Modeling and Analysis with Induction Generators, 3rd Edition

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6

Construction Features of

Induction Generators

6.1 SCOPE OF THIS CHAPTER

Induction machines are of robust construction and relatively low manufacturing cost.

They are more economical than synchronous machines. For high-power applications,

this difference is less perceptible because those units are custom made. However, for

medium and small sizes, the difference in price is dramatic, as much as 80%. For the

same kVA rating, though, induction machines are larger than synchronous machines

because their magnetizing current circulates through the stator winding. Induction gen-

erators connected to the grid need no voltage regulation, less constraints on the turbine

speed control, and practically no maintenance. These advantages plus the fact that induc-

tion machines are readily available from several manufacturers make them very competi-

tive for just-in-time installation. The U.S. National Electrical Manufacturers Association

(NEMA) has standardized the variations in torque–speed characteristics and frame

sizes, assuring physical interchangeability between motors of competing manufacturers,

thereby making them a commercial success and available for integral horsepower ratings

with typical voltages ranging from 110 to 4160 V. As discussed in Chapter 2, the only

perceived weaknesses of induction machines are lower efciency—the rotor dissipates

power—and the need for reactive power in the stator. This chapter will discuss the physi-

cal construction of various types of induction machines, emphasizing their features in

generator mode. An electrical generator used for a wind turbine system has efciency

imposed by three main issues: (1) stator losses, (2) converter losses, and (3) gearbox

losses, as depicted in Figure 6.1. Stator losses are considered in this chapter by the proper

design of the machine for the right operating range; the converter losses are given by

proper design of the power electronic circuits. On-state conduction losses of transistors

and diodes, plus their frequency proportional switching losses, are not considered in this

book because they must be approached in a specic power electronic topology that best

ts the nal design. The third main factor responsible for a noticeable power loss is the

use of a gearbox, as indicated in Figure 6.1 for a typical wind turbine system.

It can be considered that mechanical viscous losses due to a gearbox are propor-

tional to the operating speed, as indicated by

PP

gear gear rated

mech

mech rated

=

,

,†

ω

ω

(6.1)

where

P

gear,rated

is the loss in the gearbox at rated speed (on the order of 3% of rated power)

ω

mech

is the rotor speed (rotation/min)

ω

mech,rated

is the rated rotor speed (rotation/min)