Powerful Savings
POWERFUL Savings
Understanding energy costs and calculations can lead to big savings for some operators.
by Daniel J. Meyer, P.E., Ph.D. oes your operation pay bills for diesel fuel on portable engine/generator sets to power electrical motors or pay for electrical bills on motors at stationary sites that are connected to the electrical grid? If so, placing capacitors, which correct power factors, on your electrical system can offer returns such as 200 percent per year. To achieve these savings, however, an operator must first understand how power works. When there is an electrical load involving inductance (magnetic elements), there is a new term beyond amps and volts called power factor (PF) that enters into the basic unit of measurement called the kilowatt-hour. There are two components of electricity flow. One portion is absorbed to do useful work (at the shaft end), which is called real power (KW). The other portion is literally borrowed from the power company or the engine/gen set and used to magnetize the magnetic portions of the circuit (KVAR), which are known as the motor windings. Due to the reversing nature of AC power, this borrowed power is subsequently returned to the power system when the AC cycle reverses. This borrowing and returning occurs on a continuous basis (60 cycles per second). PF then becomes a measurement of the amount of real power that is used, divided by the total amount of power, both borrowed and used (KVA). The PF is variable and changes with the amount of load that is applied to the motor. It increases steadily with the horsepower loading that is applied to the motor. Energy savings increase even more when there is no load or a light load. The concept is best explained by looking at the "power triangle." It involves KVA (total or apparent power), KW (true power), and KVAR (reactive power). See Figure 1. The square root of the sum of the KW squared and the KVAR squared gives KVA. When the PF is 100 percent, the KVAR is 0, and the KVA is equal to the KW. Since this seldom happens, capacitors can offer significant savings. A capacitor is a passive electrical component that can store an electrical charge or energy on its plates. The amount of energy stored equals voltage squared times the capacitance (KVAR) divided by 2. The motor has no way of intelligently adjusting the amount of electricity it consumes in relation to the job or work it does. Most AC motors operate in the 65 to 80 percent PF range under full load and drop further under partial loading, especially anything with low horsepower motors. For example, a large saw in a sawmill can run as low as 8 percent when idling and as high as 85 percent when cutting
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AGGREGATES MANAGER April 2010