Stepper Torque
Estimate holding torque and power.
Understanding Stepper Motors
A stepper motor divides one full rotation into a fixed number of equal steps — usually 200 steps of 1.8° per step, though 0.9° (400-step) motors are also common in higher-precision applications. Each electrical pulse sent to the driver advances the rotor by exactly one step, which is what makes steppers fundamentally different from DC motors: position is tracked open-loop, purely by counting pulses, with no encoder or feedback required.
This open-loop control is the main reason steppers dominate low-cost automation. A DC motor needs an encoder and a PID loop to know where it is; a stepper motor "just knows," as long as it never skips a step (loses synchronization due to excessive load or acceleration). The trade-off is holding torque at standstill: because the driver keeps current flowing through the windings even when the motor isn't moving, a stepper locks itself in place with real mechanical resistance, something a de-energized DC motor cannot do.
Steppers are the mechanical backbone of the desktop "Maker" movement. Almost every FDM 3D printer on the market — RepRap-derived kits, Creality Ender machines, Prusa i3 and MK-series printers — uses stepper motors for X, Y, Z, and extruder axes. The same is true for hobby CNC routers, laser engravers, camera sliders, and small robotic arms, where precise, repeatable positioning matters more than raw speed.
Stepper Motor Types
| Type | Step Angle | Wiring | Common Use |
|---|---|---|---|
| NEMA 17 Bipolar | 1.8° (200 steps/rev) | 4-wire or 8-wire | 3D printers, hobby CNC |
| NEMA 23 Bipolar | 1.8° | 4-wire or 8-wire | Larger CNC, robotic arms |
| NEMA 14 / 11 | 1.8° | 4-wire | Compact printers, camera sliders |
| 5-Wire Unipolar | 1.8° or 7.5° | 5 or 6-wire | Legacy/low-cost projects |
| High-Res Steppers | 0.9° (400 steps/rev) | 4-wire or 8-wire | Precision positioning, medical devices |
Torque and Power Formulas
Exact torque curves require detailed magnetic simulation (accounting for pole geometry, back-EMF, and driver chopping behavior), but the two relationships below are enough to estimate the numbers that matter for real builds: how much power supply you need, and roughly how much holding torque to expect.
Power (W) = 2 × (Iphase × Vsupply)
The factor of 2 accounts for both motor phases being driven simultaneously in a typical bipolar chopper drive. This is the number to use when sizing a power supply — not the motor's rated wattage alone, since a driver running multiple motors needs headroom for all of them at once.
Holding Torque ∝ N × I × Φ
Holding torque scales with the number of winding turns (N), phase current (I), and magnetic flux (Φ, largely set by magnet strength and pole geometry). In practice this means: more current per phase generally means more torque, up to the thermal and magnetic saturation limits of the motor — which is why datasheet-rated current, not driver maximum current, is the real ceiling.
Driver note: Stepper drivers such as the A4988, DRV8825, or TMC2209 are current choppers — they regulate winding current with PWM rather than applying supply voltage directly. This is why a 12V supply can safely drive a motor rated for far less than 12V per phase. The power figure from the formula above is what you use for supply sizing, e.g., specifying a 24V 10A PSU for a printer running 5 stepper motors plus a heated bed.
Quick Reference: Typical NEMA 17 Values
| Parameter | Typical Range | Notes |
|---|---|---|
| Phase Current | 1.2 – 2.0 A | Higher current = more torque, more heat |
| Phase Resistance | 1.0 – 3.0 Ω | Lower resistance = faster current rise |
| Phase Inductance | 2 – 5 mH | Higher inductance = better low-speed torque |
| Holding Torque | 30 – 65 N·cm | Varies by motor length/body size |
| Rated Voltage | 2.5 – 4.2 V | Not the driver supply voltage |
Worked Example
A common NEMA 17 (e.g. 42BYGH or similar) rated at 1.5 A per phase, driven from a 12V supply:
- Power = 2 × (1.5 A × 12 V) = 36 W
- With a phase inductance around 3.0–3.5 mH and typical winding/magnet specs, estimated holding torque lands around 45–52 N·cm, in line with common NEMA 17 datasheet values.
This matches the example values shown in the calculator above — a useful sanity check when comparing your own motor's datasheet numbers against the estimate.
Practical Applications
- 3D Printers: The NEMA 17 is the workhorse standard across nearly all consumer FDM printers. Higher-inductance motors suit slow, high-torque Z-axis leadscrews, where torque at low speed matters more than top speed. Lower-inductance motors suit fast X/Y movement, where current needs to rise and fall quickly to keep up with rapid step pulses.
- Camera Sliders: Steppers provide the smooth, jitter-free, and precisely repeatable motion needed for timelapse and motion-control photography, where a DC motor's speed variation would show up as visible stutter in the footage.
- Robotic Arms: Joint control benefits from a stepper's inherent holding torque and absence of hunting/jitter that closed-loop DC servo systems can exhibit near a target position.
- CNC and Laser Machines: Open-loop positioning is acceptable because cutting forces are known and controllable, making steppers a cost-effective choice compared to closed-loop servo systems.
Stepper vs. DC Servo Motor
| Factor | Stepper Motor | DC Servo Motor |
|---|---|---|
| Positioning | Open-loop (step counting) | Closed-loop (encoder feedback) |
| Holding torque at rest | High, built-in | Requires active current draw |
| Cost | Lower | Higher (encoder + driver) |
| Top speed | Lower, torque drops off with speed | Higher |
| Missed-step risk | Yes, if overloaded | No (self-correcting) |
| Typical use | 3D printers, CNC, sliders | Industrial robots, CNC at high speed |
Common Mistakes to Avoid
- Undersizing the power supply. Sizing the PSU off a single motor's rated current instead of the total system draw (all axes plus heaters/electronics) is the most common cause of brownouts and missed steps.
- Confusing driver current limit with motor rated current. Setting the driver's current-limit trim pot above the motor's datasheet rating causes overheating and premature demagnetization.
- Ignoring inductance when choosing motors for speed. A high-torque, high-inductance motor that's great for a Z-axis will struggle to reach speed on a fast X/Y axis because current can't rise fast enough between steps.
- Skipping microstepping tuning. Running at full or half-step when the driver supports 1/16 or 1/32 microstepping leaves usable smoothness and noise reduction on the table.
Video: How Stepper Motors Work
Frequently Asked Questions
Why does my motor get hot?
It's normal. Stepper motors draw close to their full rated current even when stationary, in order to maintain holding torque. Most are designed to run safely up to around 80°C case temperature — hot to the touch, but within spec. If a motor is too hot to touch for more than a second or two, or if temperatures exceed the datasheet rating, check the driver's current-limit setting.
What is microstepping?
Instead of energizing coils in a single full 1.8° step, the driver sends sine and cosine current waveforms to the two phases, positioning the rotor at intermediate points between full steps. Standard 1/16 microstepping makes the motor noticeably quieter and smoother at low speed, at the cost of a small reduction in torque per microstep and some loss of positional accuracy compared to full steps (real mechanical resolution is limited by detent torque, not the electrical microstep count).
Series vs. parallel wiring — which should I use?
This applies to 8-wire (or configurable 6-wire) stepper motors. Parallel wiring gives better high-speed torque but draws roughly twice the current of series wiring for the same torque output, demanding a beefier driver and PSU. Series wiring gives better low-speed/holding torque and lower current draw, but torque falls off more sharply as speed increases. Most NEMA 17 motors sold for 3D printers ship as fixed 4-wire bipolar motors, so this choice doesn't apply — it mainly matters when sourcing 8-wire industrial steppers.
How do I choose between a 1.8° and 0.9° stepper?
A 0.9° (400 step/rev) motor doubles angular resolution for the same driver microstepping setting, which helps in precision positioning tasks. For most 3D printing and hobby CNC work, a 1.8° motor with adequate microstepping already exceeds the mechanical precision of the belts, leadscrews, and frame, so the extra resolution offers little practical benefit while typically costing more.
Can I run a stepper motor without a driver, straight from a microcontroller pin?
No. Stepper motor phases draw far more current than a GPIO pin can safely source, and lack the flyback protection a chopper driver provides. A dedicated driver IC (A4988, DRV8825, TMC2209, etc.) is required for safe and controlled operation.