Bench Talk

(Source: SValeriia /stock.adobe.com)

Stepper motors are widely used, and as with so many basic components, they are simple in some ways but also have multiple configurations and operating subtleties. For steppers—the "motor" part of the term is often left out—there are two widely used winding and drive configurations: unipolar and bipolar. This blog explores the differences between unipolar and bipolar drive configurations for stepper motors, focusing on wiring, performance, and applications.

First, a note about terminology. Use of the terms “unipolar” or “bipolar” for steppers has a different meaning than in most general discussions about circuits, especially analog circuitry. In power-rail discussions, "unipolar" refers to a positive supply voltage and ground, without a negative supply, meaning the circuit operates solely from this single-ended supply. Conversely, a "bipolar" supply includes both positive and negative rails. Some applications, like those involving op-amps, require a bipolar or split supply to achieve full performance. However, these definitions of unipolar and bipolar do not apply to stepper motors.

A stepper motor is a type of brushless DC motor composed of interconnected coils called “phases.” The rotor aligns with these coils due to the force generated by the stator's magnetic flux, which is produced by the current flowing through the phases.

To achieve incremental and precisely controlled rotation, the drive current is directed to successive phases. This sequence creates rotational steps, resulting in partial, full, or continuous rotation as needed, and can even reverse direction if required.

Basic Phase Wiring Arrangements

The coils’ construction and wiring arrangement are broadly divided into unipolar and bipolar connection topologies (Figure 1). Unipolar and bipolar configurations differ in drive circuitry and performance attributes such as speed and torque, efficiency, and materials cost. The bipolar arrangement is sometimes called an “H-bridge” due to its resemblance to the letter.

Figure 1: Stepper motor poles can be arranged and wired in bipolar and unipolar configurations, each requiring various electronic switches (MOSFETs). (Source: Texas Instruments)

Note that unipolar and bipolar stepper motor configurations operate from a unipolar (single rail) power supply. Both configurations can rotate in both directions from that single supply.

The stepper motor driver controls the rotor position by energizing the stepper windings in a particular sequence. In this example, the microcontroller sends a pulse to the stepper driver to indicate that the stepper rotor should move to the next position. When the stepper driver receives a step pulse, it energizes one of the phase windings in the sequence.

When it receives the next pulse, it energizes the next phase so the rotor can continue moving. If the microcontroller stops sending step pulses, the rotor will remain stationary and aligned with the magnetic field of the energized phase.

Hybrid and permanent magnet stepper motors come in bipolar and unipolar winding configurations. For bipolar motors, H-bridges allow the current to flow in either direction through the winding. The current’s direction decides the polarity of the magnetic field created by that winding.

The bipolar stepper motor has a single winding per phase, making it more complex for the driving circuit to reverse the current flow and magnetic field. While the unipolar motor uses four transistors (FETs or IGBTs) in a half-bridge configuration for full control, the bipolar motor requires eight transistors arranged into two H-bridges for control.

Unipolar motors only need low-side or high-side FETs to drive the current in one direction through the windings. The winding of a unipolar motor has a center tap that can be connected to ground or the motor supply. Rather than driving current in two directions to change the polarity of the stator magnetic field, the coils for a particular phase are wound in opposite directions to achieve the change in magnetic field polarity that is needed to continue moving the rotor.

This approach allows the direction to be reversed without switching the current's direction, making reversal possible even with a single-polarity supply. Typically, the wiring has three leads per phase, resulting in six leads for a standard two-phase stepper motor. Unipolar motors can be driven like bipolar motors if the center taps are left unconnected and the coils are connected to a bipolar stepper motor driver.

The primary distinction between “unipolar” and “bipolar” stepper motors is the presence of the center tap wire, which divides the winding coils in half. This division can be achieved with either a single connection wire for the pair or two wires, one for each end of the coils. Removing the center tap converts the unipolar connection into a bipolar-series connection.

Practically speaking, motor lead colors are somewhat standardized within the industry and are consistent within a specific vendor's product line. As a result, many wiring diagrams use colors instead of numbering the leads. While some find this method useful and clear, others might find it confusing; nonetheless, this is the common practice.

However, the choice between unipolar and bipolar configurations involves more than just the wiring setup. It also impacts the electrical characteristics of the motor windings, thereby affecting voltage, resistance, inductance, velocity, acceleration, and torque.

Differences, Benefits, and Drawbacks

With unipolar control, only half of the phase winding is energized at a time, whereas bipolar drives utilize the entire copper winding per phase. This difference affects cost and weight compared to performance, with speed and torque typically being the most critical parameters (Figure 2).

Figure 2: Unipolar and bipolar pole arrangements have different speed versus torque roll-off curves. (Source: Texas Instruments)

Torque is proportional to the product of the driving current and the number of winding turns on the coil. A higher number of turns results in greater torque, but this advantage diminishes at high speeds, limiting the maximum effective speed of the stepper motor. The coil's self-inductance restricts the rate at which the drive current can change, thus reducing torque at higher speeds. On the other hand, with fewer turns, the self-inductance is less, providing less torque at lower speeds but maintaining torque at higher speeds.

It is important to note that torque also has an inverse proportional relationship concerning speed and the square root of inductance:

Torque α Motor Supply Voltage/(Motor Speed × √Motor Inductance)

Due to their more effective use of the coil winding, bipolar motors generally have more torque and are more efficient than unipolar motors. In contrast, unipolar motors use only half of each winding coil at a given time, lowering torque and efficiency.

The choice between unipolar and bipolar configurations certainly poses complexity in stepper motor designs. Figure 3 summarizes the significant attributes of basic unipolar and bipolar approaches.

Figure 3: This chart shows the tradeoffs between unipolar and bipolar stepper motor configurations. (Source: Portescap; redrawn by author from original)^[1]

Although unipolar control was commonly used in the past, the trend now is to use bipolar configurations with current drive due to the cost improvement of electronics. However, for voltage drive, unipolar is still a cost-effective option.

It might seem logical for vendors to offer a single type of motor with all possible coil ends exposed and available, allowing users to configure the stepper motor coils as needed. This would simplify the bill of materials and inventory, similar to the flexibility provided by field-programmable gate arrays (FPGAs).

After all, a six-wire motor can be wired in unipolar or bipolar series, and an eight-wire motor can be wired in unipolar, bipolar series, or bipolar parallel. In fact, this uncommitted wire-lead scheme is sometimes used, and vendors then provide a configuration chart that shows how to connect the wire leads to achieve the desired setup.

Of course, there is much more to motors in general, and stepper motors in particular. Most vendors offer extensive product lines, allowing for an objective approach to finding the right fit rather than forcing a specific solution. Recognizing that many designers may not be experts in motor-parameter tradeoffs, these vendors provide useful application notes covering stepper motor basics, sizing, selection, thermal considerations, optimization, and more. Much of this material is vendor-independent and applicable to motors from any supplier. Additionally, vendors have application engineers who specialize in both the qualitative aspects of motor selection and use, as well as the quantitative specifications.

What About the Driver?

Despite the practicality and suitability of stepper motors for precision positioning, specifically in back-and-forth motion applications, one factor that delayed their extensive adoption was the difficulty in providing the needed drivers and coil-timing management. However, the advent of ICs with high levels of embedded functionality has resolved this issue. Today, many vendors offer stepper motor driver ICs with a range of features and capabilities.

The stepper driver serves as the interface between the system microcontroller (MCU) and processor, which issues motion-related commands. The driver then executes these commands by directly managing the current and voltage to the motor (Figure 4) or by controlling external MOSFETs.

Figure 4: A stepper motor driver is the intermediary between the MCU and the motor itself. Higher-power motors usually require discrete external MOSFETs controlled by the driver for the necessary current switching. (Source: Texas Instruments)

Basic drivers come with various rating combinations to deliver up to one or two amps to the motor coils using on-chip MOSFET drivers and the MOSFETs. For stepper motors needing higher current or voltage, a different class of stepper driver is used, which functions as the MOSFET driver but does not include the actual MOSFETs that control and deliver the current to the motor.

Instead, the designer adds suitable external MOSFETs with ratings matched to the motor and project requirements. Some vendors offer broad families with a range of voltage- and current-drive capabilities, as well as some stepper motor controllers, which are rated for broader temperature ranges or automotive applications.

Conclusion

Stepper motors are used in a wide range of applications, from small, low-cost printers to critical functions like powertrain control in automobiles. Designers must balance tradeoffs in speed, torque, efficiency, size, and cost when selecting and configuring a specific motor. Controller ICs provide varying levels of management sophistication and algorithms, as well as options for internal or external MOSFETs to power the stepper coils. Additionally, vendors offer helpful application notes and guides for selecting the appropriate motor and controller combination and for proper operation.

Sources

[1]  https://www.portescap.com/-/media/project/automation-specialty/portescap/portescap/pdf/whitepapers/wp_bipolar_drives_vs_unipolar_drives_for_stepper_motors.pdf