by Ken Rohr, Applied Industrial Technologies
Published July 2006, Aggregates Manager magazine
Electric motor-driven systems are estimated to consume more than one-half of all electricity in the U.S. and more than 70 percent of all electricity in industrial applications. Therefore, a well-planned energy management program can reduce plant-operating expenses and increase profits – and there are many opportunities available to do so.
From high-efficiency motors to eliminating air leaks, energy usage can be reduced and savings can be realized in the entire operating system or in specific sections of a production line.
The initial purchase price of a motor represents only 2 percent of its total lifetime cost. Its power use represents almost all of the remaining 98 percent. Using a motor that is too large for the job at hand will further increase this power usage. Suppliers of AC and DC electric motors have straightforward sizing procedures to help in this analysis.
An energy efficiency plan focused on improved electric motor system management can result in an estimated 13 percent energy savings, according to Reliance Electric.
The National Electrical Manufacturers Association (NEMA) established a motor program to provide highly energy-efficient products that meet the needs and applications of users and OEMs. Replacing standard electric motors built prior to the Energy Policy Act of 1992 (EPAct) with NEMA Premium® efficient models can reduce energy consumption by 3 to 9 percent.
Efficiency can be further enhanced through proper motor maintenance. These steps will also improve reliability as well as prolong motor life. A basic motor maintenance program requires periodic inspection and correction of unsatisfactory conditions. Items to check during inspection include: lubrication, vibration, ventilation and the presence of dirt or other contaminants; alignment of motor and load; possible changing load conditions; belts, sheaves and couplings; and tightness of hold-down bolts.
Nema-rated Motors Improve Energy Efficiency in Five Areas.
Some electrical motors that meet the Nation Electrical Manufacturers Association’s (NEMA) rated specifications can have improved energy efficiency by reducing wattage losses as much as 50 percent over standard efficient industrial motors. This reduction results from optimized design, improved materials, and controlling the following five areas of wattage loss (as shown in the motor cut-away)
Watt Loss Area
Thinner gage, lower-loss core steel reduces eddy current losses. Longer core adds more steel to the design, which reduces operating flux densities and their associated losses.
More copper and larger conductors increases cross sectional area of stator windings. This lowers winding resistance and losses due to current flow.
Larger, specially designed rotor conductor bars increase size of cross section, lowering conductor resistance and losses due to current flow.
Low loss fan design reduces losses due to air movement while also reducing noise.
Stray Load Loss
Optimized design, advanced manufacturing and quality control procedures minimize stray load losses.
Note: Testing and quality control ensure compliance with a broad range of industrial performance standards, including the National Electrical Manufacturers Association, Institute of Electrical and Electronics Engineers, and Underwriters Laboratory.
Variable-frequency Drives and Energy Conservation
Variable frequency drives (VFDs) have long been known to save energy and help in the development of a continuous-flow manufacturing process. They help achieve energy savings through the follow methods:
Energy savings for pumps and fans: In most facilities, centrifugal pumps and fans run at fixed speeds, and fluid flow is adjusted by an automatic valve or other mechanical means. A VFD allows motor speeds to be regulated electronically, increasing speeds (and energy consumption) only when necessary. Adjusting pump or fan speed to a desired flow rate can result in energy savings many times the cost of the motor.
Improved process control: Throughput rates of most industrial processes depend on many variables. With a VFD, the time needed to adjust to changes in these variables may be significantly reduced. The results are decreased energy use and increased efficiency.
Reduced mechanical stress: Starting a motor places significant mechanical stress on other components in the system. If the start is not controlled, belts can slip and squeal and chains can jump off their sprockets.
VFDs operate with reduced-voltage and reduced-frequency starting (called soft start) to decrease this mechanical stress. At start-up, VFDs vary output voltage along with output frequency to control motor torque and speed. This produces a soft start as motor speed accelerates at a preprogrammed rate. Acceleration time in most VFDs can vary from 5 to 360 seconds, depending on the needs of the system.
Combining premium efficiency motors with highly efficient gearing can save substantial energy and operating costs. Efficiency gains of 8 to 35 percent are possible by upgrading to more efficient or properly sized gearboxes and energy-matched gear components (e.g. helical, cycloid, bevel or planetary).
Some gearmotors incorporate a helical bevel gear reducer, which can improve efficiency by as much as 30 percent as with traditional worm gears, according to Reliance Electric. Total gearing efficiency in these units is 95 percent. In addition, helical gearing generates less noise than conventional gearing.
A properly designed belt transmission system is highly efficient, quiet and requires minimal maintenance. Certain types of belts, however, are more efficient than others.
V-beltsare used in the majority of belt drives. These belts have a trapezoidal cross section that wedges into sheaves to increase friction and power transfer capability. Upon installation, v-belts are typically 95 to 98 percent efficient. But they soon stretch, and efficiency deteriorates by as much as 5 percent if the belt is not periodically re-tensioned.
Cogged belts have slots that run perpendicular to the belt length are about 2 percent more efficient than standard V-belts.
Synchronous belts are toothed and require the installation of mating toothed drive sprockets. Synchronous belts are about 98 percent efficient, operate in wet and oily environments, and run slip-free.
Additional Tools for Efficiency
Sprocket and sheave alignment plays an important role, whether one is installing new belt drives or maintaining existing ones. Alignment tools are available to assist the user with correctly positioning sheaves and sprockets, as misalignment can cause excessive wear on sheaves, resulting in a 12-percent loss in V-belt drive efficiency and up to a 50-percent reduction in belt life.
Improper tensioning also leads to inefficient power transmission. Manufacturers offer a variety of tools that help ensure proper belt tensioning, ranging in costs from $0 to $1,200. For example, one tool is a simple tension gauge that adheres to a belt during installation. As the belt is tightened, the gauge stretches and the actual tension is displayed.
Another concern is sheave condition. Eroded sheave sidewalls can cause up to 12-percent loss in V-belt drive efficiency. Rough and worn sidewalls can reduce belt life by up to 50 percent. Belt manufacturers offer sheave gauges to help assess sheave condition and make decisions about maintenance.
Daily Inspection Checklist
To be the most energy efficient, inspect the following items during your daily maintenance check:
Ventilation and the presence of dirt or other contaminants
Alignment of motor and load
Possible changing load condition
Sheaves and coupling
Tightness of hold-down bolts
Leakage is the major cause of inefficiency in compressed air systems. A typical system under constant operation with only a ¼-inch leak can cost an average of $8,300 per year in lost energy.
Thread sealants seal air lines while allowing for post-assembly adjustments. A quality sealant can withstand temperatures to 400° degrees Fahrenheit and seal at operating pressures up to 10,000 psi without cracking or shrinking. They are a simple, inexpensive way to save thousands of dollars in energy costs each year.
Thread-locking compounds have also been found to improve the efficiency of motors by preventing the loosening of electric contact lugs. In one plant, the lugs were loosening due to vibration and accounted for an energy loss of 13 percent in addition to consuming 8 man-hours per week for retightening. The annual cost per motor was calculated at $250. After applying thread-locking compound on the contact lugs, the plant saved $4,000 per year.