说明电机换向极的功用及极性？
参考答案：
直流电机的换向极是为了消除电枢反应影响、改善电机换向而设置的。换向极绕组应与电枢绕组串联，使换向极产生的磁场强度随电枢电流的增大而增强；同时还使换向极的方向与此处出现的电枢磁场方向相反。这样，就可以使任何工况下换向极产生的磁场，总是起到抵消电枢反应影响的作用，使换向元件有较好的换向条件。
换向极的极性按电枢旋转方向，发电机主磁极前方的是异极性换向极；而电动机前方的是同极性换向极。这是因为在磁极极性与电枢旋转方向相同的发电机和电动机中，电枢电流方向是相反的，电枢磁场的方向也相反，要使换向极磁场抵消电枢磁场，发电机和电动机附加极的极性也必须相反。
Passage Two
| People can be addicted to different things — e.g. alcohol, drugs, certain foods, or even television. People who have such addition are compulsive, i.e. they have a very powerful psychological need that they feel they must spend money. This compulsion, like most others, is irrational — impossible to explain reasonably. For compulsive spenders who buy on credit, charge accounts are even more exciting than money. In other words, compulsive spenders feel that with credit, they can do anything. Their pleasure in spending enormous amounts is actually greater than the pleasure that they get from the things they buy. There is special psychology of bargain hunting. To save money, of course, most people search for sales, low prices and discounts. Compulsive bargain hunters, however, often buy things they don’t need just only because they are cheap. They believe that they are helping their budgets, but they are really playing an exciting game: when they can buy something for less than other people, they feel that they are winners. Most people, experts claim, have two reasons for their behavior: a good reason for things that they do and the real one. Of course, it is not only scientists who understand the psychology of spending habits, but also business people. Stores, companies, and advertisers use psychology to increase business: they consider people’s needs for love, power, or influence, their basic value, their beliefs and opinions, and so on in their advertising and sales methods. Psychologists can often use a method called "behavior therapy" to help individuals solve their personality problems. In the same way, they can help people who feel that they have problems with money. |
According to the passage, which of the following is true
A．All people only buy things they need.
B．Both business people and advertisers increase sales through using psychology.
C．Scientists understand psychology better than business people.
D．Compulsive spenders can never get any help to solve their problems with money.