--- title: "DC servo motor and applications" subject: "Electrical Machine" chapter: 7 key: "semester_4/electrical_machine:dc-servo-motor-principle-applications" source: https://www.bctengineer.com/q/semester_4/electrical_machine/dc-servo-motor-principle-applications site: BCT Engineering --- # DC servo motor and applications Electrical Machine · Chapter 7: Fractional Kilowatt Motors - Asked in: 2069 Poush - Video: [dc servo motor | dc servo motor control system | dc servo motor used in electric vehicle | robotics](https://www.youtube.com/watch?v=GX67qZcs11Q) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/dc-servo-motor-principle-applications - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7 ## Question Explain the operating principle of dc servo motor and its applications. ## Short answer A DC servo motor uses armature control with constant field; varying armature voltage gives proportional torque and speed. Closed-loop negative feedback reduces position/ speed error to zero. Applications include robotics, CNC, antenna tracking, and printers. ## Answer A d.c. servo motor is a d.c. motor (separately excited or permanent-magnet) designed for fast, accurate speed and position control, with a low-inertia armature and high torque-to-inertia ratio. #### Construction - Armature: long and thin laminated core, light, low inertia. Often a coreless (hollow-cup or disc) armature in small motors. - Field: permanent magnet, or separately excited field winding. - A tachogenerator or an encoder or potentiometer is mounted on the shaft for feedback. #### Operating principle The torque of a d.c. motor is T = k φ I_a and the speed is N ∝ (V − I_a R_a)/φ. Two methods of control: 1. **Armature control:** the field is kept constant (or a permanent magnet) and the armature voltage is varied. The torque and speed are proportional to the armature voltage. This gives a linear torque-speed curve and is the common method. 2. **Field control:** armature current is kept constant and field voltage is changed. This has a slow response because of field inductance, so it is used rarely. ```text Ref -> (+) -> Amp -> [Armature] M -> Load ^ (-) | +---- Tachogenerator / encoder ``` The error between the reference and the feedback signal is amplified and applied to the armature. Torque is produced in proportion to the error, so the motor turns the load until the error becomes zero. Reversal of the error signal polarity reverses the direction of rotation. #### Characteristics - Torque proportional to armature current, speed proportional to voltage, a linear torque-speed curve. - High starting torque and fast response (small electrical and mechanical time constants). - Speed range is wide, from a few rpm to thousands. #### Applications Robotics, CNC machine tools, position control of antennas and radar, tape drives, disc drives, automatic cameras, printers, process control valves, aircraft control surfaces and toys. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is the common method of controlling a d.c. servo motor? A. Armature control with constant field, which gives a linear torque-speed curve B. Field control, because it has a quick response C. Varying the supply frequency D. Changing the number of poles Answer: A. Armature control keeps the field constant (or uses a permanent magnet) and varies the armature voltage. (Key point: Armature control) 2. Why is field control used rarely? A. The response is slow because of field inductance B. It gives non-linear torque only on a.c. C. It needs a tachogenerator D. It reduces the torque to zero Answer: A. The field winding inductance makes the response slow. (Key point: Why field control is rare) 3. What does reversing the polarity of the error signal do? A. Nothing B. Reverses the direction of rotation C. Doubles the speed D. Stops the feedback Answer: B. Torque is produced in proportion to the error, and reversing its polarity reverses the armature voltage and the rotation. (Key point: Error signal action) 4. Why is the armature of a d.c. servo motor long and thin (or coreless)? A. To give a low inertia and a high torque-to-inertia ratio for fast response B. To increase the armature resistance C. To reduce the number of brushes D. To increase the air gap Answer: A. A light, low-inertia armature allows fast acceleration and short time constants. (Key point: Low-inertia construction) ## Close variants set in other exams - [Servo motor operating principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aservo-motor-principle) — Electrical Machine, chapter 7, asked 2× ## Related in Electrical Machine - [Stepper motor and servo motor principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Astepper-and-servo-motor-principle) — Electrical Machines, chapter 7 - [Operating principle of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-operating-principle) — Electrical Machines I, chapter 5 - [DC motor principle and role of commutator and brushes](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-principle-commutator-brushes) — Electrical Machines I, chapter 4 - [DC motor working principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-principle-torque-equation) — Electrical Machines I, chapter 4, asked 4× - [DC motor working principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-motor-working-torque-equation) — Electrical Machines, chapter 4, asked 2× - [DC motor principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-motor-principle-torque-equation) — Electrical Machine, chapter 4 ## Related in other subjects - [Short note: DC servo motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashort-note-dc-servo-motor) — Electric Machines II, chapter 3 - [Capacitor start motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-motor-principle) — Electric Machines II, chapter 3, asked 2× - [Shaded pole motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashaded-pole-motor) — Electric Machines II, chapter 3 - [Stepper motor principle and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Astepper-motor-principle-applications) — Electric Machines II, chapter 3