Vector Control technology for Traction Drive

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Cecube technology consulting

IGBT inverter

EV Battery Bus - Vector Control Electric Traction Drive

This prototype electric vehicle (EV) originally owned by Leicestershire Council in the UK was 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 the present owners, Brush Traction, 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.

Water cooled induction motor

The motor is located at the front 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.

development FPGA - vector control on DSP microprocessor

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.

MOTOR BEARING LIFE IN IGBT DRIVES

IGBT inverters introduce a bearing reliability concern. The existence of shaft induced voltage (SIV) has been known for nearly a century. Conduction of induced bearing current through paths to ground has been minimised by careful motor design, frame earthing strategy, shaft brushes or insulated bearings. The effect has been to negate conducted currents, otherwise particularly evident at low speed, when a good electrical contact between rolling elements exist. This results in a non-arcing bearing current, which has no adverse effect on life expectancy. However, publicity has raised the profile of long term bearing damage resulting from the use of IGBT inverters.

When designers thought these issues were mastered, along came IGBT inverters in the 1990's, with higher switching frequencies to 10kHz for reduced harmonics and noise design. A high dV/dt at switching instants is inevitable, which discharges current through parasitic capacitances, firstly between the stator winding and motor frame, and secondly from stator to rotor bars and rotor core. The discharge current spikes can be positive or negative according to the direction of the switching common-mode voltage creating them.

The resulting bearing current caused by repeated discharges in IGBT inverters have the capacity to break down the bearing grease dielectric impedance, posing a risk as motor speed increases. As the grease breaks down the high energy discharge currents cause electro-erosion of the bearing, evidenced as pitting or fluting of the raceway and often associated with a grey coloring. The problem becomes more acute with increased motor size because of the corresponding increase in parasitic capacitance. Consideration to switching frequency reduction should be given in this case. Additionally ceramic coated insulated bearings can prevent discharge currents, but is expensive and not always 100% reliable.

This represents a new risk to rolling stock operators considering modernising old thyristor or GTO equipment with high frequency IGBT inverters and choppers. The higher the switching frequency the more rapid the failure mode becomes once arcing conditions are established. Motors not originally designed to work in a high switching frequency environment could have life expectancy compromised. There are also electrical interference and longitudinal voltage compatibility issues creating by discharge currents, and the capacity for premature bearing wear to adversely affect the harmonic footprint of a vehicle. These problems can be overcome by use of careful design with simulation support and endurance testing, but should not be ignored when re-engineering or renovating equipments.

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