问题 问答题 简答题

为什么A/D转换型电子式多功能电能表不仅能计量有功电能,而且还能计量无功电能?

答案

参考答案:

A/D转换型电子式多功能电能表是通过对被测电路中的电压、电流模拟量精确采样并将采样得到的模拟量转换成数字量,然后再进行数字乘法,即可获得有功功率在采样周期内的平均值。即如果将某段时间内的每一个采样周期的有功功率平均值累加,就可得到这段时间内电路所消耗的有功电能。

只要将电流取样值与延时π/2(50Hz时为5ms)的电压取样值进行数字相乘即可得到无功功率在采样周期T内的平均值。

同样,将某时间内的每一个采样周期的无功功率平均值累加,就可得到这段时间内电路所消耗的无功电能。

这样,A/D转换型电子式多功能电能表就实现了有、无功电能的计量功能。

阅读理解

阅读理解。

     A car needs gas to run and your body also needs food to work for you. Eating the right kind

of food is very important. It can help your body grow strong, so take care of what you eat.

    There are four main food groups altogether. The dairy group(乳制品) has food like milk, cheese

and sour milk. The other three groups are the meat and fish group, the fruit and vegetable group,

and the bread and rice group. Each meal should have at least one food from all four main groups.

With all these food together, you will be given enough energy during the day.

    It is easy to get into bad eating habits. You may eat your breakfast in a hurry to get to school on

time. Or you may not have time for a good lunch. It may seem easy to finish your supper with fish

and chips all the time. But you will find yourself tired in these days and you can not think quickly.

    Watching what you eat will help keep your body healthy and strong. It is also good to take some

exercise. It will help you eat more if you take a walk or play games in the open air. Having a good

eating habit with some exercise is the key to your health.

1. Which of the following diets do you think is the best one?

A. Milk, bread, cabbage and beef.      

B. Eggs, tomatoes and chicken.  

C. Corn, fish, cream and pork.         

D. Rice, beancurd, apples, fish and chicken.

2. In the passage the writer mainly tells us that________.

A. every person needs food to grow well

B. taking exercise can keep your body strong

C. right kind of food with exercise will keep you healthy

D. enough energy helps people think more quickly

3. The best title for the passage is_________.

A. The Four Food Groups                  

B. A Healthy Diet 

C. Your Body and Food                    

D. The key to Your Health

单项选择题