--- title: "Stepper motor and servo motor principle" subject: "Electrical Machines" chapter: 7 key: "semester_4/electrical_machines:stepper-and-servo-motor-principle" source: https://www.bctengineer.com/q/semester_4/electrical_machines/stepper-and-servo-motor-principle site: BCT Engineering --- # Stepper motor and servo motor principle Electrical Machines · Chapter 7: Fractional Kilowatt Motors - Asked in: 2068 Bhadra, 8 marks - Video: [Stepper Motor vs Servo Motor | What's the Right Choice](https://www.youtube.com/watch?v=_J1Rh1OZyW8) — Electrical lectures - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines/stepper-and-servo-motor-principle - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-7 ## Question Explain the operating principle of stepper motor and servo motor. ## Short answer Stepper motor rotates in fixed steps per pulse by sequentially energizing phases, step angle = 360°/(phases × rotor teeth), open-loop. Servo motor uses closed-loop control: an error signal fed to a two-phase AC motor's control winding; torque proportional to error, stops when error zero. ## Answer #### Stepper motor A stepper motor is a brushless motor that rotates in **fixed angular steps**, one step for each input pulse. A drive circuit energises the stator phases in sequence, and the rotor turns to the minimum-reluctance (or magnetic alignment) position of each energised phase. - Phases A, B, C are energised in order. The rotor moves through the **step angle** at each switching: $$ \beta = \frac{360^\circ}{m\,N_r} $$ ($m$ = number of phases, $N_r$ = number of rotor teeth; for $m=3$, $N_r=4$, $\beta = 30^\circ$). - Total rotation $=$ number of pulses $\times\ \beta$; speed (rpm) $= \frac{\beta f_p}{6}$ for pulse frequency $f_p$ in Hz. - Reversing the sequence reverses the direction. It works in open loop and holds its position while a phase is kept energised. ```text Pulse train -> [Driver / sequencer] -> A,B,C phases | [Stepper] -> step by step ``` Use: printers, plotters, CNC, robots. #### Servo motor A servo motor is a motor used in a **closed-loop** system. A sensor measures the shaft position (or speed), which is compared with the reference; the difference (error) is amplified and fed to the control winding (AC servo: two-phase induction motor with a high-resistance rotor; DC servo: armature or field controlled). - The torque is proportional to the control voltage. As the error reduces, the control voltage falls, and the motor stops when the position matches the reference. - The direction depends on the polarity or phase of the control voltage. - The rotor has a small diameter and a long length, giving low inertia and fast response. ```text Ref -> (+) -> [Amplifier] -> [Servo motor] -> load ^ - | +------- [Sensor / tacho] <-----------+ ``` | Point | Stepper | Servo | |---|---|---| | Control | Open loop | Closed loop | | Motion | Discrete steps | Continuous | | Feedback | None | Needed | | Use | Printers, CNC | Robotics, radar | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a stepper motor move? A. Phases are energised in sequence and the rotor aligns to the minimum reluctance position of each B. It follows a feedback signal C. It uses brushes D. It runs at a fixed speed Answer: A. Switching phases A, B, C in order makes the rotor step through the step angle. (Key point: Stepper principle) 2. What is the step angle for m = 3, Nr = 4? A. 30 degrees B. 90 degrees C. 12 degrees D. 120 degrees Answer: A. beta = 360/(m Nr) = 30 degrees. (Key point: Step angle formula) 3. How does the servo motor control position? A. Without feedback B. The error between reference and actual position is amplified to the control winding and the motor stops at zero error C. By the pulse count D. By constant speed Answer: B. Closed-loop: sensor feedback is compared with the reference, and the motor stops when the error is zero. (Key point: Servo principle) 4. Which statement distinguishes them correctly? A. Stepper is closed loop, servo is open loop B. Both need feedback C. Stepper is open loop with discrete steps; servo is closed loop with feedback D. Servo moves in discrete steps Answer: C. A stepper is open-loop and moves in steps; a servo is closed-loop with continuous motion. 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