What is the basic working principle of asynchronous motor?

The fundamental working principle of an asynchronous motor (also called an induction motor) is based on electromagnetic induction and the interaction of a rotating magnetic field. The core mechanism involves the stator-generated rotating magnetic field cutting through rotor conductors, inducing current in the rotor and producing torque. Below is a detailed step-by-step explanation:

1. Generation of the Rotating Magnetic Field

• Stator Windings: The stator core is embedded with a three-phase symmetrical winding (when supplied with three-phase AC) or a two-phase winding (single-phase motors require auxiliary starting).

• Rotating Magnetic Field: When three-phase AC is applied to the stator windings, it generates a magnetic field rotating at synchronous speed ($n_s=\frac{60f}{p}$ns​=p60f​), where:

  • $f$f: Power supply frequency (Hz);

  • $p$p: Number of motor pole pairs.

2. Electromagnetic Induction in the Rotor

• Rotor Structure: The rotor typically consists of short-circuited bars (squirrel cage) or wound windings.

• Induced Current: When the rotating magnetic field cuts through the stationary rotor conductors, an electromotive force (EMF) is induced in the rotor according to Faraday’s law of electromagnetic induction (direction determined by the right-hand rule). Since the rotor conductors are short-circuited, the induced EMF generates current.

• Rotor Magnetic Field: The induced current produces its own magnetic field (rotor field).

3. Generation of Electromagnetic Torque

• Interaction of Magnetic Fields: The rotor magnetic field interacts with the stator’s rotating magnetic field. According to Lorentz force law (motor principle), the rotor conductors experience an electromagnetic force, creating torque ($T$T) that drives the rotor to rotate in the same direction as the rotating field.

• Slip ($s$s): The rotor speed ($n$n) is always slightly lower than the synchronous speed ($n_s$ns​), resulting in slip ($s=\frac{n_s-n}{n_s}$s=ns​ns​−n​). Slip is essential for induction motor operation—if $n=n_s$n=ns​, no current is induced, and torque becomes zero.

4. Energy Conversion Process

• Motor Mode: The stator draws electrical energy from the grid, converting it into mechanical energy via the rotating magnetic field.

• Generator Mode: If an external force drives the rotor faster than synchronous speed ($n>n_s$n>ns​), electrical energy is fed back into the grid (negative slip).

Key Formulas & Parameters

1. Synchronous Speed:

   $$n_s=\frac{60f}{p}\text{(r/min)}$$ns​=p60f​(r/min)

2. Actual Rotor Speed:

   $$n=(1-s)n_s$$n=(1−s)ns​

3. Torque Formula:

   $$T\propto \varphi I_2\cos {\theta }_2$$T∝ϕI2​cosθ2​

   • $\varphi$ϕ: Main magnetic flux;

   • $I_2$I2​: Rotor current;

   • $\cos {\theta }_2$cosθ2​: Rotor power factor.

Key Features

• Self-Starting: Three-phase induction motors do not require external starting devices (single-phase motors need auxiliary starting).

• Speed Characteristics: Speed decreases slightly under increased load (higher slip), automatically adjusting torque.

• Simple & Reliable: No brushes or commutators, resulting in low maintenance costs.

Due to their simple structure, low cost, and high reliability, induction motors are widely used in industrial drives, household appliances, electric vehicles, and more. Understanding their working principle helps optimize motor selection and control strategies.