Blog · 2026-07-05 · Steppers
Steppers are the motors that move in counted clicks — no encoder, no feedback, just faith and physics. Here's how they work, how to read their strange spec sheets, and the honest cases for and against putting one on your robot.
Every 3D printer, most desktop CNC machines and half the world's camera sliders run on stepper motors, and for one seductive reason: a stepper turns "move exactly 12.5 mm" into a countable number of electrical pulses, with no encoder, no PID loop, and no tuning. Command 1,000 steps, get 1,000 steps — usually. The word "usually" is where all stepper engineering lives, and understanding when the promise holds (and how it breaks) is what this article is for.
Inside a hybrid stepper (the kind on your bench) are two things: a rotor of permanent magnets with fine teeth, and a stator with two electromagnet coil groups — the two phases, which is why four wires come out. Energize phase A and the rotor teeth snap into alignment with A's magnetic field; energize B and it snaps one tooth-position further; reverse A, then reverse B, and the sequence marches the rotor around in fixed increments. The geometry of a standard motor — 50 rotor teeth interacting with the phase sequence — yields 200 discrete positions per revolution: the famous 1.8° step angle. (0.9°/400-step motors double the tooth count for finer moves.)
The consequences of this mechanism define the whole personality of the motor. The rotor is held at each position by magnetic force — hence holding torque, the spec on every listing: the torque needed to drag an energized, stationary stepper out of position. Position comes from counting, not measuring — hence open-loop control and its risks. And torque comes from current in the coils regardless of motion — hence a stepper drawing full current while doing nothing, warm to the touch at standstill, which surprises everyone the first time.
NEMA numbers are faceplate sizes, nothing more. NEMA 17 means a 1.7-inch (42 mm) square mounting face; NEMA 23 is 57 mm. The number says how it bolts on — not how strong it is. Within a NEMA size, length is the torque knob: a 48 mm-long NEMA 17 offers roughly double the holding torque of a 24 mm "pancake" version, and a long 17 out-pulls a short 23. Read the torque line, not the frame number.
Holding torque (in N·cm or kg·cm — the unit conversions from our torque units guide apply) is the headline figure: typical NEMA 17s span 20–60 N·cm. But holding torque is measured at standstill, and it's all downhill from there. Rated current (per phase, typically 1–2 A for NEMA 17) is what the driver must be set to deliver — the subject of the companion article on drivers and current limits. Coil resistance and inductance matter more than beginners expect: inductance is the spec that controls how fast current — and therefore torque — can be established in a coil, which brings us to the most important curve in stepperdom.
A stepper's torque falls with speed — not gently, like a DC motor's straight line, but with a characteristic collapse. The physics: each step requires current to rise in a coil, coil inductance resists changes in current, and as step rate climbs there is simply less time per step for current to reach its target. At some speed the current barely establishes before reversing, torque plummets, and the motor skips steps or stalls outright while emitting a sound like an offended goose.
Time available per step = 1 ÷ step rate
Current rise is limited by L/R and by supply voltage
→ Higher supply voltage pushes usable torque to higher speeds
The practical numbers: at hobby voltages (12–24 V), most NEMA 17s deliver genuinely useful torque up to roughly 300–600 RPM, with 1,000+ RPM possible only lightly loaded and at higher voltage. This is why stepper systems gear downStepper Motor Calculator converts your mechanism and target speed into motor RPM and flags when you've wandered past the realistic ceiling.
Modern drivers can hold both phases at intermediate current levels, positioning the rotor between full steps — 1/16 microstepping turns 200 positions into 3,200. Microstepping is a genuine gift for smoothness and noise: full-stepped motors buzz and resonate (there's a resonance band, typically at low-mid speeds, where full-stepping can literally stall a motor that runs fine faster); microstepped motors purr. What microstepping does not deliver is sixteen times the accuracy: the incremental torque between adjacent microsteps is tiny (roughly sin(90°/16) ≈ 10% of holding torque for one microstep of deflection), so under real load the rotor settles up to a full step away from the commanded microstep. Honest rule: microstepping for smoothness, full-step math for accuracy. The calculator reports both resolutions for exactly this reason.
| Property | Stepper (open loop) | DC + encoder + PID | Hobby servo |
|---|---|---|---|
| Position control | Counted, no feedback | Measured, closed loop | Built-in closed loop |
| Torque at speed | Collapses past ~500 RPM | Broad, strong | N/A (angle device) |
| Standstill behaviour | Full holding torque, draws current | Needs active control to hold | Holds, draws when loaded |
| Efficiency | Poor (current always on) | Good | Fair |
| Overload response | Skips silently | Slows, recovers, error visible | Strips gears eventually |
| Software burden | Trivial (count pulses) | PID tuning, encoder handling | Trivial |
The pattern in the table explains the ecosystem. Steppers own positioning machines — printers, CNC, sliders, syringe pumps — where loads are predictable, speeds are modest, standstill holding matters, and skipping can be prevented by design margin. DC motors with encoders own drivetrains, where speeds are high, loads are unpredictable (carpets, walls, opponents) and a skipped-step equivalent would mean a robot that's silently lost in space — the closed-loop world our PID tuning guide serves. Hobby servos own bounded joints. Closed-loop steppers — a stepper with an encoder bolted on, sold as a unit — split the difference and are quietly taking over quality CNC builds: stepper simplicity, servo honesty.
Given the above: not the drivetrain, usually. A stepper drive base is heavy, current-hungry at all times, torque-poor at speed and vulnerable to silent skipping on the first carpet transition — a geared DC motor with an encoder beats it on every axis that matters (the Motor Sizing Calculator will show the torque comparison plainly). Where steppers shine on robots: camera and sensor gimbals (smooth microstepped motion, exact repeatability), linear axes — lifts, sliders, 3D-printer-style mechanisms grafted onto mobile platforms, turrets and indexers, and any lab-robot mechanism metering precise motion against predictable load. The 28BYJ-48 — the $2 geared stepper in every electronics kit — deserves its own mention: slow, weak and charming, it's a legitimate choice for indicator dials, small sensor turrets, and learning, with ~2048 steps per output revolution thanks to its internal gearbox.
Holding torque costs current, and the driver supplies rated current at standstill by default. It's normal up to "uncomfortably warm" (steppers are rated for 80 °C+ case temperatures); most drivers offer idle-current reduction if standstill holding isn't needed at full strength.
Four = bipolar, two coils, the modern standard. Six = unipolar with centre taps (usable as bipolar by ignoring the taps). Five = unipolar with joined taps, like the 28BYJ-48 — needs a unipolar driver or a small modification for bipolar use.
Unpowered, yes — with a distinctive cogging feel from the magnets (the "detent torque"). Powered, it resists with full holding torque until overpowered, then skips in loud protest. If unpowered holding matters, add a leadscrew or worm stage, which won't back-drive at all.
Standing in front of a listings page, check five things in order. Torque with margin: compute your load torque at the motor shaft (gearing and mechanism included) and buy at least double — remember the spec is holding torque and you'll operate well below it at any speed. Rated current versus your driver: a 2 A motor behind a 1 A-practical A4988 delivers half its paper performance; match the pair (the driver guide has the table). Inductance if speed matters: lower inductance sustains torque to higher RPM; between two similar motors, the lower-inductance one is the faster one. Shaft details: diameter (5 mm is the NEMA 17 norm), flat or round, and length — pulleys and couplers must actually fit. Wires and connector: four leads with a detachable JST-PH connector beats captive wires for serviceability. Genuine-brand motors (or reputable clones with real datasheets) cost a few dollars more than mystery listings and repay it in specs that mean something.
Counted motion, honest limits: that's the stepper deal. Turn your mechanism into steps/mm and check your speed against reality in the Stepper Motor Calculator — then give the motor the driver it deserves with How to Choose a Stepper Driver and Set the Current Limit.