Introduction
AC Induction motors are the most widely used motors in industrial motion control systems, as well as in home appliances thanks to their reliability, robustness and simplicity of control.

Until a few years ago the AC motor could either be plugged directly into the mains supply or controlled by means of the well-known scalar V/f method. When power is supplied to an induction motor at the recommended specifications, it runs at its rated speed. With this method, even simple speed variation is impossible and its system integration is highly dependent on the motor design (starting torque vs maximum torque, torque vs inertia, number of pole pairs). However many applications need variable speed operation. The scalar V/f method is able to provide speed variation but does not handle transient condition control and is valid only during a steady state. This method is most suitable for applications without position control requirements or the need for high accuracy of speed control and leads to over-currents and over-heating, which necessitate a drive which is then oversized and no longer cost effective. Examples of these applications include heating, air conditioning, fans and blowers