This challenge has been around longer than most folks have been alive— after 1950 that is— and the knowledge seems to have left the world. AC drives are significantly more user friendly and have gained in popularity. Not too long ago I remember setting up my first BBC Veritron drive and battling with the oscilloscope to get it just right so I could measure the drives operation and set up field weakening. Today, software and enhancements have made this process faster and more efficient.
We generally work on 50 to 250 DC drive systems a year and they are all retro-fits. In 15 years, only once have I consulted, applied, designed and tested a new DC drive system. The application was a very sophisticated test stand simulating a turbine engine. In most cases, especially plastics on extruders, the original drive experiences a problem and the user needs a replacement.
B&D Technologies and the Applications Engineering Group has retro-fit over 20 different brands with the ABB DCS800 and we’ve created tools to make the process easier and more efficient. As you can see from the photo, we created a 3D model of the retro-fit stand that goes into the original BBC Veritron, similar to the first one I worked on so many years ago. The design includes a bus bar and connection points for the existing furniture and power-feeds.
Here are a few tried-and-true recommendations when assessing an application:
- Application type dictates sizing and accessories
- Does the apparatus go beyond base speed of the motor into field weakening? If so, you need a feedback device.
- If the feedback device is a “tacho” or tach generator, highly recommend replacing with an encoder. The tach will generally have worn brushes and always seem to drift or create inaccuracies in speed regulation.
- The application always requires an isolation transformer or a reactor— if absent the warranty will be voided
- Fuse the incoming lines and one side of the armature circuit, or both sides if the drive is a regen drive
- The drive will require an AC side power contactor or a DC side armature contactor
- When cooling the enclosure, the original DC drive may generate less heat based on different variables
- Get your motor serviced by a motor shop that has a solid reputation for DC
Lastly, getting the motor to turn in a circle is never the problem; controls that operate the drive and integrate into the existing system can be. If the schematics or prints from the original system are missing, we generally create a set but this step requires more time.
My fastest start-up on a basic extruder has been less than two hours, yet it has also taken three days on a test stand running to 5,000 rpm with a speed control requirement of plus/minus 1 rpm.
C. Tolbert DOE