| 摘要 |
PURPOSE:To reliably perform overcurrent protective operation even under a situation that a pulse-shaped current not suitable for detecting an overcurrent flows in an inverter main circuit. CONSTITUTION:An inverter main circuit 15 switches a DC output between power-supply lines 14a, 14b, and it drives an AC motor 16. A current detection circuit 18 output a load current signal Is which represents the mean value of inverter output current detected by a current detector 17. A current correction circuit 19 output a current value DELTAI, for correction, which gives a larger value as a V/F-ratio instruction which changes the ratio of an inverter output voltage to a frequency by keeping a definite relationship is larger. An addition circuit 20 gives a result in which the load current signal Is has been added to the current value DELTAI for correction to an overload detection circuit 21 as a correction current value Ix. The overload detection circuit 21 detects the overload state of the AC motor 16 on the basis of the correction current value Ix in terms of an inverse-time characteristic, and it stops the operation of the inverter main circuit 15 when the overload state is detected. |
