Overview

Speed control of induction motors by means of variable-voltage, constant-frequency and variable-voltage, variable-frequency drives was introduced in Chapter 6. By far, the majority of variable-speed drives for industrial process control uses the latter method. Traditionally, dc drives have been used, but recent technological advancements are making ac drives (particularly induction motor drives) more popular. AC machines are generally cheaper, smaller, and more rugged and reliable with the absence of commutators and brushes, but the converter costs may be somewhat higher. Overall, ac drives are much superior to dc drives, and it appears that eventually dc drives will be totally obsolete. One of the motivations for using variable-speed drives in flow control is the energy savings, which will be described later. Higher frequency operation of machines through an inverter permits machine size reductions because of the reduced volume of magnetics, but the loss density becomes higher. In modern factories, variable-frequency drives with high performance control permit high-volume, automated production with superior product quality. Even for constant-speed applications, the variable-frequency converter can permit soft starting. Variable-frequency soft starting has...