EESMs generally are not great at city driving cycles compared to IPMSMs. They do really excel in field weakening at moderate torques.
21 karma · joined February 22, 2021
There are other startup companies developing EESMs but not Niron to my knowledge.
Advantages:
- Not subject to the price and supply chain volatility of rare earth permanent magnets.
- For highway dominant drive cycles, the cycle efficiency of EESMs can be higher than state of the art IPMSMs. EESMs tend to have their best efficiency at moderate torques and high speeds because of their excellent field weakening characteristics. I tend to think that they would be a good fit for application in class 8 trucks or as auxiliary motors in automobiles with two powered axles.
- The output torque doesn't necessarily decrease with rotor temperature. In IPMSMs the permanent magnet flux linkage decreases with rotor temperature.
- At least theoretically, with proper control, it is possible to operate EESMs with unity power factor and decrease the kVA rating of the stator inverter.
- If there is a stator inverter fault, there are schemes to denergize the rotor which have some safety implications.
Disadvantages:
- DC current needs to be transferred to the rotating field winding. For automotive applications this tends to be done either with brushes and slip rings or brushlessly using a high frequency transformer with a rotating rectifier. In either case additional power electronics and other components are needed for the field power transfer and control which reduces some of the potential cost savings of the elimination of the permanent magnets. If brushes and slip rings are used with oil spray/oil jet cooling of the rotor they need to be sealed in a separate compartment. I am a little surprised that Renault has stuck with brushes and slip rings versus an inductive high frequency transformer solution. I think this has limited their power density.
- For very torque dense machines, cooling the rotor field winding is challenging, and in my opinion is best accomplished by oil spray/oil jet cooling.
- It is difficult to reach the same maximum speeds as IPMSMs in an automotive package size. The rotor field winding retention system to keep the field turns from moving into the airgap at high speeds needs considerable attention during the design.
- The overall axial length of the non-active region of EESMs is typically longer than IPMSMs because of the field winding end turns and field excitation system.
- EESM efficiency is dominated by the manufacturable slot fill of the field winding.
- High performance current/torque regulation is considerably more difficult.
High performance EESMs have been used in aerospace generator applications for decades, albeit with a different rotor excitation system than what is used in automotive applications. Renault (and their supplier Continental) really led the commercialization of EESMs into automotive mass production. Now BMW has followed suit and multiple suppliers have EESM designs (Mahle, ZF, etc.) GM had a really nice EESM design and high frequency transformer excitation which they published back in 2014. My colleagues and I built several generations of EESMs as part of U.S. Dept. of Energy projects (https://www.osti.gov/servlets/purl/1837809) and I think they have their place as EV traction motors for certain applications.
A good survey paper of excitation systems for wound field synchronous machines or electrically excited synchronous machines is.
J. K. Nøland, S. Nuzzo, A. Tessarolo and E. F. Alves, "Excitation System Technologies for Wound-Field Synchronous Machines: Survey of Solutions and Evolving Trends," in IEEE Access, vol. 7, pp. 109699-109718, 2019, doi: 10.1109/ACCESS.2019.2933493.
An example of a high frequency transformer or inductive coupling developed by General Motors is detailed for example in the following paper.
C. Stancu, T. Ward, K. M. Rahman, R. Dawsey and P. Savagian, "Separately Excited Synchronous Motor With Rotary Transformer for Hybrid Vehicle Application," in IEEE Transactions on Industry Applications, vol. 54, no. 1, pp. 223-232, Jan.-Feb. 2018, doi: 10.1109/TIA.2017.2757019.
Below is some work done by Oak Ridge National Lab on rotating high frequency transformers for field excitation.
T. Raminosoa, R. H. Wiles and J. Wilkins, "Novel Rotary Transformer Topology With Improved Power Transfer Capability for High-Speed Applications," in IEEE Transactions on Industry Applications, vol. 56, no. 1, pp. 277-286, Jan.-Feb. 2020, doi: 10.1109/TIA.2019.2955050.
The field excitation current is typically provided using a single phase high frequency transformer (20 to 100 kHz switching frequency) which is then rectified by a rotating full bridge rectifier.
Just to clear up some of the misconceptions in the comments. In state-of-the-art electric vehicles there are basically three main machine topologies that are used for the main traction machine.
- Interior permanent magnet synchronous machines (IPMSMs): A large number of OEMs and suppliers use/supply this motor topology, e.g., General motors, Tesla model 3, Ford, etc.
- Induction machines (IMs): Also used by a large number of OEMs and suppliers, e.g., Tesla model S and X, G.M. for assist motors, etc.
- Wound field synchronous machines (WFSMs) [in Europe more commonly known as electrically excited synchronous machines (EESMs) but they are the same thing] - Currently used in Renault and some BMW models. A large number of suppliers are developing them.
Each of the machine topologies has tradeoffs compared to the others, cost, manufacturing CapEx, supply chain risk, drive cycle efficiency, difficulty of control, failure modes, cooling complexity, etc.
WFSMs or EESMs have some potential advantages over IPMSMs or IMs for some applications.
- They don't contain rare earth permanent magnets eliminating the supply chain and price volatility associated with them.
- In terms of drive cycle efficiency generally they are more efficient than IMs and if the drive cycle is dominated by moderate torques and highspeeds they can exceed the drive cycle efficiencies of IPMSMs.
- They are very easy to field weaken and theoretically can have an infinite constant power speed range.
- The field excitation can be controlled to provide unity power factor potentially allowing the stator inverter to be downsized.
The main negatives of WFSMs or EESMs compared to IPMSMs or IMs are:
- The field excitation is more complicated and requires power electronics either for brushes and slip rings or inductive power transfer approaches.
- The cooling of the field winding is difficult and typically spray/jet cooling of the end turns or at a minimum through shaft cooling.
- The control of WFSMs or EESMs is considerably more complicated than IPMSMs or IMs.