<正>利用异步电动机机械特性曲线，分析了变频调速异步电动机在拖动直线型负载和推进器型负载时转差率随频率的变化规律。结果表明：变频调速异步电动机拖动直线型负载时，频率改变而转差率不变；拖动推进器型负载时，转差率的改变不仅与频率有关，还与电机的额定转差率有关。在近似计算时，可以认为变频后的转差率与额定转差率之比，等于变频后的频率与额定频率之比的平方。

The index species diversity was used to discussing the stand or fall of intermediate cutting intensity. The result showed that, the number of the family, the genus, and the species of undergrowth vegetation species increased with thinning increasing. Species diversity, evenness index and richness index increased as well. There were some changes in the life-form spectrum and of plant community of Pinus tabulaeformis. The results also showed that the density o1" plant community of half mature P. tabulaeformis needed be further tended and thinned. The density should be controlled below 1 900-2 200 tree/hm2.