As technology tends to improve by the minute nowadays, people are inclined to totally write off some of the earlier products used.  One such a product is the DC drive.  Some so-called experts in the industry have been predicting the death of DC drives for years now.  However, this does not seem to be the case though.

The premature predictions of the death of DC drives have been greatly exaggerated in recent years.  Even though DC drives have been neglected by a lot of manufacturers in the industry, they still constitute about one third of the total European variable speed drive market.  Despite what suppliers have said over the years, there is no one single drive technology that is ideal for all applications.  As such, the DC drive has some unique benefits that differentiate it from AC drives.

The torque characteristics of DC drives include:

• DC drives are capable of producing 100% continuous torque at any speed from standstill up to base speed.  Ironically, this is something that AC drive designers have been striving for and is only just becoming available without requiring a feedback device.

• The torque loop bandwidth also tends to be higher in a DC drive than its AC drive equivalent.

The regenerative capability of DC drives is also quite impressive.  Many applications require the DC drive to be capable of controlled braking as well as motoring.  The braking capability may only have to be transient – for example, controlled deceleration of the load – or may have to be continuous, as in a drive on a paper or steel strip unwind system which continually opposes the pull of the downstream drives to maintain the required tension.

Controlled braking in a DC drive system is quite simple and straightforward, seeing as it requires only a second thyristor bridge employed in reverse polarity.  This allows the DC motor to act as a generator while the braking energy is returned to the supply as useful electrical power.