Abstract: A CVCF (Constant Voltage Constant Frequency) inverter includes a main circuit, a voltage control loop, and a frequency control loop. In a normal operating mode, constant voltage, constant frequency control is carried out by these loops. In a lap switching operation, if a comparator detects that the output current of the inverter exceeds a predetermined level, a switch is closed so that the effective power component computed by an effective power computing unit is inputted to the frequency control loop. The output frequency of the inverter is controlled by changing a reference frequency in response to the effective power. This reduces the phase difference between the output voltage of the inverter and an external power supply, thus decreasing a cross current. This makes it possible to protect the components of the inverter and to supply power reliably.