DC motors were once the best option for speed and torque control. As a matter of fact, the first variable frequency drives were only for DC applications.

While many operators function with legacy equipment outfitted for DC voltage, DC motors do have their drawbacks. Brush maintenance and high replacement costs cause many to seek an alternative.

For those interested in shedding annual maintenance duties, here is a quick guide to DC conversion using a variable frequency drive.

About DC and Modern VFD’s

DC VFD’s can work with simpler transistors than AC and were first to market.  High torque at low speed made DC perfect for winding and lifting applications. 

Early AC VFD’s could not compete with DC, but modern drives feature flux vector control (FVC).  FVC allows AC motors to provide rated torque at low speeds, bringing once DC only applications within reach.

Choosing an AC motor

Selecting an AC motor for a once DC application may seem complex, but paying attention to three key areas will lead to success:

  • Motor speed range: total RPM required.
  • Motor design: temperature ratings, horsepower, NEMA ratings.
  • Motor dimensions and orientation: spindle height, length, width, etc.

The difference in construction between DC and AC motors endows them with different speed ranges. 2500 & 3000 RPM is common for DC, while 1800 and 3600 RPM are standard for AC.

Balancing speed and torque requirements for a replacement motor may prompt the use of a gearbox. When controlled by a VFD, a motor with a higher speed range can be chosen. Making sure that torque output is adequate across the desired speed range is key.

Along with speed range, the motor design is a top concern when choosing a replacement. Temperature, environment, and power ratings will have to match.

Motor dimensions and orientation will have to match as well. If matching dimensions is not possible, then retrofits to the mechanical system will have to be made. Referencing motor data documents is a must in every DC conversion and will ensure a best-fit match.

Choosing a Variable Frequency Drive

Choosing a VFD with flux vector control is key for a DC conversion.  Word to the wise: the PowerFlex 750-series features FVC control, while the 5-series does not.  Choose a 750-series for voltage conversions. 

For best FVC performance, add an encoder to the VFD package.  The Encoder will maximize the performance of FVC and will match the tachometer/encoder often included with a DC drive.  Also, make sure that the new VFD can accept all existing IO.

In addition to choosing a drive with FVC, make sure to answer the eight drive questions.  While more general in nature, the answer to each question will prevent drive miss-application.

What Can be Gained

So, plans have been made. You have selected a motor and set a date for the conversion. At this point, most would see less maintenance and lower motor costs to be the only benefit of a conversion.

While it is true that induction motors are more cost-effective than DC motors, coupling them with a PowerFlex drive can offer much more:

  • Advanced system diagnostics opens the door to predictive maintenance. It’s more than getting rid of brushes.
  • Access parameter data easily with Datalinks.
  • Winding and lifting applications are a snap with advanced programming.

All these features and more are available in the 750-Series from Rockwell Automation.  The 755T, in particular, offers an array of advanced predictive maintenance features and adaptive tuning control.  Adaptive tuning allows for smooth mechanical operation for heavy winding applications coupled with expert speed control.  This is in addition to anti-sway technology that makes using cranes and hoists significantly more efficient.

Downtime hurts.  Preventive maintenance is better than none, but predictive maintenance allows manufacturers to maintain their systems on their schedule, not Murphy’s. 

A move from an antique DC drive to a modern drive will open the door to improved productivity and overall equipment effectiveness. 

Voltage conversion brings many benefits, but conversion with a PowerFlex drive brings expanded possibilities.

For more information about drives and drive systems, check out the Variable Frequency Drive Main Help Page