Abstract: A pump off control method comprises detecting the speed of the induction motor and an instantaneous value of secondary current of the induction motor. Down stroke time in every cycle of the pump jack is detected. An average value of instantaneous values of the secondary current of the induction motor in the down stroke time in said every cycle is calculated. An average value reference of the secondary current of the induction motor to be compared with calculated average value of the instantaneous values of the secondary current of the induction motor is set. The calculated average value of the instantaneous values of the secondary current is compared with the average value reference after the down stroke end in each cycle. An occurence of pump off is detected if the calculated average value of the instantaneous values is greater than the average value reference.

Abstract: In an inverter controlling method for an induction motor, the slip of the motor is detected and an optimum slip which is dependent on the motor constant and the frequency supplied to the motor providing the highest efficiency is calculated, and a voltage manipulated variable is determined from a comparison of the optimum slip and the slip of the motor, and then the voltage manipulated variable is used to form an output voltage instruction or a magnetic flux instruction so that running of the induction motor is controlled with the slip with which the highest efficiency is provided with respect to load variation and frequency variation.

Abstract: An object of this invention is to provide a high performance and low cost crane rope steadying control method and apparatus for which mechanical or optical swing angle detecting means are not necessary.The invention provides a rope steadying control method for a crane or the like having a trolley driving apparatus for causing a load suspended by a rope of a crane or the like to travel, wherein swinging of a load suspended by a rope is stopped by calculating a swing load signal I.sub.2W * proportional to the rope swing angle and the load by computationally estimating a motor torque estimate signal .tau..sub.M * not including load torque fluctuations caused by swinging of the rope on the basis of gain coefficients and equivalent time constants of the control system and the drive system, and comparing this estimate signal .tau..sub.M * with an actual load torque .tau..sub.M and negatively feeding back to a trolley speed command N.sub.S of the trolley driving apparatus (1) a speed signal N.sub.