Today, industries are increasing demanding process automation in all sector. Automation results in better quality, increased production and reduced costs. The varaible speed drives, which steplessly control speed of a.c/d.c motors are indispensible controlling elements in automation systems. Depending on the applications, some of them are fixed speed and some of them are variable speed.

The advent of power electronics transformed the scene completely and today we have variable drive systems which are not only smaller in size but also very efficient, highly reliable and meeting all the demands of the various industries of modern era.

D.C motors have been used in variable speed drives for a long time. The versatile control characteristic of d.c motors have contributed to their extentive use in industry. D.C motors can provides high starting torque which qre required for traction drives. Control over a large speed range both below and above the rated speed can be easily achieved. The three basic method for obtaining a variable d.c. output voltage from afixed supply voltage a.c or d.c are phase control, ON-OFF control and chopper control.

Phase locked loop control techniques are employed in some d.c drives to provide precise speed control and essentially zero speed regulation. Microcomputer control of complex drive systems can provides great operating flexibility when required.

Primarily, the drives systems can be divided into two groups d.c drive systems and a.c drive systems. The invention of power semi conductor saw the advent of d.c drive systems for most of the early variable speed requited . Based on the advantages of simple construction and ease of control this technology was taken up for further improveents all over the world till very recently. Almost perfected as for as the controller is concerned this technology has most of its drawbacks due to d.c motor.

D.C. motors have inherent disdvantages in that need regular maintainance they are tailor made hence, not readily available for replacements bulky in size commutator sparking they are simply not suitable in hazardous areas like chemical and petrochemical plants or in mines.

A.C drives systems use the a.c motors as the driven element is either induction or synchronous type. Induction motor drives find application in very high power application and synchronous motor drives power drives.

Some of the adantages of a.c drives lower maintainance higher efficincy, better reliability , lower cost weight and inertia and the ability to operate in dirty md explosive environments.

As the trends exists, applicatuon of ac drives will continue to grow. However dc drives will also continue to be used for quite some time.