predetermined reduced pressure by driving the vacuum pump 5 a surge voltage generated in a surge generator 10 is applied to a coil of the motor 1 through one of the lead wires 12 14 another of the lead wires 12 14 is grounded and the other of the lead wires 12 14 is not connected to anything there are four switch patterns of connection which is realized by the surge changeover switch 9 as shown in fig2 the surge voltage is evenly applied to the coil in accordance with the switch patterns i ii iii and iv the surge voltage is thus applied to the coil in all directions and a surge current flows in all the ways of the coil in the switch pattern i the surge current enters a tap m which is the beginning point of winding of a main coil 1b in the motor 1 fig1 and flows out from a tap c which is the common connection point of the main coil 1b and an auxiliary coil 1c a surge stress given by the surge current is applied between adjacent windings of the main coil 1b and the surge stress is also applied between the main coil 1b and the laminate core 1a surge energy becomes extinct at the tap c of the common.