May/June 2021 | Vol. 26 No. 3

by Eric Hustedt, Chief of Engineering, and Richard Meaux, Chief Marketing Officer, Exro Technologies

Mr. Hustedt has more than 20 years of experience leading innovations in all facets of automotive power electronics.

Mr. Meaux has had diverse experiences in the power conversion industry, beginning his career with GE as part of the Commercial Leadership Program.

As electric vehicles (EVs) gain acceptance, the components within automotive vehicles shift from complicated mechanical assemblies like high gear count transmissions and combustion engines to mechanically simple electric powertrains comprising a battery inverter (dc to ac converter) and an electric motor.

While hybrid systems have been available for a while now, they present a remarkable increase in vehicle complexity and powertrain weight as electric assist was added to the existing combustion engine powertrain.

Electric powertrains come with their own technical challenges, though. Typical combustion engines have a relatively narrow operating speed range than an electric machine and require multi-speed transmissions to match the road speed to the engine speed. For electric, the machines can operate effectively over a much wider speed range, and therefore, in theory at least, don’t require gearboxes. That helps reduce system costs and keeps the powertrain components simple. While an electric motor does have a vast speed range, it still has limits. The rotor structure bounds the upper-speed range, and the inverter current limits the peak torque production and, therefore, the cost of the inverter itself. For high-performance cars, where very high maximum speeds are demanded, a two-speed gearbox may still be needed with conventional three-phase drives to reach the desired peak performance.

This speed or torque trade-off is due to the nature of electric machines. The current density in the stator slots (i.e., ampere-turns) produces torque, and voltage produces speed, or more specifically, battery voltage limits speed. All electric machines, when excited, produce a voltage at their terminals proportional to their rotating speed—which at some point in the speed range reaches the battery voltage—and at that point producing meaningful torque becomes more difficult.

Machine designers are then presented with the choice of selecting appropriate turn count. One can choose to increase turn count, which easily produces large ampere turn numbers from a given inverter current output and produces high torque. However, those same turns that make it easy to produce torque also produce more voltage. This limits the high-speed capability. Alternatively, a machine with fewer turns will spin to high speeds but require a large phase current to produce torque. This trade-off invariably requires a compromise in some aspects of performance. Like high-performance vehicles or heavy-duty vehicles, some EV applications still require multi-speed gearboxes or increased motor size and quantity to reach desired performance, which inevitably adds to the vehicle’s cost and weight.

For example, an electric vehicle in Germany is expected to feel peppy in the city. Still, it must also be able to cruise at above 200 KPH on the autobahn, challenging to achieve without two-speed gearboxes. Another example would be a heavily laden commercial vehicle must have the torque to overcome friction to get moving, yet still drive efficiently at highway speeds. This electric vehicle today is likely fitted with two or even three gearboxes to maintain that performance.

The shift to pure electric powertrains presents another challenge to traditional automotive industries, as the skillset requirement is different. For example, manufacturing and servicing an electric vehicle with battery voltages in the 300-750V range requires awareness in high-voltage safety and training more aligned with an electrical technician than what might be taught to mechanics.

From an automotive production standpoint, some traditional mechanical parts will continue to be required. Wheels still need wheel bearings, steering linkages still need to connect the steering unit to the front wheels, and suspensions must still absorb road imperfections. However, many precision mechanical components will disappear, primarily the combustion engine and its complex gearboxes. Even differentials may not be needed anymore to some extent, with innovations like dual motor e-axles. This presents a significant shift in automotive design and manufacturing skill set from precise mechanical part manufacturing to complex electronics manufacturing.

As electric vehicle powertrain technology improves enough to remove the additional mechanical components, like the gearboxes and additional motors, and still enhance traditional performance, then the mass adoption of electric vehicles will accelerate the conventional automotive industry’s shift. Auto manufacturers will standardize hiring or outsourcing new skill sets more aligned with electrified components, and tier-one suppliers of electric powertrains may become more vertically integrated within their production.

The automotive industry’s electrification will drastically simplify the manufactured components required for producing standard electric vehicles while also being able to improve on traditional vehicle performance across the speed range.

The NEMA Automotive Component Council was formed to address product regulations, component qualification programs, and Standards gaps in an automotive market that is becoming increasingly electrified.

More information about the Council can be found at www.nema.org/directory/nema-councils/automotive-components-council ei