问题 问答题 简答题

异步电动机的调速方式分几种?

答案

参考答案:

异步电动机的调速方式可分为变极对数、变转差率及变定子供电频率三种。

①改变定子极对数的办法来调速则需在电机运行时改变定子接法或在定子上绕上2套、3套不同极对数的线圈,后者成本高,由于极对数是整数,这种调速方法只能是跳跃式的。通常是改变电机定子绕组接线方式实现的,得到的是二级,即调速比为2:1,叫做双速电动机,还有三速、四速电机。

②变转差率的方法又可以通过调定子电压、转子电阻、转差电压等方法来实现。

③异步电动机的变频调速是通过改变定子供电频率来改变同步转速而实现调速的,在调速中从高速到低速都可以保持较小的转差率,因而消耗转差功率小,效率高。可以认为,变频调速是异步电动机的唯一最为合理的调速方法。

单项选择题

I will never forget the year I was about twelve years old. My mother told us that we would not be (16) Christmas gifts because there was not enough money. I felt sad and thought, "What would I say when the other kids asked what I’ d (17) " Just when I started to (18) that there would not be a Christmas that year, three women (19) at our house with gifts for all of us. For me they brought a doll. I felt such a sense of (20) that I would no longer have to be embarrassed when I returned to school. I wasn’t (21) . Somebody had thought (22) of me to bring me a gift.

Years later, when I stood in the kitchen of my new house, thinking how I wanted to make my (23) Christmas there special and memorable, I (24) remembered the women’s visit. I decided that I wanted to create that same feeling of (25) for as many children as I could possibly reach.

So I (26) a plan and gathered forty people from my company to help. We gathered about 125 orphans (孤儿) at the Christmas party. For every child, we wrapped colorful packages filled with

toys, clothes, and school supplies, (27) with a child’ s name. We wanted all of them to know they were (28) Before I called out their names and handed them their gifts, I (29) them that they couldn’t open their presents (30) every child had come forward. Finally the (31) they had been waiting for came as I called out, "One, two, three. Open your presents!" As the children opened their packages, their faces beamed and their bright smiles (32) up the room. The (33) in the room was obvious, and (34) wasn’ t just about toys. It was a feeling, and the feeling I knew (35) that Christmas so long ago when the women came to visit. I wasn’t forgotten. Somebody thought of me. I matter.

31().

A. chance

B. gift

C. moment

D. reward

单项选择题