This chapter covers fundamental of electronic variable speed drives construction and working of DC motors &AC induction motors

Any variable speed electrical drive system comprises of the following components:

• An electronic actuator - the controller.

• A driving electrical machines - motor.

• A driven machine (load) - pump, fan, blower, compressor…

The task of a variable speed electrical drive is to convert the electrical power supplied by the mains into mechanical power with a minimum loss. To achieve an optimum technological process, the drive must be variable in speed. This will steplessly adjust the speed of the driven machine. This is ensured by the low loss control using solid state Technology in electronic controllers. The controllers are connected to mains supply and the electrical machine as shown in figure

The solid - state devices, which convert the AC supply to DC supply were first used as variable speed devices, in DC technology. Using these devices the armature voltage of a DC motor and therefore the speed can be adjusted, almost without losses and over a wide range of speed . Using these features the drive can be designed which start smoothly and jerk-free. This helps to maintain the desired selected speed, independently of the load and operate with good dynamic response

The DC drive needs special consideration in some applications. For example in hazardous atmosphere, vibrations and higher speeds the usage of AC motor with squirrel-Cage rotor is advantageous. The use of frequency inverters (VFD's) to supply to AC Motors resulted in a new orientation of electrical power for handling variable speeds operation is shown in figure

Every standard AC motor can be fitted with a variable speed drive using a frequency inverter. Frequency and voltage of the single - phase or three - phase mains are varied by the frequency inverter, such that the motor can be operated with varying speeds over large range settings. The operating mode of any motor connected to these variable speed drives can be classified in Four Quadrants, depending upon the Torque and Speed of the drive in figure above