问题 问答题 简答题

在三相全控桥式电路中每个晶闸管在一个周期中出现几个触发脉冲?为什么?如何实现?

答案

参考答案:

晶闸管在一个周期中出现二个间隔为60度的触发脉冲,即双脉冲。三相全控桥采用双脉冲的原因是为了保证在电路启动或电流断续时能有二个管子同时导通。双脉冲可以通过:

(1)采用带二组绕组的脉冲变压器使每个触发电路去触发二个桥臂上的晶闸管。

(2)采用电子线路使每个触发电路在一个周期中产生二个触发脉冲。

单项选择题
阅读理解

You may have never heard of Ladislao Biro, but you have certainly heard of the pen he invented—the ballpoint pen, or biro. Before Biro invented his pen, people wrote with fountain pens. The ink smudged(弄脏) and blotted and the pens sometimes leaked. In the 1930s Biro was a magazine editor in Budapest in Hungary. He noticed that the inks which the magazine’s printers used dried very quickly. Biro wondered if quick-drying inks could be used in pens. He came up with the idea of a tube of ink with a free-moving ball on the end. As a person wrote, the ball collected ink from the tube and rolled it on to the paper. The pen would be cheap and could be thrown away when the ink ran out.

Biro began to work on his invention, but before he could patent it the Second World War broke out. Biro left war-torn Europe and fled to Buenos Aires in Argentina. There, he and his brother Georg, who was a chemist, began to improve the pen. In the early 1940s Biro began to manufacture his new pen, the biro. In 1944, he sold his invention to another company, who began to mass-produce the pen for the British and American armed forces.

Biro was pleased that his pen was popular, but he did not gain much from his invention. The biro was later sold to the French firm, Bic, who now sell twelve million pens a day. Biro sank into obscurity in South America. His name, however, has become a household word.

48. The reasons for the popularity of ballpoint pens are these EXCEPT that_______.

A. the inks dried very quickly                      B. they were cheap

C. they were easy to carry around           D. they were mass-produced

49. Which of the following is the correct order of the events that happened to Biro?

a. applied for patent for first ballpoint pen

b. began to manufacture pens

c. fled from Hungary to escape Nazis

d. sold his invention

A. a c d b                    B. c d a b           C. a c b d           D. c a b d

50. The underlined part “sank into obscurity”(last paragraph) is closest in meaning to“_______”.

A. became unknown to many people       B. became popular with people

C. lost interest in business                         D. lost a lot of money

51. What does the passage mainly tell us about Biro?

A. He is successful in business.                 B. He is an important inventor.

C. He is a famous magazine editor.         D. He is a popular writer.