EV Battery Bus - Vector Control Electric Traction Drive

This prototype electric vehicle (EV) previously owned by Leicestershire Council in the UK is used to evaluate energy saving compared to similar sized diesel ICE bus drives. It is cost effectively powered by a series of conventional lead-acid cells.

The propulsion was designed by Brush Traction, who now own the vehicle, based on a 65kW AC water cooled induction motor and IGBT inverter. The electric traction drive is coupled to a conventional, well used, compliant bus transmission with significant gearbox play. Cecube provided a novel PWM vector control design to meet the exacting performance requirements for reliable gradient starting and in-traffic operation. The main benefit of vector control is to make an AC induction motor operate similarly to a conventional DC separately excited motor, with independent control of field (flux) and armature (torque). However, the brushes and commutator of the DC motor are not present on an induction motor, resulting in a smaller, more reliable and efficient drive unit.

For comparison a scalar direct torque controller based on principles described in an earlier paper was also fitted and tested on the bus. However, low speed gradient performance and the load variations that result from sudden steering movement proved too demanding in this application, despite successful use of the scalar controller in railway fleet passenger service.

The motor is located at the front of the bus behind the grill. The radiator and cooling fan are visible to the right of the water cooled induction motor.

The vector control design required a bespoke floating point DSP solution. The processor is to the left of the prototype interface electronics, which is designed around 5 FPGAs. A second smaller DSP (piggy-back mounted) is cable connected via an output port to provide real time diagnostic output.

The prototype IGBT inverter and DSP / FPGA electronics on which the vector control is implemented.

Most of the engineering challenges to automotive electric and hybrid traction are similar to those embraced by the railway traction industry, only their relative importance differs.

1. Alternative maintenance and management requirements of electric or hybrid vehicles. The dependence of vehicle availability on recharging strategies restricts flexibility of service use, particularly for the EV.
2. The optimum application of slowly improving battery technology that retains cost-effectiveness and a less deleterious environmental recharge regime.
3. Integration to achieve high space utilisation without compromising other requirements.
4. The electronic control unit (ECU) is highly optimised and designed with support of advanced simulation techniques. It is no longer an optional refinement to vehicle efficiency and operation.
5. EMC requirements necessitate careful layout and screening for AC motors and cables, high voltage inverters and electronics.
6. The reliability of electric traction drives must match the conventional vehicles they seek to replace. This requires robust, fault tolerant design of critical components such as sensors and power switching transistors.
7. The electronic hardware is subject to wide environmental operating envelope of temperature, vibration and mechanical stress.
8. The presence of high voltage cables and motors in the event of an accident are a potential fire and safety hazard, particularly when operating in traffic predominantly comprising fossil-fuelled ICE vehicles. The absence of automated collision avoidance systems on two degree of freedom road systems heightens the criticality of safety design.

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