What is a stepper motor? Operation, types and practical selection

Hello everyone. A stepper motor is an electric motor whose shaft can be controlled in defined angular increments. With a suitable driver, it provides positioning, speed control and repeatable movement. Turning a valve by a set amount, moving a printer axis or indexing a part to the next station are examples of this motion.

The defining feature is that movement can be specified as a series of small command increments. A stepper motor cannot necessarily follow every command under every operating condition. The load, speed, driver and mechanical arrangement must suit one another. Our guide to stepper motor operation explains the underlying relationship.

How does a stepper motor work?

The stationary part is the stator; the rotating part is the rotor. The driver energizes the stator's phase windings in a controlled sequence, changing the magnetic field. The rotor follows the new equilibrium position created by that field. Advancing the sequence turns the shaft; reversing it changes direction with a suitable motor and driver.

In common pulse-input systems, the controller sends motion commands to the driver, which generates the motor currents. A command pulse is different from the power applied to a winding. Use a compatible driver rather than connecting the motor directly to a control output. Read our guides to stepper motor drivers and required control signals together.

Step angle and steps per revolution

The step angle is the nominal shaft increment in full-step operation. A motor with a 1.8° step angle has 360 / 1.8 = 200 full steps per revolution. A 0.9° motor has 400 full steps per revolution. Step angles vary, so check the motor documentation.

For a 1.8° motor operating in full-step mode, 50 command steps request a nominal 90° rotation. This calculation assumes that the rotor follows the commands under the operating conditions. Calculating step count does not establish that actual position has been measured. Our stepper resolution guide covers the calculation in more detail.

Main stepper motor construction types

Permanent-magnet stepper motor: The rotor contains a permanent magnet. Changing the stator field controls rotor position. Step angle and performance depend on the design.

Variable-reluctance stepper motor: A toothed rotor made of magnetic material moves toward a position that reduces magnetic reluctance, or opposition to magnetic flux, in the circuit created by the energized stator. The basic rotor construction does not use a permanent magnet.

Hybrid stepper motor: Permanent magnets and toothed rotor/stator geometry combine principles from permanent-magnet and variable-reluctance designs. The name describes these two magnetic design principles within one motor. This construction is common in industrial positioning. Our guide to stepper motor types explains the differences.

What do bipolar and unipolar mean?

Bipolar and unipolar describe winding connections and drive arrangements. In a common two-phase bipolar connection, the driver can reverse current through each phase. A unipolar connection uses sections of centre-tapped windings. Seeing four, six or eight wires does not explain every connection detail; the wiring diagram and driver compatibility are decisive.

Bipolar does not mean bidirectional, and unipolar does not mean one-way operation. Direction is determined by the appropriate phase sequence. Construction types describe rotor and stator design rather than operating direction. When selecting a motor and driver, examine electrical specifications before relying on a marketing label.

Full steps, half steps and microsteps

Full-step operation uses the motor's basic step increment. Half stepping and microstepping create smaller command increments. In microstepping, the driver changes the ratio between phase currents to establish intermediate targets between full steps. This can produce smoother movement and less vibration. Our drive-mode guide compares the methods.

A 1.8° motor configured for an external command resolution of 1/16 microstepping requires 200 × 16 = 3200 command pulses per revolution, giving a nominal command increment of 0.1125°. This does not guarantee positioning accuracy of 0.1125° under every load. Motor tolerances, current-regulation quality, friction and load affect the actual result.

Torque at standstill and during movement

Holding torque describes the motor's ability to resist displacement while energized and stationary. Available torque during movement depends on speed, the driver, supply and motor characteristics. Stepper motor torque generally falls at higher speeds. Rated holding torque alone is insufficient for selecting a motor. Selection must account for speed–torque behaviour.

When power is removed, active holding torque disappears. The passive magnetic detent torque present in some designs must not be assumed sufficient to hold a load safely. Vertical loads require an appropriate brake and mechanical safety arrangement. A motor can also draw current and heat up at standstill, making thermal management part of the system design.

How are speed, direction and starting position defined?

With a common STEP/DIR interface, pulse count specifies nominal travel, pulse frequency specifies nominal speed, and the direction signal selects rotation direction. A PLC or another motion controller can generate these commands. Use suitable acceleration and deceleration ramps rather than an abrupt high-speed command. Our speed and position control guide explains these relationships.

A motor does not automatically know the machine's absolute zero position when power is applied. Many systems establish a starting reference through a homing routine and a home/limit switch. Returning to the starting point is another programmed motion command; an automatic return mechanism is not an inherent feature of a stepper motor.

Open loop, closed loop and “servo stepper” labels

In an open-loop stepper system, the controller counts commands but may not directly measure whether the shaft reaches each target. Excessive load or unsuitable acceleration can cause missed steps. Our guide to common stepper motor problems explains the practical symptoms.

A closed-loop stepper system evaluates position feedback using a suitable encoder and driver. Marketing names such as “servo stepper” do not establish performance on their own. Hybrid construction and closed-loop control are different attributes: a hybrid motor can also operate open loop. Our stepper versus servo comparison helps establish suitable expectations.

Where are stepper motors used?

CNC axes, 3D printers, dosing mechanisms, optical positioning and small automation systems are common examples. A screw or belt can convert rotation into linear movement. Gears and transmission mechanisms change the relationship between movement and torque; a linear encoder can measure actual load position. Our stepper applications guide gives further examples.

Which specifications matter when selecting a motor?

Consider torque at operating speed, load inertia, acceleration, phase current, winding characteristics, step angle and mechanical interfaces together. Frame/mounting designations such as NEMA 17 and NEMA 23 do not establish torque, power or driver compatibility on their own. With a current-regulated driver, the motor's rated winding voltage need not equal the driver supply voltage; supply selection must remain within manufacturer limits.

Correct wiring, cooling, coupling alignment and maintenance conditions matter after selection too. Compare the motor's advantages and limitations with the application. When assessing motor and driver prices, consider more than frame size or advertised microstep count.