How it works
Stepper motor types: Variable Reluctance (VR) — toothed soft-iron rotor, no magnets, multiple stator poles, step angle typically 7.5° or 15°; Permanent Magnet (PM) — rotor carries axially magnetised PM discs, step angle 7.5° or 11.25°; Hybrid — combines VR teeth with PM magnetisation, giving step angles of 0.9° or 1.8°, the most common industrial type. Full-step mode energises one phase at a time; half-step mode alternates between single-phase and two-phase energisation, doubling resolution to 0.9° from a 1.8° motor. Microstepping subdivides the full step by proportionally varying current in adjacent phases using a PWM driver (DRV8825 supports 1/32 microstepping), achieving smooth motion and step angles as small as 0.05625°.
Key points to remember
Holding torque is the maximum torque a stationary energised stepper can resist without losing position; detent torque is the torque in the de-energised state due to PM attraction to stator teeth. Pull-in torque is the maximum torque at which a stepper can start without missing steps; pull-out torque is higher and represents the torque limit during continuous running. Step angle θs = 360°/(Nr × Ns) for a VR motor, where Nr is rotor teeth and Ns is the number of phases × poles. At high stepping rates, the motor may miss steps because the rotor cannot mechanically follow the faster electrical pulses — this is called step loss or loss of synchronism, and is prevented by using acceleration ramps rather than sudden high-frequency starts.
Exam tip
Every Anna University and VTU exam asks you to calculate the step angle of a given stepper motor configuration and list the sequence of phase energisation for full-step and half-step modes — always start the sequence table from phase A and show at least 8 steps for half-step mode.