Ricardo develops prototype next-generation 85 kW switched reluctance EV motor; no rare earth elements
23 February 2015
|The Ricardo RapidSR switched reluctance drive motor avoids the use of rare earth elements. Click to enlarge.|
Ricardo has developed a new prototype 85 kW synchronous reluctance motor designed primarily for electric vehicle traction applications. The motor avoids the use of expensive rare earth elements, while delivering strong performance at a significantly reduced cost.
The new EV motor was designed and built in prototype form by Ricardo as part of a collaborative research and development project, RapidSR (Rapid Design and Development of a Switched Reluctance Traction Motor). (Earlier post.) Using a conventional distributed stator winding, the Ricardo synchronous reluctance electric machine is an innovative design that makes use of low-cost materials, simple manufacturing processes and uncomplicated construction.
As the market for electric vehicles grows globally, there is an imperative to explore alternatives to permanent magnet traction motors which require the use of expensive and increasingly difficult to source rare earth elements. The Ricardo prototype that we have announced today demonstrates what can be achieved by using the latest electric machine design processes in the creation of a high performing, compact, lightweight, and rare earth element free concept.—Paul Rivera, MD of the Ricardo hybrid and electric vehicle systems business
|CAD model of Ricardo RapidSR motor. Click to enlarge.|
Electric traction motors commonly employ permanent magnets made from materials such as neodymium-iron-boron and samarium-cobalt, although there are numerous efforts underway to reduce or to eliminate the use of rare earth elements in these applications (e.g., earlier post, earlier post, earlier post, earlier post). Since switched reluctance motors (SRM) do not use permanent magnets, they could provide an ideal replacement technology.
In a switched reluctance motor, torque is produced by the magnetic attraction of a steel rotor to stator electromagnets; there are no permanent magnets, and the rotor carries no windings. A controller energizes each stator winding only when it can produce useful torque. With suitable timing of the stator excitation, the machine can operate as a motor or generator. Switched reluctance motors are simple, robust and can offer very good efficiency over a wide load range.
However, while SRMs can have very high power density at low cost, they have had issues with high torque ripple when operated at low speed, and the acoustic noise caused by torque ripple.
Ricardo’s prototype has a rotor made from cut steel laminations, which are used to direct and focus the flux across the air gap. By maximizing this flux linkage between the stator and rotor, performance can be optimized within a tightly packaged, low weight and rare earth element free design.
Since its launch in 2012, the RapidSR project has been researching the design of next-generation economic electric motors that avoid expensive and potentially difficult to source rare earth elements typically used in permanent magnets.
By developing effective CAE-led design processes as well as prototype designs, the team has created a framework for the future design and manufacture of electric vehicle motors that offer the performance, compact packaging and light weight required for EV applications, but at a significantly reduced cost compared to permanent magnet machines.
Ricardo’s partners in this research include project leader Cobham Technical Services —which is developing its multi-physics CAE design software, Opera, as a part of the project—and Jaguar Land Rover. The research is being co-funded by the UK’s innovation agency, Innovate UK.
|Electromagnetic design in three dimensions. Cobham developed specialist simulation software and a design environment for electric motors to provide a virtual test bench for rapid design and optimization. Source: Cobham. Click to enlarge.|
By bringing together state-of-the-art simulation technology with advanced electric machine design we have created a highly credible next generation EV motor concept that shows considerable promise. The Ricardo prototype is now built and will be rigorously tested over the coming weeks in order to validate the extremely positive results that it has shown in simulation, as a concept that provides an exceptional balance of performance, compact package, light weight and low cost.—Dr. Will Drury, Ricardo team leader for electric machines and power electronics
Would be great to have some comparative numbers between this design and PM motors of the same torque rating; a torque curve and efficiency plot at the various rpms would be informative also.
Posted by: Lad | 23 February 2015 at 11:13 AM
Why don't they cut the length of the internals, shaft etc. and use multiple sections stacked, and offset by some angle to smooth out the torque ripple.
Just like more cylinders in an ICE is used primarily to smooth out torque.
This is even more critical to have smooth torque in electric motors to take advantage of the zero RPM capability.
Posted by: GdB | 23 February 2015 at 12:35 PM
...Which implies a distributed rotor system, one for each wheel, a transverse set for the crankshaft, maybe another for the transmission, all enough to dampen vibration at many angles and points, and better traction and braking. Come to think of it, why bother with a clutch or transmission, if you can manage small points of steering/transmission fluid at each wheel, save for a flywheel generator/regenerator behind the engine to control RPM and engine mount vibration?
Posted by: kalendjay | 23 February 2015 at 02:10 PM
Posted by: SJC | 23 February 2015 at 02:32 PM
Quieting strategies to quiet the SR machine, as you would expect, tend to push back on its efficiency and cost advantage. In general, have enough poles, use thin laminations for the rotor stack, and pay really close attention to the rotor shape and you can do reasonably well with noise and "cogging".
Did you know...
that the initial flight-test configuration of the F35 had a Switched Reluctance Starter/Generator?
The prevailing reason for the selection was that SR is "fault-tolerant": you can lose a phase and still have an operable motor/generator. The truth is not quite so neat, and so ultimately the original recommendation by the electric system supplier, a redundant wound-field synchronous machine, was selected, and is the configuration used in production. BTW, the SR was the heavier of the two.
Posted by: Herman | 23 February 2015 at 05:18 PM