问题 问答题 论述题

哪几种原因使低压电磁开关衔铁噪声大?

答案

参考答案:有以下几个原因:

(1)开关的衔铁,是靠线圈通电后产生的吸力而动作,衔铁的噪声主要是衔铁接触不良而致。正常时,铁芯和衔铁接触十分严密,只有轻微的声音,当两接触面磨损严重或端面上有灰尘、油垢等时,都会使其接触不良,产生振动加大噪声。

(2)另外为了防止交流电过零值时,引起衔铁跳跃,常采用在衔铁或铁芯的端面上装设短路环,运行中,如果短路环损坏脱落,衔铁将产生强烈的跳动发出噪声。

(3)吸引线圈上所加的电压太低,电磁吸力远低于设计要求,衔铁就会发生振动力产生噪声。

阅读理解

“Old wives tales” are beliefs lasting for many generations. For example, most of us remember our parents’ telling us to eat more of certain foods or not to do certain things. Is there any truth in these teachings? Some of them agree with present medical thinking, but others have not passed the test of time.

Did your mother ever tell you to eat your carrots because they are good for your eyes? Scientists now report that eating carrots can help prevent a serious eye disease called macular degeneration. Eating just one carrot a day can reduce the possibility of getting this disease by 40%. Garlic is good for you, too. It can kill the type of virus that causes colds.

Unfortunately, not all of Mom’ s advice passed the test of medical studies. For example, generations of children have been told not to go swimming within an hour after eating. But research suggests that there is no danger in doing so. Do sweets cause tooth problems? Well, yes. However, sticky sweets made with grains tend to cause more problems than sweets made with simple sugars.

Even though science can tell us that some of our traditional beliefs don’t hold water, there is still a lot of truth in the old wives’ tales. After all, much of this knowledge has been accumulated from thousands of years of experience in family health care. We should respect this body of knowledge even as we search for clear scientific support to prove it true or false.

小题1:Which of the following is TRUE according to the test?

A.Eating garlic is good for our eyes.

B.Sticky sweets are damaging our teeth.

C.Swimming after a meal is dangerous.

D.Carrots prevent people from catching colds.小题2: The phrase “hold water” in the last paragraph most probably means”_____”.

A.to be believable

B.to be valuable

C.to be admirable

D.to be suitable小题3:What is the author’s attitude toward “old wives tales” in the text?

A.Subjective

B.Objective

C.Dissatisfied

D.Curious

单项选择题