Why the permanent-magnet motor is the issue
Most electric vehicles use a permanent-magnet synchronous motor (PMSM). When the drive wheels rotate, the motor rotates. When the motor rotates, it generates electricity, regardless of whether the vehicle is switched on. That generated current flows back through the inverter. The inverter is designed to handle current from the battery, not from the motor acting as a generator under tow.
Towing an EV on its drive wheels, even at walking pace, puts the inverter in an operating state it was not designed for and may not have thermal protection against. The result ranges from a fault code and a limp-home mode to a permanent inverter failure requiring a battery-pack replacement, which starts at £5,000 depending on the model.
Flatbedding eliminates the risk entirely: all four wheels are off the ground, the drivetrain does not rotate, and the inverter is idle.
