Stepper Motors and Drivers
Steps and microstepping, the torque-speed curve, resonance, drivers and current settings, wiring, and when a stepper loses steps.
How a stepper works
A stepper motor has many magnetic poles (typically 50 teeth on the rotor, giving 200 full steps per revolution, a step angle of 1.8°). The driver energizes the two phase windings in sequence, and the rotor steps to each new magnetic position. With no feedback the controller simply counts pulses, trusting the rotor to follow. That is why steppers are simple and cheap, and why they are open loop: if the load exceeds the available torque the rotor skips steps and the controller never knows.
| Feature | What it means in practice |
|---|---|
| Holding torque | Torque with the windings energized at rest (the headline catalog number). Useful torque at speed is much less. |
| Detent torque | Small torque with the windings off, from the permanent magnet: a slight cogging feel. |
| Step angle | 1.8° (200 steps/rev) is standard; 0.9° (400 steps/rev) gives finer steps at a higher price. |
| Step accuracy | About ±5 percent of a full step, not cumulative: the error does not build up over many steps. |
| Rated current | Per phase. Sets the torque and the heat: the driver is set to this value. |
| Resonance | At certain speeds the rotor rings and torque dips (see the curve below). |
Steps, microsteps and resolution
In full-step mode both phases are switched between full positive and full negative current. Microstepping divides each full step by feeding the two phases with staircase approximations of a sine and cosine. The result is smoother, quieter motion and finer resolution.
Worked example: resolution and pulse rate
- A 1.8° motor has 360 ÷ 1.8 = 200 full steps per revolution; at 1/16 microstepping that is 3 200 microsteps per revolution.
- On an 8 mm lead screw one full step moves 8 ÷ 200 = 0.040 mm and one microstep 8 ÷ 3 200 = 0.003 mm (2.5 µm).
- To turn at 600 rpm (10 rev/s) the controller must send 10 × 3 200 = 32 000 steps/s (32 kHz); the screw travels 80 mm/s. Many microcontrollers cannot generate that, and many drivers cap the input frequency (commonly 200 kHz to 1 MHz): lower the microstep setting at high speed.
- Accuracy is not resolution. A microstep moves the rotor less than a full step but with less holding torque: at 1/16 the torque available to resist a load that tries to push the rotor off one microstep is only about sin(90° ÷ 16) = 9.80 percent of the full-step torque, and the position is accurate only to the step error. Microstepping gives smoothness more than precision.
The torque-speed curve
Stepper torque falls with speed because the winding inductance resists a fast change of current: the faster the steps, the less time the current has to rise, so the torque drops. The pull-out curve is the highest torque the motor can hold once running; the pull-in curve is the highest it can start at without an acceleration ramp. The dip is a resonance.
- Design inside the curve with a safety margin: use no more than about 50 to 60 percent of the pull-out torque at the operating speed.
- Accelerate gradually. Starting at high speed with no ramp works only up to the pull-in speed. Use a trapezoid or S-curve ramp that keeps the torque within the curve.
- Pass through resonance quickly or avoid it: use microstepping (smoother), a damper, a mechanical coupling with some damping, or change the current. Many drivers have anti-resonance filters.
- Higher supply voltage raises the torque curve at speed (below).
Drivers, current and supply voltage
A modern stepper driver is a chopper current source: it switches the supply voltage on and off to hold each phase at a set current, so the supply can be several times the motor’s rated voltage. That lets the current rise quickly even against the inductance, and gives torque at speed. Set the driver’s current to the motor’s rated phase current (or lower to run cooler); many drivers allow a reduced current at standstill.
Worked example: supply voltage and heat
- Supply voltage rule of thumb: V ≈ 32 × √L, with L the motor inductance in mH. For a motor with L = 3.2 mH: V ≈ 32 × √3.2 = 57 V, so a 48 V supply gives good speed. A 12 V supply would limit the same motor to low speed.
- Electrical time constant: L ÷ R = 3.2 mH ÷ 0.9 Ω = 3.6 ms; the current would take several of these to reach its value from a low voltage, which is why a high voltage chopper is used.
- Heat: with both phases at 2.8 A the winding copper loss is 2 × 2.8² × 0.9 = 14.1 W. It is dissipated even when the motor is stopped, which is why steppers run hot: lower the hold current to 50 percent (a quarter of the heat) when idle.
- Supply current: the supply current is far less than the motor current, because the chopper converts voltage to current like a buck converter (see Power Electronics); size the supply for the power, not for the motor current.
| Driver feature | What to look for |
|---|---|
| Current range and supply voltage | Above the motor’s rated current, and a supply high enough for the speed |
| Microstep settings | Up to 1/16 or 1/256; interpolation (the driver smooths coarse steps) |
| Decay mode | Mixed or automatic decay for smooth current at both low and high speed |
| Interface | Step and direction (the common one), or fieldbus (Modbus, CANopen, EtherCAT) for networked axes |
| Protection | Over-current, over-temperature, undervoltage, short circuits; fault output |
| Stall detection | Sensorless stall detection on some drivers: a hint, not a measurement |
Wiring and motor types
- Bipolar (4 wires, two coils): the common type; the driver reverses the current in each coil. Use one winding pair per phase.
- Unipolar (5 or 6 wires, center tapped): can be driven as a bipolar motor by ignoring the center taps (use the full coil for the most torque).
- 8 wires: series connection (more torque at low speed, more inductance) or parallel (less inductance, better at high speed).
- Find the coil pairs with a meter: the two wires of one coil show a low resistance; different coils show an open circuit.
- Never connect or disconnect a motor with the driver powered: the voltage spike can destroy the output stage.
- Use twisted, shielded cable for long runs and keep it away from signal wires (see Cabling, Grounding and Noise).
- Direction wrong? Swap the leads of one coil, or reverse the direction signal.
Why a stepper loses steps
| Cause | What happens | Cure |
|---|---|---|
| Load torque above the curve | The rotor cannot follow the field and slips | Lower the speed or load, a larger motor, a higher supply voltage |
| Acceleration too fast | The rotor lags during the ramp | Lengthen the ramp (see Motion Control Basics) |
| Resonance | Torque dips at a certain speed | Microstep, damp, avoid that speed |
| Current set too low | Low torque at all speeds | Set to the rated phase current |
| Supply voltage too low or sagging | The current cannot rise at speed | Higher voltage, a bigger supply or capacitor |
| Mechanical binding, belt slip, coupling slip | The shaft does not follow the rotor | Check the transmission (see Mechanical Transmission: Screws, Belts and Gearboxes) |
| Electrical noise on step or direction | False or missed pulses | Shielded cable, differential signaling, filtering, a clean ground |
| Overheating | The driver reduces current or the motor’s torque falls | Lower the current, add cooling or heat sinking |
Closed-loop steppers
Adding an encoder to a stepper lets the drive see the rotor position. A closed-loop stepper drive keeps the rotor aligned, raises the current only as needed (cooler, quieter), and signals or corrects a lost position. It keeps the simple step-and-direction interface but removes most of the weakness of open loop, at a cost between an open-loop stepper and a servo. For a heavier or fast axis, a true servo (Servo Systems and Tuning) has more torque at speed.
Troubleshooting
| Symptom | Likely cause | Check |
|---|---|---|
| Motor does not move, hums | Wrong coil wiring, too little current, too fast a start | Coil pairs, current setting, start with a slow ramp |
| Moves the wrong way | Direction polarity or wiring | Swap one coil’s leads or invert the direction |
| Stalls at higher speed | Beyond the pull-out curve | Raise the supply voltage, reduce load or speed |
| Loud buzzing, rough at low speed | Full-step mode or resonance | Use microstepping |
| Motor very hot | Current too high, or full hold current at rest | Lower the current and use idle current reduction |
| Positions drift over time | Lost steps | Add feedback (closed loop), reduce acceleration, check the mechanics |
| Driver faults | Short, over-temperature, undervoltage | Wiring, supply, cooling |
Related
- Motion Control and Actuators: the overview of this guide.
- Choosing a Motor Technology: where steppers fit among the other motors.
- Servo Systems and Tuning and Encoders, Resolvers and Feedback: closing the loop.
- Driving Motors and Actuators: the electronics of a stepper driver and a microcontroller connection.
- Motor Torque and Inertia Matching Calculator.
FAQ
How much microstepping should I use?
Use 1/8 or 1/16 for smooth motion at ordinary speeds. Higher settings give little extra accuracy, and need a very high pulse rate at speed. At high speed reduce the microstepping so that the pulse rate stays within the driver and controller limits.
Why does my stepper get so hot?
The windings are energized at full current even at rest. A temperature of 60 to 80 °C on the case is normal for rated current; reduce the idle current, lower the run current if the torque is not needed, or add cooling.
Can I run a stepper from a 12 V supply?
Yes, for low speeds and small motors. For torque at higher speed use a higher voltage (24 to 48 V) with a chopper driver; check the driver’s maximum supply voltage.
Do I need an encoder?
Not for a light, well-margined axis. If a lost step is costly (a CNC machine, a long positioning stage), or the load varies, use a closed-loop stepper or a servo.
References
- Oriental Motor, Technical Reference: Stepper motors; Applied Motion, Stepper motor basics.
- Texas Instruments, Stepper motor drive application notes (DRV88xx family); Trinamic, Microstepping and StealthChop notes.
- Acarnley, P. P., Stepping Motors: A Guide to Theory and Practice, 4th ed., IET.
Last reviewed: October 2026. Values are typical examples; use the motor and driver datasheets for real ratings.