问题 问答题 简答题

为什么变压器的空载损耗可近似看成铁损耗,而短路损耗可近似看成为铜损耗?

答案

参考答案:变压器铁损耗的大小决定于铁心中磁通密度的大小,铜损耗的大小决定决定于绕组中电流的大小。变压器空载和短路时,输出功率都为零。输入功率全部变为变压器的损耗。即铜损耗与铁损耗之和。空载时,电源电压为额定值,铁心中磁通密度达到正常运行的数值,铁损耗也为正常运行时的数值。而此时二次绕组中的电流为零,没有铜损耗,一次绕组中电流仅为励磁电流,远小于正常运行的数值,它产生的铜损耗相对于这时的铁损耗可以忽略不计,因而空载损耗可近似看成为铁损耗。短路试验时,输入功率为短路损耗。此时一次、二次绕组电流均为额定值,铜损耗也达到正常运行时的数值,而电压大大低于额定电压,铁心中磁通密度也大大低于正常运行时的数值,此时铁损耗与铜损耗相比可忽略不计。因此短路损耗可近似看成铜损耗。

判断题
单项选择题

Some of the concerns surrounding Turkey’s application to join the European Union, to be (1) on by the EU’s Council of Ministers on December 17th, are economic--in particular, the country’s relative poverty. Its GDP per head is less than a third of the average for the 15 pre-2004 members of the EU. (2) it is not far off that of Latvia--one of the ten new members which (3) on May 1st 2004, and it is much the same as (4) of two countries, Bulgaria and Romania, which this week concluded (5) talks with the EU that could make them full members on January 1st 2007.

(6) , the country’s recent economic progress has been, according to Donald Johnston, the secretary-general of the OECD, stunning. GDP in the second quarter of the year was 13.4% higher than a year earlier, a (7) of growth that no EU country comes close to (8) . Turkey’s (9) rate has just fallen into single figures for the first time since 1972, and this week the country (10) agreement with the IMF on a new three-year, $10 billion economic program that will help Turkey (11) inflation toward European levels, and enhance the economy’s resilience.

Resilience has not historically been the country’s economic p point. (12) , throughout the 1990s growth oscillated like an electrocardiogram (13) a violent heart attack. This (14) has been one of the main reasons why the country has failed dismally to attract much-needed foreign direct investment. Its stock of such investment is lower now than it was in the 1980s, and annual (15) have scarcely ever reached $1 billion.

One deterrent to foreign investors is due to (16) on January 1st 2005. On that day, Turkey will take away the right of virtually every one of its citizens to call themselves a millionaire. Six zeros will be removed from the face value of the lira (里拉,土耳其货币单位); one unit of the local (17) will henceforth be worth what 1 million are now--ie, about £ 0.53 (0.53 欧元). Goods will have to be (18) in both the new and old lira for the whole of the year, (19) foreign bankers and (20) can begin to look forward to a time in Turkey when they will no longer have to juggle mentally with indeterminate strings of zeros.

14()

A.mobility

B.flexibility

C.stability

D.irregularity