问题 问答题 简答题

一台定子绕组为Y接法的三相笼形异步电动机轻载运行时,若一相绕组的引出线突然断线,问电动机还能否继续运行?若电动机停止运行后能否重新起动?为什么?

答案

参考答案:

电动机轻载运行时,三相绕组中的一相引出线断掉,则电动机能继续运行,但是电停止后则不能重新起动。这是因为三相异步电动机当一相绕组中的一相引出线断掉则三相电动机变成单相电动机,该单相电动机有工作能力而无起动转矩,其工作能力较三相电动机差的很多,尚可带轻负载继续运行,若是带重负载则会发生堵转现象:而停车后。则由于无起动转矩而无法重新起动。

单项选择题
阅读理解

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     America hopes to start building a base that can continue to exist all the time on the moon within 20 years.

By the mid-2020s, astronauts will be staying there for six months a time. So what do scientists think food

and water will be like for those first moon residents?

     In the short term, food would have to be brought from Earth. The dream, however, is to grow things in

greenhouses on the moon. "You'd grow things in the liquid-there's no reason why that wouldn't work," says

Pillinger, a professor. This would mean floating plants in a nutrient-rich solution of water. "Soil is not a

necessary thing for plants; it's only to keep them standing upright," Pillinger adds.

     It would take at least a decade of experiments after the base was built before such a plan would work,

however. The main problem is working out how to get nutrients from the lunar soil. "Getting all that to work

requires a minimum level of infrastructure (基础设施) present before you can start experimenting with

greenhouses," says Pillinger.

     Finding strong plants to grow in space will also be hard, though scientists are already working on it:

scientist Amy Grunden of North Carolina State University has been working on genetically engineering food

crops that can be grown in harsh, off-planet environments.

     Water is a somewhat easier prospect (前景). One of the reasons that America wants to put its base at

the moon's south pole is that it thinks that, in the permanently shaded craters, there are large pools of frozen

water. If that's true, it could easily be mined and used to drink and to create oxygen for the habitants.

1. The passage is mainly about _____.

A. life on the moon by 2020

B. America's first moon base that exists forever

C. food and water on the future moon base

D. housing for settlers on the future moon base

2. How do people plant on the moon in the future?

A. By burying plants in the lunar soil.

B. By growing plants in water.

C. By keeping plants standing straight.

D. By making use of the moonlight.

3. What difficulty will scientists meet in solving the food problem?

A. How to find water to irrigate plants.

B. Where to experiment with plants.

C. How to grow plants in the moon soil.

D. What plants are suitable to grow on the moon.

4. Scientists want to build the base on the moon's south pole because _____.

A. there is oxygen for people to breathe

B. people have found the frozen water that can be drunk there

C. it is a place where water will possibly be found and supplied

D. there are large pools that can let people have a swim