CURRENT DETECTOR FOR MOTOR DRIVING INVERTER

PURPOSE:To reduce the size of a current transformer (CT) for detecting a large current by applying a circuit for correcting a voltage determined by its winding resistor and the secondary current. CONSTITUTION:A current i2 detected by CT 21 is inverted by an operational amplifier 22, further inverted by an operational amplifier 23, and converted to a voltage vx. The output Vx of the amplifier 23 becomes as an equation I, where the resistance values of resistors 24, 25, 26 are R24, R25, R26. An output voltage V0 becomes as an equation II, where the resistance value of a resistor 27 of an equivalent circuit is R27, the primary current of CT is i1, the number of turns of a detecting current in n1, the number of turns of the winding of CT is n2. When thus set, a voltage dropped in the current transformer is corrected by an internal winding resistance of the transformer to be applied to the transformer.