Connect one phase winding to a low voltage (3~6V) DC power supply (battery, dry cell or low voltage rectifier), and use a compass to move along the inner circumference of the stator. If the winding is connected correctly, the compass will change alternately from north to south when passing through each pole phase group in sequence; if the compass needle points uncertainly when passing through a certain pole phase group, the coil of the pole phase group may be connected in reverse. By testing the three-phase winding in sequence, the fault point can be found. For the star-connected winding, it is not necessary to disassemble the connection point during the test. Just connect the two ends of the power supply to the neutral point and the output end of a certain phase winding respectively; for the triangle-connected winding, the connection points of the three-phase winding must be disassembled and then tested separately.