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1、1,Chapter 9,Kouhei Ohnishi,Nobuyuki Matsui,Yoichi Hori,Estimation,Identification,and Sensorless Control in AC Drives,2,AC drives became more and more economical and popular.,Since the early years of the 20th century,electric motors for variable speed drives have been widely applied in large-capacity
2、 applicationsthe steel industry the automobile industryIn the early stage,DC motors were widely used for adjustable speed control Since the late 1960s,AC motors have been replacing DC motors in a wide area of industry applications Since AC drives required more complicated controllers in the beginnin
3、g,they were not so economically feasibleAllied to advances both in digital control technology and power semiconductor devices,AC drives became more and more economical and popular.In almost all areas,DC drives are now replaceable with AC drives,3,However,there still exist some areas which are not su
4、itable for AC drive applications.One of these is the area which requires precise torque control.For instance,injection machines.AC motors sometimes generate torque error or torque pulsation due to some parameter variations.To overcome such problems,more sophisticated techniques are necessary in the
5、controller.These techniques are based on estimation or identification of motor parameters Employ the recent developments in digital control,including high-speed digital signal processors(DSPs)and parallel processing,To overcome torque problems.,4,9.2 PARAMETER ESTIMATION IN AC DRIVES,9.2.1.Parameter
6、 Identification In Brushless Motors 9.2.2.Parameter Identification In Induction Motors,5,The control scheme of brushless motors with trapezoidal flux distributions(BLDM,brushless DC motor)is relatively simple.Usually it does not need parameter identification.For brushless motors with sinusoidal flux
7、 distribution(PMSM,permanent magnet synchronous motor),generally identification of the parameters is necessary for precise control.(3 electric parameters)Armature resistance,armature inductance,and EMF coefficient.,Parameters of Brushless Motors,6,Two effective approaches are presented here:self-tun
8、ing regulator(STR)which has a tuning ability to make output-error zero inside the controller;model reference adaptive system(MRAS)which has a referred model in the controller.Direct applications of STR and MRAS to parameter identification do not always lead to successful results,because of the limit
9、ation of the processing time of the controller CPU.Since identification should be performed in parallel with current and speed control,it is essential to reduce the processing time for identification by a simple algorithm.,7,STR-Based Parameter Identifier,At steady-state,the PMSM has the simple equi
10、valent circuit just like a DC motor.Terminal voltage,line current,and armature resistance are measured to identify the circuit parameters.Figure 9-1 shows an experimental evaluation of the influence of such parameter variations in armature current error at steady state.A current-regulated voltage so
11、urce inverter supplies almost sinusoidal current.In the figure,the ordinate is the current control error due to the parameter variation,8,the parameter variation coefficient is defined as,9,10,since the estimation of the armature inductance uses division and the relatively small d-axis voltage,the e
12、rror is a little larger compared to that of the EMF coefficient.,11,MRAS-Based Parameter Identifier,Figure 9-4 displays a MRAS-based identification approach,which includes a voltage-based motor model as a reference model.,The input of the identifier is a current difference between model and actual m
13、otor.The current difference is decomposed into two elements,from which the armature inductance and the EMF coefficient are identified.,12,the experimental results of identification of armature inductance and EMF coefficient for a tested motor.,13,the experimental estimation error,14,Application of P
14、arameter Identification to Torque Control,An interesting application example for the parameter identification of the brushless motor is a torque sensorless torque control.Fig(a),the conventional current-based torque control system.The torque reference is divided by a torque constant(=EMF coefficient
15、 in SI unit)to generate a current reference.,15,16,Parameters of Induction Motors,Basically induction motor in steady state is represented by the equivalent circuit in Figure 9-7.The classical no-load test,locked rotor test,and electrical quantity measurement test give identified parameters in Figur
16、e 9-7.,17,A sample of flowchart is shown in Figure 9-8 4.,18,9.3 Sensorless Drives of AC Motors,19,sensorless drives of AC motors,basically,the vector-controlled AC motors require speed or position sensorsthese sensors bring several disadvantages drive cost reliability machine size noise immunityit
17、is necessary to achieve the precise control of torque and speed without using position and speed sensors(so-called sensorless drives of AC motors),20,9.3.1 Sensorless Drives of Brushless Motors,As stated,there are two kinds of brushless motors:the motor with a trapezoidal flux distribution and that
18、with a sinusoidal flux distribution.The approaches to sensorless drive of the brushless motor vary,depending on the rotor flux distribution.the motor with a trapezoidal flux distributionThe first one provides an attractive candidate,because two of the three stator windings are excited at a time.As a
19、 result,the unexcited winding can be used as a sensor 10,11;that is,the speed EMF induced in the unexcited winding is used to determine the rotor position and speed.the motor with a sinusoidal flux distributionexcites three windings at a time and the sensorless control algorithm becomes complicated.
20、,21,Since the actual rotor position is not known without a position sensor,the aim is to make the angular difference between the fictitious and actual rotor positions converge to zero.,the motor with a sinusoidal flux distribution,the d-q axis corresponds to an actual rotor position the-axis is a fi
21、ctitious rotor position,22,two approaches,Both are the estimation of the angular difference by using the detected state variables and the estimated state variables which are obtained from a motor model in the controller.The approaches differ according to the motor model,i.e.Voltage model-based drive
22、 12Current model-based drive 13These two are basically the model-based control and generally require on-line identification of the motor parameters if higher performance is required.However,it is interesting to note that the second method has robust control characteristics against the motor paramete
23、r variation.,23,the voltage model-based sensorless drive,the voltage equation is,the voltage equation under the ideal condition that the fictitious and actual axes are coincident is,24,the angular difference can be made to converge to zero by the following rule if vr0,then decreasesif vr 0,then incr
24、eases(for clockwise rotation),Taking a difference between-axis voltage assuming is small,25,(whereT is a sampling period and is the motor speed of the model),then,the current model-based sensorless drive,The current model is,the current difference is,26,torque-speed characteristics under a current m
25、odel-based algorithm.,The motor rating is 1.2(kW),6-poles,1200(rpm),98(kgf cm).The maximum speed is 1500(rpm),the minimum speed is 60(rpm),and a steady-state maximum speed error is within 0.4%.,27,9.3.2 Sensorless Drives of Vector Controlled Induction Motors,the basic idea is estimation of speed by
26、using applied voltage,line current,and frequency Slip frequency control approach field orientation control approach,28,Slip frequency control approach,29,field orientation control approach,not only the speed but also the rotor flux are simultaneously estimated for the sensorless drive in a wide spee
27、d range.The stator equation is used for correction of adjustable current reference model.The speed is estimated by a kind of error of flux components which is derived from Popovs stability criteria.,30,The rotor flux observer based on the motor voltage model estimates the rotor flux by using the sta
28、tor current transformed to the stationary coordinates(-axis)systemthe vector control is carried out on the basis of the estimated rotor flux the motor speed is directly estimated through MRAS by using the flux and the stator current the estimated speed is used to adjust the motor model,31,Figure 9-1
29、6,an example of the sensorless control characteristics of 2.2(kW),4-poles induction motor.The maximum speed is 2400(rpm)under the field-weakening control and the minimum speed is 15(rpm).It is noted here that the speed control accuracy is 0.4%under the tuned condition;however,it is 1.4%under the detuned condition.Like the conventional vector control with sensors,the identification of the rotor resistance is important and difficult.,32,End of Chap9,
