Electric vehicles (EV) are a relatively new development in the automotive world and there are currently three main types of motors used in these cars.
These are an asynchronous induction motor (ASM), a permanently-excited synchronous magnet motor (PSM), and an electrically-excited synchronous motor (ESM).
All three have their own distinct benefits and drawbacks and all three are used by various major manufacturers, and there’s no definitive answer as to which one is best.
We go into more detail on what sets these EV engines apart below.
How an electric motor works
All variants of electric motors have two main parts namely a stator and a rotor, according to Arena EV.
Usually, the stator is a steel cylinder with slots and copper coils on the inside that are weaved in a specific pattern to create a magnetic field when an alternating current (AC) runs through them. As the rotating magnetic field builds it influences the rotor that is housed within the stator, which consequently starts to turn and drive the wheels.
A key differentiating factor between the various motors is therefore the type of rotor they use.
BMW Gen5 eDrive Stator
Asynchronous induction motor
In an ASM, the rotor is made up of a cage of bars in a highly-conductive material such as aluminium or copper in what is known as a “squirrel cage” setup.
In a “slip ring” configuration, the bars are replaced by aluminium/copper windings that connect to resistors on the outside of the rotor to perform the same duty as the cage bars, as stated by EV engine specialists Oswos.
In either application, the stator’s magnetic field “pulls” the rotor into motion, which will then constantly try to “catch up” with the rotating magnetic field created by the stator, said Renault. This difference in speed lends the type of motor the “asynchronous” designation.
Benefits of an ASM
- Reliable
- Relatively cheap to build
- Little need for rare earth materials
Drawbacks of an ASM
- More cooling needs
- Relatively low power density
- Lower efficiency compared to other electric motors
This type of motor is found in EVs including the Audi e-tron SUVs, Mercedes-Benz EQC, and certain VW ID. cars.
Audi Q8 e-tron slip-ring ASM
Permanently-excited synchronous magnet motor
In a PSM, the rotor features magnets that create their own magnetic field instead of relying on the stator for this purpose. This means the motor requires less power to get the rotor spinning, writes ThomasNet.
The rotor’s magnetic field matches that of the stator when a current flows through it resulting in “synchronous” rotations and near-zero slippage, and thus, better efficiency than asynchronous engines.
Benefits
- High efficiency
- Low cooling needs
- High power density
Drawbacks
- Expensive to produce
- Needs rare earth materials
This type of motor is found in EVs including the Audi e-tron GT, Jaguar I-Pace, and Porsche Taycan.
Porsche Taycan PSM
Electrically-excited synchronous motor
An ESM works similarly to a PSM but requires fewer rare earth materials to produce, as the rotor is agitated by an electrical current from the main battery rather than by the stator or magnets.
Instead of magnets in the rotor, the ESM uses metal brushes to transfer electricity from the commutator – an electric switch on the rotor itself that periodically reverses the direction of the incoming current – to the rotor’s electromagnetic windings, according to MotorTrend.
Again, this induces a magnetic field in the rotor whose rotations match that of the stator.
Benefits
- High efficiency
- Cheaper to make than PSMs
- Little need for rare earth materials
Drawbacks
- Long-term reliability not the best
This type of motor is found in EVs including BMW’s i4, iX3, and iX.
BMW iX3 ESM
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