问题 问答题 简答题

什么是电流跟踪型PWM逆变器?有何特点?

答案

参考答案:

电流跟踪型PWM又称电流控制型电压源PWM逆变器(CRPWM),它兼有电压型和电流型逆变器的优点:结构简单、工作可靠、响应快、谐波小,采用电流控制,可实现对电机定子相电流的在线自适应控制,特别适用于高性能的矢量控制系统。其中滞环电流跟踪型PWM逆变器还因其电流动态响应快,系统运行不受负载参数的影响,实现方便,而得到了广泛的重视。

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阅读并选择正确答案.(15分)       

     Mobile phones have become a problem for middle schools. Some middle schools in Australia have

banned (禁止) students from carrying mobile phones during school hours.

     Mobile phones use among children has become a problem for the school this year. Several children

have got mobile phones as Christmas gifts, and more students will want them.

     Mary Bluet, an official, said mobile, phones use is a distraction (分心的事) to students during school

hours and it also gives teachers so much trouble in their classrooms. Teachers were also saying that

sometimes students might use phone message to cheat during exams. She said some schools had tried to

ban mobile phones. Some parents felt unhappy because they couldn't get in touch with their children.

     Many teachers said students should not have mobile phones at school, but if there was a good reason,

they could leave their phones at school office. They also said there were many reasons why the students

should not have mobile phones at school. They were easy to lose and were a distraction from studies.

     Many people say that they understand why parents would want their children to have phones, but they

think schools should let the students know when they can use their mobile phones.

1. Some middle schools in Australia have banned students from carrying mobile phones _____.

A. because they are students            

B. when they are free

C. when they are at school              

D. because they are children

2. We know from the passage that some children get mobile phones from _____.

A. the makers and sellers              

B. the passers by and strangers

C. their parents and friends            

D. some mobile phone users

3. What does the underlined word mean in the passage?

A. 聊天      

B. 核对        

C. 查询        

D. 作弊

( )

4. Some parents felt unhappy because they couldn't _____, hours.        

A. use their mobile phones

B. leave their mobile phones at school office

C. help the teachers with their work

D. get in touch with their children

5. The passage tells us that _____ during school

A. students shouldn't have mobile phones at school except for some special reasons

B. it is impossible to ban students from using mobile phone at school

C. some parents felt unhappy because they couldn't use their phones at school

D. parents should teach their children how to use mobile phones during school hours.

多项选择题