We never seriously considered line regen braking on our centrifuge systems (VFD-driven ac motors) because of initial cost. We dump the energy to choppers. We're being told that partly because of hybrid car advances and demands in industries with high energy consumption, it has become much more cost-effective. There's interest in both new installations and refit projects. What can anyone tell me about ROI these days to use regen drives?
—From December '12 Control Design
Document the ROI
Equipment suppliers in many industries wrestle with how to sell efficiency in the face of higher upfront costs — the reality of losing an order to a less expensive competitor makes the case against upgrading to energy-efficient equipment every time.
So what has happened to pricing for variable-frequency drives (VFDs), particularly regenerative drive units? The concept of putting the power back onto the line instead of dumping it into brake choppers is a slam dunk, right?
Depending on the last time you shopped for drives, the rate of price compression in the market may or may not leave you underwhelmed, particularly with respect to regenerative drives. The key reason for this is that the number of vendors serving the regenerative market has remained relatively constant in comparison to the number of vendors that have gotten into the "regular" VFD business one way or another. Many VFD suppliers have entered by way of expansion outside of their domestic market, while others have arrived on the scene as a brand-labeled VFD.
As a result of this increased competition in the "regular" drives business, machine builders have undoubtedly seen an influx of attention from those ready to offer up their VFDs as a better mousetrap, and the price for these regular, ordinary drives has consequently gone down.
This means the go/no-go decision to switch to regenerative drives from an inverter plus chopper set is unlikely to look more attractive than it did in the past. The reality is absolute cost has gone down for all drive types. As long as the brake chopper is still less expensive than the converter section, the regenerative drive will fail to deliver a lower initial price tag.
So where does that leave the demand for regenerative drives in a society where going green seems to always better both business and the planet? What is the ROI on a regenerative drive anyhow? If the absolute cost has come down, we know ROI must be more attractive.
And it is. The challenge in trying to quantify the ROI in this situation is the number of variables. Machine variables, like the fluctuating inertia of the centrifuge, are just one piece of the challenge. To be frank, I would be wary of blanket statements regarding ROI from VFD manufacturers unless the application is a fan or pump. My advice for a machine builder that wants to market or position itself as a green player is to document energy consumption in identical end user installations — one machine with a regenerative solution, and the other not.
Demonstrate success with a single case study, and even the most reluctant customers warm up to paying a little more up front. The energy czars are out there and they are ready to listen!
senior product manager,
Mitsubishi Electric Automation
Where Does the Energy Go?
A high-inertia load traveling at high speed contains significant kinetic energy. When this load is decelerated, this energy needs a place to go. As you might suspect, this issue affects not only VFDs, but also brushed and brushless dc motors, servo systems and, to a lesser extent, stepper motor systems.
To understand the issue of regenerative energy (usually referred to simply as "regen"), it is important to understand what happens to that energy during a hard deceleration.
Modern pulse-width modulation (PWM) motor drives efficiently transfer energy from the power source into the motor to produce high-performance, high-efficiency motion.
PWM drives also do a great job returning energy from the motor and load. But where does that energy go? In the case of an ac-powered system, the power source is your local electric utility, and the power bus is quite stiff; it is not usually possible to return the energy to the power grid without highly specialized (and very expensive) equipment.
This leaves the drive's internal bus capacitor as the most likely recipient of regenerated energy. That's OK to an extent, but as the recovered energy charges the bus cap, the bus voltage increases. If it goes too high, the drive will either shut down unexpectedly, or it will be damaged.
DC-powered systems typically get their power from a switching power supply, which can absorb some regen, but will shut down to protect itself when its bus cap exceeds the safe voltage.
Just to get a feel for the numbers, let's consider a 400 W servo system driving a 5X inertial load. The rotor inertia of an Applied Motion M0400 Series servo motor is 340 g-cm2, so the total inertia will be 2,040 g-cm2. For this example, assume the motor max speed is 3,000 rpm (314 rad/s). We can easily calculate the kinetic energy: