Problems of actual-value measurement and vector orientation
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This chapter aims to explain principles of the actual-value measurement, to highlight its problems and to answer some related questions of the field orientation. The current measurement technique influences decisively the controller design and thus also the dynamic characteristic of the inner current control loop, which in turn is the prerequisite for the superimposed speed control. So the actual-value measurement is an important interface of every drive control system which must be taken into account very carefully for the controller design. Similarly, for the design of the speed control loop the speed measuring is an important issue to consider. Either an incremental encoder or a resolver can be used to measure the speed. Also the alternative possibility of sensorless capture of the speed will be discussed, and possible ways to solve this problem will be shown. The second problem of this chapter is the field orientation, which is very closely connected to the speed measurement. Field orientation means namely, 1. that the field angle -s and respectively the location of the field coordinate system (dq- coordinates) must be calculated, and 2. that the un-measurable rotor flux, which will be used for calculating the rotor frequency or the slip and therefore also the field angle -s, has to be estimated. The estimated value of the rotor flux can be used as actual value in the flux control loop, which is – for example – of decisive importance for field-weakening operation. The estimation of the rotor flux, which can be realized either by flux models or by flux observers, and the calculation of the field angle require actual values of current and speed.
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Problems of actual-value measurement and vector orientation
4.1 Acquisition of the current The measurement of the currents can be performed as shown in the figure 1.3. Depending on the coordinate system the inner current control loop is realized in – field synchronous or stator-fixed – actual values isD, isE or isd, isq are obtained after the transformation of the measured phase currents isu and isv. What could not be indicated in this figure are: 1. The technical realization of the measurement and 2. the fact, that for the current control only the instantaneous value of the fundamental wave is relevant. From the technical view, two possibilities to measure the currents exist: 1. The most advanced technique is the measurement of instantaneous values using A/D converters (ADC: Analog to Digital Converter) and 2. The integrating measurement using V/f converters (VFC: Voltage to Frequency Converter). a) Measurement of instantaneous values using an ADC This method is frequently applied because of the simplicity of its technical realization and the possibility of a high resolution. The inherent current harmonics have to be suppressed, for example by an additional filter. This however, would result in an additional delay of the measured values. This delay is unwanted, and therefore has to be avoided if possible, to maintain the dynamics of the current control loop, part
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