December 2007 Electrical Apparatus

December 2007 Electrical Apparatus

This is a summary of the Electrical Apparatus December 2007 featured technical article, by Richard L. Nailen, P.E.    

The Nola device, named for its inventor who received a patent in 1977, acts to reduce voltage applied to an a-c motor whenever the driven machine demands less power. Using semiconductors to control voltage, the circuit senses the motor power factor. As in-phase current decreases with reduced shaft torque, a corresponding decrease in motor voltage maintains the same power factor. Nola’s invention was therefore widely known as a power factor controller. “Voltage controller” is perhaps more appropriate.

Since then, several variations have been developed. All, however, produce the same result: lower motor voltage to suit reduced load. Because the motor’s voltage-dependent core loss then dominates, the result is an energy saving. The smaller the motor, and the larger the number of poles, the more pronounced is the effect.

Regardless of motor design, however, such control is cost-effective only for motors subject to long periods of operation at half load or less. Typical applications are industrial sewing machines, moving stairways, or certain machine tools, for which frequent starting and stopping is impractical. Some loads, such as mixers, may exhibit wide swings in load, but predicting the cost saving from voltage control may be quite difficult.

With the advent of solid-state starters, little extra cost is involved in adding voltage control circuitry to the basic starter electronics, and such additions are commonly made.

Although exaggerated claims of energy savings were made for many of the early Nola controllers, the principle remains sound. As with any energy-saving measure, the motor user needs to consider both motor and load characteristics carefully before making an economic decision..

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