Unipolar vs Bipolar Stepper Motors

A unipolar stepper motor splits each winding into two half-coils using a centre tap, allowing current to flow in only one direction per half-coil. A bipolar stepper motor uses the full winding on each phase and reverses current direction to change magnetic polarity. This single design difference shapes everything that follows: driver complexity, torque output, wiring and the applications each type suits best.

This article explains how unipolar and bipolar stepper motors differ in construction and operation, compares their practical strengths and weaknesses and offers guidance on choosing between them.

Contents:

• How a Bipolar Stepper Motor is Wired
• How a Unipolar Stepper Motor is Wired
• Torque Comparison
• Driver Complexity
• Wiring and Lead Configurations
• Converting a Unipolar Motor to Bipolar
• When to Choose Each Type
• Wrap-up
• FAQs

How a Bipolar Stepper Motor is Wired

A bipolar stepper motor has the simpler internal layout of the two. Each phase consists of a single, undivided solenoid coil. The motor typically has four leads comprising two per phase, with no centre tap.

To reverse the magnetic polarity of a phase, the driver reverses the direction of current through the coil. This requires an H-bridge circuit for each phase: a switching arrangement that can push current through the winding in either direction. Because the full length of each coil is energised every time a phase is active, bipolar motors generate stronger magnetic fields per phase than their unipolar equivalents.

How a Unipolar Stepper Motor is Wired

A unipolar stepper motor adds a centre tap to each winding, effectively splitting one coil into two smaller half-coils that can be powered independently. The centre tap connects back to the power source, giving the motor five, six or eight leads depending on whether the taps and coil ends are brought out separately.

By switching current through one half-coil or the other, the driver can change the effective magnetic polarity of the phase without ever reversing current direction. This eliminates the need for H-bridge circuitry entirely. A simple transistor or MOSFET per half-coil is enough to drive the motor.

However, because only one half of each winding is active at any given time, the effective coil area producing the magnetic field is halved. This is the fundamental trade-off at the heart of the unipolar design.

Torque Comparison

Bipolar motors produce more torque from the same frame size. Two factors drive this advantage.

First, the full winding is energised on every step. A bipolar NEMA 17 motor uses 100% of its coil copper to generate the magnetic field, while an equivalent unipolar NEMA 17 uses roughly 50% at any given moment. More active copper means a stronger electromagnetic pull on the rotor teeth.

Second, H-bridge driving allows both phases to operate at full field strength simultaneously. Combined with the ability to reverse polarity dynamically, this gives bipolar motors noticeably higher holding torque and better torque retention at higher speeds.

For applications where maximising torque within a given motor envelope is the priority — CNC routers, 3D printer extruders, robotic joints — bipolar is the stronger choice.

Driver Complexity

Unipolar motors win on simplicity. Because current only flows in one direction through each half-coil, the driver circuit needs nothing more than a switching transistor per winding. This makes unipolar drivers cheaper to build, easier to debug and less prone to shoot-through faults (a failure mode in H-bridge circuits where both switches in a leg conduct simultaneously, creating a short circuit).

Bipolar motors require a dedicated H-bridge per phase. While integrated H-bridge driver ICs have become inexpensive and widely available, making this less of a barrier than it was a decade ago, though the circuit is inherently more complex. For prototyping, education or projects where driver simplicity matters more than peak torque, unipolar motors remain a practical choice.

Wiring and Lead Configurations

The number of leads coming out of a stepper motor tells you what configurations it supports.

• Four-lead motors are bipolar only. Two leads per phase, no centre tap, no option to run in unipolar mode.
• Five-lead motors are unipolar only, with the two centre taps internally joined to a single common wire. This reduces lead count but prevents bipolar conversion.
• Six-lead motors bring out both ends of each coil plus separate centre taps. This is the most versatile configuration: connect the centre taps to the supply and you have a unipolar motor; leave them disconnected and wire the full coils to an H-bridge driver and you have a bipolar motor. Six-lead motors give engineers the flexibility to test both configurations on the same hardware.
• Eight-lead motors bring out every coil end independently, offering the most wiring options. They can be configured as unipolar (with external centre-tap connections), series bipolar (higher inductance, better low-speed torque) or parallel bipolar (lower inductance, better high-speed performance).

Converting a Unipolar Motor to Bipolar

Any unipolar motor with accessible centre taps can be run as a bipolar motor simply by leaving the centre-tap leads disconnected and driving the full windings through an H-bridge. This is a common upgrade path: engineers prototype with a unipolar driver for simplicity, then switch to bipolar driving when they need more torque from the same motor.

The reverse is not true. A four-lead bipolar motor cannot be converted to unipolar operation because it has no centre tap to split the windings.

When to Choose Each Type

Choose bipolar when:

• Maximum torque from a given frame size is the priority
• The application uses a modern microstepping driver (most off-the-shelf drivers are bipolar-compatible)
• Space or weight constraints rule out stepping up to a larger NEMA size

Choose unipolar when:

• Driver simplicity and low component count matter more than peak torque
• The project is educational or a rapid prototype where ease of wiring speeds up iteration
• Legacy equipment or existing driver boards only support unipolar operation

In practice, the industry has shifted heavily toward bipolar configurations. Integrated driver ICs like the A4988, DRV8825 and TMC2209 have made bipolar driving almost as simple as unipolar from a wiring perspective, while delivering substantially better torque and efficiency. Most modern 3D printers, CNC machines and robotic platforms use bipolar motors as standard.

Accu supplies stepper motors across NEMA 17 and NEMA 23 classifications in both unipolar and bipolar configurations, along with stepper motor drivers rated from 0.3 A to 2.0 A. If you are unsure which configuration suits your application, Accu's engineering support team can help match motor and driver to your torque, speed and mounting requirements.

Wrap-up

The choice between unipolar and bipolar comes down to a simple trade-off: torque versus driver simplicity. Bipolar motors deliver more torque from the same frame size because they energise the full winding on every step. Unipolar motors sacrifice that torque for simpler drive circuitry. With modern integrated driver ICs closing the complexity gap, bipolar has become the default for most precision applications, but unipolar still earns its place in prototyping, education and legacy system maintenance.

Further reading

• How a Stepper Motor Works - anatomy, stepping methods and motor selection fundamentals
• Calculating Voltage, Current and Resistance - electrical theory for sizing motors and drivers
• Stepper Motor Drivers (0.3 A–2.0 A) - Accu's compatible driver board range

FAQs

Q: Can I use a bipolar driver with a unipolar motor?

A: Yes, provided the motor has six or eight leads. Leave the centre-tap wires disconnected and wire the full coil ends to the bipolar driver. The motor will behave as a bipolar motor, delivering higher torque than it would in its unipolar configuration.

Q: Is a unipolar motor less reliable than a bipolar motor?

A: No. Reliability is determined by bearing quality, thermal management and operating conditions, not by winding configuration. Both types use the same fundamental construction. The difference is purely in how the coils are tapped and driven.

Q: Why do most 3D printers use bipolar stepper motors?

A: 3D printers need high positional accuracy and consistent torque across a range of speeds, often within tight space constraints. Bipolar motors deliver more torque per frame size and pair naturally with widely available microstepping driver boards like those based on the TMC2209 chipset. The driver complexity penalty is negligible in this context because the driver IC handles everything on a single chip.

Q: What does the number of leads on a stepper motor tell me?

A: Four leads means bipolar only. Five leads means unipolar only (internally joined centre taps). Six leads mean the motor can run as either unipolar or bipolar, depending on how you wire it. Eight leads offer the most flexibility, supporting unipolar, series bipolar and parallel bipolar configurations.

Q: Will I get more torque by switching from unipolar to bipolar?

A: In most cases, yes. Running the full winding instead of half-coils produces a stronger magnetic field and higher holding torque. The exact improvement depends on the motor's construction, but gains of 30–40% are typical when converting a six-lead unipolar motor to bipolar operation.