The design of large machines for variable speed drives

The advantages of the squirrel cage induction motor are well documented and known to users; its only disadvantage is that it is fixed speed. In many applications this is a serious setback and has been the main reason for “hanging on” to DC machines with their higher first cost and maintenance costs. The author describes how this limitation has been overcome with the introduction of the AC variable speed drive, the most common being the voltage source inverter. Variable speed drives can reduce energy costs, improve process control, increase productivity and improve product quality. The inverter cost is usually several times that of the motor, which emphasises the importance of variable speed to many users, but also means that the induction motor designer has to be prepared to design around the shortfalls of the inverter, as the motor is the lower cost item in the drive