问题 问答题 简答题

脉宽调制型逆变器有哪些优点?

答案

参考答案:脉宽调制型(PWM)逆变器是随着全控、高开关频率的新型电力电子器件的产生,而逐步发展起来的,其主要优点是:

主电路只有一个可控的功率环节,开关元件少,简化了结构;

使用不可控整流器,使电网功率因数与逆变器输出电压的大小无关而接近于1;

由于逆变器本身同时完成调频和调压的任务,因此与中间滤波环节的滤波元件无关,变频器动态响应加快;可获得比常规阶梯波更好的输出电压波形,输出电压的谐波分量极大地减少,能抑制或消除低次谐波,实现近似正弦波的输出交流电压波形。

阅读理解

阅读理解。

      One day when Jack was walking in the park, he saw a woman, who lived a few miles away, sitting on a

bench with a dog beside her. The dog was looking up at the woman.

      Jack walked up to the woman and said, "Hello, Sue, how are you? May I sit and talk with you for a while?"

"Of course, Please sit down", Sue said. Jack sat down next to Sue on the bench, and they talked quietly together.

The dog continued to look up at Sue, as if waiting to be fed.

      "That's a nice dog, isn't it he?" Jack said, pointing at the animal.

      "Yes, he is. He's handsome. He's a bit of a mixture, but that's not a bad thing. He's strong and healthy."

      "And hungry," Jack said, "He hasn't taken his eyes off you. He thinks you've got some food for him."

      "That's true," Sue said, "But I haven't." 

      They both laughed and then Jack said, "Does your dog bite?"

      "No," Sue said, "He's never bitten anyone. He's always gentle and friendly."

      Hearing this, Jack decided to hold out his hand and touched the animal's head. Suddenly it jumped up and

bit him.

      "Hey!" Jack shouted. "You said your dog didn't bite." Sue replied in surprise. "Yeah, I did. But this is not my

dog. My dog's at home."

1. The dog looked at the woman because _______. [ ]

A. the woman wanted to feed him

B. the woman was friendly

C. he was strong and healthy

D. he was hungry

2. Jack and Sue were _______. [ ]

A. friends

B. nest-door neighbors

C. strangers

D. in the same family

3. Jack touched the dog because he believed _________. [ ]

A. the dog was handsome

B. Sue's dog was unfriendly

C. the dog belonged to Sue

D. Sue's dog was at home

4. We can infer from the passage that _______.[ ]

A. Sue gave a wrong answer

B. Jack made a mistake

C. the dog wasn't dangerous

D. both Jack and Sue like the dog

5. Which of the following can be the best title of the passage? [ ]

A. A Wrong Question

B. Sue's Dog

C. A Pleasant Meeting

D. Sue's Friend

填空题