The main goal of the work on the bike during this month was to test the motorcycle. As the bike had already done a few laps on a circuit, one and a half month earlier, it was deemed a good idea to see what performance could really be achieved with the current state of the bike, as it was designed last year by the previous team.
A first indication should be obtained by testing the bike on the dynamometer. The dynamometer, from now on referenced by its cute nickname “dyno”, is a device that consists of two rather heavy rolls placed horizontally behind each other, and an electrical motor being able to apply a torque on one of these rolls. The rear wheel of the bike is then placed between these rolls for the power output of the motor to be measured.
Up until now, the output power of the motor has always been limited to 40% of its full potential. The performance of the bike at this percentage has been measured thoroughly on the dyno, but this time we wanted to make it work a little harder to see how this influences parameters like voltage drop of the battery, current drawn and heat generated in the battery pack and motor controller. The power limit was increased by us to 60%.
During the second run with this setup, the velocity was increased up to 180 km/h, according to the dyno software. This velocity was maintained for a few seconds so a stable voltage and current reading could be acquired.
Then suddenly the motorbike starts to shake vigorously and a loud humming noise can be heard from the dyno. For a few seconds only one of the dyno rolls was still spinning, but not making contact with the rear wheel. The emergency button of the bike was pressed just to be safe and the dyno was slowed down by applying the rear brake.
When investigating the wheel, a clear abraded area could be seen on the tire. The dyno roll also contained a similar track of burnt rubber. These marks and a recording of the test helped us to conclude that the motor must have blocked suddenly.
The motor that makes the wheel turn is a synchronous AC motor, which as you might know can also act the other way around, as a voltage generator. This happens when the motor is spinning around while not receiving any electrical power. When this perceived voltage source is then short-circuited, a huge current is drawn resulting in a similarly large torque working against the spinning of the rotor. This could have stopped it so all of a sudden, together with the rear wheel.
Now we need to investigate how this short-circuit could have happened. This is actually a very interesting task, because it gives insights in a wide range of subjects. I have learned a lot about, for example, the behaviour of DC fuses and the three-phase operation of a synchronous AC motor by using an inverter. Furthermore, we were also given the, although slightly frightening, opportunity to perform an emergency shutdown of the bike.
Some repairs are also required. For example the battery pack, which received a shock as well when the failure happened. However, working on the high voltage battery pack is an exciting task and deserves a blog post on its own.
A close-up of the synchronous AC motor and the three-phase shielded power cables, a subject of investigation