Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Patent
1998-08-05
2000-07-25
Han, Y. J.
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
363 37, 318807, H02M 114
Patent
active
060943645
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an inverter arrangement for simultaneous control of a stator flux and a torque of an electric machine, the arrangement comprising an inverter connected to a direct voltage intermediate circuit for generating an alternating voltage that has the same number of phases as the electric machine and a control system for controlling an instantaneous value of the alternating voltage generated by the inverter; the control system receiving, as input variables, reference values for the torque and stator flux and measured values for the stator current and stator voltage or for variables comparable to them, and comprising means for defining, for the torque and the stator flux, error variables that are based on input variables and describe the difference between the reference values and actual values, and means for controlling the switches of the inverter on the basis of these error variables.
The basic idea of a conventional DTC inverter (DTC=Direct Torque Control) of the type described above is that the stator flux and torque of an electric machine are controlled simultaneously such that the instantaneous voltage supplied to the machine corresponds as closely as possible to the instantaneous need of controlling the flux and torque. A block diagram of such a conventional DTC inverter is shown in FIG. 1 of the accompanying drawings and is also previously known, for example, from the article `A New Quick-Response and High-Efficiency Control Strategy of an Induction Motor` by Takahashi and Noguchi in IEEE Transactions on Industry Applications, Vol. IA-22, No. 5, September/October 1986.
The actual values of the flux and torque needed in the control are usually computed on the basis of the voltage and current supplied to the machine. Normally, a stator voltage vector u.sub.s is computed from a direct voltage U.sub.DC measured in a direct voltage intermediate circuit DC and from positions S.sub.a, S.sub.b and S.sub.c of switches SWa, SWb and SWc used in an inverter AC, whereas a current vector i.sub.s is obtained on the basis of two measured phase currents.
The stator voltage and current are input variables for a flux computing block VL in which a stator flux vector .PSI..sub.s is computed on the basis of the basic equations of the machine. Computation of a vector product of the stator flux and current in block R gives an instantaneous torque T. When the so obtained actual value of the torque is subtracted from the reference value T.sub.ref of the torque in block V1, an error variable T.sub.err is obtained for controlling the torque. Correspondingly, when an absolute value .PSI..sub.s computed in block IT for the actual value of the flux is subtracted from a reference value .PSI..sub.ref of the stator flux in block V2, an error variable .PSI..sub.err is obtained for controlling the torque. When the difference value is positive, control measures should be taken to increase the actual value, and vice versa. The means associated with the computation of the error variables are indicated in FIG. 1 by a common reference EC.
A DTC inverter also needs to know in which sector (in which sixth of a circle) a stator flux vector is located at a given moment. The sector number n (=0 . . . 5) is determined in a separate sector computation block SL, to which the stator flux vector is thus inputted.
The sector number n and the error variables .PSI..sub.err and T.sub.err of the flux and torque controls are input variables for a switch reference selection block KV, the function of which is to select the switch position (i.e. voltage vector) that maintains the actual values of the torque and flux within certain hysteresis limits. A zero vector and six voltage vectors that appear from FIG. 2 of the accompanying drawings are available; from these is selected the vector that best suits the instantaneous control situation. The essential feature is then the phase angle of the voltage u.sub.s in relation to the flux .PSI..sub.s. This is illustrated in FIG. 3 of the accompanying drawings. If the torque is to grow (T.sub.err >
REFERENCES:
patent: 5107438 (1992-04-01), Sato
patent: 5541488 (1996-07-01), Bansal et al.
"A New Quick-Response and High-Efficiency Control Strategy of an Induction Motor", Isao Takahashi and Toshihiko Moguchi, Transactions on Industry Application, (no month) 1986.
"Principles and Implementation of Direct Torque Control" by Stator Flux Orientation of an Induction Motor, Y.S. Chapuis, D. Roye and J. Davoine, Mar., 1995.
"Direct Torque Control of AC Motor Drives", Mika Aaltonen, Pekka Tiltinen, Jarkko Lalu, and Samuli Heikkila, Mar. 1995.
ABB Industry OY
Han Y. J.
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