Control of Induction Machine Drives
In this chapter, attention is given to the control concepts that can be used to achieve independent torque and flux linkage control of induction machines over a wide speed range. Following the machine model inversion principles, the machine models introdu
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Control of Induction Machine Drives
In this chapter, attention is given to the control concepts that can be used to achieve independent torque and flux linkage control of induction machines over a wide speed range. Following the machine model inversion principles, the machine models introduced in the previous chapter will again be used for deriving suitable controller structures. First, classical V/f control concepts with very limited dynamic performance capabilities will be discussed to illustrate that these simple controllers, which are based on steady-state models, can become unstable. Next, field-oriented control techniques will be discussed which build on the fundamental model given in Sect. 4.2.2. A key role is assigned to the presentation of a universal field-oriented (UFO) model which is used in conjunction with a current source induction machine model. At a later stage, this model is replaced with a voltage source induction model which is connected to a synchronous current-controlled converter. Finally, attention is given to the operational aspects of the drive which takes into account the maximum supply voltage and current constraints. In this context field weakening techniques are also discussed. A set of tutorials is again provided to further familiarize the reader with the concepts presented in this chapter.
9.1 Voltage-to-Frequency (V/f) Control In low-performance adjustable speed drives the issue of controlling torque or speed of a mechanical load connected to an induction motor is not critical in terms of required dynamic performance. For these applications, the classical voltage-tofrequency (V/f) control can be used resulting in a very simple drive control which can easily be implemented in analog hardware. It was therefore extensively used in inverter controllers before digital hardware was available. The key concepts of the
© Springer Nature Switzerland AG 2020 R. W. De Doncker et al., Advanced Electrical Drives, Power Systems, https://doi.org/10.1007/978-3-030-48977-9_9
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9 Control of Induction Machine Drives
V/f control are based on the steady-state model of the induction machine and are presented in the following. The drive structure in question does not utilize closed loop current control as discussed in Chap. 3. Instead, only the stator voltage uˆ s and the stator frequency ωs are controlled in a way that ψs = const.
(9.1)
The lack of a current control in V/f control is a key issue which distinguishes this low-dynamic drive concept from, for example, the dynamic field-oriented control concepts to be discussed in the subsequent section of this chapter. Furthermore, it will be shown in this section that the omission of current control can in some applications lead to instability. The ensuing discussion is undertaken for a two-pole machine to facilitate the readability of the figures and equations. Steady-State Equations for V/f Control Prior to examining the algorithms and generic models which may be invoked for this type of control it is helpful to consider the steady-state re
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