Thermal Cut off question

Hi Mad Scientist

Looks like you are entering into the spirit of innovation, fantastic! I have discussed your question with the team and will respond in two parts, first answering the question and then sharing some of their wisdom for you to discuss with students.

We have no electrical concerns and will let you use the 40A breaker on the 48V battery.
For safety sake you will need a fuse on their 12V battery as well.

However, the team were keen to find out what research/ design the students have conducted that have led them down this path? We certainly don't want to dampen student's enthusiasm to try new things but as you asked, here are Warren's initial thoughts:

Car alternators are unfortunately very inefficient and probably even less efficient when run as a motor.
A very important efficiency quality is the no-load current.
Ideally the 48V no-load current should be less than 1 amp. This includes current to the field winding.
Also
Controlling the normal motor speed over a wide speed range introduces even less efficiency. It is generally only practical to increase the base speed 20%. Use the brushless speed controller from zero to normal full speed. If you want a bit more speed on top of that, reduce the field current slightly.
Also
The stall torque from sensorless drives is unreliable. Often they blow up. Some need a push to get started. It is best to have 3 hall sensors fitted to the motor to give rotor position feedback.

I have some 24V brushless motors so I may be able to lend RNL one. They would probably blow up if 48V is put on them though.

Happy to connect your students to Warren as he may be willing to share some of his experience.