Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1992-08-12
1994-03-15
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318809, 318807, H02P 540
Patent
active
052948765
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an improved method and apparatus for controlling an AC induction motor by indirect measurement of the air-gap voltage of the AC induction motor and using said air-gap voltage for generating an optimal modulation frequency for the conversion of control signals from field coordinates to drive signals in stator coordinates for the AC induction motor. The modulation frequency is generated so that the magnetizing current is constant, which is a necessary condition for obtaining Natural Field Orientation.
PRIOR ART
Many qualified systems for AC induction motor control are based on the concept "Field Orientation". This concept has been known for twenty years. Conventional field orientation requires measurement of the magnetic field. However, the magnetic field is difficult to measure.
Today there are systems for "Indirect Field Orientation". The field measurement has been replaced, for example, by methods which are based on measurement of the rotor frequency plus calculation of the slip frequency. Much research is still going on.
Swedish patent No. 8000118 (same as European Patent No. 0049241 "Method and apparatus for controlling an AC induction motor") discloses a control system which creates field orientation without field measurement, and is presently called "Natural Field Orientation". The motor contributes to the field orientation in a natural way, thanks to the inherent properties of all AC induction motors. Said system operates with voltage control, and requires a speed sensor on the motor shaft. The present invention is a development and an improvement of said system and eliminates the speed sensor. The present invention can be used with voltage as well as current control.
In all field oriented control systems, also in the system according to the present invention, the motor is controlled by a control vector in field coordinates. The externally generated control vector is converted from field coordinates to stator coordinates by a first vector rotator and connected to the motor via frequency inverters. An airgap voltage appears in the motor.
DISCLOSURE OF THE INVENTION
The airgap voltage has a central role in the AC induction motor. The present invention is based on indirect measurement of the airgap voltage. Then, the measured airgap voltage vector is transformed to field coordinates by a second vector rotator. An optimal rotation frequency is generated, proportional to the amplitude of the measured airgap voltage and both the vector rotators are controlled or rotated with this frequency, resulting in a constant ratio between the amplitude of the airgap voltage and the modulation frequency, thereby obtaining a constant amplitude of the magnetizing field current during all operating conditions. Quadrature between the field current and the rotor current in each phase is automatically obtained, thanks to the constant amplitude of the magnetizing field current. The resistive rotor current is frequency independent and can be controlled without disturbing the field current of being disturbed by the field current. This can be defined decoupled control of the two currents. Thus the major features of field oriented control, decoupling and quadrature, are naturally obtained, thanks to the optimal modulation frequency.
Thus, there is provided a control system for an AC induction motor, comprising a stator, a rotor, at least two stator windings and rotor windings, said control system comprising control signals, a first vector rotator for rotating said control signals with a first angle (.alpha.) for providing output signals connected to frequency inverters for providing drive signals to each stator winding of the motor, said first angle (.alpha.) being the time integral of a rotation frequency (w) which is controlled essentially for maintaining the amplitude of the magnetizing currents of each stator winding constant. According to the invention, the control system comprises a means for measuring the voltages and/or currents of said drive signals, at least a second vec
REFERENCES:
patent: 3796935 (1974-03-01), Blaschke
patent: 4282473 (1981-08-01), Dreiseitl et al.
patent: 4447787 (1984-05-01), Schwesig et al.
patent: 4626761 (1986-12-01), Blaschke
patent: 4763058 (1988-08-01), Heining et al.
patent: 4885519 (1989-12-01), Vogelmann
patent: 5001411 (1991-03-01), Heinle
Cabeca John W.
Shoop Jr. William M.
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