I don't know what is GTB but assume it is an ESC that can control both brush DC motor and brushless motor. I guess, if it operates in "brushless mode", its internal 3ph inverter will produce 3 phase AC currents to the 3 wires.
If it operates in "brush mode", its internal 3ph inverter will control the return current (-) only. You can see that, from the above picture, the (+) has been directly connected to the motor. So, you could say,用有刷摩打時表三條線都係負 |