Читать книгу Analysis and Control of Electric Drives - Ned Mohan - Страница 23

The block diagram of Fig. 1-1b points to various fields, which are essential to electric drives: electric machine theory, power electronics, analog and digital control theory, real‐time application of digital controllers, mechanical system modeling, and interaction with electric power systems. A brief description of each of the fields is provided in the following subsections.

1 Theory of electric machinesFor achieving the desired motion, it is necessary to control electric motors appropriately. This requires a thorough understanding of the operating principles of various commonly used motors, such as dc, synchronous, induction, and stepper motors. The emphasis in an electric drives course needs to be different from that in traditional electric machines courses, which are oriented toward the design and application of line‐fed machines.

2 Power electronicsIn Fig. 1-1b, voltages and currents from a fixed form (in frequency and magnitude) are converted to the adjustable form best suited to the motor. It is important that the conversion takes place at a high‐energy efficiency, which is realized by operating power semiconductor devices like switches.Today, power electronics is being simplified using “Smart Power” devices, where power semiconductor switches are integrated with their protection and gate‐drive circuits into a single module. Thus, the logic‐level signals (such as those supplied by a digital signal processor) can directly control high‐power switches in the converter. Such power‐integrated modules are available with voltage handling capability approaching 4 kV and current handling capability above 1000 A. Paralleling such modules allows even higher current handling capabilities. The progress in this field has made a dramatic impact on PPUs by reducing their size and weight, while substantially increasing the number of functions that can be performed. Recently, there has been a quiet revolution where transistors based on wide bandgap materials such as SiC and GaN are commercialized. These devices have many superior characteristics compared to original Si‐based devices, thus increasing the efficiency of power converters and reducing the system cost.

3 Control theoryIn the majority of applications, the speed and position of drives need not be controlled precisely. However, there is an increasing number of applications, for example, in robotics for automated factories, where accurate control of torque, speed, and position is essential. Such control is accomplished by feeding back the measured quantities, and by comparing them with their desired values, in order to achieve a fast and accurate control. In most motion control applications, it is sufficient to use a simple proportional‐integral (PI) control, as discussed in this book. The task of designing and analyzing PI‐type controllers is made easy due to the availability of powerful simulation tools.

4  Real‐time control using DSPsAll modern electric drives use microprocessors and digital signal processors (DSPs) for flexibility of control, fault diagnosis, and communication with the host computer and other process computers. The use of 8‐bit microprocessors is being replaced by 16‐bit and even 32‐bit microprocessors. DSPs are used for real‐time control in applications which demand high performance or where a slight gain in the system efficiency more than pays for the additional cost of sophisticated control.

5 Mechanical system modelingSpecifications of electric drives depend on the torque and speed requirements of the mechanical loads. Therefore, it is often necessary to model mechanical loads. Rather than considering the mechanical load and the electric drive as two separate subsystems, it is preferable to consider them together in the design process. This design philosophy is at the heart of Mechatronics.

6 SensorsAs shown in the block diagram of electric drives in Fig. 1-1b, voltage, current, speed, and position measurements may be required. For thermal protection, the temperature needs to be sensed.

7 Interactions of drives with the utility gridUnlike line‐fed electric motors, electric motors in drives are supplied through a power electronic interface (see Fig. 1-1b). Therefore, unless corrective action is taken, electric drives draw currents from the utility that are distorted (non‐sinusoidal) in their wave shape. This distortion in line currents interferes with the utility system, degrading its power quality by distorting the utility voltages. Available technical solutions make the drive interaction with the utility harmonious, even more so than line‐fed motors. The sensitivity of drives to power system disturbances such as sags swells and transient overvoltages should also be considered. Again, solutions are available to reduce or eliminate the effects of these disturbances.