AC Asynchronous Motor

1. What is an AC Asynchronous Motor

An AC asynchronous motor, commonly known as an induction motor, is one of the primary types of drive motors used in new energy vehicles. The term "asynchronous" signifies that the rotor's rotational speed is always lower than the speed of the stator's rotating magnetic field—meaning the rotor never catches up with the rotating field, resulting in a persistent speed difference between the two.

Unlike permanent magnet synchronous motors, an AC asynchronous motor relies on both its stator and rotor being energised to generate magnetic fields, with no permanent magnets embedded in the rotor. This fundamental difference defines its distinct positioning in terms of performance, cost, and practical application.

2. Working Principle

The operating principle of an AC asynchronous motor is grounded in electromagnetic induction. When three-phase alternating current flows through the stator windings, a rotating magnetic field is generated. The closed conductor windings on the rotor sit within this field, effectively cutting through magnetic flux lines. In accordance with Faraday's Law of Electromagnetic Induction, an induced electromotive force and induced current are produced in the rotor windings. Under the influence of the rotating magnetic field, this induced current experiences an Ampere force, producing electromagnetic torque that drives the rotor to spin.

The entire process can be summarised as: Stator is energised to generate a rotating magnetic field → Magnetic field cuts rotor conductors → Rotor produces induced current → Induced current interacts with the magnetic field to generate torque → Rotor rotates. Because the rotor speed lags behind the magnetic field speed, it is termed "asynchronous".

3. Basic Construction

An AC asynchronous motor consists primarily of two core components: the stator and the rotor, separated by an air gap.

• Stator (Stationary Component) : Comprises the stator core (laminated from silicon steel sheets), stator windings (three-phase coils), and the frame. Evenly spaced slots are stamped along the inner circumference of the stator core to house the stator windings.

• Rotor (Rotating Component) : Comprises the rotor core and rotor windings. Based on the winding design, it is categorised into two types: squirrel-cage and wound-rotor. The squirrel-cage rotor features conductive copper or aluminium bars inserted into the core slots with both ends short-circuited by end rings—resembling a rodent cage. Offering a simple construction and low manufacturing cost, it represents the mainstream configuration for automotive traction motors.

Additionally, the motor assembly includes ancillary components such as end covers, bearings, and the outer casing.

4. Core Strengths

• Zero risk of demagnetisation, high reliability: With no permanent magnets in the rotor, there is no risk of high-temperature demagnetisation. Motor characteristics remain largely unaffected by ambient conditions, ensuring dependable operation under demanding driving cycles.

• Cost-effective: Eliminating the need for expensive rare-earth permanent magnet materials (such as NdFeB) keeps raw material costs substantially lower than those of permanent magnet synchronous motors. Compared with a direct current (DC) motor of equivalent output and speed, an induction motor weighs roughly half as much and costs only a third as much.

• Outstanding high-speed performance: Delivers a high top-speed ceiling and a broad rev range. It maintains high electrical efficiency even during sustained high-speed driving.

• Simple construction, straightforward maintenance: With no direct physical contact between the stator and rotor, manufacturing, operation, and servicing are straightforward, resulting in exceptional operational reliability.

5. Key Drawbacks

• Lower power density: For a given power and torque output, an asynchronous motor is noticeably bulkier and heavier than a permanent magnet synchronous unit.

• Lower energy conversion efficiency: Because the rotor relies on induced currents to create its magnetic field, copper losses (rotor winding losses) are unavoidable. Energy conversion efficiency typically hovers between 85% and 90%, roughly 5% lower than that of permanent magnet synchronous motors.

• Reduced efficiency in low-speed driving: In urban driving scenarios involving frequent stop-and-go traffic, acceleration, and deceleration, energy consumption is higher than in permanent magnet synchronous motors.

• Requires dedicated thermal management: The rotor heats up rapidly under load, necessitating a robust cooling system for prolonged high-speed running.

6. Applications in the New Energy Vehicle Sector

Within the new energy vehicle landscape, AC asynchronous motors are predominantly deployed across the following scenarios:

• High-performance models: Capitalising on their strong high-rev capabilities and potent output, they are well suited for performance-oriented models prioritising rapid acceleration and top-speed performance. Early iterations of the Tesla Model S and Model X adopted AC asynchronous motors.

• Commercial and heavy-duty vehicles: Thanks to lower unit costs, zero rare-earth dependency, thermal resilience, and rugged durability, induction motors are widely utilised in electric buses and commercial vehicles.

• Secondary traction in dual-motor setups: Many carmakers adopt a layout pairing a front permanent magnet synchronous motor with a rear AC asynchronous motor—the front motor ensures daily cruising efficiency, while the rear induction motor engages when extra performance is summoned.

Notably, European and American carmakers (such as Tesla and Mercedes-Benz) initially favoured AC asynchronous motors due to limited access to rare-earth resources. Conversely, Chinese carmakers, backed by abundant domestic rare-earth reserves, more widely adopted permanent magnet synchronous motors. However, this dynamic is evolving—the Tesla Model 3 has switched to permanent magnet synchronous motors, illustrating that the two technical approaches are not mutually exclusive, but rather selected flexibly based on vehicle positioning and market demands.

7. Summary

With key strengths such as immunity to demagnetisation, low production costs, and exceptional high-speed capability, the AC asynchronous motor commands an essential role in the new energy vehicle sector. Although its power density and overall efficiency fall short of permanent magnet synchronous motors, it remains irreplaceable in performance cars, commercial transport, and dual-motor secondary drive setups. Each technical route offers distinct advantages—there is no absolute "better" option, only what is "more suitable", reflecting the rich engineering diversity powering modern electrified vehicles.