| 摘要 |
PROBLEM TO BE SOLVED: To provide a control system for an induction motor by a voltage-type PWM inverter, by which an efficiency drop can be suppressed at a light load irrespective of the kind of a load. SOLUTION: A voltage-type inverter is used as the driving power supply of an induction motor 8, the gate of a main circuit 7 is controlled by an approximately sinusoidal wave PWM signal generator 6, and the motor 8 is driven at a variable speed. In this case, two-phase current instantaneous values iu , iw are detected as a primary current. The current instantaneous values iu , iw and phase data θ on output-voltage instruction sine waves from the approximately sinusoidal wave PWM signal generator 6 are used, an active-current component Iα whose phase is identical to that of the output-voltage instruction sine waves and a reactive-current component Iβ having a π/2 phase difference are separated in a coordinate converter 10. Iα /Iβ =Adet is found, and it is used as the judgment value of a load torque. The Adet is compared with a reference set value Aref . When Adet >Aref , a voltage is controlled so as to be increased. When Adet <Aref , a voltage is controlled so as to be decreased. At a light load, a lower-limit value in which 50% of a value by a V/f constant control system is a minimum value is set. |
