Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2001-03-21
2003-07-15
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S189000, C702S190000, C318S434000
Reexamination Certificate
active
06594592
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for determining the torque of an induction machine, in which the stator terminal voltages and the stator terminal currents are used. This is preferably done by means of integration and subsequent high-pass filtering, to determine a filtered space vector, from which the DC component has thus been removed, of the concatenated stator flux. In addition, the invention relates to an associated device for carrying out a method, having a measurement device for detecting the stator terminal voltages and the stator terminal currents, and having a computation device for determining a filtered space vector of the concatenated stator flux.
BACKGROUND OF THE INVENTION
For drive tasks with induction machines, it is important to know the torque developed by the induction machine, since such knowledge is the only way to specifically regulate the torque of the induction machine. By way of example, devices for determining the torque in such induction machines are known from DE 42 29 554 C2 and JP 56-79223 A, with the torque being formed by means of electronic components in what is known as a computation circuit. In both publications, electronic high-pass filters are used to suppress DC components when determining the concatenated stator flux.
Indirect torque control is known from EP 0 621 680 A1, in which the power in at least one of the three phases is regulated at a constant value. A disadvantage in this case is that no direct relationship can be specified between the power and the torque, so that the torque can necessarily be regulated only inaccurately using the known method.
A further method for determining the torque from the concatenated stator flux and the stator current, that is to say by measuring purely electrical variables and thus not using a separate torque sensor, is disclosed, for example, in DE 195 32 477 A1. The aim in this case is to regulate a torque of 0 by synchronizing the stator and rotor flux. In this case, the space vector of the concatenated stator flux revolves at the rotor angular velocity, so that the speed of the machine can be determined indirectly from the fundamental frequency of the supplying frequency-converter voltage. The cited document does not, however, describe how the concatenated stator flux is determined without accumulating errors which necessarily occur in the recording and processing of measured values and which lead to intolerable discrepancies when determining the torque.
SUMMARY OF THE INVENTION
The invention is now based on the object of specifying a method which is as simple and accurate as possible for determining the torque developed by the induction machine using only electrical variables, in which case the errors caused by numerical integration and the necessary high-pass filtering are largely corrected.
In addition, the invention is based on the object of specifying a device for carrying out the method.
According to the invention, the first-mentioned object is achieved by means of a method having the features of patent claim 1 for example. In the method according to the invention for determining the torque of an induction machine, the stator terminal voltages and the stator terminal currents are used, by means of numerical integration and subsequent high-pass filtering, to determine a filtered space vector of the concatenated stator flux. Further, the torque is calculated from this by multiplication by a complex correction factor. The multiplication by the complex correction factor corrects the amplitude and phase errors which are caused by numerical integration and high-pass filtering and are each related to the fundamental of the power supply system voltage. As such, a value of the flux concatenation is calculated which is substantially exact with respect to the amplitude and phase of the fundamental of the power supply system voltage.
The aforementioned tasks are achieved according to the invention in the case of a method of the type mentioned initially in that the space vector is multiplied by a complex correction factor and, after the multiplication, the torque is calculated by means of the correction factor. The associated device has a computation device by means of which, in addition to determining the filtered space vector, this is multiplied directly by a complex correction factor. Thus, it is once again possible to calculate the torque from the filtered space vector multiplied by the complex correction factor.
The high-pass filtering makes it possible to compensate for errors which occur firstly in the measurement of the current and voltage values and which, secondly, result from the fact that, since the temperature varies with the operating conditions, the value of the electrical resistance of the stator winding, which is required to calculate the stator flux, is not known exactly. Furthermore, the high-pass filtering compensates for any residual error in the integration, thus avoiding any remaining discrepancy or drift in the calculated concatenated stator flux, which would lead to a remaining and/or increasing error in the determination of the torque.
In detail, the following steps are carried out in one advantageous refinement of the method according to the invention:
a) The stator terminal voltages and the stator terminal currents are measured at predetermined time intervals,
b) the measured values of the stator terminal voltages and the stator terminal currents are used to calculate the space vector of the stator voltage and, respectively, the space vector of the stator current,
c) the space vector of the stator voltage and the space vector of the stator current together with an initial value for the space vector of the concatenated stator flux are used to determine, by numerical integration, an unfiltered space vector of the concatenated stator flux,
d) the unfiltered space vector of the concatenated stator flux is multiplied by a predetermined filter factor for high-pass filtering,
e) the filtered space vector, determined in this way, of the concatenated stator flux is used as the initial value for the next numerical integration step, and
f) is multiplied by a complex correction factor in order to calculate the present torque of the induction machine.
Thus, in this refinement of the method, a numerical integration method is used to determine the concatenated flux from current and voltage signals sampled at predetermined time intervals.
In one preferred refinement of the invention, the computation device includes a first computation unit for calculating and for storing the space vector of the stator voltage and the space vector of the stator current. It further includes a second computation unit for calculating the space vector of the concatenated stator flux by numerical integration from the stored space vectors of the stator voltage and of the stator current and from an initial value, which is stored in an initial value memory, for the space vector of the concatenated stator flux. A third computation unit is included, connected downstream therefrom, for multiplying the calculated space vector of the concatenated stator flux by a filter factor. Finally, a fourth computation unit is included for calculating the torque from the space vector of the stator current, the space vector of the corrected concatenated stator flux, the number of poles and a correction factor, with the filtered space vector of the concatenated stator flux being passed to the input of the initial value memory.
In a further advantageous refinement of the invention, a control device is provided for controlling the torque of the induction machine as a function of the actual value, which is present at the output of the fourth computation device, of the torque.
The invention is based on equation (1), which is also used, by way of example, in DE 195 32 477 as the basis for the motor control system disclosed there and which makes it possible to calculate the torque from the concatenating stator flux and the stator current:
m
=3/2
·p
·(&PSgr;−×
i
−
) (1)
m
Torque
p
Nu
Griepentrog Gerd
Runggaldier Diethard
Barlow John
Cherry Stephen J.
Harness & Dickey & Pierce P.L.C.
Siemens Aktiengesellschaft
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