A stepper motor is a special type of synchronous machine that converts electrical pulses into precise mechanical rotation. Unlike conventional motors that rotate continuously, a stepper motor moves in discrete angular steps — making it ideal for applications requiring accurate position control without feedback sensors.
In this article, you will learn what a stepper motor is, how it works, the concept of step angle, types of stepper motors, and the working of a variable reluctance stepper motor.
Table of Contents
What is a Stepper Motor?
A stepper motor is a brushless, pulse-driven motor in which the rotor rotates in discrete angular steps. Each input pulse produces exactly one step of angular movement. The total rotation is the sum of all individual steps — hence the name "stepper motor."
Key characteristics:
- Rotation is in discrete steps, not continuous
- Angular position is determined by the number of pulses applied
- Speed is controlled by the pulse frequency
- Direction is controlled by the switching sequence
- No feedback sensor is needed for position control (open-loop operation)
Working Principle
The basic principle of a stepper motor is that the rotor aligns itself with the magnetic field produced by the energized stator winding. When the stator windings are energized in a specific sequence, the rotor moves step by step to follow the changing magnetic field direction.
The operation can be summarized as:
- Stator has multiple poles (phases) that can be individually energized
- When a phase is energized, the rotor moves to align with that phase's magnetic axis
- Switching to the next phase causes the rotor to move one step
- The sequence of switching determines the direction of rotation
Step Angle
The step angle is the angle through which the rotor rotates for each input pulse applied to the stator. It is the fundamental parameter of a stepper motor.
Where:
- m = Number of stator phases
- Nr = Number of rotor teeth (or poles)
Alternatively:
Resolution is the number of steps the motor takes to complete one full revolution:
Smaller the step angle, higher the resolution and more precise the positioning. Common step angles are 1.8° (200 steps/rev), 7.5° (48 steps/rev), and 15° (24 steps/rev).
Types of Stepper Motors
Based on rotor construction, stepper motors are classified into three types:
- Variable Reluctance (VR): Uses a soft iron rotor with salient poles. Torque is produced by the tendency of the rotor to align with the minimum reluctance path. No holding torque when de-energized.
- Permanent Magnet (PM): Uses a permanently magnetized rotor. Provides holding torque even when windings are not energized. Larger step angles.
- Hybrid: Combines features of both VR and PM types. Has a permanent magnet rotor with toothed iron caps. Provides high torque, small step angle, and holding torque. Most commonly used in industry.
Variable Reluctance Stepper Motor
The variable reluctance (VR) stepper motor operates on the principle that magnetic flux always follows the path of minimum reluctance. The rotor moves to a position where the magnetic reluctance between the stator and rotor is minimized — i.e., where the rotor teeth align with the energized stator poles.
Construction features:
- Stator has salient poles with windings (multiple phases)
- Rotor is made of soft iron (no permanent magnets, no windings)
- Rotor has fewer teeth than the stator
- Can be single-stack or multi-stack type
Single Stack VR Motor — Working
Consider a simple VR stepper motor with 4 stator poles (A, B, C, D) and 2 rotor poles, as shown below:
The stator coils are excited from a DC source through semiconductor switches. The switching sequence determines the direction of rotation.
Step-by-Step Operation
- Step 1: Phase A is energized → rotor aligns with the axis of pole A (minimum reluctance position)
- Step 2: Phase A is de-energized, Phase B is energized → rotor rotates 90° to align with pole B
- Step 3: Phase B off, Phase C on → rotor rotates another 90° to align with pole C
- Step 4: Phase C off, Phase D on → rotor rotates 90° to align with pole D
The rotor completes one full revolution (360°) in 4 steps, with a step angle of 90°.
Direction reversal: To reverse the direction, simply reverse the switching sequence (A → D → C → B → A instead of A → B → C → D → A). The direction does not depend on the polarity or magnitude of the current — only on the switching sequence.
In practical VR stepper motors, the number of stator and rotor poles is much higher (e.g., 8 stator poles and 6 rotor teeth), giving smaller step angles (e.g., 15°) and finer positioning.
Advantages and Disadvantages
Advantages
- Precise position control without feedback (open-loop)
- Excellent low-speed torque
- Simple digital control — just count pulses
- Brushless — no wear, long life
- Holds position firmly when energized (holding torque)
- Highly reliable — no feedback sensor to fail
Disadvantages
- Limited high-speed performance (torque drops at high speeds)
- Can lose steps under excessive load (no feedback to detect)
- Resonance issues at certain speeds
- Higher power consumption compared to servo motors at equivalent output
- Noisy operation at low speeds
Applications
- CNC machines: Precise axis positioning for milling, drilling, and cutting
- 3D printers: X, Y, Z axis movement and extruder control
- Robotics: Joint positioning and arm movement
- Printers and plotters: Paper feed and print head positioning
- Medical equipment: Syringe pumps, CT scanners, blood analyzers
- Textile machinery: Pattern control and fabric positioning
- Disk drives: Read/write head positioning (older drives)
- Camera systems: Focus and zoom lens control
FAQs
What is the difference between a stepper motor and a servo motor?
A stepper motor operates in open-loop (no feedback) and moves in fixed steps. A servo motor uses closed-loop control with a feedback sensor (encoder) for continuous, precise positioning. Servos are better for high-speed applications; steppers are simpler and cheaper for low-to-medium speed positioning.
Why is a stepper motor called a synchronous machine?
Because the rotor speed is directly proportional to the input pulse frequency — it rotates in exact synchronism with the applied pulses. There is no slip. Each pulse produces exactly one step, making it a discrete synchronous device.
What happens if a stepper motor misses steps?
If the load torque exceeds the motor's holding torque or dynamic torque at that speed, the rotor cannot follow the switching sequence and "misses" steps. Since there is no feedback, the controller doesn't know the actual position — leading to positioning errors. This is the main limitation of open-loop stepper systems.
What is holding torque in a stepper motor?
Holding torque is the maximum torque that can be applied to the shaft of an energized (but stationary) stepper motor without causing rotation. It represents the motor's ability to hold its position against external forces. PM and hybrid types have holding torque even when de-energized (detent torque).
How do you increase the resolution of a stepper motor?
By using microstepping — a technique where the stator currents are varied in small increments (sine/cosine pattern) instead of full on/off switching. This divides each full step into smaller microsteps (e.g., 1/16 or 1/32), greatly increasing positioning resolution.