Energy Management: First Steps Toward Greater Efficiency
This article series provides specific examples of how engineers can identify and address --at relatively little or no cost --the principal sources of energy waste in their production facilities. Part one focuses on facility and machine power use and motors and drives. Part two will focus on energy reuse and application-specific energy savings.
David Greenfield, Control Engineering -- Control Engineering, 1/1/2009
|
As an engineer, you’re faced with countless areas to target with the goal of gaining greater efficiencies. Despite the recent plunge in some energy prices, there’s still really no better place to focus your efforts than on your facility’s energy use. Those prices won’t stay low for long, which means the steps you take now to cut energy costs will actually increase in value to the business as time goes on.
This article series is designed to highlight the more straightforward and easily remedied energy concerns in both discrete and process manufacturing plants. Basic advice on how to identify and address these issues is provided for each area covered.
Facility and machine power usage
An important first step for engineers interested in managing their facility’s energy usage is to understand how much energy they are consuming and how they are using the energy, says Corey Morton, product manager with B&R. “This can easily be done by working with local utilities, using portable measurement devices, or installing energy measurement devices,” he says.
|
| Because the industrial sector accounts for the largest portion of energy consumption in the U.S., the inherent potential for significant savings is also greater. |
Morton says one of the easiest ways to address energy usage is for engineers to observe the way their plant operates to enact an energy management discipline known as “load shedding.” “For example, consider a plant that runs two production shifts,” he says. “Many of their operations may be pneumatic, and are supplied from a central air compressor. Because there are only two shifts, there is no need to run the compressor during the third shift. This same train of thought can easily be expanded to HVAC systems, plant lighting, and water systems.”
Another easy way to address energy consumption is to observe “running demands.” If a plant also uses a great deal of water as part of the production process, the amount of water used will vary based on the number of machines being run at any one time. “If the motors driving the pumps are operated across the line (i.e., they are turned on/off with contactors at line voltage), the same amount of power is being consumed independent of the actual demand,” Morton notes. This issue can be addressed by measuring demand (flow) and varying the pump speed with a drive based on demand.
An additional benefit to addressing running demands can be an improvement in “power factor,” as a drive/motor system will typically have a higher power factor than the motor alone. In general terms, power factor is a measure of how well the plant is using its power. “A lower power factor means the utilities’ and plants’ distribution system must be sized to supply the power being used to produce work (real power), as well as the apparent power,” Morton says. “As a result, utilities may pass along a surcharge to plants with a low power factor.”
The pump application mentioned above can also serve as an example of “peak demand.” Each time the pump is started, depending on the motor size, it can result in a large inrush of current. This, in turn, will result in a momentary increase in the plant’s power consumption. Many plants contract rates with their utility based on typical demand. Any peak in demand may be enough to push the plant into a higher rate bracket for the entire billing cycle. “An easy method for limiting this inrush current is through the use of a soft starter or drive, which ramps the motor to running speed,” says Morton, “thereby reducing the starting current.”
Even applications that already use drives may provide opportunities for energy management. For example, an application may have motor-driven sections that unwind material, process the material, and then rewind the finished material. “In these cases, the unwind typically runs in a regenerative mode, meaning power is being pushed from the motor back to the drive,” Morton says. “The drive must then do something with this energy; in many cases it just discharges the energy with resistors—often referred to as a braking resistor. Therefore, the regenerated energy is wasted. One way of utilizing this energy would be to share the dc bus of each drive (see dc bus sidebar for more information). This allows the drives consuming power to utilize the regenerative power of the unwind drive.”
Morton notes that although sharing the dc bus of each drive can reduce the amount of wasted energy, it may not completely eliminate the waste. “A braking resistor may still be required to dissipate the excess energy,” he adds.
Motors and drives
Considering that motor systems are one of the top three sources of industrial energy consumption (see “U.S. Industrial Energy Consumption” graphic), it makes sense for automation engineers to focus a considerable amount of attention on this area.
“Surprisingly, only 30% of the manufacturing industries use variable frequency drives (VFDs) with their three-phase motors,” says Ron Koehler, director of next generation products at Yaskawa Electric. However, applying VFDs to three-phase motors is one area where engineers can directly affect the company’s bottom line.
Koehler suggests engineers consider the following four drives-related issues to save both energy and money over time by using drives. “The best thing about most of these suggestions is that they don’t cost anything to implement,” says Koehler, “so the benefits can only be positive in return.”
-
Stop oversizing drives and motors. Check the load requirements needed for a particular project to maximize the potential of the drive. If you reduce the speed and torque of a drive, it will obviously reduce the energy consumed to accomplish the job. Engineers should be able to look at the application being performed and determine what size drive is needed to accomplish the job.
-
Finless drives. These specially designed drives can be beneficial, as they can reduce the need for additional cooling methods. This is possible as a special heatsink feature allows for the release of heat out the back of the drive. Overall, this reduces the size of the cabinet needed and limits the amount of cooling needed for the drive.
-
Sequencing of operation by use of on/off heatsink fan control. The use of an automatic on/off fan control will reduce the use of unnecessary power, as you will be saving energy and money when it’s not being used. This may sound like a simple idea, but many times it is not enforced and energy is lost. Engineers should make sure that a drive has this option and that it is being used.
-
Permanent magnet control. Motors with permanent magnet (PM) control utilize the magnetic field to the motor’s advantage. These motors are capable of increasing energy savings even higher than the NEMA standards, says Koehler. “Sometimes they can be a couple of percent better than a high-efficiency motor,” he adds.
To access Part Two of this article (originally published in February 2009), click here.
Reusing energy via a common dc bus
Applications where at least two variable frequency drives are combined on a coordinated system can take advantage of having several inverters being connected to a common dc bus, according to Ron Koehler, director of Next Generation Products for Yaskawa Electric. Connecting VFDs via a common dc bus allows for power load sharing on the dc bus, because different inverters simultaneously drive the motor and generate power.
“Here’s how it works,” says Koehler: “The regenerative energy produced during deceleration can be reused and recycled right back to the line for best system energy efficiency, rather than wasting it by getting rid of it. Engineers can reconfigure their systems to make sure that this energy is being utilized wisely so that less power from the rectifier is needed or used, making the system more efficient.”
Because dc bus terminals are standard on most VFDs, the material cost is minimal; however, Koehler adds, “engineer involvement is necessary.”
|
| Common bus connections provide the ability to transfer energy form a regenerating load back to the main input line. The regenerating load power can be used for driven loads, thus reducing the power required from the main input line. Otherwise, regenerative energy is lost or dissipated into a dynamic braking resistor as heat. Source: Yaskawa Electric |
View All Control Engineering Webcasts from CTL
