How Rotating Magnetic Field is Produced in Induction Motors

What is Rotating Magnetic Field in Induction Motors?

In an induction motor, when AC supply is given to the stator, a magnetic flux is produced. This magnetic flux revolves at synchronous speed (Ns), therefore it is called as Rotating Magnetic Field or RMF. This rotating magnetic field is the fundamental principle that enables induction motor operation by inducing current in the rotor.

Production of Rotating Magnetic Field in 2-Phase Supply

Two phase revolving magnetic field sine wave diagram — Figure 1: Two-phase supply waveform with 90° phase difference

For a 2-phase supply system, the stator is wound with two phases positioned 90 spatial degrees apart. Let Φ₁ and Φ₂ represent the instantaneous flux values produced by phase 1 and phase 2 respectively.

Step-by-Step Analysis of Magnetic Field Rotation

Revolving magnetic flux two phase diagram showing different positions — Figure 2: Magnetic field positions at different angles in 2-phase system

i. At θ = 0° (Origin Position)

Phase 1 flux magnitude: 0
Phase 2 flux magnitude: Maximum (negative direction)
Resultant flux Φᵣ = Φₘ

ii. At θ = 45° (Position 1)

Phase 1 flux: √2/2 × Φₘ
Phase 2 flux: √2/2 × Φₘ
Resultant flux Φᵣ = Φₘ (shifted 45° clockwise)

iii. At θ = 90° (Position 2)

Phase 1 flux: Φₘ
Phase 2 flux: 0
Resultant flux Φᵣ = Φₘ (shifted 90° from initial position)

iv. At θ = 135° (Position 3)

Phase 1 flux: √2/2 × Φₘ
Phase 2 flux: √2/2 × Φₘ
Resultant flux Φᵣ = Φₘ (shifted 135° from initial position)

v. At θ = 180° (Position 4)

Phase 1 flux: 0
Phase 2 flux: Φₘ
Resultant flux Φᵣ = Φₘ (shifted 180° from initial position)

Key Conclusion: The magnitude of the resultant flux remains constant at Φₘ, but its direction continuously rotates clockwise, creating a uniform rotating magnetic field.

Synchronous Speed of Rotating Magnetic Field

Synchronous speed formula for rotating magnetic field — Synchronous Speed Formula: **Nₛ = 120f/P**

The speed at which the rotating magnetic field revolves is called synchronous speed (Nₛ) and is calculated using the formula:

Where:

f = frequency of the AC supply (Hz)
P = number of poles in the motor
Nₛ = synchronous speed (RPM)

Production of Rotating Magnetic Field in 3-Phase Supply

3-Phase System Advantages

Three-phase induction motors are more commonly used in industrial applications due to their superior performance characteristics. The three phases are spatially displaced by 120° and create a more uniform rotating magnetic field. The principles of vector addition are similar to the 2-phase system, resulting in a resultant magnetic flux with a constant magnitude that rotates in space.

Rotating magnetic field 3 phase diagram showing winding arrangement — Figure 3: Three-phase rotating magnetic field configuration

Key Differences: 2-Phase vs 3-Phase Systems

Parameter	2-Phase System	3-Phase System
Phase Displacement	90° electrical	120° electrical
Magnetic Field Uniformity	Good	Excellent
Starting Torque	Lower	Higher
Industrial Application	Limited	Widespread

FAQ on Rotating Magnetic Field

What is a rotating magnetic field?: A magnetic field that maintains constant magnitude but changes direction continuously, rotating around the stator.
Why is a rotating magnetic field important in induction motors?: It induces current in the rotor, producing torque and enabling motor rotation.
How does synchronous speed affect motor performance?: Synchronous speed determines the speed at which the magnetic field rotates. The actual rotor speed is slightly less (slip) in induction motors.
Which is better for motors: 2 phase or 3 phase supply?: 3 phase supply is better, as it produces a smoother rotating magnetic field and makes the motor self-starting without external arrangements.

Also Read: Starting methods of three phase induction motors.