PURPOSE: To suppress noises and to prevent heating of high capacity machine by providing a tap switching output circuit for closing a relay contact connected with a resistor onto which a voltage equal to power supply voltage, among voltages divided through resistors, is applied.

CONSTITUTION: The rotational speed controller is divided into two system outputs of a tap switching output circuit 125 and an inverter output circuit 111 wherein the tap switching output circuit 125 is employed in place of the inverter circuit 111 for an induction motor having high current capacity which can not be driven with inverter output. A voltage level regulated through a triac 50 in a voltage regulating circuit 110 is detected through a voltage detector 124 in the tap switching output circuit 125 and thus detected voltage is then fed through voltage dividing resistors 114, 115, 116 to a voltage comparator 117. A voltage level fed from the voltage detector 124 is then compared with a reference voltage level of a timing oscillation circuit and any one of relay switching transistors 118, 119, 120 is turned ON to turn the contact of any one of relays 121, 122, 123 ON.