| Abstract |
The induction motor drive described in this paper consists of a voltage source PWM transistor inverter with current control loops producing approximately sinusoidal impressed stator currents in steady state. The amplitude and frequency of the current references are derived from a steady state machine model. In order to obtain good dynamic performance of the speed control loop the references are calculated with the assumption of constant air gap flux. Subsequently this condition is modified with the view of improved efficiency of motor and inverter by varying the air gap flux as a function of the load, causing the lightly loaded motor to operate at reduced flux. The feedback variables are two stator currents and speed which is measured by a digital sensor. All signal processing is performed by a single board microcomputer (CPU 80186) with an analog/digital interface card. As a result, a four quadrant drive with torque limit and good dynamic performance over a wide speed range is obtained, the response time of which is somewhat larger than in case of an optimal field orientated control scheme. However, the computational complexity is considerably reduced. In order to eliminate the effects caused by changing rotor temperature and desaturation in the field weakening region, the varying motor parameters are detected by on-line Fourier transformation of a stator current and a line to line voltage and subsequent updating of the controller parameters. This task is also performed by the microcomputer. The paper covers the underlying theory and demonstrates the design of an experimental 7.5 kW drive. |
