Power electronics for contemporary motorsport
The new era in motorsport of energy recovery from exhaust gases, combined with regenerative braking, will hopefully make our ‘industry’ more attractive to sponsors, who worry about our less-than-exemplary environmental credentials, and to car manufacturers who in recent years have not seen the relevance of some of our engines. For purely electric drives as well as applications where electric hybrid systems are combined with an engine and exploit regenerative braking, there are three main components. Where exhaust energy recovery is concerned, the picture is more complex, but most of the components remain similar.
First there is a motor/generator unit, which is used either as a propulsion motor to augment the engine’s torque or, in the case of purely electric vehicles, to provide all the motive effort. Then there is a battery, or possibly a supercapacitor, which is where the electrical energy is stored. It is possible to connect a motor to a battery without any other components, but this is generally not the case, especially not in automotive propulsion, where we require precise control. The third component, generally called the ‘power electronics’, carries out a number of very important functions.
The battery supplies a voltage and a direct current (dc); the voltage varies depending on a number of factors, as does the current. It is highly likely though that the propulsion motor is a multi-phase alternating current (ac) device, which is not compatible with a dc supply, so one of the important roles of the power electronics is to turn the ac output of the motor/generator unit, when it is acting as a generator, into dc, which can be fed to the battery. When the motor/generator is acting as a motor, the power electronics acts to convert the dc output of the battery to ac, switching it between phases very precisely so that the motor propels the vehicle forward with peak efficiency.
In the Formula One cars of 2014, the electric motors running at the same speed as the turbocharger; the speed at which the switching of current takes place is incredible, and any inaccuracy in its timing can mean a serious loss of power conversion efficiency. Where dc machinery is being run from an ac supply in a workshop, the device responsible for the conversion is referred to as a rectifier, and where an ac device is run from a battery, it is called an inverter. The power electronics combines both of these roles and has to switch very swiftly from one to the other.
Another important job for the power electronics is to supply from the battery an almost constant voltage to a number of other electronic components. This process of converting a high dc voltage to a lower one allows us to dispose of the traditional 12 V battery and run everything electronic on the car from the main propulsion battery. Components that we might need to run include starter motors, electrically powered pumps, lights and injectors, among others.
Written by Wayne Ward