Bipolar stepper motors produce approximately 40% more torque than an equivalent unipolar motor using only the outer windings, because the full copper of each phase is energised on every step [S6].
The trade is real: a unipolar drive needs just four low-side switches and recirculating diodes, while a bipolar drive requires two full H-bridges (eight switches) to push and pull current through a coil that has no center tap [S1][S2].
Winding construction and lead count
A unipolar phase is built from two coil halves sharing a center tap; a bipolar phase uses one continuous coil. Practically, that means a four-wire motor is always bipolar, a six-wire motor exposes both center taps and the four phase ends, and an eight-wire motor carries two center taps plus four phase ends with no internal connection [S1][S3].
The center tap is the structural difference: it splits the full coil in half, so a unipolar driver only ever energises half the copper at any instant [S4]. Removing or ignoring the center tap (tying it off) converts a six-wire unipolar into a four-wire bipolar-series equivalent on the same lamination stack [S2][S4].
Drive topology and component count
For each phase the unipolar voltage drive uses one transistor per half-coil: transistors Q1 and Q2 cannot close simultaneously, otherwise both halves of the phase would short the supply through the center tap [S1]. A bipolar phase needs an H-bridge of four switches, so a complete two-phase bipolar drive uses eight transistors where unipolar uses four [S1][S4].
Current control is the natural fit for bipolar: a chopper stepper drive modulates the H-bridge with PWM to hold phase current constant against back-EMF, which is how the 40% torque advantage is realised in practice. Running a unipolar phase in current mode is possible but loses the simplicity advantage without recovering full torque [S1][S2].
Torque, inductance, and the speed-torque curve
Bipolar wins at low speed and at hold, because both halves of the winding contribute ampere-turns at the same time, generating a larger magnetic field than a single unipolar half [S5][S6]. The torque equation ties this directly to inductance: output torque is proportional to motor supply voltage divided by the product of speed and inductance, so halving inductance roughly doubles usable torque at a given speed [S2].
The flip side is that the bipolar-series connection uses the full winding, so phase inductance is roughly four times a single unipolar half. Higher inductance limits current slew rate, so torque falls off faster as rpm climbs: the same NEMA 23 frame run as bipolar holds more torque at low speed but drops off sooner than the same lamination stack run as unipolar [S2][S4]. For high-rpm positioning axes, that curves trade-off is the central engineering decision, and it pairs naturally with coupling selection for torsional stiffness as covered in Bellows vs Beam vs Oldham coupling stiffness.
Decision matrix: bipolar vs unipolar vs bipolar-series on a unipolar frame
Three concrete options line up against four selection criteria. Holding and low-speed torque: bipolar highest, bipolar-series on a unipolar frame next, stock unipolar lowest (the 40% rule of thumb applies to the first vs third) [S2][S6]. Driver complexity: unipolar is four transistors, bipolar is eight transistors in two H-bridges, bipolar-series reuses the bipolar H-bridge on a re-wired unipolar frame [S1][S2]. High-speed torque retention: unipolar better, because lower per-phase inductance lets current rise faster each step [S2][S4].
Wire count and field-service flexibility: six-wire and eight-wire unipolar frames can be reconfigured in the panel as bipolar-series by insulating the center taps, which is the cheap retrofit path when a legacy machine needs more low-end torque without swapping the stepper motor [S2][S4]. A four-wire bipolar motor cannot be driven as unipolar; there is no center tap to exploit, so the choice is locked at purchase [S3][S4].
Where each topology fits
Pick bipolar (with a chopper drive) for CNC feed axes, 3D printer extruders and motion stages that hold position against gravity or cutting load, where the extra low-speed torque directly increases usable feed force [S1][S3]. Pick unipolar for low-cost, low-torque loads such as small valves, indicator drives, and educational kits, where a four-transistor voltage drive on a 5 V or 12 V rail is enough and BOM cost matters more than torque density [S1][S4].
Pick bipolar-series on a re-wired six-wire unipolar when a legacy part number must stay in service but more holding torque is needed and the application stays below a few hundred rpm, accepting the earlier high-speed torque roll-off as a known cost [S2]. Avoid unipolar voltage drive on any axis that needs precise current regulation or microstepping at high resolution, because the lack of current chopping and the half-winding energisation both limit smoothness and dynamic torque [S1][S7].
Standards, sourcing, and failure modes
There is no IEC or NEMA standard that pins the bipolar-vs-unipolar split; the distinction is in the winding topology and the drive, not a rated performance class. What does matter for sourcing is the NEMA frame size (17, 23, 34 are the most common) and the phase current rating, which together with supply voltage set the chopper stepper drive current limit [S1][S4].
The dominant failure modes are driven by that same physics: bipolar phases with high inductance lose steps at high rpm if the supply voltage is too low to force current rise in one step interval, and unipolar phases overheat at the center tap if both halves are energised simultaneously or if the return path is mis-wired to a stiff voltage source without current limiting [S1][S2]. For new builds, the safe default is bipolar on a current-controlled chopper drive; revisit unipolar only when the cost of the extra four switches exceeds the mechanical value of the 40% torque gain.
Component reference pages worth checking: ac motor.