The serial-parallel connection requires nine terminals in the electric motor, admitting external access to the half of the coil of each phase Gates A73. In series coils, each half of the coil is connected in series, being subject to only half of the phase voltage of the network, ie if the allowable voltage in the coil is 220V, with the serial connection we can start the motor with voltage of 440V (phase)
In the parallel coils each half of the coil is connected in parallel, being subject to the same phase voltage of the network and we use the series-parallel connection whenever the voltages are one double of the other. Example: 220 / 440V – 230 / 460V.
Since the star-triangle connection requires six terminals in the electric motor, allowing external access to the two ends of the coil of each phase, it serves for any double nominal voltage, provided that the second is equal to the first multiplied. Examples: 220 / 380V – 380 / 660V – 440 / 760V. If the acceptable voltage in the winding is 220V, connecting star can power the motor with 380V (line voltage), already in the triangle connection we can connect the motor with 220V (line voltage).