功率(lv)是表(biao)征電信號特(te)性的(de)一個重(zhong)要參數(shu)。在(zai)直流和低頻范(fan)圍(wei),可以(yi)通過(guo)測量電壓和電流計算功率(lv),功率(lv)的(de)瞬時(shi)值可用下式表(biao)示:
對(dui)于周(zhou)期信號,一(yi)個周(zhou)期內的瞬時功率的平均值(zhi),稱為有功功率。有功功率按下式計算:
對(dui)于正弦電(dian)路,下式成立:
上式中,U、I分別為正(zheng)弦交流電(dian)的有效值,φ為電(dian)壓與電(dian)流信號的相位差。
在(zai)(zai)(zai)超高頻(pin)(pin)和微波(bo)(bo)頻(pin)(pin)段(duan),有(you)TEM波(bo)(bo)和非(fei)TEM波(bo)(bo)之(zhi)分。在(zai)(zai)(zai)TEM波(bo)(bo)的(de)同軸系統中,電(dian)(dian)(dian)(dian)壓和電(dian)(dian)(dian)(dian)流(liu)雖有(you)確切含意(yi),但測(ce)量其絕對值(zhi)很困(kun)難。在(zai)(zai)(zai)波(bo)(bo)導系統中,因為存在(zai)(zai)(zai)不同的(de)電(dian)(dian)(dian)(dian)磁模式,電(dian)(dian)(dian)(dian)壓和電(dian)(dian)(dian)(dian)流(liu)失去(qu)唯一性。在(zai)(zai)(zai)個(ge)頻(pin)(pin)段(duan)和各傳輸系統中,功率是單值(zhi)表(biao)征信號強度的(de)重要(yao)方(fang)法。在(zai)(zai)(zai)射(she)頻(pin)(pin)范圍直(zhi)接測(ce)量功率代替了(le)電(dian)(dian)(dian)(dian)壓和電(dian)(dian)(dian)(dian)流(liu)的(de)測(ce)量。
功率定義為(wei)(wei)單位(wei)時間內所做的(de)功。基(ji)本單位(wei)為(wei)(wei)瓦(wa)(W),1W等于(yu)在1秒內做1焦耳的(de)功。常用(yong)的(de)功率單位(wei)還有兆瓦(wa)(1MW=10^6W)、千瓦(wa)(1KW=10^3W)、毫瓦(wa)(1mW=10-3W)、微瓦(wa)(1μW=10-6W)、皮瓦(wa)(1Pw=10-12W)。
另(ling)一種(zhong)常用(yong)的功率(lv)(lv)單(dan)位以分貝毫瓦(dBm)表示。它以1毫瓦為基準電平P0=1mW,實(shi)際功率(lv)(lv)值P(mW)與P0比較(jiao)后取(qu)對數。這是功率(lv)(lv)的絕對單(dan)位。也可用(yong)分貝瓦(dBW)作為功率(lv)(lv)單(dan)位,此時P0=1W,即1dBW=3dBm。
根據被測信號頻率分類
功(gong)(gong)率(lv)(lv)(lv)計(ji)可分(fen)為(wei):直流功(gong)(gong)率(lv)(lv)(lv)計(ji)、工(gong)(gong)頻(pin)(pin)功(gong)(gong)率(lv)(lv)(lv)計(ji)、變頻(pin)(pin)功(gong)(gong)率(lv)(lv)(lv)計(ji)、射頻(pin)(pin)功(gong)(gong)率(lv)(lv)(lv)計(ji)和(he)微波(bo)功(gong)(gong)率(lv)(lv)(lv)計(ji)。由于(yu)(yu)直流功(gong)(gong)率(lv)(lv)(lv)等于(yu)(yu)電(dian)壓和(he)電(dian)流的簡(jian)單乘(cheng)積,實際測量(liang)中,一般采用電(dian)壓表(biao)(biao)和(he)電(dian)流表(biao)(biao)替代。工(gong)(gong)頻(pin)(pin)功(gong)(gong)率(lv)(lv)(lv)計(ji)是應用較普遍的功(gong)(gong)率(lv)(lv)(lv)計(ji),常說的功(gong)(gong)率(lv)(lv)(lv)計(ji)一般都是指工(gong)(gong)頻(pin)(pin)功(gong)(gong)率(lv)(lv)(lv)計(ji)。變頻(pin)(pin)功(gong)(gong)率(lv)(lv)(lv)計(ji)是21世紀變頻(pin)(pin)調速技(ji)術高速發展的產物。其測量(liang)對象為(wei)變頻(pin)(pin)電(dian)量(liang),變頻(pin)(pin)電(dian)量(liang)是指用于(yu)(yu)傳輸功(gong)(gong)率(lv)(lv)(lv)的,并且(qie)滿足下(xia)述(shu)條件之一的交流電(dian)量(liang):
1、信號頻(pin)譜僅包含一(yi)種頻(pin)率成(cheng)分,而(er)頻(pin)率不局限于工頻(pin)的交流(liu)電信號。
2、信號頻譜包含兩種或(huo)更多的被關(guan)注(zhu)的頻率成分(fen)的電(dian)信號。
變頻電(dian)(dian)量包括(kuo)電(dian)(dian)壓(ya)、電(dian)(dian)流以及電(dian)(dian)壓(ya)電(dian)(dian)流引出的有功(gong)功(gong)率(lv)(lv)、無功(gong)功(gong)率(lv)(lv)、視(shi)在功(gong)率(lv)(lv)、有功(gong)電(dian)(dian)能、無功(gong)電(dian)(dian)能等(deng)。
除了變頻(pin)器輸(shu)出的PWM波,二極管整(zheng)流(liu)(liu)的變頻(pin)器輸(shu)入(ru)的電(dian)流(liu)(liu)波形,直流(liu)(liu)斬波器輸(shu)出的電(dian)壓波形,變壓器空載的輸(shu)入(ru)電(dian)流(liu)(liu)波形等(deng),均含有(you)較大(da)的諧波,右圖(tu)中為常(chang)見(jian)變頻(pin)電(dian)量的波形及(ji)相關頻(pin)譜(pu)圖(tu)。
由于變(bian)頻(pin)(pin)(pin)電量的頻(pin)(pin)(pin)率(lv)(lv)成分復雜,變(bian)頻(pin)(pin)(pin)功率(lv)(lv)計(ji)(ji)的測量一般(ban)包括基(ji)波(bo)(bo)(bo)有(you)功功率(lv)(lv)(簡稱(cheng)基(ji)波(bo)(bo)(bo)功率(lv)(lv))、諧波(bo)(bo)(bo)有(you)功功率(lv)(lv)(簡稱(cheng)諧波(bo)(bo)(bo)功率(lv)(lv))、總(zong)有(you)功功率(lv)(lv)等,相比工頻(pin)(pin)(pin)功率(lv)(lv)計(ji)(ji)而言(yan),其功能較(jiao)多,技術較(jiao)復雜,一般(ban)稱(cheng)為變(bian)頻(pin)(pin)(pin)功率(lv)(lv)分析(xi)(xi)儀或寬頻(pin)(pin)(pin)功率(lv)(lv)分析(xi)(xi)儀,部分高精度功率(lv)(lv)分析(xi)(xi)儀也(ye)適用(yong)于變(bian)頻(pin)(pin)(pin)電量測量。
變(bian)頻功率分析儀可(ke)以作為(wei)工頻功率分析儀使用,除此之外,一般還(huan)需滿足下(xia)述要求:
1、滿足必要(yao)的帶(dai)寬要(yao)求,并且采樣(yang)頻率(lv)應高于儀器帶(dai)寬的兩(liang)倍。
2、要(yao)求(qiu)分(fen)析儀(yi)在較寬(kuan)的(de)頻率(lv)范圍之內,精(jing)度均能(neng)滿足(zu)一定的(de)要(yao)求(qiu)。
3、具備傅(fu)里葉變(bian)換功能(neng),可以分離信號的(de)基波(bo)和諧波(bo)。
射頻或微波功率計按照在測試(shi)系統中的(de)連接(jie)方式(shi)不同分類(lei)
有終端式(shi)(shi)和通過式(shi)(shi)兩種(zhong)。終端式(shi)(shi)功(gong)(gong)(gong)(gong)率(lv)計(ji)把功(gong)(gong)(gong)(gong)率(lv)計(ji)探頭(tou)作為測試系統的終端負載,功(gong)(gong)(gong)(gong)率(lv)計(ji)吸(xi)收全部待(dai)測功(gong)(gong)(gong)(gong)率(lv),由功(gong)(gong)(gong)(gong)率(lv)指示器直接(jie)讀取功(gong)(gong)(gong)(gong)率(lv)值。通過式(shi)(shi)功(gong)(gong)(gong)(gong)率(lv)計(ji)利(li)用某種(zhong)耦(ou)合(he)(he)裝置,如定(ding)向耦(ou)合(he)(he)器、耦(ou)合(he)(he)環、探針等(deng)從傳(chuan)輸的功(gong)(gong)(gong)(gong)率(lv)中按一(yi)(yi)定(ding)的比例(li)耦(ou)合(he)(he)出一(yi)(yi)部分功(gong)(gong)(gong)(gong)率(lv),送入(ru)功(gong)(gong)(gong)(gong)率(lv)計(ji)度量,傳(chuan)輸的總功(gong)(gong)(gong)(gong)率(lv)等(deng)于功(gong)(gong)(gong)(gong)率(lv)計(ji)指示值乘(cheng)以比例(li)系數。
射頻或(huo)微波功率計(ji)按的測量(liang)原理分類
測(ce)熱電(dian)阻(zu)型功率計使用(yong)熱變(bian)(bian)(bian)電(dian)阻(zu)做(zuo)功率傳感元(yuan)件。熱變(bian)(bian)(bian)電(dian)阻(zu)值的(de)溫度(du)系數較大(da)。被(bei)測(ce)信號的(de)功率被(bei)熱變(bian)(bian)(bian)電(dian)阻(zu)吸收(shou)后產生(sheng)熱量,使其自身溫度(du)升高,電(dian)阻(zu)值發生(sheng)顯(xian)著(zhu)變(bian)(bian)(bian)化(hua),利(li)用(yong)電(dian)阻(zu)電(dian)橋測(ce)量電(dian)阻(zu)值的(de)變(bian)(bian)(bian)化(hua),顯(xian)示功率值。
熱(re)(re)電偶型(xing)功(gong)(gong)率(lv)計(ji)(ji)熱(re)(re)電偶型(xing)功(gong)(gong)率(lv)計(ji)(ji)中的(de)熱(re)(re)偶結直(zhi)接吸(xi)收(shou)高頻(pin)信(xin)號(hao)功(gong)(gong)率(lv),結點(dian)溫度升高,產生溫差電勢(shi),電勢(shi)的(de)大小正(zheng)比于吸(xi)收(shou)的(de)高頻(pin)功(gong)(gong)率(lv)值(zhi)。
量熱式功(gong)率(lv)(lv)(lv)計(ji)典型的(de)(de)熱效應功(gong)率(lv)(lv)(lv)計(ji),利用隔熱負載吸(xi)收高(gao)頻信(xin)號功(gong)率(lv)(lv)(lv),使負載的(de)(de)溫度升高(gao),再利用熱電(dian)偶元件測量負載的(de)(de)溫度變(bian)化量,根據產(chan)生的(de)(de)熱量計(ji)算高(gao)頻功(gong)率(lv)(lv)(lv)值。
晶體(ti)檢波式功率計晶體(ti)二(er)極管檢波器將高(gao)頻(pin)(pin)信號變換為低(di)頻(pin)(pin)或直流電信號。適當選擇工(gong)作點,使檢波器輸出信號的(de)幅度正比于高(gao)頻(pin)(pin)信號的(de)功率。
射頻或微波功率計按被測信號連續性分類有連續波功率計(ji)和(he)脈沖峰值功率計(ji)。
以下是變頻功率分析儀(yi)的典型技術指(zhi)標
帶寬:50kHz~100kHz;
采(cai)樣(yang)頻(pin)率:大(da)于(yu)帶寬的2倍;
電(dian)壓、電(dian)流準確級(ji):0.02級(ji)、0.05級(ji)、0.1級(ji)、0.2級(ji)、0.5級(ji);
功率準確級(ji)(ji):0.05級(ji)(ji)、0.1級(ji)(ji)、0.2級(ji)(ji)、0.5級(ji)(ji)、1級(ji)(ji);
準確級適用基波頻(pin)率范圍:DC,0.1Hz~400Hz;
準(zhun)確(que)級適(shi)用電壓范圍:0.75%Un~150%Un;
準確級適用(yong)電流范圍:1%In~200%In;
準確級適用功率因數(shu)范圍(wei):0.05~1。
以(yi)下是(shi)射頻功率計的(de)典(dian)型技術(shu)指標
功率范圍
保證(zheng)測量精度的可測功(gong)(gong)率值的范(fan)圍(wei)。功(gong)(gong)率計的功(gong)(gong)率范(fan)圍(wei)決(jue)定于功(gong)(gong)率探頭(tou)。
最大允許功率
探(tan)頭不(bu)(bu)被損(sun)壞的(de)最大(da)輸入功率(lv)值(zhi)(zhi),通(tong)常指平均功率(lv)。在測量大(da)功率(lv)峰值(zhi)(zhi)信號時,注意峰值(zhi)(zhi)電(dian)壓和峰值(zhi)(zhi)功率(lv)不(bu)(bu)能超(chao)過一定值(zhi)(zhi),否(fou)則會造成功率(lv)探(tan)頭燒毀。
頻率范圍
能保證測量(liang)精(jing)度和性能指標的被測信號的頻率范圍。
測量精度
指(zhi)功率探頭(tou)校(xiao)準修正后的精度。不(bu)包括測試系統的失配(pei)誤差。
穩定性
功率計(ji)的(de)穩定(ding)性取決(jue)于功率探(tan)頭的(de)穩定(ding)性和指示器的(de)零漂及噪聲干擾。
響應時間
也稱功(gong)率(lv)(lv)傳感元件(jian)的時間常數。通常指功(gong)率(lv)(lv)指示(shi)器上升到穩定值的64%所需的時間。
探頭的型號、阻抗
選用功率(lv)計(ji)探頭時(shi),功率(lv)探頭的使用頻率(lv)、功率(lv)范圍必須(xu)與(yu)被測信號一致,探頭傳輸線的結構和阻抗應(ying)與(yu)被測傳輸線相(xiang)互匹配。
用于(yu)測(ce)量(liang)(liang)(liang)絕對(dui)光(guang)功(gong)率(lv)(lv)或通(tong)(tong)過(guo)一段光(guang)纖的(de)(de)(de)(de)光(guang)功(gong)率(lv)(lv)相對(dui)損耗。在光(guang)纖系統中,測(ce)量(liang)(liang)(liang)光(guang)功(gong)率(lv)(lv)是(shi)最基本的(de)(de)(de)(de),非常像電(dian)子學中的(de)(de)(de)(de)萬用表(biao)。在光(guang)纖測(ce)量(liang)(liang)(liang)中,光(guang)功(gong)率(lv)(lv)計(ji)是(shi)重(zhong)負(fu)荷常用表(biao)。通(tong)(tong)過(guo)測(ce)量(liang)(liang)(liang)發(fa)射端機(ji)或光(guang)網絡的(de)(de)(de)(de)絕對(dui)功(gong)率(lv)(lv),一臺光(guang)功(gong)率(lv)(lv)計(ji)就(jiu)能(neng)夠評價光(guang)端設備的(de)(de)(de)(de)性能(neng)。用光(guang)功(gong)率(lv)(lv)計(ji)與穩定(ding)光(guang)源組合使用,則能(neng)夠測(ce)量(liang)(liang)(liang)連接(jie)損耗、檢驗連續性,并幫助(zhu)評估光(guang)纖鏈路傳輸質(zhi)量(liang)(liang)(liang)。
變(bian)頻功率分(fen)析儀(yi)適用(yong)于電力(li)推進(jin)、電機、風(feng)(feng)機、水泵、風(feng)(feng)力(li)發電、軌道交通、電動汽車、變(bian)頻器(qi)、特(te)種(zhong)變(bian)壓器(qi)、熒光燈、LED照明等領域的(de)產品檢試(shi)驗、能效評測及電能質量分(fen)析。
選(xuan)擇(ze)(ze)(ze)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表(biao)的(de)(de)量(liang)(liang)(liang)程(cheng)就是(shi)選(xuan)擇(ze)(ze)(ze)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表(biao)中的(de)(de)電(dian)流量(liang)(liang)(liang)程(cheng)和電(dian)壓(ya)(ya)量(liang)(liang)(liang)程(cheng)。使(shi)用時(shi)應使(shi)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表(biao)中的(de)(de)電(dian)流量(liang)(liang)(liang)程(cheng)不(bu)小于(yu)(yu)負(fu)載(zai)(zai)(zai)電(dian)流,電(dian)壓(ya)(ya)量(liang)(liang)(liang)程(cheng)不(bu)低于(yu)(yu)負(fu)載(zai)(zai)(zai)電(dian)壓(ya)(ya),而不(bu)能僅從功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)量(liang)(liang)(liang)程(cheng)來考慮。例如,兩只功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表(biao),量(liang)(liang)(liang)程(cheng)分別(bie)是(shi)IA、300V和2A、150V,由(you)計(ji)算可知其功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)量(liang)(liang)(liang)程(cheng)均為300W,如果要(yao)(yao)測量(liang)(liang)(liang)一負(fu)載(zai)(zai)(zai)電(dian)壓(ya)(ya)為220V、電(dian)流為IA的(de)(de)負(fu)載(zai)(zai)(zai)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)時(shi)應逸用IA、300V的(de)(de)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表(biao),而2A、150V的(de)(de)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表(biao)雖功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)量(liang)(liang)(liang)程(cheng)也大于(yu)(yu)負(fu)載(zai)(zai)(zai)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv),但是(shi)由(you)于(yu)(yu)負(fu)載(zai)(zai)(zai)電(dian)壓(ya)(ya)高于(yu)(yu)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表(biao)所(suo)能承受的(de)(de)電(dian)壓(ya)(ya)150V,故不(bu)能使(shi)用。所(suo)以,在(zai)測量(liang)(liang)(liang)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)前要(yao)(yao)根(gen)據負(fu)載(zai)(zai)(zai)的(de)(de)額(e)定電(dian)壓(ya)(ya)和額(e)定電(dian)流來選(xuan)擇(ze)(ze)(ze)功(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表(biao)的(de)(de)量(liang)(liang)(liang)程(cheng)。
電(dian)(dian)動系測量機(ji)構的(de)轉動力矩方向(xiang)和兩線(xian)圈(quan)中的(de)電(dian)(dian)流方向(xiang)有(you)關(guan),為了防止電(dian)(dian)動系功(gong)率表(biao)(biao)的(de)指針反偏,接線(xian)時功(gong)率表(biao)(biao)電(dian)(dian)流線(xian)圈(quan)標(biao)有(you)“·”號的(de)端鈕(niu)必須接到電(dian)(dian)源的(de)正極(ji)端,而電(dian)(dian)流線(xian)圈(quan)的(de)另(ling)一(yi)端則(ze)(ze)與負載相連,電(dian)(dian)流線(xian)圈(quan)以(yi)串聯形(xing)式接入電(dian)(dian)路中。功(gong)率表(biao)(biao)電(dian)(dian)壓(ya)線(xian)圈(quan)標(biao)有(you)“·”號的(de)端鈕(niu)可以(yi)接到電(dian)(dian)源端鈕(niu)的(de)任一(yi)端上,而另(ling)一(yi)電(dian)(dian)壓(ya)端鈕(niu)則(ze)(ze)跨接到負載的(de)另(ling)一(yi)端。
當負載(zai)(zai)(zai)電(dian)阻(zu)遠遠大于電(dian)流(liu)(liu)(liu)線(xian)(xian)圈(quan)(quan)(quan)(quan)的(de)電(dian)阻(zu)時(shi),應采用電(dian)壓線(xian)(xian)圈(quan)(quan)(quan)(quan)前接法。這時(shi)電(dian)壓線(xian)(xian)圈(quan)(quan)(quan)(quan)的(de)電(dian)壓是負載(zai)(zai)(zai)電(dian)壓和(he)(he)(he)電(dian)流(liu)(liu)(liu)線(xian)(xian)圈(quan)(quan)(quan)(quan)電(dian)壓之(zhi)和(he)(he)(he),功(gong)率(lv)表測量的(de)是負載(zai)(zai)(zai)功(gong)率(lv)和(he)(he)(he)電(dian)流(liu)(liu)(liu)線(xian)(xian)圈(quan)(quan)(quan)(quan)功(gong)率(lv)之(zhi)和(he)(he)(he)。如果負載(zai)(zai)(zai)電(dian)阻(zu)遠遠大于電(dian)流(liu)(liu)(liu)線(xian)(xian)圈(quan)(quan)(quan)(quan)的(de)電(dian)阻(zu),則可以略去(qu)電(dian)流(liu)(liu)(liu)線(xian)(xian)圈(quan)(quan)(quan)(quan)分壓所造成的(de)影響(xiang),測量結(jie)果比較接近負載(zai)(zai)(zai)的(de)實際功(gong)率(lv)值。
當負(fu)載電(dian)(dian)(dian)(dian)阻(zu)遠(yuan)遠(yuan)小(xiao)(xiao)于電(dian)(dian)(dian)(dian)壓線(xian)(xian)圈(quan)(quan)(quan)電(dian)(dian)(dian)(dian)阻(zu)時,應采(cai)用電(dian)(dian)(dian)(dian)壓線(xian)(xian)圈(quan)(quan)(quan)后(hou)接法。這時電(dian)(dian)(dian)(dian)壓線(xian)(xian)圈(quan)(quan)(quan)兩端的(de)電(dian)(dian)(dian)(dian)壓雖然等于負(fu)載電(dian)(dian)(dian)(dian)壓,但電(dian)(dian)(dian)(dian)流(liu)(liu)線(xian)(xian)圈(quan)(quan)(quan)中(zhong)(zhong)的(de)電(dian)(dian)(dian)(dian)流(liu)(liu)卻等于負(fu)載電(dian)(dian)(dian)(dian)流(liu)(liu)與功率(lv)(lv)表電(dian)(dian)(dian)(dian)壓線(xian)(xian)圈(quan)(quan)(quan)中(zhong)(zhong)的(de)電(dian)(dian)(dian)(dian)流(liu)(liu)之(zhi)和(he),測(ce)量時功率(lv)(lv)讀(du)數(shu)為負(fu)載功率(lv)(lv)與電(dian)(dian)(dian)(dian)壓線(xian)(xian)圈(quan)(quan)(quan)功率(lv)(lv)之(zhi)和(he)。由于此時負(fu)載電(dian)(dian)(dian)(dian)阻(zu)遠(yuan)小(xiao)(xiao)于電(dian)(dian)(dian)(dian)壓線(xian)(xian)圈(quan)(quan)(quan)電(dian)(dian)(dian)(dian)阻(zu),所以(yi)電(dian)(dian)(dian)(dian)壓線(xian)(xian)圈(quan)(quan)(quan)分(fen)流(liu)(liu)作用大大減小(xiao)(xiao),其對測(ce)量結果的(de)影(ying)響也可(ke)以(yi)大為減小(xiao)(xiao)。
如界被測負載(zai)本身功(gong)(gong)(gong)率(lv)較(jiao)大,可(ke)以不(bu)考(kao)慮功(gong)(gong)(gong)率(lv)表(biao)本身的(de)功(gong)(gong)(gong)率(lv)對測量結果的(de)影響,則兩種接(jie)法可(ke)以任(ren)意(yi)選(xuan)擇。但最好選(xuan)用電(dian)(dian)壓線圈前接(jie)法,因為功(gong)(gong)(gong)率(lv)表(biao)中電(dian)(dian)流線圈的(de)功(gong)(gong)(gong)率(lv)一(yi)般(ban)都小于電(dian)(dian)壓線圈支路的(de)功(gong)(gong)(gong)率(lv)。
一般安裝式功率表為直讀單量程(cheng)(cheng)式,表上(shang)的(de)示數(shu)(shu)(shu)(shu)(shu)即為功率數(shu)(shu)(shu)(shu)(shu)。但便攜式功率表一般為多量程(cheng)(cheng)式,在(zai)(zai)表的(de)標度尺上(shang)不直接標注(zhu)示數(shu)(shu)(shu)(shu)(shu),只標注(zhu)分格(ge)。在(zai)(zai)選用不同(tong)的(de)電(dian)(dian)流與(yu)電(dian)(dian)壓量程(cheng)(cheng)時,每一分格(ge)都可以表示不同(tong)的(de)功率數(shu)(shu)(shu)(shu)(shu)。在(zai)(zai)讀數(shu)(shu)(shu)(shu)(shu)時,應先根據(ju)所(suo)選的(de)電(dian)(dian)壓量程(cheng)(cheng)U、電(dian)(dian)流量程(cheng)(cheng)I以及標度尺滿量程(cheng)(cheng)時的(de)格(ge)數(shu)(shu)(shu)(shu)(shu)&,求出每格(ge)瓦數(shu)(shu)(shu)(shu)(shu)(又稱(cheng)功率表常數(shu)(shu)(shu)(shu)(shu))C,然后再乘(cheng)上(shang)指針偏轉(zhuan)的(de)格(ge)數(shu)(shu)(shu)(shu)(shu)夕(xi),就可得到所(suo)測功率P
例題
例:有一只電壓量(liang)程為(wei)250V,電流量(liang)程為(wei)3A,標度尺分(fen)格數為(wei)75的功(gong)率(lv)表,現用它來測量(liang)負載的功(gong)率(lv)。
當指針偏轉(zhuan)50格時負載功率為多(duo)少?
解:先計算功率表常數C
C=UI/a,=250V×3A/75格=10W/格
故被測功率為
P=C色=10W/格×50格=500W
對大多(duo)數從事(shi)電氣方面工(gong)作的(de)(de)人(ren)員(yuan)來說(shuo),功(gong)率(lv)表(biao)(biao)的(de)(de)使(shi)(shi)用(yong)并非難事(shi)。但真正做到正確使(shi)(shi)用(yong)功(gong)率(lv)表(biao)(biao),即在準確度一定的(de)(de)情(qing)況下確保測(ce)量(liang)的(de)(de)精度及儀(yi)表(biao)(biao)的(de)(de)使(shi)(shi)用(yong)壽命又并非易事(shi)。以(yi)單相(xiang)電動系功(gong)率(lv)表(biao)(biao)為(wei)例,就功(gong)率(lv)表(biao)(biao)的(de)(de)使(shi)(shi)用(yong)及使(shi)(shi)用(yong)中應注意(yi)的(de)(de)問題(ti)作一介紹。
1、要遵守“發電機端守則”
由電(dian)(dian)動系功率(lv)表(biao)的(de)(de)(de)原(yuan)理(li)可知,功率(lv)表(biao)的(de)(de)(de)轉(zhuan)矩與流過表(biao)內線圈(quan)(quan)的(de)(de)(de)電(dian)(dian)流方(fang)向(xiang)有(you)關,一旦其中一個線圈(quan)(quan)的(de)(de)(de)電(dian)(dian)流方(fang)向(xiang)改變,轉(zhuan)矩方(fang)向(xiang)也會改變。為此,在功率(lv)表(biao)兩個線圈(quan)(quan)對(dui)應于電(dian)(dian)流流進的(de)(de)(de)端(duan)鈕上,都(dou)注有(you)稱為發電(dian)(dian)機(ji)端(duan)的(de)(de)(de)“*”標志。功率(lv)表(biao)在接線時,應使(shi)電(dian)(dian)流或電(dian)(dian)壓線圈(quan)(quan)帶“*”標志的(de)(de)(de)端(duan)鈕接到電(dian)(dian)源同極性(xing)的(de)(de)(de)端(duan)子上,以保證兩線圈(quan)(quan)的(de)(de)(de)電(dian)(dian)流方(fang)向(xiang)都(dou)從發電(dian)(dian)機(ji)端(duan)流入(ru)。這(zhe)就是功率(lv)表(biao)接線的(de)(de)(de)“發電(dian)(dian)機(ji)端(duan)守(shou)則”。
2、合(he)理選擇電壓線圈(quan)的前、后(hou)接方式
盡(jin)管(guan)電(dian)壓線圈不論前接(jie)還是后接(jie),功率表(biao)都能正偏,對于某些(xie)負載(zai)來說,測量的(de)結果相(xiang)(xiang)差較小,這時(shi)兩種接(jie)法采(cai)用哪種均可。但對于那些(xie)電(dian)阻(zu)(或阻(zu)抗(kang))過大或過小的(de)負載(zai)來說,兩種接(jie)法所得結果相(xiang)(xiang)差較大,有時(shi)甚(shen)至(zhi)出現與(yu)理論相(xiang)(xiang)矛盾的(de)結果。
這(zhe)種方式(shi)的(de)接線(xian),功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表電(dian)(dian)流(liu)(liu)(liu)線(xian)圈(quan)(quan)的(de)電(dian)(dian)流(liu)(liu)(liu)雖然(ran)等于負(fu)載電(dian)(dian)流(liu)(liu)(liu),但功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表電(dian)(dian)壓支(zhi)路兩(liang)端電(dian)(dian)壓卻等于負(fu)載電(dian)(dian)壓與功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表電(dian)(dian)流(liu)(liu)(liu)線(xian)圈(quan)(quan)的(de)電(dian)(dian)壓之和,在功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表讀數中(zhong)多了電(dian)(dian)流(liu)(liu)(liu)線(xian)圈(quan)(quan)的(de)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)消耗。這(zhe)種接線(xian)方式(shi)適用于負(fu)載電(dian)(dian)阻(zu)(或阻(zu)抗(kang))遠(yuan)比(bi)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表電(dian)(dian)流(liu)(liu)(liu)線(xian)圈(quan)(quan)電(dian)(dian)阻(zu)(或阻(zu)抗(kang))大得(de)多的(de)情況(kuang),這(zhe)樣才能(neng)保(bao)證功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)表本(ben)身(shen)的(de)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)消耗對測量結果的(de)影響比(bi)較小。
這種方式的(de)接線,功(gong)(gong)率表電(dian)(dian)壓(ya)支(zhi)(zhi)路(lu)兩端的(de)電(dian)(dian)壓(ya)雖然等(deng)于負載(zai)電(dian)(dian)壓(ya),但電(dian)(dian)流線圈(quan)的(de)電(dian)(dian)流卻等(deng)于負載(zai)電(dian)(dian)流與(yu)功(gong)(gong)率表電(dian)(dian)壓(ya)線圈(quan)支(zhi)(zhi)路(lu)電(dian)(dian)流之和(he),功(gong)(gong)率表讀(du)數中多了電(dian)(dian)壓(ya)支(zhi)(zhi)路(lu)的(de)功(gong)(gong)率消(xiao)耗(hao)。因此(ci),這種接線適用于負載(zai)電(dian)(dian)阻(zu)(或阻(zu)抗(kang))遠比功(gong)(gong)率表電(dian)(dian)壓(ya)支(zhi)(zhi)路(lu)電(dian)(dian)阻(zu)(或阻(zu)抗(kang))小得多的(de)情(qing)況,這樣才能保證功(gong)(gong)率表本身的(de)功(gong)(gong)率消(xiao)耗(hao)對測量結果(guo)的(de)影響比較小。