Stepper Motor Basics

Stepper Motor Basics

• Stepper motors convert electrical power into mechanical power as electromechanical devices
• They are brushless, synchronous electric motors
• Stepper motors use magnets to turn the motor shaft a precise distance per electrical pulse
• Example motor specs: 8 stator poles, 6 rotor poles with 24 pulses needed for a full revolution
• The rotor moves precisely 15° per electrical pulse

Working Principles

• Stepper motors have a stationary stator and a moving rotor
• The stator has teeth with wired coils
• The rotor is either a permanent magnet or a variable reluctance iron core
• Energizing stator phases generates a magnetic field, aligning the rotor
• Sequential phase activation rotates the rotor to a final position
• When coil B is energized the motor rotates clockwise 60 degrees

Rotor Types

• Three rotor types exist: permanent magnet, variable reluctance, and hybrid

Permanent Magnet Rotor

• The rotor has a permanent magnet that aligns with the stator's magnetic field
• Good torque and detent torque are guaranteed
• The motor will resist changes in position
• Lower speed and resolution than other rotor types

Variable Reluctance Rotor

• The rotor has an iron core shaped to align with the magnetic field
• Higher speed and resolution are easier to achieve
• The torque is often lower, and there is no detent torque

Hybrid Rotor

• This rotor combines permanent magnet and variable reluctance designs
• Provides high resolution, speed, and torque
• Has more complex construction, resulting in higher cost
• Teeth on the N-magnetized cap align with the S-magnetized tooth of the stator when coil A is energized
• High tooth count enables small step sizes, down to 0.9°

Stator Characteristics

• The stator creates the magnetic field for rotor alignment
• Stator characteristics include the number of phases, pole pairs, and wire configuration
• The number of phases refers to the number of independent coils
• Pole pairs indicate how main pairs of teeth are occupied per phase
• Two-phase stepper motors are most common, while three- and five-phase motors are less so

Advantages

• Stepper motors do not need a sensor to detect motor position
• Position is determined by counting steps
• Simple motor control, lower control effort, and no complex tuning are needed
• Microstepping achieves high position accuracy, up to approximately 0.007°
• Good torque at low speeds and a long lifespan are offered

Disadvantages

• They can miss steps if the load torque is too high, impacting control due to unknown motor position
• Microstepping increases the likelihood of missing steps
• High current draw even when still, reduces efficiency, and can cause overheating
• Low torque and noisy operation at high speeds
• Low power density and torque-to-inertia ratio are other disadvantages