Читать книгу Analysis and Control of Electric Drives - Ned Mohan - Страница 13

Electric machines and electric drives are shown by their block diagrams in Fig. 1-1a and b. Electric machines were invented more than 150 years ago and have been in use ever since in increasing numbers in a variety of applications. As shown in Fig. 1-1a, electric machines convert energy from the electrical system to the mechanical system, and vice versa. In their motoring mode, where the machine is called a motor, the electric power P_elect from the electrical system at the certain voltage/current magnitude and frequency get converted to the mechanical power P_mech to the mechanical system at corresponding torque and speed. The opposite is true for a machine in its generator mode, where power from the mechanical system gets converted and is supplied to the electrical system. In machines, as shown in Fig. 1-1a, some of the quantities (voltage/current, torque/speed) are dictated by external sources, and no attempt is made to control the others.

Fig. 1-1 Block diagrams of (a) electric machines and (b) electric drives (motoring mode shown).

However, in certain applications, it is required that for given quantities on the electrical or the mechanical side, the other quantities be controlled, as in a wind turbine. This is made possible in electric drives shown by their block diagram in Fig. 1-1b. It should be noted that in the literature and in trade publications, electric drives sometimes refer only to the power electronic converter and its control, excluding the motor. In this textbook, however, electric drives refer to the entire block, which is shown dotted in Fig. 1-1b, that includes power electronic converter (power processing unit – PPU) and its control, as well as the electric machine, whether it is in its motoring or the generating mode. We should also note that we will be looking at only the ac machines, hence the title of the book is ac drives.