---
title: "Synchronous speed and why rotor lags stator field"
subject: "Electrical Machines I"
chapter: 5
key: "semester_4/electrical_machines_i:synchronous-speed-rotor-direction-slip"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/synchronous-speed-rotor-direction-slip
site: BCT Engineering
---

# Synchronous speed and why rotor lags stator field

Electrical Machines I · Chapter 5: Three-Phase Induction Machines

- Asked in: 2075 Baisakh, 2+3+3 marks
- Video: [Synchronous Speed: What is it?](https://www.youtube.com/watch?v=RpkGoeUCmcM) — Electrical4U
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/synchronous-speed-rotor-direction-slip
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/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 is the speed of the stator rotating magnetic field, given by 120f/P. The rotor turns in the same direction because induced currents oppose relative motion. It cannot reach synchronous speed because zero slip means no induced emf, current, or torque.

## Answer

#### Synchronous speed

**Synchronous speed** is the speed at which the rotating magnetic field produced by the 3-phase stator winding rotates. It depends only on supply frequency and number of poles:

$$
N_s = \frac{120 f}{P}\ \text{rpm}
$$

Example: $f = 50$ Hz, $P = 4$: $N_s = 1500$ rpm; $P = 6$: $N_s = 1000$ rpm.

#### Why the rotor turns in the direction of the rotating field

1. The stator field rotates at $N_s$ and cuts the stationary rotor conductors, inducing emf and current in them.
2. By **Lenz's law**, the induced current opposes the cause producing it. The cause is the **relative motion** between the field and the rotor conductors.
3. The rotor cannot stop the field, so it reduces the relative speed by **chasing the field**, i.e. rotating in the same direction.
4. In force terms, the rotor current in the field gives a force $F = BIl$ on each conductor in the direction that drags the rotor along with the field.

To reverse the motor, the field direction is reversed by interchanging any two supply leads.

#### Why the rotor can never reach synchronous speed

1. If the rotor ran at $N_s$, there would be **no relative motion** between rotor conductors and the field.
2. Then no flux is cut, so rotor emf $= 0$, rotor current $= 0$ and torque $= 0$.
3. But the rotor always needs some torque to overcome friction, windage and load. So it slows down until enough relative speed exists to induce the required current.
4. Hence the rotor always runs slightly below $N_s$. The difference is expressed as **slip**:

$$
s = \frac{N_s - N}{N_s}
$$

Typical full-load slip is 2–5% (about 0.5% at no load). This is why the machine is called an **asynchronous** motor. Running exactly at $N_s$ is possible only if an external prime mover drives it; above $N_s$ it becomes a generator.

## Check yourself

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

1. On what does synchronous speed depend?
   A. Supply frequency and number of poles, Ns = 120f/P
   B. Rotor resistance and load
   C. Supply voltage and rotor speed
   D. Slip and number of phases
   Answer: A. Ns = 120f/P depends only on frequency and poles. (Key point: Synchronous speed)

2. Why does the rotor turn in the direction of the rotating field?
   A. Because the rotor has permanent magnets
   B. Because Lenz's law makes the rotor reduce the relative speed between field and conductors by chasing the field
   C. Because the rotor is driven by friction
   D. Because the field drags air with it
   Answer: B. The induced current opposes its cause, the relative motion, so the rotor follows the field. (Key point: Rotor direction)

3. Why can the rotor never reach synchronous speed?
   A. Friction makes it impossible to rotate that fast
   B. Rotor emf would be maximum at Ns
   C. At Ns there is no relative motion, so no emf, current or torque, and the rotor would slow down
   D. The supply frequency drops at Ns
   Answer: C. Zero relative speed gives zero torque, but torque is needed for losses and load, so a small slip always remains. (Key point: Why not Ns)

4. How is the direction of rotation of the motor reversed?
   A. Increase the supply voltage
   B. Add rotor resistance
   C. Change frequency
   D. Interchange any two supply leads
   Answer: D. Swapping two leads reverses the phase sequence and the direction of the rotating field. (Key point: Reversal of rotation)

## Related in Electrical Machines I

- [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
- [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
- [No-load and blocked rotor tests of induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-noload-blocked-rotor-test-explain) — Electrical Machines I, chapter 5
- [Effect of five-times rotor resistance on 40 kW slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-40kw-rotor-resistance-5x) — Electrical Machines I, chapter 5
- [Construction and working of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-construction-working) — Electrical Machines I, chapter 5

## Related in other subjects

- [Synchronous speed; rotor follows rotating field](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Asynchronous-speed-rotor-direction) — Electrical Machines, 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 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
- [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×