Drives for Electric Vehicles: Which Electric Motor is the Best? #Update | springerprofessional.de

10-06-2024 | Electric Engine | In a Nutshell | Article

Author: Christiane Köllner

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Various electric motor concepts are competing for market share. But which technology is best for which application? An overview of the advantages and disadvantages of different types of electric motors.

Electrically excited synchronous machine (EESM) from Renault. The motor, called E7A, is currently in the prototype stage. The unit is due to be launched on the market in 2027.

Renault Group

Compact, powerful and energy-efficient: Permanently excited synchronous machines are now the standard in vehicle electrification. Externally excited synchronous machines and asynchronous machines have increasingly fallen behind. In the meantime, however, the framework conditions for electric drives are changing. This is now giving one electric motor concept in particular a comeback. An overview of the technology, types, advantages and disadvantages of electric motors.

The following types of motor are generally used to drive electric vehicles: The permanently excited synchronous machine (PSM) or hybrid synchronous machine (HSM), the asynchronous machine (ASM) and the externally excited synchronous machine (EESM) - often referred to in German as stromerregte Synchronmaschinen or SSM. For example, Porsche uses a permanently excited synchronous motor on both the front and rear axle in the Taycan. The new Audi Q8 E-Tron has electric motors on both axles that work according to the asynchronous machine principle. And the Renault Zoe is driven by a separately excited synchronous motor.

01-05-2024 | Cover Story

There is a lot of movement in the field of electric drives. The availability of raw materials and rare earths require a new approach and more technological diversity, especially for the targeted mass market. Prof. Dr. Andreas Docter is Head of Development at Magna Powertrain. In this interview, he talks about the prospects for future and current electric motor technologies and the importance of integrating them into the overall system.

There are also other topologies such as axial flux motors or reluctance and transverse flux machines, which are either relevant for niche applications or are still at the concept stage. “Overall, the different types of electric motors all have their advantages and disadvantages, which must be taken into account depending on the requirements and application scenario,” explains Professor Dr. Andreas Docter, who heads up development at Magna Powertrain, in the MTZ interview “Electric motors are continually improving in competition”.

The three types of motor used by the vehicle industry are similar in their technical design, but differ in their properties. In the PSM, the rotor magnetic field is generated by the use of permanent magnets. As a result, this drive variant achieves very high levels of efficiency even in the partial load range. The fact that PSMs are now standard in vehicle electrification is due to the high efficiency of these electric motors in city traffic or at medium ranges. “Permanently excited synchronous machines are very well suited for vehicle drives due to their high power density, their particularly high efficiency (up to 94%), their simple mechanical and electrical design (no brushes, no sliding contacts, no complicated windings) and their electronic commutation,” says Springer author Peter Hofmann in the german book chapter Energy converters for hybrid vehicles.

However, the PSM requires special technologies for the assembly of the rotor and stator, which can overcome the enormous magnetic forces. The PSM is also more expensive than an asynchronous motor, whose special feature is the short-circuit winding in the rotor. Here, the magnetic field in the rotor is generated by electromagnetic induction; permanent magnets are not required. The efficiency of ASMs is again lower than that of synchronous motors, they are somewhat heavier than synchronous machines, but very robust. The advantage of asynchronous machines is that they are simple and inexpensive to manufacture. However, they have a poor power factor and low efficiency. Another disadvantage is the decreasing power curve at high speeds.

Externally excited motors are technically complex, as they require their own power source to drive the rotor. However, they have one major advantage: their rotor does not require permanent magnets - instead of magnets, the rotor has coils - and they are highly efficient. The reason for this is the option of being able to adjust the rotor current and thus the magnetization of the rotor. However, the precise control of the rotor current increases the complexity of the overall system. The advantage, however, is that the electric motor can now be optimized for a wider range of applications with higher efficiency.

Permanently Excited Synchronous Motor

Current-Excited Synchronous Motor

Asynchronous Motor

Advantages:+ high efficiency+ Low installation space requirement+ high effectiveness in recuperation

+ high efficiency

+ lower manufacturing costs

+ robust+ cost-effective

Disadvantages:- Higher costs and possible availability of the magnetic material- Drag losses when idling

- Slip ring transmissionnecessary- Larger installation spacerequired

- Efficiency especiallylower at lower speeds

Note: There is another variant of permanently excited synchronous motors, the combination of reluctance motor and permanently excited synchronous motor. This is also referred to as a “hybrid motor”.

Table 5.1 Overview of three-phase motors from: Elektrifizierter Antriebsstrang, 2022, page 59 (Source in German).

Car manufacturers have not yet decided on one type or another. Some groups are changing their drive concepts completely from PSM to ASM and others from PSM to EESM. Some are also opting for a combination of technologies. However, a trend is emerging: magnet-free electric machines such as the EESM or the asynchronous machine are “less sensitive to raw material prices, more environmentally friendly and good alternatives with a wide range of applications”, writes Mahle in the german MTZ article Magnet-free HV traction drives with contactless power transmission. In the long term, separately excited motors in particular could prevail. This is because PSMs require expensive rare earth magnet materials, which increase the drive's CO2 footprint, and ASMs have poor efficiency. EESMs, on the other hand, use a copper coil to generate the magnetic field, a metal that is available in sufficient quantities. As Vitesco Technologies explains, separately excited synchronous machines show their advantage on long distances with fast highway driving. They are more efficient than PSMs, especially at higher speeds.

EESM will play an important role for Vitesco in the near future: “Replacing the magnets in the rotor with controllable electromagnetic fields opens up potential for OEMs in the areas of CO2 footprint, long-term price stability and global supply security with similar technical characteristics,” says Gunter Mühlberg, Head of Product Management HV Drives at Vitesco Technologies, in an interview with MTZ, “System optimization at vehicle level has great potential. Accordingly, Vitesco has turned its fully integrated EMR4 electric axle drive platform into a separately excited synchronous machine. In this new variant, the PSM rotor with permanent magnets is replaced by a new rotor with electromagnets - the EMR4 thus becomes an EESM. The separately excited machine is said to save one watt-hour of electricity per kilometer, as there is no permanent magnetic field to slow down the rotor.

One example of a car manufacturer currently working on the EESM is Renault. The French car manufacturer is currently developing a new electrically excited synchronous motor with its partner Valeo. The E7A electric motor is due to be launched on the market in 2027.

This is a partly automated translation of this german article.