Setpoint Systems builds custom automation solutions and in the process there are problems that come up where a creative solution can come in quite handy. Recently we used a creative solution to fix a problem of excess energy with servo motors. Many people don’t understand that one of the issues faced with running motors and servos at high speed and high cycle rates is the deceleration. As if accelerating to speed were not bad enough, you have to be able to stop the thing as well. Accelerating requires extra current, but in decelerating, there is a lot of energy that has to be disposed of somewhere. In fact, the energy available in an object in motion increases with the square of the velocity. If the velocity doubles you increase the energy by 4X. All that energy has to be absorbed by something.
Commonly in servo drives and frequency drives that excess energy shows up in the form of excess DC voltage on the DC bus. If this voltage gets high enough, the drives are designed to protect themselves, usually by declaring a fault and shutting down. Well, now that’s really convenient, eh? So the question that begs to be answered is: What do you do with all that extra DC voltage? Most drives have some sort of internal method of absorbing the extra energy, frequently in the form of a resistor circuit. This feature allows the excess voltage to bleed off to ground at a reasonable rate. If the DC voltage climbs too high or too fast, such that the bleed off circuit can not absorb all of it, then the drive faults. Let’s add more resistors! That will usually work.
However, on a machine that I was working on recently we sized a resistor to handle the excess energy of a VERY LARGE servo press that had to stop VERY fast. The resistor recommended by the vender was 48” long. No that’s not a misprint. That is four feet long, for a resistor! We didn’t like that option. So our vender recommended that we look at a product from a company named Bonitron. They make several sizes and flavors of devices that take excess DC energy, chop it up and spit it back out onto the three phase AC line. They call them Line Regen Modules. By using a diode module, also from Bonitron, we were able to hook multiple drives onto a single DC bus without back feeding into each other and feed it into the Line Regen Module. So far, it’s working great. I am quite impressed with the capability of these units. Check them out the next time you see a “DC BUS OVERVOLTAGE” fault, it was a great solution for us.
