Variable Frequency Drive Systems

This chapter discusses the basic design and construction of variable frequency drives (VFD). VFD and variable speed drives (VSD) are used to control the speed and/or torque of induction and synchronous motors at the power generation facility when speed control or torque control is required for the application. The types of VFD technologies available are varied and each has its own unique advantages and disadvantages. While details of construction vary from manufacturer to manufacturer, the three main types of VFD are the voltage source inverter (VSI), the current source inverter (CSI) and the pulse width modulated inverter (PWM). Harmonic currents can have several adverse effects on the electrical distribution system feeding the drive. Some of the effects are the possible overheating of distribution equipment due to nonlinear current waveforms and increased copper and iron losses in motors and transformers.