 It is therefore desirable to control the point on the wave at which to switch, the ideal point is when the motor amperes to the lowest value in all the active windings of the autotransformer.A 3-coil autotransformer requires a 3 pole switch to control 3 phase voltages, an examination of figure 9 shows three sine waves displaced at 120 degree intervals.There appears there is no real point on the cycle where all three phases can be switched simultaneously at any equal low motor amperage without resorting to complex staggered switching methods.