---
title: "Synchronous speed; rotor follows rotating field"
subject: "Electrical Machines"
chapter: 5
key: "semester_4/electrical_machines:synchronous-speed-rotor-direction"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/synchronous-speed-rotor-direction
site: BCT Engineering
---

# Synchronous speed; rotor follows rotating field

Electrical Machines · Chapter 5: Three Phase Induction Machines

- Asked in: 2075 Baisakh (old course), 2+3+3 marks
- Video: [How Does an Induction Motor Spin Without Powering the Rotor?](https://www.youtube.com/watch?v=gApo7eeUASg) — LifeAda English
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/synchronous-speed-rotor-direction
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-5

## Question

Define synchronous speed of three phase induction motor. Why does the rotor of a three phase induction motor rotate in the same direction as the rotating magnetic field? Why rotor can never reach the speed of stator field?

## Short answer

Synchronous speed Ns=120f/P is the rotating field speed. Rotor follows the field direction by Lenz's law: induced current creates torque opposing relative motion. If rotor reached Ns, slip s=0, no EMF, no current, no torque, so it must run below Ns.

## Answer

#### Synchronous speed
Synchronous speed is the speed of the rotating magnetic field produced by the stator winding of a three-phase induction motor:
$$
N_s = \frac{120 f}{P}\ \text{rpm}
$$
where $f$ is the supply frequency and $P$ the number of poles. For $f = 50$ Hz and $P=4$, $N_s = 1500$ rpm.

#### Why the rotor rotates in the direction of the field
1. The rotating field cuts the stationary rotor conductors and induces an emf (Faraday's law).
2. The closed rotor circuit carries a current, so each conductor in the field experiences a force.
3. By **Lenz's law**, the effect of the induced current opposes its cause, which is the relative motion between the field and the rotor conductors. The rotor can reduce this relative motion only by moving in the same direction as the field.
4. So the torque on the rotor acts in the direction of the field and the rotor follows it.

#### Why the rotor cannot reach synchronous speed
- If the rotor ran at $N_s$, the relative speed between field and rotor would be zero.
- No emf would be induced in the rotor, so no rotor current would flow.
- Without current there is no torque, and the rotor would slow down due to friction and windage losses and load.
- It therefore settles at a speed $N_r < N_s$ where the induced current gives the torque needed for the load. The difference $N_s - N_r$ is the slip speed, and slip $s = \frac{N_s - N_r}{N_s}$ is 2 to 5 % at full load.

For this reason the induction motor is also called an **asynchronous** motor.

## Check yourself

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

1. Synchronous speed of a 4-pole, 50 Hz machine is:
   A. 750 rpm
   B. 1000 rpm
   C. 3000 rpm
   D. 1500 rpm
   Answer: D. 120 x 50/4 = 1500 rpm. (Key point: Synchronous speed)

2. Why does the rotor turn in the field direction?
   A. Lenz's law: it reduces relative motion by following the field
   B. The field pulls rotor by magnetic poles
   C. The supply fixes the direction
   D. Friction
   Answer: A. The induced current opposes relative motion. (Key point: Direction)

3. At N_r = N_s the rotor would have:
   A. Maximum torque
   B. No relative motion, so no emf, current or torque
   C. Double current
   D. Negative slip
   Answer: B. With zero relative speed nothing is induced. (Key point: Cannot reach N_s)

4. The rotor settles at a speed where:
   A. N_r = N_s
   B. N_r = 0
   C. Torque is zero
   D. N_r < N_s so current gives the torque the load needs
   Answer: D. Slip provides the needed rotor current. (Key point: Slip speed)

## Related in Electrical Machines

- [Synchronous speed and why rotor lags stator field](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Asynchronous-speed-rotor-direction-slip) — Electrical Machines I, chapter 5
- [Production of rotating magnetic field](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Arotating-magnetic-field-production) — Electrical Machines, chapter 5
- [Rotating magnetic field, slip and standstill torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Armf-synchronous-speed-slip-standstill-torque) — Electrical Machines I, chapter 5
- [Delta induction motor: slip, rotor emf, turns ratio](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aim-delta-440v-1440rpm-rotor-emf) — Electrical Machines, chapter 5, asked 2×
- [Rotor copper loss and efficiency of 35 kW induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-4pole-1440rpm-35kw-losses) — Electrical Machines I, chapter 5
- [Induction motor equivalent circuit from no-load and blocked rotor tests](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-noload-blocked-400v-equivalent) — Electrical Machines I, chapter 5

## Related in other subjects

- [Rotating field in 3-phase IM](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Arotating-field-3ph-im-vector) — Electrical Machine, chapter 5
- [Unequal stator/rotor slots and output equation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aslots-unequal-output-equation) — Electric Machine Design, chapter 4
- [Output equation of three phase induction motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainduction-motor-output-equation) — Electric Machine Design, chapter 4, asked 6×