Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1994-08-24
1995-12-19
Wieder, Kenneth A.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
G01R 2726
Patent
active
054771625
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of determining a short-circuit inductance in an asynchronous machine, wherein the short-circuit inductance is determined by means of a stator voltage and a stator current derivative of the machine.
Control of an asynchronous machine usually aims at a desired behaviour of the torque created by the machine when the current and voltage supplied to the machine are known. One thus attempts to affect the electric torque, the relative value of which as a function of the stator flux and stator current is:
Proper torque control thus requires that not only the current i.sub.s but also the stator flux of the machine or a parameter proportional to it (such as the rotor or air gap flux) is known.
Methods for calculating the stator flux are based on the well-known differential and current equations of the stator and rotor of an asynchronous machine, which are as follows in the coordinate system of the stator: ##EQU1##
The object is to calculate the stator flux by means of the measured stator current and stator voltage, and so the rotor flux and rotor current have to be eliminated from the above equations. Using Eq. 4 and 5, the rotor flux and rotor current are first solved as a function of the stator flux and stator current: ##EQU2## where ##EQU3## and .sigma.L.sub.s =short-circuit inductance.
Using Eq. 6 and 7, Eq. 2 and 3 are reduced into the following form: ##EQU4## where ##EQU5##
Most prior art stator flux calculation methods utilize either Eq. 8 or Eq. 9 or both of them. When using Eq. 8 alone, it is not possible to effect proper control at very low frequencies, wherefore the best methods always use either Eq. 9 or both equations.
One crucial parameter required in Eq. 9 is the short-circuit inductance. To deduce one method for calculating it, the derivative of the rotor flux is discussed first, which is on the basis of Eq. 2 and 6: ##EQU6##
Introducing Eq. 10 into Eq. 3 gives: ##EQU7## where u.sub.0 is a voltage depending on the state of the machine: ##EQU8##
At the starting time of the asynchronous machine (indicated with the reference t.sub.0) the stator and rotor currents and fluxes are zero, so that u.sub.0 (t.sub.0) =0, and it follows from Eq. 11 that time t.sub.0.
Accordingly, the stator current starts to increase in the direction of the stator voltage with a slope u.sub.s (t.sub.0)/.sigma.L.sub.s when a predetermined voltage u.sub.s (t.sub.0) is supplied to an unmagnetized machine. The situation is illustrated in FIG. 1, which shows a voltage u.sub.s and a current i.sub.s as a function of time in a starting situation described above.
One prior art way of determining .sigma.L.sub.s is, in fact, based on measuring the stator voltage and the derivative of the stator current of the machine at the starting time, and so the short-circuit inductance can be calculated on the basis of Eq. 13 directly as a ratio between them: ##EQU9##
A drawback of the above-described method is that .sigma.L.sub.s is determined only at the starting time, whereafter it is assumed to remain constant. Like the other inductances of a machine, the short-circuit inductance may actually vary considerably during operation due to the fact that the saturation state of the magnetic flux of the stator or the rotor varies with the operating point of the machine.
The object of the present invention is to provide a method of determining a short-circuit inductance estimate, which does not have the limitations and problems described above and which may be applied during the operation of the machine as well. This is achieved by means of a method according to the invention, which is characterized in that it comprising the steps of causing a step change in the stator voltage; measuring both the stator voltage and the stator current derivative both before and after said step change in the stator voltage; determining the difference between the measured stator voltages and the difference between the measured stator current derivatives; and determining the quotient of the difference between the stator voltages and the diffe
REFERENCES:
patent: 4649341 (1987-03-01), Ulbrich et al.
ABB Industry OY
Wardas Mark
Wieder Kenneth A.
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