A closer look at our motor controller
Here at DNV GL Fuel Fighter, we implement and design an electric car. In essence, usage of battery as the main energy source and the operation of an electric motor for our car are the key concepts behind our electronic works. This week we are taking a closer look at our motor controller and show you its importance and functionality on our electric car for the Shell eco-marathon 2018 competition.
A motor controller is an electronic circuit that controls the amount of energy sent to the motor to have the same behaviour as in a thermal engine. As with thermal engine, we cannot directly connect the energy source to the motor as this would result in the motor operating at maximum power at all times! Therefore, this is where the motor controller come into play.
Of course, the first command in a fueled car is given by the driver with the gas pedal. In our car however, the same principle applies except there is a lever behind the steering wheel. This information is “read” by an electronic circuit in the steering wheel where it passes on a message to the motor controller. The motor controller will read that message, and that is where the magic happens!
It is an intelligent module: a current controller. The current going through the motor is directly proportional to the force with which it pushes the car. It therefore constantly measures the current in the motor, compares it to the target current (how hard the driver pressed the lever) and adapts if something slows the car down e.g. a wind or a hill or if something accelerates it. This is in every electric car, and it feels just as if you are cruising on a fueled car.
With a gasoline engine, we can’t produce gasoline as we brake! This is where electric cars are of an advantage. We can charge the batteries when we are braking. This concept is known as regenerative braking. We have a second lever behind the steering wheel that sends braking messages, and the motor controller sends a negative current in the motor, resulting in decelerating the car.
To go deeper in details, here is how the controller works:
The circuit is a full bridge DC/DC converter. It uses four electric switches (MOSFETs) and sends alternatively the battery voltage or its negative. This concept is called “chopping” or Pulse Width Modulation (PWM). By changing how long we sent the battery voltage versus how long we send its negative, we can get any voltage average between +50V (maximum speed) and -50V (maximum speed backwards).
There are other interesting functionalities that we chose to add. Our circuit measures the speed of the car, checks that the temperature of the motor doesn’t increase too much and measures how much energy is being used. This last functionality is very important to us as our aim is to use as least amount of energy as we can.