How a Stepper Motor Works

A stepper motor is a brushless electric motor that divides a full rotation into a series of equal, discrete steps. Unlike conventional DC motors that spin freely when powered, stepper motors move in precise angular increments, giving engineers direct control over position, speed and direction without the need for feedback sensors. This makes them one of the most widely used motor types in modern precision engineering, from CNC machines and 3D printers to robotic arms and automated laboratory equipment.

This article breaks down the anatomy of a stepper motor, explains how each component contributes to its operation and covers the stepping methods that give engineers fine control over motion.

Contents

• NEMA Sizing Standards
• The Stator: Generating the Magnetic Field
• The Rotor: Converting Magnetic Force Into Motion
• Bearings: Supporting the Shaft
• The Stepper Motor Driver
• Stepping Methods
• Unipolar Vs Bipolar Configurations
• Selecting the Right Stepper Motor
• Wrapping Up
• FAQs

NEMA Sizing Standards

Before examining the internal workings of a stepper motor, it helps to understand how they are classified. NEMA sizes are a standardised set of housing dimensions established by the US National Electrical Manufacturers Association. The NEMA number represents the motor's faceplate width in inches, multiplied by ten. A NEMA 14 motor has a 1.4-inch faceplate, a NEMA 17 measures 1.7 inches and a NEMA 23 comes in at 2.3 inches.

NEMA sizing standardises the mounting interface so bolt hole patterns and faceplate dimensions remain consistent within each classification. However, body length varies significantly across motors in the same NEMA size. A longer body generally houses larger coils and a bigger rotor, which translates to higher torque output. When selecting a stepper motor, matching the NEMA size to your mounting constraints and the body length to your torque requirements is the first practical decision.

The Stator: Generating the Magnetic Field

The stator is the stationary outer section of the motor's internal mechanism. It houses several electromagnetic phases, each made up of multiple solenoid windings arranged around the inside of the motor housing.

Windings

A winding (or coil) is the basic electromagnetic building block of every stepper motor. Each coil consists of two parts: a tightly wound length of conductive wire and a magnetic core. When current passes through the wire, it generates a magnetic field around the core. This field pulls the teeth of the rotor into alignment — the fundamental action that produces motion.

Phases

A phase is a group of several windings wired in series so they magnetise in unison. Most stepper motors are either two-phase or four-phase designs. The number of phases determines the number of distinct electromagnetic states the motor can cycle through, which in turn affects step resolution and torque characteristics. Two-phase motors (the most common configuration) offer a good balance of simplicity, cost and performance for the majority of applications.

The Rotor: Converting Magnetic Force Into Motion

The rotor sits at the centre of the motor and is the component that actually turns. In a hybrid stepper motor, the most common type in precision applications, the rotor is a permanently magnetised cylindrical column lined with a large number of evenly spaced teeth.

These teeth are arranged into multiple offset rings known as laminations. The slight misalignment between lamination rings is a critical design feature: it encourages the rotor to transition smoothly from one step to the next. Without this offset, the motor would lock up or jerk violently between positions rather than rotating in a controlled manner.

The interplay between stator and rotor is straightforward. The driver energises a phase, the stator coils generate a magnetic field and the rotor teeth snap into alignment with that field. Energise the next phase and the rotor advances by one step. Repeat this thousands of times per second and you get continuous, precisely controlled rotation.

Bearings: Supporting the Shaft

Rotary bearings centralise the motor shaft and reduce friction during rotation. Improving rotational efficiency extends both the operating life and the energy efficiency of the motor. Because stepper motors are brushless assemblies with very few internal sources of friction, the bearings are typically the first (and often the only) component to wear out over extended service. Replacing worn bearings is one of the most common maintenance tasks for stepper motor assemblies.

The Stepper Motor Driver

A stepper motor cannot run directly from a power supply. It requires a driver, which is an electronic circuit that acts as both a control system and a current amplifier.

The driver directs current to each motor phase in rapid, precisely timed sequences, potentially thousands of times per second. By controlling which phases are energised and in what order, the driver dictates the motor's speed, direction and step resolution. The complexity of a driver can range from a simple breakout board for hobbyist projects to a sophisticated closed-loop controller for industrial automation.

Stepping Methods

The method by which a driver energises the motor's phases determines step resolution and smoothness. Three methods are commonly used.

Full stepping

In full-step mode, the driver energises one phase at a time (wave drive) or two phases simultaneously (full-step drive). Each pulse advances the rotor by one complete step, typically 1.8° for a standard 200-step motor, yielding 200 discrete positions per revolution. Full stepping delivers maximum torque per step but produces the most vibration.

Half stepping

Half stepping alternates between energising a single phase and energising two adjacent phases simultaneously. When two phases are active, the rotor settles at a position halfway between the two full-step positions. This doubles the number of steps per revolution to 400, halving the step angle to 0.9°. The trade-off is a slight reduction in torque during the single-phase portions of the cycle, but the improvement in positional resolution and smoothness is significant.

Microstepping

Microstepping takes this principle further by varying the current ratio between two active phases rather than simply switching them on and off. By applying proportionally more current to one phase than the other, the driver can position the rotor at intermediate points between full steps. Common microstepping resolutions include 1/4, 1/8, 1/16 and 1/32 of a full step, with some high-end drivers supporting 1/256 microstepping. This produces far smoother motion and quieter operation, which is why microstepping is the default mode in most CNC and 3D printing applications.

Unipolar Vs Bipolar Configurations

Stepper motors fall into two electrical configurations, each with distinct wiring and performance characteristics.

• Unipolar motors use a centre-tapped winding on each phase, effectively splitting each coil in two. Current only ever flows in one direction through each half-coil, which simplifies the driver circuit. The trade-off is that only half the winding is active at any time, reducing available torque.
• Bipolar motors use the full winding on each phase and reverse current direction to change the magnetic polarity. This requires a more complex driver (an H-bridge circuit) but delivers significantly higher torque from the same motor frame size. For most precision applications, bipolar configurations are the preferred choice. Accu's technical article on unipolar vs bipolar stepper motors covers this comparison in detail.

Selecting the Right Stepper Motor

Choosing a stepper motor for a given application involves balancing several factors: the torque required at operating speed, the available mounting envelope (NEMA size), the positional accuracy needed and the electrical compatibility with your driver. Higher NEMA sizes house larger stators and rotors, increasing torque capacity but also increasing weight and power consumption. Accu supplies stepper motors across NEMA 17 and NEMA 23 classifications in both unipolar and bipolar configurations, along with compatible stepper motor drivers rated from 0.3 A to 2.0 A.

Wrap-up

Stepper motors earn their place in precision engineering because they offer open-loop position control and accurate, repeatable motion without the cost and complexity of encoder feedback. Understanding how the stator, rotor, bearings and driver work together to produce discrete, controllable steps is the foundation for specifying the right motor for any project. The stepping method and electrical configuration you choose will then fine-tune that selection to match your resolution, torque and smoothness requirements.

Further reading

• Unipolar Stepper Motors vs Bipolar Stepper Motors - a detailed comparison of the two winding configurations
• Calculating Voltage, Current and Resistance - foundational electrical theory for motor selection
• Stepper Motor Drivers (0.3 A–2.0 A) - Accu's range of compatible driver boards

FAQs

Q: What is the difference between a stepper motor and a servo motor?

A: A stepper motor moves in fixed angular increments and relies on open-loop control, meaning the driver sends step pulses and trusts the motor to follow. A servo motor uses a closed-loop system with a feedback sensor (typically an encoder) to continuously correct its position. Servos excel at high-speed, high-torque applications where dynamic load changes are common. Stepper motors are better suited to applications that need precise positioning at moderate speeds without the added cost of feedback hardware.

Q: How many steps does a stepper motor have per revolution?

The most common configuration is 200 steps per revolution, giving a step angle of 1.8°. Some motors offer 400 steps per revolution (0.9° step angle). Microstepping can subdivide these further, for example, a 200-step motor running at 1/16 microstepping effectively delivers 3,200 positions per revolution.

A: Can a stepper motor run continuously like a DC motor?

Yes. Although stepper motors are often associated with precise positioning tasks, they can run continuously at constant speed. The driver simply continues sending step pulses at a fixed rate. However, stepper motors lose torque at higher speeds and generate more heat during continuous operation than intermittent duty, so thermal management and torque curves should be checked during the specification process.

Q: Why does my stepper motor vibrate or make noise?

A: Vibration in stepper motors is caused by the discrete nature of each step. The rotor snaps from one position to the next and at certain speeds these impulses can resonate with the mechanical structure. Switching from full-step to half-step or microstepping mode significantly reduces vibration and ensuring the motor is securely mounted and properly loaded also helps dampen resonance.