ISSUE 012 Winter 2021 Sigma Powertrain EMAX transmission dossier l In conversation: David Hudson l 48 V systems focus l 2021 Battery Show North America and Cenex-LCV reports l Everrati Porsche 911 digest l Switching insight l Motor laminations focus

electric rickshaws and minivans. For example, a 9 kg lithium-ion 48 V battery pack gives a range of 50 km and a power of 10 kW. It can be detached simply and charged externally. Multiple packs can be connected and controlled via a dedicated battery management system, doubling the range to 100 km by using two batteries. 48 V is also opening up the use of gallium nitride (GaN) switches. For example, for smaller 1 kW motors for e-scooters, a three-phase 48 V brushless DC motor drive inverter uses an 80 V GaN transistor running at 100 kHz. The higher efficiency of running at this frequency, and the lower on- resistance of 2.2 mΩ of the GaN switch, supports 15 A of steady-state current with a 50 C temperature rise in natural air convection, and can reach 20 A rms (28 A peak) with a heat sink attached. The design has been tested with a 60 V input and 50 A peak current. Switching in the 100 kHz range also eliminates the need for electrolytic capacitors and reduces the motor losses. Reducing the ‘dead’ time of the GaN switch to about 20 ns provides higher torque per amp, as well as providing lower distortion for a quieter design, lower current ripple for reduced magnetic losses, lower torque ripple for improved precision and less need for filtering, leading to lower cost. All this allows a complete motor drive inverter in only 130 x 100 mm including the connector, enabling the inverter to be integrated inside the motor. Commercial vehicles In the commercial vehicle sector, 48 V motors can provide the start- stop function for light trucks as well as supporting the mild hybridisation of heavy trucks. There are three main drivers for commercial vehicles, starting with reducing diesel and nitrous oxide emissions to meet increasingly tight regulations. This is achieved by using the 48 V supply for a heater for the IC engine’s catalyst to provide more efficient operation, especially at low load. For just generating power for an electric heater, the P0 topology is the simplest and has the lowest cost, as it doesn’t need energy storage with a DC- DC converter to the 12 V base electrical system. There are 48 V alternators being developed that could provide 12-13 kW – 10 kW for the heating and 2-3 kW for base load. The second driver is to reduce CO 2 emissions, by using the motor/ generator to capture energy when slowing down and to accelerate the vehicle, and to power accessories such as an engine cooling fan, air conditioning and exhaust gas recirculation pumps. These require 48 V energy storage, which depends on duty cycle but typically entails a battery capacity of 5-10 kWh, although 35 kW is the upper limit for 48 V systems. In some cases the key metrics are the charging rate, or C-rate, and the energy absorbed, so a supercapacitor can be used. This is particularly useful in vehicle designs such as buses that have numerous stop-start cycles. For battery-electric systems, that means there are three electric buses, handling the high voltage from the battery pack at 400 or 800 V, the 48 V bus and the 12 V bus. This then requires 48 to 12 V DC-DC buck- often called a boost recuperation machine, and an inverter. This converts kinetic energy into electrical energy during braking and stores it in the 48 V battery. It also supports the IC engine with up to 12 kW of power. In addition, safety-relevant functions such as driver assistance functions or electric brake boosters can be reliably supplied with 48 V. There are other topologies for the 48 V motor in other parts of the powertrain. The P1 configuration sees the motor installed between the IC engine and the transmission, but this is rarely used in practice. The P2 approach indicates an integration directly on the side of the main transmission or connected via a belt, while P3 has the motor sitting directly behind it on the driveshaft. Both topologies are mechanically much more complex than P0. For example, the e-motor needs to be installed in the transmission as individual components, and air cooling is not possible. In addition, a starter motor is usually still needed in this set- up, which increases costs. Integrating one or two 48 V motors on the rear axle via a differential gear provides the full-hybrid, P4 option. This has the lowest frictional losses in the powertrain, with emissions savings of up to 25%. The use of 48 V motors, battery packs and inverters is also opening up applications such as e-scooters, The circuit requirements of the P0 mild hybrid topology (Courtesy of Yageo) Winter 2021 | E-Mobility Engineering 33 Focus | 48 V systems