Although new advances and technologies are nice, is it really necessary to replace all DC drives with AC drives?  Should one not rather select the most appropriate motor and drive solution for your particular application?  This is an important question being raised quite often about this topic.  Both DC drives and AC drives have their benefits and drawbacks.

DC drive replacement costs less than AC system replacement; this is especially significant at more than 200 hp.  However, when replacing the whole system, AC drive replacement costs less, especially below 1,000 hp.  The DC drives available on the market nowadays provide performance and ease-of-use that are comparable to high-performance AC drives because they’re easy to install, and they’re not difficult to repair in the field.  Integrating a DC drive into an existing system is thus easier than you think due to DC drives being feature-rich, and because AC and DC often can work together on the same line.

The main advantages of DC drives are that they cost less, produce less heat, have smaller footprints and are easier to fix in the field.  They also can perform high torque at low speed and handle high-impact loads.  However, the main knocks against DC drives are that the motor costs more and requires some maintenance, and they need a field supply, which brings about complexity.  DC drive systems also require accessory equipment, such as inductors, contactors and fusing, and many users are increasingly unfamiliar with DC technology.

The advantages of AC drives are that they’re used with AC induction motors, so there are no brushes to maintain, and the price of an AC drive system, including the motor, is more cost-effective than a DC drive.  AC drives also have better power factors, are easy to install and start up, are available from multiple vendors and can run motors efficiently across the whole speed range.  However, the disadvantage of AC is that the drive is fundamentally more complex, involving volts and amps and hertz.  Also, line regeneration is possible, but expensive, and brake choppers are sometimes necessary, but they waste energy and give off heat.