15. Variable Frequency Voltage Supply of Rotating Field Machines

Traditional rotating field machines have no (for synchronous motors) or poor and limited (for induction motors) possibilities for variable speed operation when power is supplied from a fixed frequency source. In this chapter, we will discuss the open-loop variable frequency voltage supply of rotating field machines. These kinds of drives are commonly denominated as V/f drives, because both the voltage and the frequency are controlled together in open loop, so as to control both the speed and the flux. The aim of the open-loop variable frequency operation of rotating field machines is to obtain a cheaper and more reliable variable speed drive than the expensive and high-maintenance DC drive. Drives that are even more demanding in terms of speed, position or torque control will be discussed in later chapters.