Variable speed drives are being increasingly used to operate high speed compressors and pumps as a result of the high efficiency and reliability that they offer in electric motors used in variable frequency drives various excitation forces exist due to the presence of mechanical and electromagnetic forces electromagnetic forces depend very strongly on the air gap between the rotor and the . Mechanical variable speed drives which have the motor operating at full speed can increase or decrease speed but cannot provide the energy savings and efficiencies of todays electronic controls mechanical drives are typically 70 80 efficient while an ac inverter and motor combination can be 85 90 efficient in certain applications. Logically structured electronic variable speed drives 3e begins with fundamental drive circuits and builds to an understanding of more complex drive circuits for improved learning troubleshooting and maintenance are emphasized throughout and abundant waveform schematics help guide the readers understanding of the theory of operation of the different types of drives and their circuitry . Variable speed drives may be either electric hydraulic mechanical or even electronic they control the speed at which a motor operates in a precise and constant manner otherwise the machines that use these mechanisms and motors would work as per their own characteristics and power supply which would be highly ineffective variable speed drives are devices used in electromechanical drives