Understanding internal combustion engine technology often involves navigating numerous complex concepts, such as naturally aspirated and turbocharged engines, direct and indirect fuel injection, and a variety of cycles like Atkinson, Miller, and Otto, among others. This diversity exists because no single design perfectly optimizes efficiency.
When it comes to electric vehicles (EVs), however, the landscape is much simpler. There are primarily three types of electric motors used in e-mobility: asynchronous induction motors, synchronous permanent magnet motors, and electrically excited synchronous motors. Let’s explore each of these.
Asynchronous Induction Motor - A Brief History
The asynchronous induction motor is a well-established invention credited to Nikola Tesla and Galileo Ferraris. While Ferraris developed his design in 1885, it was Tesla who first patented the motor in 1888.
This invention marked a significant milestone in the use of electricity for powering machines. Today, asynchronous induction motors are ubiquitous, powering countless industrial machines and everyday electric devices.
Nikola Tesla’s historic patent of the induction motor
How Does an Asynchronous Induction Motor Work?
Electric motors consist of two main components: a stationary stator and a rotating rotor. The stator, typically a steel cylinder with copper windings, receives three-phase alternating current (AC) converted from direct current (DC) supplied by the vehicle’s battery. This energizes the windings to create a rotating magnetic field known as synchronous speed.
The rotating magnetic field induces voltage in the rotor, generating a current that produces its own magnetic field. This rotor magnetic field lags behind the stator's, and the interaction between these fields creates the Lorentz force that turns the rotor. The rotor’s motion is then transmitted to the vehicle’s wheels through the drivetrain.
This motor is termed "asynchronous" because the rotor’s magnetic field does not rotate in perfect sync with the stator’s magnetic field. The difference in speed, called "slip," is typically up to 5% depending on design. During acceleration, the rotor field lags behind; during regenerative braking, the rotor field leads the stator field.
Asynchronous induction motors typically offer around 90% efficiency. Their robustness, simplicity, longevity, and lack of rare earth material requirements make them popular in industrial settings and in front axles of all-wheel-drive electric vehicles.
Advantages
- Reliable and durable with minimal exotic materials
- Cost-effective manufacturing
- Good efficiency in various operating conditions
Disadvantages
- Higher cooling requirements
- Lower power density compared to other motor types
- Lower maximum efficiency
Vehicles featuring asynchronous induction motors include the Audi e-Tron SUV, Mercedes-Benz EQC, Tesla Model S, 3, X, and Y (used on front axles), as well as Volkswagen Group’s MEB platform cars with front-axle motors.
Induction motor used in Mercedes-Benz EQC
Synchronous Permanent Magnet Motor
Unlike induction motors, synchronous permanent magnet motors generate a native rotating magnetic field in the rotor via permanent magnets. This locks the rotor and stator fields in sync, eliminating slip.
The use of permanent magnets—typically made from rare earth materials—increases power density and efficiency significantly. These motors deliver high power in a compact form, making them ideal for plug-in hybrids where space is limited.
However, the reliance on rare earth elements raises ethical and supply-chain concerns, as China controls most of these resources. Despite this, synchronous permanent magnet motors remain the most efficient type, achieving efficiencies of 94-95%.
Advantages
- Exceptional efficiency
- Compact size with high power density
- Reduced cooling requirements
Disadvantages
- Higher production costs
- Dependence on rare earth materials
- Potential, albeit low, risk of demagnetization
Hyundai Ioniq 5 permanent magnet motors
Examples of vehicles using permanent magnet motors include the Hyundai Ioniq 5, Tesla Model S, 3, X, Y (rear axles), Volkswagen Group MEB cars (rear axles), Jaguar I-Pace, Audi e-tron GT, and Porsche Taycan.
Electrically Excited Synchronous Motor
To address concerns related to rare earth materials, some manufacturers such as BMW, Renault Group, and Smart use electrically excited synchronous motors. These motors achieve high efficiency similar to permanent magnet variants without relying on rare earth magnets.
Instead of permanent magnets, these motors use brushes and slip rings to supply current to the rotor’s electromagnetic windings. BMW reports efficiencies up to 93% for this design.
While promising, the presence of brushes means potential maintenance issues over the long term, as brushes may wear out and require replacement.
BMW electrically excited synchronous motor
Advantages
- High efficiency
- Lower production cost than permanent magnet motors
- No reliance on rare earth materials
- No risk of demagnetization
Disadvantages
- Brushes may limit long-term reliability
This motor type is found in vehicles such as the BMW iX3, iX, i4, Renault Megane E-TECH, and Smart EQ.
