The growing cost of energy for manufacturing puts stress on maintaining a competitive edge in the marketplace. The increasing costs soak up profits, causing plant management to look for ways to control these expenditures and still meet production objectives. These objectives have been addressed by different technologies, including lean principles, agile manufacturing, six sigma and other waste reduction methods. Typical lean principles attack wastes related to overproduction, waiting for materials, parts movement, inventory/work-in-progress, motion and quality problems. Now, another lean element can help reduce the energy waste that processes generate.
Electric motors drive a great percentage of plant processes. Motor-driven equipment accounts for 64% of the electricity consumed in the U.S. manufacturing sector. That’s approximately 290 billion kilowatt hours (kWh) of power per year. Plants can reduce energy consumption by about 5% with a simple electric motor energy optimization program and little capital investment. Applying this motor energy optimization program could easily reduce consumption by 14.5 billion kWh. At an electric rate of $0.05/kWh, the annual savings to the manufacturing sector would be $725 million.
Electric motors drive a number of different loads, from pumping fluids to transporting materials from one process to another. Energy optimization should be the number one strategy for plants trying to reduce energy waste. Each motor is rated by the work that it can accomplish, its horsepower (hp). It converts electrical energy into mechanical energy and the inefficiency of that conversion can drive electrical costs far above the budget plan for the year.
You can take several measures to ensure your electric motors are operating at their peak efficiency. Maintenance can put motors on a simple energy diet by addressing voltage imbalance, aligning shafts and installing energy-efficient belts.
This condition is characterized by unequal voltage on the three phases coming into a plant. Generally, only about 66% of the three-phase line voltage feeders are balanced. Each year, this problem costs U.S. industry between $40 billion and $150 billion in energy consumption, lost production and failed motors, according to some estimates. Imbalance can be traced to problems inside the facility or to outside influences that are more difficult to control and fix. The causes are numerous and may include:
- Lack of symmetry in transmission lines
- Large single-phase loads
- Faulty power factor correction capacitor banks
- Unidentified single-phase ground fault
- Open delta or wye connections
The most apparent effect of voltage imbalance is a decrease in motor efficiency and performance, both of which increase energy consumption. Operating with larger imbalance increases the I[+]2[+]R loss. NEMA standard MG-1 indicates that motors must meet their efficiency ratings with a voltage imbalance of 1%. For example, if the measured three-phase line voltages are 462, 463 and 455, the average is 460 volts. The maximum imbalance is 100 * (460-455)/460, or 1.1%. Voltage unbalance is probably the leading power-quality problem that results in overheating the windings and premature motor failure.
Maintenance should ensure that any motor being replaced or added be capable of operating on an imbalanced voltage supply. Check any older rewound motors to ensure they won’t waste energy after being placed back into service. Table 1 shows the efficiency of a rewound, 1,800-rpm, 100-hp motor as a function of the voltage unbalance and the motor load. If this motor operates fully loaded under a 2.5% imbalance for 8,000 hours using electricity that costs $0.05/kWh, it would waste:
Loss = (100 hp) * (0.746 kW/hp) * (8,000 hrs) * (100/93 -100/94.4) * $0.05/kWh
This calculation shows that tolerating imbalance exacts a price.
The objective of motor shaft alignment is to ensure efficient power transmission from motor to driven equipment. Ideally, the motor shaft and the shaft on the driven equipment are perfectly coaxial. Misalignment between the shafts reduces efficiency because it promotes vibration, noise, overheated couplings and bearings, as well as premature bearing and coupling failure.
Angular misalignment occurs if the shafts are set at an angle to each other. Parallel misalignment occurs when the shaft centerlines are parallel but not colinear. They can be offset horizontally, vertically or both. Combination misalignment suffers from both angular and parallel misalignment.
Rigid couplings can’t compensate for misalignment. While flexible couplings have some limited tolerance for misalignment, it’s a mistake to think they can compensate fully. Using a flexible coupling to correct the misalignment stresses the motor and driven equipment, reduces efficiency, leads to premature bearing failure and increases energy costs. To ensure that the shafts are properly aligned, use dial indicators or laser alignment tools to check and correct misalignment issues.
About one-third of industrial electric motors power belt drives. Belt drives offer flexibility in placing the motor relative to the load and in speed ratios. A properly designed belt system can provide high efficiency, decreased noise and lower maintenance costs.
Using certain high-efficiency belts can help you realize potential energy cost savings. Most belts are V-shaped to fit into the sheave and provide the friction necessary to transfer power from motor to load. When installed, these belts have a peak efficiency of 95% to 98%, depending on the size of the sheaves, driven torque, under- or over-belting, and V-belt construction. Over time, slippage and poor maintenance can decrease belt efficiency by as much as 5%. But other belt types can minimize this loss.
A cogged belt, for example, has slots running perpendicular to the belt’s length, which reduces the belt’s bending resistance. Cogged belts can be used in the same type of sheaves as the equivalently rated V-belt. Cogged belts typically run cooler, last longer and have efficiency about 2% higher than the standard V-belt.
Synchronous belts (also called timing, positive-drive or high-torque drive belts) are toothed and require mating toothed driven sprockets. These belts generally offer an initial 98% efficiency and can maintain this rating over a wide range of loads. In contrast, the V-belt, under high torques, presents a sharp reduction in efficiency because of slippage.
Synchronous belts have lower maintenance and retensioning requirements and can operate in wet and oily environments while running slip-free. The downside is that synchronous belts become noisy in operation, are unsuitable for shock loads and transmit vibration.
Consider an example. A continuously operating, 100-hp motor has a 93% efficiency rating and operates at an average load of 75% while annually consuming 527,000 kWh. The plant is considering replacing the 93% (ETASUB1) efficient V-belt with a 98% (ETASUB2) efficient synchronous belt. Does this proposed change make any economic sense? Once again, simple math provides the answer.
Savings = (527,000 kWh/yr) * (1-ETASUB1/ETASUB2) * (0.05 $/kWh)
= $1,344 per year
Energy-saving diet plan
Develop a long-term and short-term energy-reduction plan that can gain management support. It will need to encompass five essential points.
Point 1. Collect and analyze data from high-waste areas in the facility to generate a current and future-state lean energy map. Data can be collected using many different types of technology such as vibration analysis, ultrasound and thermography.
Point 2. Complete an engineering analysis on equipment on the current-state lean energy map as a high area of consumption. Look at electric motors, fans, pumps or other assets that could be re-engineered to reduce energy use. Many engineering tools can be used to determine the most cost-effective direction for the energy-efficient designs.
Point 3. Calculate a life-cycle energy cost trade-off to determine whether the capital investment to reduce energy use will produce the necessary return on investment.
Point 4. Integrate the energy-saving operations, such as line voltage measurements, belt tensioning and mechanical inspection, into the overall maintenance strategies. Develop a long-term plan to replace inefficient electric motors with better units.
Point 5. The final point in energy reduction is called stability. This is a long-term data collection process covering areas that have been upgraded to reduce energy use. Monitor these key performance indicators and report them to management in monthly energy meetings to provide the necessary momentum to keep the plant’s energy reduction program in place.
The energy diet is a long-term goal that management and plant personnel need to support a collaborative effort to convert waste into profitability. In many cases, the U.S. Department of Energy has cost-sharing grants available to help manufacturing facilities reduce energy costs. The areas described in this article attack only a small section of an overall plan that needs to be developed to achieve the necessary cost savings.
Douglas R. Malcolm is president of Riverside Storage Technology Group in Shelby Twp, Mich. Contact him at [email protected] and 586-781-5080.