---
title: "Servo motor construction and working"
subject: "Electrical Machines"
chapter: 7
key: "semester_4/electrical_machines:servo-motor-construction-working"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/servo-motor-construction-working
site: BCT Engineering
---

# Servo motor construction and working

Electrical Machines · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2077 Chaitra, 8 marks; 2071 Magh, 4 marks (asked 2 times)
- Video: [Servo motor Construction and working principle #motor #servo #automation](https://www.youtube.com/watch?v=84gwMAI1wHA) — General Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/servo-motor-construction-working
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-7

## Question

Explain the construction and working of a servo motor.

## Short answer

Servo motor is a closed-loop control motor. Its stator has reference and control windings 90° apart; the high-resistance, small-diameter, long-length rotor gives fast response. Torque depends on control voltage magnitude, direction on phase; motor stops when error is zero.

## Answer

A servo motor is a small, high-performance motor used in a **closed-loop control system** for accurate control of position, speed or torque. It is fast-responding and its shaft position follows a control signal. Two main types are the AC (two-phase) servo and the DC servo motor.

#### Construction (two-phase AC servo motor)
- **Stator:** a laminated stator carries two windings placed $90^\circ$ apart in space: the **reference (fixed) winding**, fed from a constant ac source (often through a capacitor), and the **control winding**, fed with the variable ac error signal from the servo amplifier.
- **Rotor:** a squirrel-cage rotor (or drag-cup rotor) of **high resistance** ($R/X$ large), with **small diameter and long length** for low inertia and fast response.
- A tacho-generator and a position sensor (potentiometer or encoder) are fitted on the shaft for feedback.

```text
 Ref. -> [Controller] -> error -> [Amplifier]
 input       ^                       |
 (set)       | feedback              v
        [Sensor] <--- shaft <--- [Servo motor] --> load
```

#### Working
1. The reference winding gets a fixed voltage $V_r$. The control winding gets the amplified error voltage $V_c$ (difference between desired and actual position), shifted $90^\circ$ in time from $V_r$.
2. The two currents with $90^\circ$ space and time displacement produce a **rotating magnetic field** in the air gap, which drags the rotor.
3. The torque and speed depend on the **magnitude** of the control voltage. A larger error gives a larger torque and speed. The direction of rotation depends on the **phase** of $V_c$ relative to $V_r$ ($\pm90^\circ$).
4. When the shaft reaches the desired position, the error becomes zero, $V_c = 0$, and the motor stops.

The high rotor resistance gives a **negative-slope torque-speed characteristic** over the full range, which provides damping and stability and prevents single-phase running when the control signal is removed.

#### Applications
Robotics, machine tools, radar and antenna positioning, aircraft control surfaces, and printers.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In what kind of system is a servo motor used?
   A. Open-loop
   B. Closed-loop control with feedback
   C. Uncontrolled
   D. Open-loop with no sensor
   Answer: B. The position or speed is compared with the reference and the error drives the motor. (Key point: Control system)

2. Why is the rotor of an AC servo motor of high resistance and small diameter with long length?
   A. High resistance for damping and negative slope; small diameter and long length for low inertia and fast response
   B. To raise speed above Ns
   C. To reduce the cost
   D. To increase starting current
   Answer: A. High R/X gives a negative-slope torque-speed curve and damping, and low inertia gives fast response. (Key point: Rotor design)

3. How do torque and direction depend on the control voltage?
   A. Torque depends on its phase; direction on its magnitude
   B. Torque depends on its magnitude; direction on its phase relative to the reference voltage
   C. Both depend only on frequency
   D. Neither depends on it
   Answer: B. A larger error voltage gives larger torque, while the phase (plus or minus 90 degrees) decides the direction. (Key point: Control voltage)

4. Why does the motor stop at the desired position?
   A. The reference winding is removed
   B. The tacho opens the field
   C. The error becomes zero so Vc = 0
   D. The rotor resistance becomes zero
   Answer: C. At the target the error is zero, there is no control voltage, and the motor stops. (Key point: Stopping)

## Related in Electrical Machines

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- [Stepper motor construction and working](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Astepper-motor-construction-principle) — Electrical Machines, chapter 7, asked 3×
- [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
- [Servo motor length and diameter reason](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aservo-motor-long-small-diameter) — Electrical Machines, chapter 7
- [DC generator construction, working, emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-construction-emf) — Electrical Machines, chapter 3
- [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×

## Related in other subjects

- [Schrage motor construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aschrage-motor-construction) — Electric Machines II, chapter 3
- [Short note: AC servo motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashort-note-ac-servo-motor) — Electric Machines II, chapter 3, asked 3×
- [Servo motor operating principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aservo-motor-principle) — Electrical Machine, chapter 7, asked 2×
- [DC servo motor and applications](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-servo-motor-principle-applications) — Electrical Machine, chapter 7, asked 2×