar and unipolar motors.

Unipolar Motors

In a four-step, permanent magnet stepper motor, the rotor has a one pole-pair magnet, and the stator has two phases — Phase A and Phase B. In unipolar configurations, the current always flows in the same direction and coils are never powered together.                                                                                                                           

 Each motor phase is composed of two coil windings. With a two-phase motor composed of phases A and B, the motor has four coil windings. The control system for the voltage drive only has one switch, or transistor, per coil. When the transistor is closed, the coil is powered. For commutation the transistors are alternately closed and opened. Only half of the phase is powered at a time, so the current only uses half of the copper volume. With voltage drives, serial resistances are typically applied to decrease the electrical time constant.

 Bipolar Motors

Bipolar motors need only one coil winding per phase. The current can go in both directions per coil, and the two phases are powered together. Eight transistors with two H-bridges are required to control bipolar motors. The transistors are alternately closed and opened to provide commutation. 

The drives are used either in the motor’s voltage drive or in current source, and they have the advantage of using all the copper per phase. For the current source, the current in each phase is controlled with a pulse width modulation (PWM). Two techniques are used for PWM: slow decay or fast decay, depending on whether the current is supposed to decrease quickly or slowly through the motor phase during the PWM “off” time. 

Advantages and Limitations
Unipolar and bipolar motor assemblies each have their advantages and limitations: 

  • Voltage drive. A simple circuit with four transistors provides cost-effective unipolar control. A voltage drive for bipolar motors requires two H bridges. Note: Due to the inductance effect, the current needs some time to rise in the coil.
  • Current drive. A bipolar mode is preferable for current drives because unipolar technology requires more complex electronics to achieve less motor performance.
  • Holding torque. Defined as the maximum torque that the motor can hold at stall, holding torque is proportional to the torque constant and the current in the phase. When the same amount of joule losses are dissipated, the bipolar motor can produce more torque than the unipolar drive. And, for the same electrical power, the bipolar drive gets better results than the unipolar drive.
  • Dynamic mode. At high speed in voltage drive, the unipolar motor can deliver higher torque than the bipolar motor because the current can flow faster in the coil. 

Make Your Comparison

With an understanding of bipolar and unipolar motors and their advantages, you’ll be able to determine the best drive strategy for your stepper motor application. Unipower control was popular in the past, but bipolar in current drive has become more prevalent thanks to cost improvements. For voltage drive, unipolar control is still a cost-effective option.

 Read the complete white paper or contact an engineer.