Utilizing new drive technology has enabled more applications for AC machines where DC machines have typically been used. In the past, DC machines have been implemented for their good torque characteristics throughout a variable speed range. However, some of these older DC drive systems are becoming obsolete and the availability of replacement parts is becoming a concern. This sort of scenario will often be accompanied by consideration for a drive retrofit project. This article covers some key points and comparisons between the two different technologies that may help make that decision a bit easier.
There are benefits to going AC (from an original DC application), but there can also be significant drawbacks. How much each weighs into the equation is based in part on the existing infrastructure and how feasible it is to change. With this in mind, take a look at the general pros and cons of each drive technology.
DC Drives/Motor Combination
Pros |
Cons |
|
Drives are generally less expensive |
Motors are generally more expensive |
|
Drives produce less heat |
Motors require routine maintenance |
|
Drives have smaller footprints |
Motors require separate field supply source |
|
Perform better when high torque at low speeds are required |
Users are increasingly unfamiliar with DC technology |
|
Can handle high impact loads |
AC Drives/Motor Combination
Pros |
Cons |
|
Motors require little to no maintenance |
Drive hardware is more complex |
|
Drives have better power factor |
Line Regen is possible but adds cost |
|
Overall AC system is generally more cost effective than a DC system |
Brake choppers are an option but waste energy and increase heat |
|
Can run motors more efficiently across the whole speed range |
Despite these differences, there are some areas where both DC and AC drives are equal. When looking at state-of-the-art drives, such as the Siemens Sinamics family of drives, both DC and AC drives have comparable features that ease the pain of commissioning. In addition, both drive types are reliable and relatively maintenance-free.
When looking to migrate an existing DC application to AC, there are some engineering considerations that should be taken into account as well:
- Compare the relative footprint of each drive.
- Compare the incoming power requirements.
- Compare cooling requirements.
- Check communication protocols and the ability to interface with the existing process line controls.
- On the motor side, look at the machine footprint and shaft height.
- What about speed - is a gear reduction required since most DC systems are direct drive?
Electrically, care must be taken to how the new drive is tuned because AC drives can respond much quicker than DC drives and this can lead to mismatched response times with existing equipment or other instabilities. To get true DC performance you may need to install an encoder (a must on hoist applications). Also, proper cabling and grounding need to be taken into account because electrical noise may cause issues with the existing equipment - especially instrumentation.
Are considering replacing your DC equipment with AC equipment and have questions about your decision?
