Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1995-08-24
1996-12-31
Cabeca, John W.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318808, H02P 528
Patent
active
055897541
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for determining an estimate for the stator flux of an asynchronous machine when the stator current, stator voltage, supply frequency, stator inductance, stator resistance or an estimate therefor, and short-circuit inductance of the machine are known. A stator resistance estimate for the machine can also be determined by the method.
In frequency converter-based control of an asynchronous machine, the object is often to make the torque generated by the machine to behave in a desired way when the current and voltage supplied to the machine are known. In that situation, one attempts to influence the electric torque, which in terms of the stator flux and stator current is:
Controlled torque regulation therefore requires that besides the current i.sub.s, the stator flux or a commensurate variable (such as the rotor flux or air gap flux) of the machine is known. This will not present any problem with operation at high frequencies, in which situation integration of the voltage supplied to the machine is known to give a good estimate for the stator flux: ##EQU1## where u.sub.s =stator voltage, and
.psi..sub.s is easy to calculate from equation 2 when the supply voltage and its frequency are known.
It can also be seen from this equation that when .omega..sub.s diminishes below a specific nominal frequency the voltage must be reduced in order for the flux not to increase too much and the machine not to become saturated.
Yet equation 2 is not practicable with low frequencies, since in reality the voltage to which the windings of the machine are subjected deviates from the supply voltage to the extent of the voltage loss developed in the winding resistances. Thus the relative proportion of the loss component in the voltage increases when u.sub.s has to be reduced as .omega..sub.s diminishes. With low frequencies the loss component should thus be taken into account, i.e., the flux estimate should be calculated from the equation:
The accuracy of the flux estimate calculated by means of this equation is, however, strongly dependent on the accuracy of the R.sub.s estimate employed and on the operating frequency, such that the error in the steady slate of the flux estimate increases in direct proportion to the error in the R.sub.s estimate and in inverse proportion to the frequency. On the other hand, the R.sub.s estimate must always be distinctly smaller than the actual stator resistance to enable stable control by the integrating method according to equation 3. Therefore, with the mere integrating method one can in practice hardly attain frequencies below 10 Hz without a significant steady state error in the flux estimate.
This problem related to the integrating method can be solved with the use of either direct or indirect vector control. In the first case, the stator flux is measured directly with a measuring element incorporated in the machine, whereas in the latter method it is calculated indirectly on the basis of the stator current and speed information obtained from a tachometer disposed on the shaft of the machine. In both cases, the torque of the machine can also be controlled at zero frequency, but bot methods require an extra measuring element which is relatively costly and diminishes reliability.
The above problems can be avoided without any need for extra measuring elements incorporated in the machine by using the method of the present invention. In this method, the stator flux estimate is calculated by means of equation 3 in such a way that corrections are made in the voltage estimate to be integrated, allowing compensation of errors in the flux estimate produced in the integration. The corrections of the voltage estimate are selected depending on the supply frequency and torque in such a way that on account of said corrections the stator current is set at a reference current value that the stator current should have in a steady state, if the machine had a stator flux of the magnitude of the flux estimate and a torque of the magnitude of the torque estimate, calculat
REFERENCES:
patent: 4441065 (1984-04-01), Bayer et al.
patent: 4447787 (1984-05-01), Schwesig et al.
patent: 5194797 (1993-03-01), Kahkipuro
patent: 5371548 (1994-12-01), Heikkila
ABB Industry OY
Cabeca John W.
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