Lightweight Electric/Hybrid Vehicle Design

Fig. 1.11 Controller for 34 kW shunt field DC motor.
Current EV design approaches 13
In the power supply area, there are four components: first is the battery charger, in this case a
CUK converter, or a boost/buck chopper is also a possibility to make the mains current look like
a sine wave for ensuring IEC555 compliance. Control of battery charging conditions is one of
the most important considerations in extending battery life in deep discharge. For lead–acid
batteries the level of float voltage is critical as well as maintaining cell temperature. The battery
charger could incorporate a 20 kHz isolating transformer if costs permit. Experiments are under
way with inductive power transfer which isolates the car and makes it necessary to plug in for
charging. Another possibility is an automatic self-aligning connector which the car drives into
when parking. The next consideration is the auxiliary 13.6 V battery supply. The vehicle seems
likely to retain a separate 12 V battery for lighting and control functions. A 300 W DC/DC
converter will satisfy this requirement. The third consideration is the control system power.
This is a small (20 W) DC/DC converter which provides the control power for the chopper. It is
likely to be incorporated with the main control PCB and could also be supplied from the 13.6 V
battery. The final factor is the field controller. This is a 4 quadrant chopper which provides the
motor field supply. It has to be able to reverse the current so that the motor can reverse without
contactors in the armature circuit. If the motor has a tachometer fitted, this may be used for
braking control and blending with electromechanical brakes. The important issue with this
controller is that the power switching is contained in a single unit so that all the DC components
are kept in one place. This is important for another reason to meet IEC555 RF interference
legislation. Therefore all insulated systems will require an isolated conductive casing which
can be connected to vehicle chassis.
1.2.12 CONTROLLER FOR A 45 kW AC MOTOR (BRUSHLESS DC OR INDUCTION)
This is illustrated in Fig. 1.12. The drive consists of a 3 phase PWM Drive which feeds the 3 phase
motor. The beauty of this arrangement is that the motor can be disconnected and the mains fed to
the inverter arms to give a high power battery charger, by phase locking the PWM to the mains.