Interesting. Is there no way to design the circuitry so that when given no input, it defaults to a fully brakes-engaged mode? I’ve only worked with PM motors, so I know how that sort of h-bridge could be designed, but my knowledge of non-PM motors is fuzzy at best.
Well, it is certainly a very shaky proposition, and the car will probably be a total loss.
But there are ways you could walk away from this.
You see some pretty crazy crash survivals in race driving, for example.
And I remember stories from a few years ago, when Ford had a recall on F150 tires because they tended to blow out at highway speeds, with an effect similar to full wheel lockup-- yes, people died, but others didn't.
So looking into it a bit there are what is called Eddy current brakes. In the section of disk eddy current brakes it talks about the power tool implementation where there is just a metal disk and the magnetics are pulled away from the disk when the device is powered on and passively return to the disk when power is removed. Something like that could work, but I am unsure of the size of magnets or disks necessary to adequately stop a vehicle. Most likely you could also add a normally closed relay hooking the motor to a resistor load. So when power is applied the resistor load is disconnected, and in a loss of power event the motor is directly tied to the resistor load.
Eddy currents develop in proportion to the speed of the moving magnetic field. So if you used eddy currents for braking, it would be most effective at high speed and gradually have no stopping power at low speed.
That is mostly true about regenerative braking as well. Once you hit a certain speed the vehicle has to use some energy to get the motors to actually stop the vehicle. Usually EVs and such will just rely on the friction brakes for the last bit but some cars like the Bolt will automatically use some energy to slow the car to a stop so you can use do one pedal driving.
I'd imagine this was introduce adverse failure modes like - slamming to a stop without warning within seconds on the highway if power is lost, rather than coasting to a halt... and then being untowwable as the car can't be put into neutral.
Tesla's that lose battery are already a challenge to tow as the parking brake cannot be disengaged without power.
Hmmm. So you're travelling along the interstate at night, you lose power and your car screeches to a stop in the middle of high-speed traffic and your lights are off.
I would love to be present at the DFMEA (Design Failure Modes Effects Analysis) meeting when this came up!
