---
title: "Short note: AC servo motor"
subject: "Electric Machines II"
chapter: 3
key: "semester_5/electric_machines_ii:short-note-ac-servo-motor"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/short-note-ac-servo-motor
site: BCT Engineering
---

# Short note: AC servo motor

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2076 Chaitra, 3 marks; 2072 Chaitra, 3 marks; 2071 Shrawan, 4 marks (asked 3 times)
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/short-note-ac-servo-motor
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Write a short note on AC servo motor.

## Short answer

A two-phase induction motor with high-resistance rotor; torque proportional to control voltage. Linear torque-speed curve gives fast response and stability. Stops when control voltage is zero, used in position control systems.

## Answer

An **AC servo motor** is a two-phase induction motor designed to give torque proportional to a control voltage, used in feedback control systems for accurate position and speed control.

#### Construction

- **Stator:** two windings displaced by $90^\circ$ electrical:
  - **Reference (fixed) winding**, supplied with a constant AC voltage $V_r$.
  - **Control winding**, supplied from a servo amplifier with variable voltage $V_c$, $90^\circ$ out of phase with $V_r$.
- **Rotor:** squirrel cage (or drag-cup) with **high resistance** and **small diameter, long length**, giving low inertia and fast response.

```text
   Vr (fixed) --> [Reference wdg]
                       |  90 deg
   Vc (from   --> [Control wdg] --> rotor --> load
   amplifier)
```

#### Working

- The two quadrature fluxes produce a rotating field; torque is proportional to $V_c$.
- When $V_c = 0$ the field is only pulsating, and because of the high rotor resistance the motor stops (it cannot keep running as a single-phase motor).
- Reversing the phase of $V_c$ reverses rotation.
- High $R_2/X_2$ gives an almost **linear torque-speed curve with negative slope**, which gives positive damping and stability.

```text
 T |\
   | \  Vc = rated
   |\ \
   | \ \  Vc = 0.5 rated
   +--\-\---- speed
```

#### Applications

Position control in radar and antenna drives, instrument servos, X-Y recorders, robotics and computer peripherals.

## Check yourself

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

1. The two stator windings of an AC servo motor are:
   A. Both supplied with the same variable voltage in phase
   B. A reference winding at constant voltage and a control winding at variable voltage, 90 degrees apart in phase
   C. A field winding and an armature winding
   D. Two windings connected in series to the same supply
   Answer: B. The reference winding has a fixed AC voltage; the control winding gets variable voltage from the servo amplifier, 90 degrees out of phase. (Key point: Stator windings)

2. Why is the rotor of an AC servo motor made with high resistance?
   A. To increase efficiency
   B. To get a linear torque-speed curve with negative slope and to stop when control voltage is zero
   C. To reduce the rotor inertia only
   D. To increase the speed above Ns
   Answer: B. High R2/X2 gives a nearly linear, negative-slope curve (damping, stability) and prevents single-phase running when Vc = 0. (Key point: High rotor resistance)

3. How is the direction of rotation of an AC servo motor reversed?
   A. By reversing the reference voltage frequency
   B. By reversing the phase of the control voltage Vc
   C. By increasing Vc
   D. By adding a capacitor
   Answer: B. Reversing the phase of Vc reverses the rotating field, hence the rotation. (Key point: Control of direction)

4. The rotor of an AC servo motor is typically:
   A. Large diameter and short length
   B. Small diameter and long length for low inertia
   C. A salient-pole rotor
   D. A wound rotor with slip rings
   Answer: B. Small diameter and long length give low inertia and fast response. (Key point: Rotor shape)

## Related in Electric Machines II

- [Short note: Servo motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashort-note-servo-motor) — Electric Machines II, chapter 3
- [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

## Related in other subjects

- [Short note: AC servo motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashort-note-ac-servo-motor) — Electrical Machine, chapter 7
- [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×
- [Servo motor construction and working](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aservo-motor-construction-working) — Electrical Machines, chapter 7, asked 2×