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.