Value of variable speed ac drives goes far beyond energy savings

It is vital for industrial facility managers to understand their total value, and not just the value achieved from reduced energy consumption.

08/03/2009


Variable speed ac drives can be powerful tools for industrial facility managers to significantly improve energy efficiency. But while that is a critically key benefit for every industrial facility, that's just the beginning. For example, an extended period of process downtime may be avoided due to the predictive capabilities built into an ac drive. The benefits include avoiding missed deadlines, pointed customer questions, perhaps diminished credibility and potentially huge costs.
Thus, in order to reap the full rewards ac drives offer, it is vital for industrial facility managers to understand their total value, and not just the value achieved from reduced energy consumption. This is particularly important for industries that do not use variable torque loads such as centrifugal pumps and fans, where energy savings are not as readily achievable. But the alternate benefits can be just as enticing.

Energy savings, process improvement
The energy savings potential of ac drives can't be denied, and is especially significant with variable torque loads such as pumps and fans, where energy consumption increases with the cube of speed. In these applications, a small change of speed results in very large changes in energy consumption. Put simply, the more time a system runs at less than full speed, the more energy is saved. Those savings can be augmented by optimizing a plant's energy rate structure, usage and operating conditions over the course of time.
In applications such as hoisting, material handling, manufacturing, woodworking, machining and grinding and polishing, small changes in speed result in relatively modest reductions in power consumption. While not immediately apparent, energy savings for applications operating at higher powers or for long periods can produce significant savings over time.
For non-variable torque applications, energy savings from speed control commonly take a back seat to performance improvements. In other words, the ability to control speed and torque allows manufacturers to improve quality, reduce waste and increase production output - all of which can translate to marketplace advantages.
A great example of this is crane and hoisting applications. AC drives can control motor speed over the entire range of movement, making it possible to reduce damage potential for both the hoisting equipment and the cargo. New specialized drives provide anti-sway capabilities that adjust the speed of hoisting equipment based on variables such as the load weight, position and environmental conditions. This is critical to help prevent damage and increase throughput of variable weight loads. In some cases the unloaded return speeds can be faster than full speed by adjusting drive output above 60 Hz.

Only the beginning
But there are many more benefits that can be accrued by incorporating ac drives within a given application.
Reduced maintenance costs - Variable frequency drives reduce the need for mechanical or hydraulic gearboxes and clutches, thus reducing the cost of maintaining and repairing these systems.
Reduced downtime - AC drives can be used to monitor load changes to predict mechanical wear such as worn bearings, and may avoid an extended period of downtime. Equipment repair and maintenance can then be scheduled before excessive mechanical wear, thus preventing costly emergency shutdown and repairs.
Reduced equipment cost - Because ac drives are connected directly to driven loads, they are typically less expensive than dc drives or mechanical drive systems.
Distributed control - AC drives that possess programmable logic control applications reduce the need for external controls and PLCs to handle processes that are associated with the drive.
Manual control - Local controls allow motor operation when control equipment is not required or not operating properly.
Reduced wiring and installation time - AC drives with serial communications capabilities can be controlled and monitored by communication networks over a single set or pair of cables. These cables are simple to connect compared to traditional I/O wiring.
Reduced personnel costs - As part of an automation and control solution, ac drives may be used to reduce the number of personnel required for many processes, meaning a facility's workforce can be more effectively allocated.
Incentives and rebates - Utilities often offer cash rebates and other incentives for implementation of energy efficiency measures. There are also state and federal tax incentives and programs that can be leveraged as well.
Simplification - As ac drive technology continues to improve performance, it is possible to use them to perform jobs that used to require much more complex dc motor drives, servo systems, wye-delta or reversing starters and wound-rotor motors.
Peak shaving or curtailment - In addition to charges for energy use (kWh), utilities charge for peak demand (kW). Peak charges may be based on instantaneous peak use or 15-minute average demand periods. Curtailment incentives are sometimes offered to reduce electrical demand during heavy-use periods. Rather than coordinating shutdowns, ac drives can be used to avoid these peaks by operating at reduced speed.

Making the choice
As with any piece of automation and control equipment, proper installation and commissioning of a variable frequency ac drive is critically key, so an application is as reliable as possible and all the benefits listed previously can be accrued. The best rule of thumb is to make contact with a trusted OEM panel builder, system integrator or automation and control solutions supplier armed with specifics about a given application. In turn, the best application of automation and control systems will be implemented to meet those needs.

Jacob Kimball is an ac drives product specialist for Schneider Electric.





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