What happens if the controller is turned off during flux weakening?
Turning off the controller during flux weakening should be avoided at all costs. Because it can lead to permanent controller or BMS damage.
Situation 1: Controller is turned off using POWER input
In this situation, the controller is turned off by POWER input. This means, that the battery is still connected, only the logic of the controller is turned off.
In this situation, the motor speed will rapidly (uncontrolable) decreases to the nominal speed. Inrush current will flow into the battery. This inrush current can not be controlled, because the controller is turned off. This inrush current may damage the controller or BMS.
If due to the inrush current BMS turns off the ports, you get the same situation as in Situation 2.
Nominal speed is the maximum speed of the motor without flux weakening. This is the speed you get if you set /driver/dac/fwc to 0.
Solution
To avoid this, do not use kill-switch that turns off the controller. Instead, set the driver to freewheeling mode. Then turn off the controller later, when the motor speed is low.
For this purpose, the LYNX application implemented seatswitch feature. This feature disables the ride, but the controller is still powered.
Situation 2: The controller battery is disconnected
In this situation, the controller battery is disconnected. It can be done by BMS, disconnecting the battery, or by fuse.
In this situation, the overvoltage from the motor immediately blows the MOSFETs. This leads to a phase short circuit and the motor will stop immediately.
Solution
To prevent this:
- Do not disconnect the battery during the motor run.
- Use 2 port BMS. Two port BMS allows current to flow from the motor into the battery.
- If you use 1 port BMS, ask for the implementation of communication protocol. In this case, the BMS must send a warning about reaching some level of protection and the controller will reduce power.
For this, the LYNX implements the BMS communication feature.