Stepper motors — VR, PM, hybrid + drive modes

Reference notes

Stepper motors convert each electrical pulse into a fixed mechanical angle, enabling open-loop position control without an encoder. Use Next → to compare variable-reluctance, permanent-magnet, and hybrid constructions, the three drive modes (full / half / micro), and the pull-in vs pull-out torque-speed envelope that governs missed-step risk.

What makes a stepper a stepper

One input pulse from the controller advances the rotor by exactly one step angle. Send N pulses, the rotor moves N · step_angle of mechanical motion. This makes a stepper a digital actuator — open-loop position control without an encoder, as long as load torque stays below the motor's pull-out at the commanded step rate.

• Typical applications — 3D printers, CNC routers, camera autofocus, paper-feed in printers, semiconductor probe stations, optical positioners, syringe pumps.

Variable-reluctance (VR) stepper

• Stator: typically 8 salient poles in 4 phase pairs.
• Rotor: soft-iron tooth wheel — no permanent magnet.
• Energizing each phase in sequence pulls the nearest rotor teeth into alignment (minimizes magnetic reluctance), stepping the rotor.

where m = number of phases, N_r = number of rotor teeth. Typical step: 15°. Cheap, no magnets, but zero holding torque when de-energized.

Permanent-magnet (PM) stepper

• Rotor: multi-pole permanent magnet.
• Stator field aligns rotor magnet — N of rotor seeks S of stator.
• Typical step: 7.5° or 15°. Coarser than VR but with much higher torque-per-amp and significant detent (holding) torque even with no current.
• Driven bipolar via H-bridges. Has more resonance issues than VR — can stall at certain step rates.

Hybrid stepper — the industrial workhorse

• Rotor: two iron tooth-discs (typically 50 teeth each) separated by an axial permanent magnet; one disc magnetized N, the other S, offset by half a tooth pitch.
• Stator: 8 poles, each with multiple tooth-faces facing the rotor air gap.
• 200 unique alignment positions per revolution → 1.8° per full step.
• NEMA 17 / 23 / 34 frames standardize sizing. NEMA 23 typically produces 1–2 N·m of holding torque at standstill, ample for most low-power positioning.

Drive modes

• Full step — one phase at a time → 1.8°/step. Two phases at a time → same angle, ~1.4× the torque.
• Half step — alternate 1-phase and 2-phase energization → 0.9°/step (400 steps/rev). Torque varies between half-steps.
• Micro-stepping — PWM current controller shapes phase currents as sin(θ)/cos(θ), so the stator field rotates smoothly rather than jumping. Common settings: 16× (0.1125°/μstep), 32×, 64×, even 256×. Reduces audible noise and mechanical resonance.

Holding torque per micro-step drops as the micro-step count increases. At 1/16 micro-step, the per-micro-step incremental torque is only ~10 % of full-step holding torque, and practical mechanical resolution is limited by detent torque, bearing friction, and elasticity.

Torque-speed envelope

• Holding torque — maximum static torque the motor can resist while energized at standstill.
• Pull-in torque (start–stop torque) — maximum torque at which the motor can START at a given step rate from a complete standstill, without missing steps.
• Pull-out torque (slew torque) — maximum torque the motor can SUSTAIN at a given step rate once it has been accelerated up to that rate. Pull-out > pull-in.
• Between pull-in and pull-out is the slew range — the motor can run there, but the controller must ramp up via an acceleration profile.

Both pull-in and pull-out fall with step rate because phase inductance limits how quickly current can build in each winding (V = L·di/dt). At high step rates, the current cannot reach its commanded value before the next phase commutates.

Missed-step risk & closed-loop variants

Open-loop steppers assume the rotor follows every pulse. If load exceeds the motor's pull-out at the commanded rate, the rotor falls behind — one or more missed steps — and the controller cannot detect the position error. For demanding applications, closed-loop steppers add an encoder for missed-step detection and correction, bridging the gap to true servo motors at lower cost.

Keyboard shortcuts

• The cross-section panel shows VR, PM, or hybrid rotor construction depending on the active step.