It will come as an interesting fact for those unfamiliar with variable frequency drives that the first electric motors available on the market were designed and built for operation from direct current power.  As such DC motors have been available for nearly 100 years.  However, AC motors have surpassed DC motors as the preferred motor of choice in the industry.  AC motors will from now on remain the primary motor for the fixed speed requirements of industry.  The basic simplicity, dependability and ruggedness make AC motors the natural choice for the vast majority of industrial drive applications. 

Many of you will thus be wondering where DC drives fit into the industrial drive picture of the future.  In order to supply an answer, some of the basic characteristics obtainable from DC motors and their associated solid state controls need to be examined more carefully. 

Basic characteristics obtainable from DC motors include the following:

• Low cost;
• Low maintenance;
• Good speed regulation;
• Wide speed range;
• Ease of control; and
• Compact size and light weight.

In order to understand the DC drive’s capability to provide the characteristics listed above, the DC drive need to be analyzed as two separate elements that make up the package.  These two elements are the (i) DC motor and the (ii) control (also known as the regulator).