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For Some Applications, Variable-frequency Drives are Just Right

POSTED 01/07/2009

 | By: Kristin Lewotsky, Contributing Editor

Engineering is all about trade-offs. 

When it comes to motion, the application determines the form of the solution. These days, motion and automation technologies can be thought of as Venn diagrams, the increasing levels of intelligence allowing each component to offer broader functionality -- smart drives are taking over the tasks of motion controllers, PCs are operating as human machine interfaces (HMIs), and so on.

In some cases, this overlapping trend is blurring the lines between what is motion control and what is not, as with AC motors fitted with intelligent variable-frequency drives. Like servo motors, they incorporate feedback to offer speed, torque and position control. No, they don't offer the sophistication of servo motors or stepper motors, but not all applications need that. To borrow an analogy from Goldilocks and the Three Bears, sometimes an application doesn't need the highest performance solution or the lowest performance solution. Sometimes mid-level performance is just right.

Figure 4 - This variable frequency drive (upper left) for an HVAC application includes the keypad for ease of control. (Courtesy of Mitsubishi Electric Automation Inc.)The variable-frequency value proposition
The simplest way to apply power is with a motor starter that runs a motor at 100% speed. With no intelligence, the hardware is economical -- or is it? A motor starter operates a motor at a constant rate. That’s fine for an application like raw materials handling conveyors that only need one speed, but what about applications that don’t? Consider a fan that needs to run at 100% flow for short bursts, but primarily needs to operate at 50% flow. One option is to keep the motor operating at 100% speed and reduce airflow using dampers. The problem is that although this approach supplies the desired performance, it squanders energy. A more effective option is to control the motor with a variable-frequency drive that will actually slow the motor down to achieve 50% flow, then speed it up during periods of 100% flow.

The savings that can be achieved here go beyond the obvious. “In constant-torque applications, the voltage to frequency ratio is linear,” says Rick Roberson, variable frequency drive product marketing manager at Mitsubishi Electric Automation Inc. (Vernon Hills, Illinois). “At 60 Hz, a variable-frequency drive would output 230 V; at 30 Hz -- one half of top speed -- it would output 115 V. When we get into fan and pump applications, however, we do a voltage-frequency curve.” Because of the operating characteristics of fans and pumps, the voltage-frequency curve goes as the square of frequency and the required power is the cube of the speed. “If we run the drive at 30 Hz -- 50% of top speed,” Roberson says, “ we would be only using 25% of the current we would use at 60 Hz.” In other words, running the fan at half speed requires only one quarter of the energy.

There are other benefits beyond energy savings. Consider a start/stop pump application in which the motor either runs at near top speed or at zero. Starting and stopping the motor continuously introduces losses caused by the need to shift the kinetic energy in the system from zero to full speed. Substituting the start/stop for variable-frequency operation removes much of the wear and tear on belting and gearing caused by starting torque. That, in turn, means reduced maintenance and downtime.

For applications like conveyors or indexing tables that have modest performance requirements, variable-frequency drives can also reduce inventory and simplify replacement. Increasingly, servo-motor manufacturers are matching drives to motors for optimum performance. “They’re a matched set,” Roberson says. “If that motor fails, I have to hope that my distributor or the vendor has that model sitting on their shelf so I can get my system back up and running. If I can use AC induction drives that can do positioning, [instead], I can use a standard AC motor. If that AC motor was to fail, depending on where I’m located there could be 10 different motor suppliers within a 20 mile radius, so I could get a replacement by the next day and have my system up and running.”

Figure 1 - Variable-frequency drives (inside cabinet at upper left) feature keypads for ease of control; the motor is at the feet of the operator. (Courtesy of Baldor Electric Co.)Understanding variable-frequency drives
Variable-frequency drives run the gamut of intelligence, says Phil Camp, product marketing specialist at Baldor Electric Co. (Greenville, South Carolina). The simplest type, which Camp refers to as a knob box, features a potentiometer on the front of the drive to control frequency and a toggle switch to turn it off and on. The problem is that they’re manual devices without any sort of interface. This isn’t necessarily a problem for applications in which the drive is easily accessible and changes are occasional but for frequent changes or machines with difficult-to-access equipment, it can be a problem.

At the other end of the range are sophisticated drives with feedback capabilities. Their resolutions may be 1024 or 4096 pulses per revolution versus a million, as offered by a high-performance servo motor, but for certain applications, that’s quite good enough. Intelligent variable-frequency drives can provide movement to specific locations, though they can’t accomplish tightly-coordinated motion á la e-camming or drawing a circle. They do, however, allow operation at a distance, courtesy of keypads. “We don't call it an HMI but that's what it is,” says Camp. “In a lot of cases it would just be left on the face of the drive for the maintenance personnel to be able to monitor the drive or have the ability to start and stop the control locally. Or you could mount it several hundred feet away from the drive in order to provide a control station remote from the drive itself.”

The keypads typically aren’t programmable in the sense of traditional HMIs but some applications don’t need that capability. What the keypads can offer is the ability to toggle between several different programmable sets of parameters. “Each table within the drive gives a whole new set of parameters to the drive for it to operate on,” says Camp. “You could use that for drives that are not necessarily operating the same motor every day, maybe on a pump cart or something that gets moved around within a facility. You could plug it into a different motor and switch to a table that has the parameters set up just for that motor.”

Variable-frequency drives require some sort of processor but their capabilities are basically implemented in software and firmware. “Software is what differentiates the two,” says Roberson. “On a low-end drive, you don't need a very fast clock rate with your CPU. You can go with a very low-cost processor because the instruction set is very small. For complex tasks like positioning, you need a very fast processor, so that drives up the cost of your hardware.”

Figure 2 - AC motors with variable frequency drives (gray, at left) power the pumps (blue) in this multipart unit. (Courtesy of Baldor Electric Co.)Picking the right application
As always, of course, there are tradeoffs. Variable-frequency drives are more expensive than their fixed-frequency counterparts and more complex to install. Over time, however, the energy savings win out. The time to return on investment (ROI) is typically two to three years, says Robeson. In the case of fans and pumps on oil drilling platforms, Craig Resnick, research director at ARC Advisory Group, says ROI can be as short as six months.

Variable-frequency drives also cannot match the performance of motion control. It all comes down to the application. “I think of motion control as primarily used for applications that require non-continuous motion, for instance moving from point A to point B or synchronizing multiple axes together,” says Camp. “Variable-frequency drives are typically used in continuous processes like web handling applications, or in high-horsepower applications requiring less precision.”

Consider an injection molding machine in which a ram pushes material into a mold. “You would typically use a variable-frequency drive or a variable-speed AC drive for that as opposed to a motion controller because it’s just kind of brute force type of movement,” Camp notes. At the same time, the application requires more flexibility than a fixed-speed motor can supply. “You may have different plastics that require different injection speeds based on temperature and pressure. You might be regulating the injection process with a pressure sensor telling the drive speed up, slow down. Variable-speed AC drives are more suited to that type of application.”

Figure 3 - Web handling processes are ideal applications for variable-frequency drives. (Courtesy of Baldor Electric Co.)Another example of a good application for variable-frequency drives is an indexing table with loose positioning specifications. “Maybe I just want to do a 90 deg increments or 180 deg increments where I’m loading a part on one side of the table and indexing it to another location,” says Roberson. Like a robot welding with the parts being loaded on the other side, I don't need high accuracy for kind of a loose tolerance welding machine, as long as I’m close enough.”

Roberson says variable-frequency drives are also used for shaftless printing presses in which individual modules are sometimes ganged together for certain processes and operated independently for others. Here again, the lower-performance technology can substitute for motion control to provide a cost savings in the mid five figures for a four-level press with four stations.

Ultimately, the sweet spot applications are those that balance flexibility with cost, though the performance tradeoffs are dropping all the time. “Variable-frequency AC drives have come a long way over the years,” says Camp. “The programmability of motion control does not totally exist in the AC drives, but they have quite a bit of flexibility nowadays that can allow them to fit into these near-motion-control applications.”

Sometimes motion control provides too much sophistication for an application; sometimes a fixed-frequency drive provides too little. And sometimes a variable-frequency drive is just right.