Alternative energy

Electrical Apparatus, Feb 2005 by Nailen, Richard L

Alternative energy Renewable Energy Systems: Design and Analysis with Induction Generators. By M.G. Simocs & F.A. Farret. CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Fla. 33431; (800) 272-7737. $99.95 (hardbound). 358 pages.

Although many technical papers have covered parts of this ground, Simoes and Farret bring together here in one volume the nature, construction, operation, and control of induction generators that produce on-site power from energy sources outside conventional utility supplies. The emphasis is on wind and water flow. Little is said concerning industrial process heat recovery applications involving steam or gas systems.

Not for the mathematically challenged, this is also not an easy read, with language sometimes difficult to follow. Both authors specialize in industrial electronics and power conversion rather than electric machinery, which may account for such unfamiliar terminology as Foucaull current (eddy current), disrupting point (pushover), or stator and rotor dents (apparently meaning either slots or vents).

Several chapters cover basic concepts of energy conversion, induction machine construction, and system economics. Most of the text, however, explains control principles for machines not connected to a utility grid, involving the complex coordination of machine speed, frequency, and terminal voltage to maintain optimum power output.

Chapters 2, 3, and 4 deal with steady-state and transient modeling of the generator. In Chapters 7 through 9, the authors concentrate on the principles of both "scalar" and vector control and the electronic circuitry involved. Chapter 10 delves deeply into such practices as "fuzzy logic" (a 16-page appendix goes into more detail) and "hill climbing" control." Chapter 12, aimed primarily at the self-excited wind turbine generator,, describes various computer simulation tools using PSpice, Pascal, Excel, PSim, and MATLAB.

All of Chapter 11 is devoted to woundrotor machines, which the authors contend are in general use for "large" power ratings (although "large" is undefined, the book cites two size ranges-up to 100 kilowatts, and above that). However, many squirrel-cage induction generators have been built in sizes from 700 to 15,000 horsepower, whereas large wound-rotor generators are unheard of in the U.S.

The authors attach great importance to machine efficiency. However, their approach sometimes appears unsound. For example, they suggest that friction loss can be reduced by using "more expensive bearings"-which simply doesn't work for either anti-friction or sleeve bearings. They consider even small improvements significant. In the U.S., however, that's often ignored, because in renewable energy applications the "fuel" is free-energy that would otherwise be wasted. A variation of one or two percent in the amount recovered is unimportant.

The authors deserve congratulations, though, on making one point clear by repetition: the induction generator always requires some external source of lagging volt-amperes to maintain stator magnetization, and therefore always operates at a lagging power factor. More than one "authoritative" reference has erroneously described induction generators as "leading" power factor machines.-RLN