32.1.1 Introduction

In every industry there are industrial processes of some form, which require adjustments either for normal operation or optimum performance. Such adjustments are usually accomplished with a variable speed drive (VSD) system. They are an important part of automation. They help to optimize the process, reduce investment costs, energy consumption, and energy cost.

There are three basic types of VSD systems: electrical drives, hydraulic drives and finally mechanical drives. This chapter focuses mainly on electrical drives.

A typical electric VSD system consists of three basic components. The electric motor, the power converter, and the control system, as illustrated in Fig. 32.1. The electric motor is connected directly or indirectly (through gears) to the load. The power converter controls the power flow from an AC supply (often via a supply transformer), to the motor by appropriate control of power semiconductor switches (part of the power converter).

Figure 32.1: VSD schematic diagram

With recent advances of power semiconductor and converter topologies, electric variable speed drives are witnessing a revolution in applications including computer peripheral drives, machine tools and robotic drives, test benches, fan pumps and compressors, paper mill drives, automation, traction and ship propulsion, cement mill and rolling mill drives.

For a proper control system, the VSD system variables, both mechanical and electrical, are required for control and protection. Signals are usually derived from sensors, whose outputs are very much dependent on the control strategy...