问题 问答题 简答题

双速电梯的主电路串入电阻和电抗有何作用?结合此电梯的起动加速和减速平层过程分别叙述?

答案

参考答案:

异步电机起动电流很大,起动时,定子电流为额定电流的4~7倍,过大的电流会影响电网电压,串入电阻和电抗可以起到在起动电流和对电网的影响,也可以限制起动时的加速度,加速度被限制在某一较小的范围,在制动时,可以使其制动转矩大大降低,使电梯降速匀滑,防止产生冲击,其作用能改善舒适感,又能延长机器使用寿命。

为了限制起动电流,以减小对电网电压波动的影响,在起动时,一般按时间原则,串电阻和电抗一级加速或二级加速;减速制动是在低速绕组中按时间原则进行二级或三级再生发电制动减速,以慢速绕组(24极)进行低速平层。

选择题
阅读理解

Street art can be found on buildings, sidewalks, street signs and even trash cans from Tokyo to Paris to New York City. This special kind of art can take the form of paintings, sculptures, cloth or even stickers (张贴物). Street art has become part of a global visual culture. Now, even art museums and galleries are collecting the works of street artists.

It is not easy to provide an exact history of the street art movement. This kind of art has developed in many kinds of ways in places all over the world. Also, because it is illegal to paint public and private property without permission, street artists usually work secretly. This secretive nature of street art and its countless forms make it hard to define exactly. And people have different opinions about the movement. Some think street art is a crime and destroys property. But others see this art as a rich form of non-traditional cultural expression.

Many experts say the movement began in New York City in the 1960s. Young adults would use paint in special cans to spray their “tag” on walls and train cars around the city. This tag was a name they created to identify themselves and their artwork. This colorful style of writing is also called graffiti (涂鸦). It is visually exciting and energetic. Some graffiti paintings were signs marking the territories of city gangs or illegal crime groups.

Graffiti also became a separate movement expressing the street culture of young people living in big cities. Graffiti art represented social and political rebellion (叛逆). This was art that rejected the accepted rules of culture and power. These artists could travel around areas of the city making creative paintings for everyone to see. Sometimes this street art created a conflict between artists and city officials. Graffiti artists created their images and city officials quickly painted over them.

During the 1980s, two New York painters — Keith Haring and Jean Michel began showing their work in art galleries and museums. This is when street art started to become part of the more general popular culture.

小题1:What can we infer from the first paragraph?

A.Street art has become popular and is accepted as a kind of culture.

B.Street art is very special compared with other forms of popular art.

C.Many people are beginning to learn street art.

D.Street art is better developed in big cities than in rural areas.小题2:Which of the following statements is TRUE according to the passage?

A.“TAG” was a word created by people to distinguish street artists and the other artists.

B.There are three reasons explaining why it is hard to define the history of street art.

C.The movement of street art began in the 1980s in New York City.

D.All the people don’t think that the street art is a rich form of cultural expression.小题3:The city officials used to get rid of graffiti probably because _____________.

A.they caused arguments between people

B.they were ugly in someone’s eyes

C.they were not painted under proper guidance

D.they didn’t express traditional culture