Optimal capacitor sizing for induction motors

In the constant search to minimize the cost of operations, plant engineers have applied capacitors to individual motors, groups of motors, and sometimes the entire plant. The benefits of doing so are: (1) improvement in power factor, which either eliminates, or reduces the demand charges imposed by the utility, (2) reduces the energy loss in electrical conductors by reducing the required current, (3) capacitors can also provide voltage support at various busbars, (4) release of valuable system capacity, (5) increases the useful life of pieces of distribution equipment. Generally, the best location for the capacitor bank is at the terminals of the motor; this best achieves the objectives (2) through (5) listed above. However, there are some exceptions, e.g., if the capacitor bank is required in a wet location it may require some sort of protective covering which would add to the capital cost, a better location may be the busbars which feeds the motor. The difference in correcting the power factor from 0.93 to 0.94 on a large motor can save or waste significant amounts of money. The purpose of this paper is to give a brief overview of the methods of power factor correction, and to demonstrate a method for selecting the optimal size capacitor for almost any motor. The problem is formulated as a single variable optimization problem with the user free to select variables such as payback period, utility rates, and motor load factor. Included in this paper is a typical industrial application followed by some discussion on points such as diversity factor and finally some conclusions.