Stepper motors are essential in precision motion control, used in everything from 3D printers to industrial robots. Unlike standard DC motors, they move in discrete “steps,” offering precise positioning without feedback systems. In this guide, we’ll cover:
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How Stepper Motors Working Principles
As all with electric motors, stepper motors have a stationary part (the stator) and a moving part (the rotor). On the stator, there are teeth on which coils are wired, while the rotor is either a permanent magnet or a variable reluctance iron core. We will dive deeper into the different rotor structures later. Figure 1 shows a drawing representing the section of the motor is shown, where the rotor is a variable-reluctance iron core.
Figure 1: Cross-Section of a Stepper Motor
The basic working principle of the stepper motor is the following: By energizing one or more of the stator phases, a magnetic field is generated by the current flowing in the coil and the rotor aligns with this field. By supplying different phases in sequence, the rotor can be rotated by a specific amount to reach the desired final position. Figure 2 shows a representation of the working principle. At the beginning, coil A is energized and the rotor is aligned with the magnetic field it produces. When coil B is energized, the rotor rotates clockwise by 60° to align with the new magnetic field. The same happens when coil C is energized. In the pictures, the colors of the stator teeth indicate the direction of the magnetic field generated by the stator winding.
Figure 2: Stepper Motor Steps
Stepper Motor Construction
The performance of a stepper motor — both in terms of resolution (or step size), speed, and torque — is influenced by construction details, which at the same time may also affect how the motor can be controlled. As a matter of fact, not all stepper motors have the same internal structure (or construction), as there are different rotor and stator configurations.
Rotor
For a stepper motor, there are basically three types of rotors:
- Permanent magnet rotor: The rotor is a permanent magnet that aligns with the magnetic field generated by the stator circuit. This solution guarantees a good torque and also a detent torque. This means the motor will resist, even if not very strongly, to a change of position regardless of whether a coil is energized. The drawbacks of this solution is that it has a lower speed and a lower resolution compared to the other types. Figure 3 shows a representation of a section of a permanent magnet stepper motor.
Figure 3: Permanent Magnet Stepper Motor
- Variable reluctance rotor: The rotor is made of an iron core, and has a specific shape that allows it to align with the magnetic field (see Figure 1 and Figure 2). With this solution it is easier to reach a higher speed and resolution, but the torque it develops is often lower and it has no detent torque.
- Hybrid rotor: This kind of rotor has a specific construction, and is a hybrid between permanent magnet and variable reluctance versions. The rotor has two caps with alternating teeth, and is magnetized axially. This configuration allows the motor to have the advantages of both the permanent magnet and variable reluctance versions, specifically high resolution, speed, and torque. This higher performance requires a more complex construction, and therefore a higher cost. Figure 3 shows a simplified example of the structure of this motor. When coil A is energized, a tooth of the N-magnetized cap aligns with the S-magnetized tooth of the stator. At the same time, due to the rotor structure, the S-magnetized tooth aligns with the N-magnetized tooth of the stator. Real motors have a more complex structure, with a higher number of teeth than the one shown in the picture, though the working principle of the stepper motor is the same. The high number of teeth allows the motor to achieve a small step size, down to 0.9°.
Figure 4: Hybrid Stepper Motor
Stator
The stator is the part of the motor responsible for creating the magnetic field with which the rotor is going to align. The main characteristics of the stator circuit include its number of phases and pole pairs, as well as the wire configuration. The number of phases is the number of independent coils, while the number of pole pairs indicates how main pairs of teeth are occupied by each phase. Two-phase stepper motors are the most commonly used, while three-phase and five-phase motors are less common (see Figure 5 and Figure 6).
Figure 5: Two-Phase Stator Winding (Left), Three-Phase Stator Winding (Right)
Figure 6: Two-Phase, Single-Pole Pair Stator (Left) and Two-Phase, Dipole Pair Stator (Right). The Letters Show the Magnetic Field Generated when Positive Voltage is Applied between A+ and A-.
Stepper Motor Control
We have seen previously that the motor coils need to be energized, in a specific sequence, to generate the magnetic field with which the rotor is going to align. Several devices are used to supply the necessary voltage to the coils, and thus allow the motor to function properly. Starting from the devices that are closer to the motor we have:
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- A transistor bridge is the device physically controlling the electrical connection of the motor coils. Transistors can be seen as electrically controlled interrupters, which, when closed allow the connection of a coil to the electrical supply and thus the flow of current in the coil. One transistor bridge is needed for each motor phase.
- A pre-driver is a device that controls the activation of the transistors, providing the required voltage and current, it is in turn controlled by an MCU.
- An MCU is a microcontroller unit, which is usually programmed by the motor user and generates specific signals for the pre-driver to obtain the desired motor behavior.
Figure 7 shows a simple representation of a stepper motor control scheme. The pre-driver and the transistor bridge may be contained in a single device, called a driver.
Figure 7: Motor Control Basic Scheme
Types of Stepper Motors
A. Permanent Magnet (PM) Stepper Motors
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Rotor: Permanent magnet.
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Pros: Low cost, good torque at low speeds.
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Cons: Lower resolution, higher vibration.
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Uses: Simple positioning (printers, small robotics).
B. Variable Reluctance (VR) Stepper Motors
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Rotor: Non-magnetic, toothed iron core.
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Pros: High speed, no detent torque.
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Cons: Lower torque, rare in modern use.
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Uses: Older industrial machines.
C. Hybrid Stepper Motors
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Combines PM + VR designs for best performance.
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Pros: High precision (0.9°–1.8° steps), smooth motion.
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Cons: More expensive.
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Uses: CNC machines, medical devices, high-end automation.
Comparison Table
| Type | Resolution | Torque | Cost | Best For |
|---|---|---|---|---|
| Permanent Magnet | Low | Medium | $ | Basic positioning |
| Variable Reluctance | Medium | Low | $$ | High-speed systems |
| Hybrid | High | High | $$$ | Precision automation |
Common Stepper Motor Sizes
1. NEMA Standard Sizes
(NEMA = National Electrical Manufacturers Association)
| NEMA Size | Frame Size (mm/inches) | Typical Torque Range | Common Uses |
|---|---|---|---|
| NEMA 8 | 20×20mm (0.8″×0.8″) | 0.01–0.1 Nm | Small robotics, cameras, lab equipment |
| NEMA 11 | 28×28mm (1.1″×1.1″) | 0.1–0.3 Nm | 3D printers (small), CNC spindles |
| NEMA 14 | 35×35mm (1.4″×1.4″) | 0.2–0.5 Nm | Desktop 3D printers, scanners |
| NEMA 17 | 42×42mm (1.7″×1.7″) | 0.3–1.2 Nm | Most common (3D printers, CNC routers, robotics) |
| NEMA 23 | 56×56mm (2.3″×2.3″) | 1.2–4.5 Nm | Industrial CNC, large 3D printers |
| NEMA 24 | 60×60mm (2.4″×2.4″) | 1.5–5.0 Nm | High-torque applications (conveyors) |
| NEMA 34 | 86×86mm (3.4″×3.4″) | 4.0–15 Nm | Heavy-duty CNC, industrial automation |
| NEMA 42 | 110×110mm (4.2″×4.2″) | 10–30 Nm | Large machinery, presses |
2. Metric Sizes (IEC Standards)
| Size (mm) | Equivalent NEMA | Torque Range | Applications |
|---|---|---|---|
| 20×20 | NEMA 8 | 0.01–0.1 Nm | Miniature devices |
| 28×28 | NEMA 11 | 0.1–0.3 Nm | Small automation |
| 42×42 | NEMA 17 | 0.3–1.2 Nm | Consumer-grade CNC |
| 60×60 | NEMA 24 | 1.5–5.0 Nm | Industrial use |
Key Specifications to Consider
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Holding Torque (e.g., 0.5 Nm for NEMA 17) – Determines load capacity.
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Step Angle (e.g., 1.8° or 0.9°) – Affects precision.
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Shaft Diameter (e.g., 5mm for NEMA 17) – Compatibility with couplings.
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Voltage/Current (e.g., 12V/1A) – Matches driver requirements.
Choosing the Right Size
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Industrial Machines: NEMA 34+ for high torque.
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Compact Devices: NEMA 8/11.
Common Applications
- 3D Printers – Precise filament control.
- CNC Machines – Accurate tool positioning.
- Robotics – Controlled joint movements.
- Medical Devices – Syringe pumps, scanners.
- Automation – Conveyor belts, pick-and-place.
3D Printers
CNC Machines
Robotics
Conclusion
Stepper motors are ideal for cost-effective, precise motion control in DIY projects and industrial systems.
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