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
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
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
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.
- 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.
- The optimum application of slowly improving battery technology that retains
cost-effectiveness and a less deleterious environmental recharge regime.
- Integration to achieve high space utilisation without compromising other
- 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.
- EMC requirements necessitate careful layout and screening for AC motors and
cables, high voltage inverters and electronics.
- 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.
- The electronic hardware is subject to wide environmental operating envelope of
temperature, vibration and mechanical stress.
- 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
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
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