Cooking Recipe: Fried Electronics with a Sprinkle of Stupidity

Many stories on this blog are success stories. This one, unfortunately, is not.

Two weeks ago, we fitted our motors on the rover and wired up the electronic speed controllers (esc). These controllers are needed to power the motors and are indispensable when working with brushless DC motors.
To explain it simply (don’t quote me on any of this), an esc has two switches for each of the three motor coils, which it can then open and close to let the right amount of current flow through the stator coils. These switches are called MOSFET’s and are the backbone of any electronic device you have ever held in your hands, from a TI-30 calculator to a modern Formula 1 car. To accurately control the motor, the esc has to know the position of the turning part of the motor, also called the rotor. This is done via sensors, or a whole lot of math.

Now that you understand the very basics of brushless DC motors and its controls, let me tell you where we went wrong. You see, a brushless motor is as much a generator as it is a motor. As soon as you turn the rotor, the magnets will induce a current in the coils, which will then be fed through a diode (think of a diode like a one way street), that is built into the MOSFETs and will finally end up in the power supply rail. Problem is, if the power supply rail is not currently (no pun intended) fed by a actual power supply or battery, the current cannot go anywhere. When this is the case, the voltage on the rail will keep rising and rising until at a certain point some component will release the magic smoke and will break. And that is exactly what happened with our motor controllers when we spun the wheels during a wheel bearing test. The aftermath: two dead motor controllers, a dead battery management system and an annoyed me. Annoyed that I didn’t think about overvoltage issues when the electronics are not running and annoyed that I was to lazy to short the power supply before the test.

Learn and move on. With electronics, stuff breaking is always a possibility. Obviously it’s no fun, but there is nothing else to do but to ensure the same error cannot happen again and then rebuild. And rebuild better.
And that is exactly what we will do. Which is why I hope I can give you an update about our new esc’s and their performance in the coming week.

Best,
Joel