What is a stepper motor?
A stepper motor is a brushless electric motor that divides a full revolution into a fixed number of equal steps and turns exactly one step per control pulse, so its position can be determined by counting pulses rather than measuring it. A standard hybrid stepper motor moves 1.8° per step, which is 200 steps per revolution. This is why a stepper motor usually runs open loop: without an encoder the controller knows where the shaft is, as long as the motor does not lose steps. Stepper motors are found in 3D printers, CNC machines, labellers, dosing units (including precision dosing in the pharmaceutical industry), laboratory equipment, ticket machines, printer mechanisms and small positioning axes in pick-and-place systems.
🧠 How does a stepper motor work?
The stator carries several phase windings, two in the common two-phase motor. The rotor is a permanent magnet, a toothed soft-iron core or a combination of both. A driver energises the windings in a fixed sequence. At each switch the magnetic field shifts and the rotor is pulled to the next equilibrium position: one step.
Three properties follow directly from this principle:
- Position by counting - 200 pulses make one revolution; with a lead screw of 5 mm pitch that is 0.025 mm per full step.
- Speed by frequency - the pulse frequency sets the speed. 1,000 pulses per second at 200 steps per revolution gives 5 revolutions per second (300 rpm).
- Holding torque at standstill - with current flowing through the windings, the motor holds its position. Without current only a small detent torque remains, and only in motors with a permanent magnet.
The step error of a standard hybrid motor is around ±3 arc minutes (0.05°) and does not accumulate: after 10,000 steps the deviation is no larger than after a single step. However, available torque falls sharply at higher speeds, because the winding inductance slows the build-up of current and the motor generates back-EMF. A higher driver supply voltage therefore improves torque at high speed.
🧱 What types of stepper motors are there?
Stepper motors are classified by rotor construction and by how the windings are arranged. These are two separate classifications.
| Construction type | Rotor | Typical step angle | Application |
|---|---|---|---|
| Permanent magnet (PM, “tin can”) | Magnetised cylinder | 7.5° to 15° (48 to 24 steps) | Low-cost, simple mechanisms such as valves and printers |
| Variable reluctance (VR) | Toothed soft-iron rotor without magnet | Coarse, depends on the toothing | Historic and niche; no detent torque |
| Hybrid | Toothed rotor with an axial permanent magnet | 1.8° (200), 0.9° (400) or 0.72° (500 for five-phase) | Industrial standard for positioning |
| Winding | Drive method | Consequence |
|---|---|---|
| Unipolar | Centre tap per coil; current always flows in one direction | Simple driver, but only half of the winding is used: about 70% of the bipolar torque |
| Bipolar | Current direction is reversed through an H-bridge | More torque per volume; virtually standard with modern chopper drives |
There are also closed-loop stepper motors (also called hybrid servo): a hybrid stepper motor with an encoder or resolver whose driver corrects position deviations and raises an alarm when the motor falls out of synchronism. This is not a different motor type but a different way of controlling it.
🔧 How is a stepper motor driven?
A stepper motor always needs a driver, either as a separate module or integrated into the motor. Among drives, the stepper driver is the simplest type. Modern drivers are chopper drives: they regulate the current per phase by rapidly switching the supply voltage (typically 24 to 48 VDC, sometimes up to 80 VDC) on and off.
- Step/Direction - the classic interface (also called pulse train): a PLC with high-speed pulse outputs or a motion controller sends one pulse per step plus a direction signal. In IEC 61131 environments the programmer usually calls the standardised PLCopen motion function blocks for this, such as MC_MoveAbsolute for an absolute move.
- Microstepping - the driver feeds the two phases with sinusoidal currents shifted by 90°. A full step is divided into 2 to 256 microsteps. This gives smoother motion and less vibration and noise, but the torque per microstep is small, so real positioning accuracy does not improve proportionally.
- Fieldbus - integrated stepper motors with a built-in controller communicate over EtherCAT, CANopen, ProfiNET or Modbus RTU. On EtherCAT and CANopen they use the CiA 402 drive profile, with modes such as profile position, homing and cyclic synchronous position, just like a servo drive.
- Smart drivers - chips such as Analog Devices’ Trinamic line offer silent current control and sensorless load detection. This lets an axis home against a hard stop without a limit switch.
⚖️ What is the difference between a stepper motor and a servo motor?
| Aspect | Stepper motor | Servo motor |
|---|---|---|
| Control principle | Usually open loop (counting steps) | Always closed loop via encoder or resolver |
| Torque at low speed | High, also at standstill | Good, depends on sizing |
| Torque at high speed | Drops sharply | Stays flat up to rated speed |
| Overload | Loses steps unnoticed | Short torque peaks possible; following error is reported |
| Cost and commissioning | Low, little tuning | Higher, the controller must be tuned |
| Typical application | Small, slow positioning tasks | Fast, dynamic and synchronised axes |
A stepper motor is therefore not a cheap servo motor. It excels at low speeds and short, repeated moves. For high speeds, varying loads and synchronising several axes in motion control, a servo system is the better choice. For a conveyor running at constant speed, an induction motor with a variable frequency drive is sufficient.
📏 How do you select and size a stepper motor?
The NEMA frame size system only defines the flange and mounting dimensions, not the torque. Within one frame size, torque is mainly determined by the length of the motor stack.
| NEMA frame size | Flange (approx.) | Typical holding torque | Examples |
|---|---|---|---|
| NEMA 17 | 42 mm | 0.2 to 0.6 Nm | 3D printers, laboratory equipment, small dispensers |
| NEMA 23 | 57 mm | 0.5 to 3 Nm | Small CNC mills, adjustment axes, labellers |
| NEMA 34 | 86 mm | 3 to 12 Nm | Heavier axes, format changeover in packaging machines |
A practical approach:
- Define the motion profile - distance, time, acceleration and the number of moves per minute.
- Calculate the required torque - load, friction and above all the acceleration torque from the mass inertia.
- Read the torque-speed curve - it applies to one specific driver and supply voltage. Choose a safety margin of 1.5 to 2 over the calculated torque.
- Watch the inertia ratio and resonance - suppliers often recommend a load-to-rotor inertia ratio of no more than about 10:1. Microstepping helps damp mid-range resonance.
- Choose open or closed loop - if a missed step leads to rejects or damage, choose an encoder or a closed-loop stepper motor.
A diagnostic tip from practice: a two-phase stepper motor always loses steps in groups of four. If the position is off by a different number, the cause is probably lost pulses or electrical noise, not overload.
🔐 What does a stepper motor mean for safety and OT?
A stepper motor is an actuator that can injure people. Machines that use one fall under the Machinery Directive and, from 20 January 2027, the Machinery Regulation. Holding torque is not a safe lock: it disappears on power loss, so a vertical axis needs a brake or a self-locking transmission. For safe stopping, some stepper drivers offer Safe Torque Off; the required safety level (PL or SIL) follows from the functional safety analysis.
Integrated stepper motors with a fieldbus or Ethernet interface are also network devices on the OT network. Industrial protocols usually lack authentication, so anyone with access to the segment can command motion or change parameters. Segmentation, configuration file management and change management are therefore just as relevant as for a PLC. Internal diagnostics such as current, temperature and load value can also feed condition monitoring.
❓ Frequently asked questions
How many steps per revolution does a stepper motor have?
A standard hybrid stepper motor has 200 full steps per revolution, or 1.8° per step. There are also versions with 0.9° (400 steps) and five-phase stepper motors with 0.72° (500 steps). With microstepping the driver subdivides each full step further, up to 256 microsteps.
Why does a stepper motor lose steps?
A stepper motor loses steps when the required torque exceeds the available torque, for example through accelerating too fast, too heavy a load or resonance. In open loop the controller does not notice, so the position is permanently off afterwards. An encoder or a closed-loop driver does detect this.
Does a stepper motor need an encoder?
A stepper motor does not need an encoder for its basic function, because the position follows from the number of pulses. An encoder is advisable when a missed step leads to damage, rejects or a safety problem. Closed-loop stepper motors combine the low cost of a stepper with position monitoring.
What is the difference between unipolar and bipolar stepper motors?
In a unipolar stepper motor, current always flows in one direction through half of a centre-tapped coil. A bipolar stepper motor reverses the current direction through an H-bridge and uses the whole winding. As a result, bipolar delivers roughly 40% more torque from the same motor.
Why does a stepper motor get hot at standstill?
A stepper motor holds its position at standstill by driving current through its windings, and that current causes heat loss. A housing that is too hot to touch for long is therefore normal; the permitted maximum temperature is stated in the datasheet. Many drivers automatically reduce the current when the motor is stationary.
Can a PLC drive a stepper motor directly?
A PLC cannot energise a stepper motor directly, but it can control one through a stepper driver. The PLC then sends step/direction pulses through high-speed outputs or controls an integrated stepper motor over a fieldbus such as EtherCAT or CANopen. The driver supplies the current to the windings.
📌 In summary
A stepper motor turns a fixed angle per control pulse, usually 1.8°, enabling accurate positioning without position feedback as long as the load stays within the available torque. It is inexpensive, simple and strong at low speeds, but loses torque at high speed and can miss steps unnoticed in open loop. For critical or dynamic axes, choose a closed-loop stepper motor or a servo motor.