While scouting around for an electric car, I came across an Audi with three motors. How would this work? What are the advantages and disadvantages?
The triple-motor set-up is unique. Electric vehicles (EVs) usually have one motor that drives either the front or rear wheels, or two motors in an all-wheel-drive.
In the Audi you came across, one motor powers the front wheels while the other two motors power the rear. The three motors are not mechanically connected - they are located either between the left and right rear wheels or between the front and any of the rear wheels.
What this means is that the front wheels are fully controlled by one motor which distributes power to the left and right wheels via an epicyclic differential.
Some performance cars with internal combustion engines use electronic control of a differential together with selective braking and claim that this allows for "torque vectoring". Essentially, these systems use multi-plate clutches and individual braking to apportion power to the left or right wheel as the situation demands. The total available torque at any point must be shared.
For example, if 200Nm is delivered to the differential, the torque split may be 100Nm on each wheel. Or if one wheel receives 150Nm, then the other must have 50Nm.
With twin rear motors, each wheel can be powered anywhere from 0 to 100 per cent of the motors' output. They do not split or share a fixed quantum of torque and power, making this set-up a real torque-vectoring system.
The main advantage of this is precise control of the car's dynamic behaviour, especially at higher speeds. As with any EV, electronic power control delivers instantaneous and accurate responses.
One disadvantage is a slight increase in weight and thus a marginal increase in energy consumption.