Abstract: In a variable frequency power source unit, a series circuit of a contactless semiconductor switching element and a resistor is provided in parallel to a d.c. smoothing capacitor, and a control circuit is provided for receiving a signal for turning on the switching element selectively at any time after the power source is interrupted, thereby discharging the electric charge on the d.c. smoothing capacitor.

Abstract: An inverter apparatus using a microcomputer control system suitable for use in driving an ac rotary electric machine is concerned. The inverter apparatus includes a device for detecting an output current or an output voltage of the inverter apparatus or a control signal supplied to the inverter apparatus and, from the detected result, detecting an output frequency of the inverter apparatus and a switch structure having memory function by means of change-over and selection of lines for holding data for setting therein an upper limit of the output frequency of the inverter apparatus in hardware manner. Thus, the device for detecting the output frequency of the inverter apparatus produces the output frequency of the inverter apparatus and the switch structure produces the exact upper limit of the output frequency which is not influenced by noise or the like.

Abstract: In a motor speed controller for controlling an AC motor to be driven at a variable speed, a signal proportional to a load torque and a signal proportional to an output torque of the AC motor are found by current detecting means for detecting the primary current of the AC motor, and integrated over the speed changing period, and the integrated values are divided by the values corresponding to the amounts of speed change during the speed changing period so that the moment of inertia can be computed. In addition, there is provided means for changing the rate of output frequency change caused by the output frequency command into a value according to the calculated moment of inertia. Moreover, there is provided a circuit for detecting a sudden change of the calculated moment of inertia, so that when a sudden change occurs, an abnormality signal is generated.