Offline Parameter Identification of an Induction Machine Supplied by Impressed Stator Voltages

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1 POSTER 2016, PRAGUE MAY 24 1 Offline Parameter Identification of an Induction Machine Supplied by Impressed Stator Voltages Tomáš KOŠŤÁL Dept. of Electric Drives and Traction, Czech Technical University, Technická 2, Praha, Czech Republic kostatom@fel.cvut.cz Abstract. Field oriented control (FOC) became nowadays probably the most popular control method of electric drives with induction machines. For it's proper operation, knowledge of certain machine parameters is needed, otherwise the performance of the control can be significantly lowered. Thus there is a demand for methods of parameter identification methods that can observe required parameters of the machine before the start of the drive. This article concerns an offline parameter identification method that is suitable for on-site drive commissioning, that is nowadays very often needed, as drives and converter may come from different producers. This method does not need any special equipment so it could be realized in industrial applications with no additional cost. Keywords Offline parameter identification, on-site methods, self-commissioning methods, induction machine, impressed stator voltages, converter. 1. Introduction Field oriented control (FOC) is nowadays a very popular control method for electric drives with induction machines. For it's proper and robust operation, it requires the knowledge of certain parameters of the induction machine. If the incorrect parameter values are deployed, it can lead to decrease of efficiency, or dynamic properties or even total detune of the drive operation. Parameters of the induction machine can vary with temperature, frequency and saturation and these changes should be respected. Fig 1 shows the situation of rotor flux oriented FOC if the controller of the drive uses different parameter values than the actual ones in the machine. The rotor flux assumed by the controller (d*-q*) differs from the actual one (d-q) which leads to a loss of flux and torque control. This example shows that the parameters of the machine have to be obtained before operation of the drive starts. These can be obtained either in a non-experimental way from the producer or by searching in other information Fig. 1. Difference between actual rotor flux (d-q) and rotor flux estimated by controller (d*-q*) [3] sources. It should be mentioned, that parameters of the same type of the machine can vary up to % from the manufacturing process [1],[2], so if the manufacturer provided these data, it would imply that every single unit would need to be tested. Searched parameters usually involve a stator resistance R s, a rotor resistance R r, a stator leakage inductance X sσ, a rotor leakage inductance X rσ and a magnetizing inductance X m. Further, iron losses represented by R Fe can be demanded as well as a magnetizing curve. From these parameters, only stator resistance R s is directly measurable. 1.1 Classification of parameter identification methods Because of those facts, only experimental methods are viable in most of cases. These can be divided into following groups [3],[4],[5]: conventional methods (no-load, locked rotor and conventional DC test) on-site methods offline parameter identification methods self-commissioning methods commissioning methods online parameter identification methods

2 2 T. KOŠŤÁL, OFFLINE PARAMETER IDENTIFICATION OF AN INDUCTION MACHINE SUPPLIED BY IMPRESSED STATOR VOLTAGES It should be noted, that some authors use different classification (e.g. [6]), but in general they follow an idea of distinguishing methods suitable for parameter identification before and after the drive is put into the operation. In general it can be said, that all of the authors generally follow the basic idea of distinguishing between methods suitable for parameter identification before the drive is put into the operation, and during the operation. In this work, the above described classification will be maintained. Now let's give a short description of the presented groups Conventional methods Under the term conventional methods, we usually understand laboratory tests like no-load, locked rotor and conventional DC test. They can be characterized by usage of a sinusoidal voltage sources and special-purpose jigs (e.g. for blocking the rotor of a machine). They are widely used for development and research as well as type tests of manufacturers, but they are not very suitable for commercial usage because of material, time and personnel costs On-site methods On-site methods are thus a response to the issues of conventional methods that makes them unfavourable for an in-field application. The main characteristic could be named as minimizing requirements of any additional devices such as measurement or control and simplicity of operation with still giving results with suitable accuracy. They are divided into two subcathegories, the offline and online parameter identification Offline methods Offline parameter identification methods are used to obtain required values before the drive is put into the operation during installation and commissioning or after a shutdown. These could be further divided into selfcommissioning methods that should be able to gather the required parameters at standstill. This permits a situation, that the load is already connected to the drive but it is not desired to move with it before good operation of the drive can be assured. The commissioning methods allows the drive to rotate and some sources points out a greater mathematical difficulty of these methods [7]. The difference between the names of these two groups correspond with their possible field of usage. The selfcommissioning methods can be applied without any special operational intervention so they can be used after the whole machinery is assembled or on an existing installation. The commissioning methods needs the operators to disconnect the drive from the load or to ensure, that moving the load will not cause any problems Online methods Online methods are used for identification of parameters during the operation of the drive. This should ensure, mainly in case of drives with high demands on its dynamic properties, that the controller respects parameter changes. Several methods have been developed based on injected test signal, mathematical observers or Model Reference Adaptive System. These methods serves as well for increasing accuracy of parameter values obtained by offline methods but on the other hand, they consumes some resources of the controller. This is not a big problem in case of current controllers that boasts with large computational power at low cost, but it can be an obsatcle in case of some older existing hardware where parameter identification is introduced additionally. This paper concerns an offline parameter identification method that does not requires to rotate with the drive and thus can be classified as suitable for selfcommissioning. 2. Description of the monitoring procedure It is very common to work in case of field oriented control with a so called inverse Γ equivalent scheme of an induction machine that is shown in Fig. 2. Fig. 2. Inverse Γ equivalent circuit of a squirrel cage induction machine axis α at standstill. This equivalent circuit can be described by an equation u sα =R s d dt Ψ rαref (1) where = L μ (2) is the rotor time constant, Ψ rαref = L μ i μα is the referred value of the rotor flux linkage space phasor. Inductance L μ can be from the T equivalent scheme values expressed as L μ = L rσ (3) Inductance is from the T equivalent scheme values defined as = L sσ L L rσ (4) rσ

3 POSTER 2016, PRAGUE MAY 24 3 where inductances, L sσ, L rσ are inductances in a common marking of the T equivalent circuit, e.g. in [6]. Similarly is defined as follows 2 L rσ R r (5) = where R r is the rotor resistance of the T scheme. 2.1 Monitoring of inductance Stator current increases linearly if a constant voltage u sα = U sα (in this case impressed by the inverter) is applied and if starting conditions are = 0 and Ψ rαref =0 the equation for inductance can be written as = U sα d d t Method based on idea presented in [6] uses voltage waveform as shown in Fig. 3. (6) The same could be done also with the values between time instants t 1 and t 2, but the chosen interval is longer as the current difference is higher and thus it is ensuring a higher accuracy. However, the resulting inductance is smaller than the real one because the procedure assumes ideal conditions like neglecting the effects of magnetic saturation. Accuracy can be increased by carrying measurements for all three combinations of stator terminals. 2.2 Monitoring of stator resistance R s In this case, current is to be maintained constant as shown in Fig. 4. The inverter, that is usually a voltage source is controlled so that it behaves as a current source which can be achieved by software. The test can fluently follow after previously described one. Time intervals shown here are much longer than those in previous cases and can last for units of seconds compared to milliseconds in previous case. Fig. 3. Terminal voltage and current variation during monitoring of. Only two terminals of the machine are connected which means the voltage U sα is equal to one half of the DC link voltage of the inverter. Timeflow of the test is shown in Fig. 3. Voltage is applied by the inverter at time t 1 which leads in the increasing of current. This increasing should be terminated whet the current reaches the peak rated value (I sn 2) by short-circuiting of the motor terminals by the inverter. The the current is let to decrease to the value I 2 = ½ I 1. At this point, the decreasing is accelerated by applying a negative voltage between time instants t 3 and t 4. Again the current is let to reach its peak value(i sn 2) of the opposite polarity and from time t 4 it is let to decrease naturally. From this test, the inductance, can be expressed according to (6) as: Fig. 4. Terminal voltage and current variation during monitoring. The value of the DC current I 2, that is impressed by the inverter should be maintained on approximately one third of the rated current I sn to prevent the effects of magnetic saturation that could influence the test [6]. The voltage has to sharply increase to a peak value from which it has to decline slowly. A steady-state value of the voltage is reached in time t 6. In this time instant, the controller is adjusted so that the current has an opposite polarity. The voltage reaches a steady-state in time instant t 7. At this moment, opposite polarity of the current is applied again, but now with the amplitude equal to a peak rated value (I 3 = I sn 2). Voltages are sampled (better to be said that their calculated values are recoded for the particular time instant as there is no measurement of voltage) in time instants t 6, t 7 and t 8 which means steady-state values are sampled. In a steady-state, the equation (1) can be simplified into a form u sα =R s I sα. (8) t =U sα 4 t 3 I sα t 3 I sα t 4 (7)

4 4 T. KOŠŤÁL, OFFLINE PARAMETER IDENTIFICATION OF AN INDUCTION MACHINE SUPPLIED BY IMPRESSED STATOR VOLTAGES When the small voltage drop across the switching devices of the inverter is neglected, the value of the stator resistance can be calculated as an average from the three values as: inductance was estimated as = 15,5 mh which is smaller than the value obtained as expected (see chapter 2.1). The difference is about 2,5 %. R s = 1 3 u sα t 6 u sα t 7 u sα (9) t 6 t Monitoring of rotor time constant We choose time interval t 6 t 7 (Fig. 4). The referred value of the rotor flux Ψ rαref changes exponentially and the time constant of this change is equal to the rotor time constant. We can express it by following relation: t = 7 ln u R I sαt 6 s 2 2 u sαt 7 R s I (10) Fig. 5. Terminal voltage and current variation during first part of the test. 2.4 Monitoring of referred rotor resistance Data of the previously described test can be used also for calculation of referred value of the rotor resistance. Considering equations (1) and (5) we can express the initial value of the of the rotor resistance at the time instant t 7 as U sα t 7 =R s t 7 so the.can be obtained as = U sα t 7 R s t 7 (11) (12) Knowledge of the stator resistance R s is based on (9). 3. Simulation results For simulation of the method, Matlab Simulink environment have been used. Parameters of a real machine that were obtained by conventional methods were used. This machine was a 3,5 kw type 1AY112L-6 from EM Brno. Measured and calculated data from the conventional tests were: c = 15,9 mh c = 989 mω L μc = 97,8 mh R sc = 1,11 Ω Fig. 5 shows the waveforms of terminal voltage between two terminals and corresponding current response. Values in the charts are in per unit system which is common in modeling of such systems. However the resulting parameter values are shown in absolute for better illustrative nature. From the test, the value of the Fig. 6. Terminal voltage and current variation during second part of the test. Results from the second part of the test were as follows: R s = 1,08 Ω; L μ = 95,4 mh and = 968 mω. All those results differ no more than 4% from the values obtained by conventional method. 4. Conclusion This paper concerns an offline parameter identification method for estimating parameters for a mathematical model of an induction machine that is required for modern was of controlling such machines. Induction machines are nowadays the most common types of devices for transforming electrical to mechanical energy and thus there is a high demand for robust and efective ways of control. Field oriented control (FOC) is one of the most common one. As mentioned before, this control method need a mathematical model of the machine for

5 POSTER 2016, PRAGUE MAY 24 5 which it needs some certain parameters. Usually, so called inverse Γ equivalent scheme is considered for the purposes of such a model which needs following parameters: inductances and L μ and resistances R s and. From these, only the stator resistance R s is directly measurable, so the other parameters have to be estimated by indirect methods. The paper gives an overview of classification of different types of methods used parameter identification and then presents an offline method that does not requires to rotate with the drive and thus can be classified as suitable for self-commissioning. Objectives of such a method are that should be able to gather the required parameters at standstill and that it should not require any additional measurement devices or special purpose jigs. The presented method have been simulated and the obtained results seem to be enough accurate. The next step should be the laboratory verification of the method with a standard three phase two level inverter. It could be expected that that accuracy of measurement will be a cornerstone. About Author... Tomáš KOŠŤÁL is a graduate student at the Dept. of Electric Drives and Traction since In his bachelor studies, he studied the Heavy Current Engineering program and continued in his master studies with program Electrical Engineering, Power Engineering and Management. He graduated in 2014 and continues with his doctoral studies at the same department. Currently he is focusing on digital control of semiconductor converters and parameter identification of induction machines. References [1] MICHALIK, W. Parameter estimation methods at three-phase induction machines. In: 2005 European Conference on Power Electronics and Applications. IEEE, 2005, s. 10 pp.-p.10 [cit ]. DOI: /EPE ISBN [2] MICHALIK, W. Anwendung modemer Verfahren zur Parameterbestimmung an Asynchmnmaschinen. Habilitationsschrift TU Dresden [3] TOLIYAT, H.A., LEVI, E., RAINA, M. A review of RFO induction motor parameter estimation techniques. IEEE Transactions on Energy Conversion. 2003, 18(2): DOI: /TEC ISSN [4] How Online Estimation Differs from Offline Estimation. Mathworks Documentation [online]. [cit ]. Available on: requesteddomain= [5] SCHIERLING, H. Self-commissioning A novel feature of modern inverter-fed induction motor drives. In Proceedings of Third International Conference on Power Electronics and Variable-Speed Drives, 1988, pp [online] [cit ]. Available on: [6] VAS, P. Parameter estimation, condition monitoring, and diagnosis of electrical machines. New York: Oxford University Press, 1993, xviii, 360 p. ISBN [7] TOLIYAT, H.A., LEVI, E., RAINA, M. A review of RFO induction motor parameter estimation techniques. IEEE Transactions on Energy Conversion. 2003, 18(2): DOI: /TEC ISSN [8] KOŠŤÁL, T. Overview of parameter identification methods of induction machines. Technical study. Praha, CTU in Prague.