Method and apparatus for controlling an AC induction motor by indirect measurement of the air-gap voltage

A control system for an AC induction motor (M) for field oriented control, comprising a stator (3), a rotor (4), at least two stator windings (1, 2) and rotor windings (5, 6), said control system comprising control signals (Sm, Sr), a first vector rotator (12) for rotating said control signals with a first angle (.alpha.) for conversion from field coordinates to stator coordinates for providing output signals (V1, V2) connected to frequency inverters (19, 20) for providing drive signals to each stator winding (1, 2) of the motor, said first angle (.alpha.) being obtained by integrating a rotation frequency (w) by means of an integrating means (22), said rotation frequency (w) being controlled essentially for maintaining a magnetizing current (Im) of each stator winding (1, 2) constant,

• characterized by
• a means (24, 25, 26, 27) for measuring at least one of voltages of said drive signals, currents of said drive signals, and an airgap voltage (E);
• at least a second vector rotator (29, 32, 33) for counterrotating said measured voltages, currents, and airgap voltage (E) with a second angle (-.alpha.) which is the inverse of said first angle (.alpha.) for conversion from stator coordinates to field coordinates,
• a means for determining an estimated air-gap voltage (E*) on the basis of said counterrotated measured voltages, currents, and airgap voltage (E);
• a means (23) for determining said rotation frequency (w) on the basis of said estimated air-gap voltage (E*) essentially for maintaining the amplitude of said magnetizing current (Im) of each stator winding constant.