Parameter adaptation of a sensorless controlled induction machine by measurement of the slip frequency

In this paper a method is depicted, that enables a highly precise speed control of an induction machine, designed as squirrel cage rotor, without use of a sensor. The speed of an induction machine depends on the rotary field assigned as well as on the load of the machine. In general, the load and with it the slip is unknown. So, the fundamental wave model does not reveal any information about the exact speed. It is impossible to find this information by observing the machine's ideal equivalent circuit. This leads to the fact that, as a rule, in an industrial environment sensorless controlled induction machines do not gain more than 0.5 % speed precision in relation to the rated operating point [1]. In case of need for more precise speed control, the use of a sensor has been essential up to now. As an alternative to the common sensor, this paper is showing a method by which the exact slip frequency and thus the exact speed of the induction machine can be detected by evaluation of anisotropies. This approach is making use of rotor anisotropies. Such anisotropies enable the measuring of the slip frequency, being the base of highly precise speed assessment. The slip frequency as well as update rate of the slip frequency measurement are low in the chosen analysis method. In order to prevent a reduction of the dynamics of the speed control by a low update rate of the slip frequency, the measured value of the rotor slip is not used directly in the evaluation of the rotation speed actual value. The measured value is used advantageously in adapting the model parameter rotor resistance to the reality. Based on this, a highly precise speed control can be ensured without any reductions in dynamics.

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