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1、电气专业毕业设计外文翻译 外文翻译 English Control of Induction Machine Drives 11.1 Introduction 11.2 Scalar Induction Machine Control 11.3 Vector Control of Induction Machines 11.3.1 Vector Formulation of the Induction Machine 11.3.2 Induction Machine Dynamic Model 11.3.3 Field-Oriented Control of the Induction Mac
2、hine 11.1 Introduction Induction machines have become the staple for electromechanical energy conversion in todays industry; they are used more often than all other types of motors combined. Several factors have made them the machine of choice for industrial applications vs. DC machines, including t
3、heir ruggedness, reliability, and low maintenance . The cage-induction machine is simple to manufacture, with no rotor windings or commutator for external rotor connection. There are no brushes to replace because of wear, and no brush arcing to prevent the machine from being used in volatile environ
4、ments. The induction machine has a higher power density, greater maximum speed, and lower rotor inertia than the DC machine. The induction machine has one signicant disadvantage with regard to torque control as compared with the DC machine. The torque production of a given machine is related to the
5、cross-product of the stator and rotor ux-linkage vectors . If the rotor and stator ux linkages are held orthogonal to one another, the electrical torque of the machine can be controlled by adjusting either the rotor or stator ux-linkage and holding the other constant. The eld and armature windings i
6、n a DC machine are held orthogonal by a mechanical commutator, making torque control relatively simple. With an induction machine, the stator and rotor windings are not xed orthogonal to one another. The induction machine is singly excited, with the rotor eld induced by the stator eld, further compl
7、icating torque control. Until a few years ago, the induction machine was mainly used for constant-speed applications. With recent improvements in semiconductor technology and power electronics, the induction machine is seeing wider use in variable-speed applications . This chapter discusses how thes
8、e challenges related to the induction machine are overcome to effect torque and speed control comparable with that of the DC machine. The rst section involves what is termed volts-per-hertz, or scalar, control. This control method is derived from the steady-state machine model and is satisfactory fo
9、r many low-performance industrial and commercial applications. The rest of the chapter will present vector-controlled methods applied to the induction machine. These methods are aimed at bringing about independent control of the machine torque- and ux-producing stator currents. Developed using the d
10、ynamic machine model, vector-controlled induction machines exhibit far better dynamic performance than those with scalar control. 11.2 Scalar Induction Machine Control Induction machine scalar control is derived using the induction machine steady-state model shown in Fig. 11.1. The phasor form of th
11、e machine voltages and currents is indicated by capital letters. The stator series resistance and leakage reactance are R1andX1, respectively. The referred rotor series resistance and leakage reactance areR2andX2, respectively. The magnetizing reactance isXm; the core loss due to eddy currents and t
12、he hysteresis of the iron core is represented by the shunt resistanceRc.The machine slip s is dened as : s=we-wr wewhere weis the synchronous, or excitation frequency, and wr is the machine shaft speed, both in electrical radians-per-second. The power supplied to the machine shaft can be expressed a
13、s Pshaft=1-sR2i22 sSolving for i2 and using Eq. (6-2), the shaft torque can be expressed as Te=22 we(sR1+R2)+s2(X1+X2)3|Vin|2R2swhere the numeral 3 in the numerator is used to include the torque from all three phases. This expression makes clear that induction machine torque control is possible by v
14、arying the magnitude of the applied stator voltage. The normalized torque vs. slip curves for a typical induction machine corresponding to various stator voltage magnitudes are shown in Fig. 11.2. Speed control is accomplished by adjusting the input voltage until the machine torque for a given slip
15、matches the load torque. However, the developed torque decreases as the square of the input voltage, but the rotor current decreases linearly with the input voltage. This operation is inefcient and requires that the load torque decrease with decreasing machine speed to prevent overheating. In additi
16、on, the breakdown torque of the machine decreases as the square of the input voltage. Fans and pumps are appropriate loads for this type of speed control because the torque required to drive them varies linearly or quadratically with their speed. Linearization of Eq. (6-3) with respect to machine sl
17、ip yields 23|Vin|2s3|Vin|(we-wr)Te= weR2we2R2The characteristic torque curve can be shifted along the speed axis by changing e with the capability for developing rated torque throughout the entire speed range given a constant stator voltage magnitude. An inverter is needed to drive the induction mac
18、hine to implement frequency control. One remaining complication is the fact that the magnetizing reactance changes linearly with excitation frequency. Therefore, with constant input voltage, the input current increases as the input frequency decreases. In addition, the stator ux magnitude increases
19、as well, possibly saturating the machine. To prevent this from happening, the input voltage must be varied in proportion to the excitation frequency. From Eq. (11.4), if the input voltage and frequency are proportional with proportionality constant kf ,the electrical torque developed by the machine
20、can be expressed as and demonstrates that the torque response of the machine is uniform throughout the full speed range. Te=3k2fR2(we-wr) The block diagram for the scalar-controlled induction drive is shown in Fig. 11.3.The inverter DC-link voltage is obtained through rectication of the AC line volt
21、age. The drive uses a simple pulse-width- modulated (PWM) inverter whose time-average output voltages follow a reference-balanced three-phase set, the frequency and amplitude of which are provided by the speed controller. The drive shown here uses an active speed controller based on a proportional i
22、ntegral derivative (PID), or other type of controller. The input to the speed controller is the error between a user-specied reference speed and the shaft speed of the machine. An encoder or other speed-sensing device is required to ascertain the shaft speed. The drive can be operated in the open-lo
23、op conguration as well; however, the speed accuracy will be reduced signicantly. Practical scalar-controlled drives have additional functionality, some of which is added for the convenience of the user. In a practical drive, the relationship between the input voltage magnitude and frequency takes th
24、e form |Vin|=kfwe+Voffset where Voffest is a constant. The purpose of this offset voltage is to overcome the voltage drop created by the stator series resistance. The relationship (11.6) is usually a piecewise linear function with several breakpoints in a standard scalar-controlled drive. This allow
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