There are two methods to detect Back-EMF. Starting with the conventional method first, which is based on neutral point voltage. When two phases are conducting current, the third winding is floating. So the back-EMF of the floating winding can be detected. Usually, the back-EMF voltage is referred to the motor neutral point. For this method, the terminal voltage is compared to the neutral point, and the zero crossing of the back-EMF can be obtained. In most cases however, the motor neutral point is not available, so a virtual neutral point is built by resistors. It looks pretty nice; however, it has a few problems.