When vehicles that include electric powertrains outnumber fossil-fuel vehicles, it will in fact be the second time in history this has happened. Interest in electrifying horse-drawn carriages goes back to the 1820s, with electric vehicles accounting for a third of all traffic by the turn of the 20th century. New York even had a fleet of more than 60 electric taxis.
However, the introduction of the mass-produced Ford Model-T, coupled with the discovery of new sources of oil, made the fossil-fuel vehicle cheaper to produce. The end of the first era of electric motoring was inevitable, handing over instead to the internal combustion engine (ICE).
Some 60 or so years later, the Arab Oil Embargo of the 1970s, along with the NASA electric Lunar rover on the moon, re-awoke interest in electric vehicles. Unfortunately, the disparity in energy density between fossil-fuel and available battery technology, coupled with a fledgling semiconductor industry, meant that electric vehicles were miles behind ICE technology.
In our previous blog on batteries, we discussed how the availability of new battery chemistries brought us far enough forward to support a viable electric vehicle (EV) and hybrid-electric vehicle (HEV) industry. However, ensuring vehicles convert as much of this stored energy into traction as possible requires highly efficient electronic circuitry. Today’s EVs manage to transfer around 60% of the battery’s energy to the tyres, compared to the ~21% achieved by conventional ICE technology.
Electric Drive Implementation
The electric drive implementation typically takes two approaches: one places the electric motor near the vehicle’s axle, together with a gearbox, while the second method places the motors directly into the wheels.
In the first approach, a single motor at one axle can be used for two-wheel drive, while two motors, one at each axle, can be used for four-wheel drive. This approach can easily be coupled with traditional ICE technology to implement a hybrid powertrain. Solutions such as the Bosch eAxle combine motor, power electronics and transmission into a scalable and compact solution. Weighing around 90kg, the power range scales from 50 to 300 kW, delivering between 1000 and 6000 Nm torque, making it suitable for everything from passenger cars to light commercial vehicles. Of similar construction is the High Voltage Axle Drive from Continental.
The second method - placing a motor in each wheel - is known as a wheel-hub motor and is the approach taken by Protean Electric. Their ProteanDrive Pd18 packs electric motor, brake disc and the inverter electronics into a 36kg design that fits inside an 18” wheel rim. Providing peak power of 80 kW (continuous 60 kW) per wheel, they distribute the vehicle’s weight to its outer corners. Applying the drive to each wheel individually also simplifies the implementation of torque vectoring - something that improves handling. With the motor and electronics around the outside of the vehicle, it leaves more space front and rear for either luggage or locating batteries.
The electric motor drive uses switching inverters to efficiently convert the battery voltage (typically around 400 Vdc) down to that required for traction - similar to the approach used in today’s digital power supplies. Given the powertrain is a safety-critical system, the components used in each motor drive must have a high level of reliability. It is easy to determine the dangers that could occur should a car travelling in traffic at speed suddenly lose power.
Until now, the well-established IGBT technology has been the switching device of choice, reliably handling the high voltages and powers involved. However, attainment of higher efficiencies requires higher switching frequencies, something that pushes IGBTs to their limits. Wide bandgap (WBG) technology, such as silicon carbide (SiC), is slowly displacing IGBTs, thanks to their lower on-resistance, higher operational temperature, and low-loss transient switching. Concerns about their reliability have been holding back their use, together with the stresses higher switching speeds could cause on motor windings6. However, as this technology becomes better understood, these concerns should dissipate.
Commercial vehicles are also impacted by green initiatives to curb carbon output. Research studies by Siemens have seen the introduction of overhead wiring, known as a catenary system, installed on some highways around the world. The aim here is to provide hybrid trucks with a method to draw electrical power via a pantograph for both propulsion and charging while on the move. A system such as this could be implemented on streets around logistics hubs too, reducing localised emissions between business parks and highways in towns and cities. Vendors such as Infineon already provide high-power IGBT switching solutions in ruggedised packaging, such as PrimePACK, that can cope with the enormous electrical and mechanical stresses of such applications.
It seems that all the required pieces are in place to make a success of the electric powertrain, from private passenger vehicles to all types of commercial vehicles. The recent advances in power electronics will improve range while maintaining reliability and, above all, safety. While pantographs may resolve the charging challenges for commercial vehicles, private road users remain a long way removed from the refuelling convenience of the service station. The developments in EV charging to address this will be examined in our next blog.
The next blog in the series will look at: Charging
- To read the previous blogs in this series follow the links below.
