功率是表(biao)征(zheng)電信號(hao)特性的一個重要參數(shu)。在直流和低頻范圍，可(ke)以通過測(ce)量電壓(ya)和電流計算(suan)功率，功率的瞬時值可(ke)用下式(shi)表(biao)示(shi)：
對于周期信號，一個周期內的瞬(shun)時功(gong)率(lv)(lv)的平(ping)均值，稱(cheng)為有功(gong)功(gong)率(lv)(lv)。有功(gong)功(gong)率(lv)(lv)按下式計算：
對于正弦電路，下式成(cheng)立：
上式中，U、I分別為(wei)正弦交流電(dian)的(de)有效值(zhi)，φ為(wei)電(dian)壓與電(dian)流信(xin)號的(de)相位差。
在(zai)超高頻(pin)(pin)(pin)和(he)微波(bo)(bo)頻(pin)(pin)(pin)段(duan)，有TEM波(bo)(bo)和(he)非TEM波(bo)(bo)之分。在(zai)TEM波(bo)(bo)的同(tong)軸系統中(zhong)，電(dian)壓(ya)和(he)電(dian)流(liu)雖有確切(qie)含意，但測量(liang)其(qi)絕對值很困難。在(zai)波(bo)(bo)導系統中(zhong)，因為存在(zai)不同(tong)的電(dian)磁模式，電(dian)壓(ya)和(he)電(dian)流(liu)失去唯一性。在(zai)個頻(pin)(pin)(pin)段(duan)和(he)各傳輸系統中(zhong)，功(gong)率是單值表征信號強度的重要方法。在(zai)射頻(pin)(pin)(pin)范(fan)圍直接測量(liang)功(gong)率代替(ti)了電(dian)壓(ya)和(he)電(dian)流(liu)的測量(liang)。
功率定義為(wei)單位(wei)時間內所(suo)做的功。基本單位(wei)為(wei)瓦(wa)（W），1W等于在(zai)1秒內做1焦耳的功。常(chang)用的功率單位(wei)還有兆瓦(wa)（1MW=10^6W）、千瓦(wa)（1KW=10^3W）、毫瓦(wa)（1mW=10-3W）、微(wei)瓦(wa)（1μW=10-6W）、皮瓦(wa)（1Pw=10-12W）。
另(ling)一(yi)種(zhong)常用的(de)功(gong)率(lv)(lv)單位(wei)以分(fen)貝毫(hao)瓦（dBm）表示。它以1毫(hao)瓦為基準電平P0=1mW，實際功(gong)率(lv)(lv)值P（mW）與P0比(bi)較(jiao)后取對(dui)(dui)數(shu)。這是功(gong)率(lv)(lv)的(de)絕對(dui)(dui)單位(wei)。也可用分(fen)貝瓦（dBW）作為功(gong)率(lv)(lv)單位(wei)，此時P0=1W，即1dBW=3dBm。
根(gen)據被(bei)測(ce)信號頻(pin)率分類
功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)可分為：直流功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)、工(gong)頻(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)、變(bian)頻(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)、射頻(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)和(he)微波功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)。由(you)于直流功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)等(deng)于電(dian)(dian)壓和(he)電(dian)(dian)流的(de)簡(jian)單乘積，實際(ji)測(ce)量(liang)中，一般采(cai)用電(dian)(dian)壓表和(he)電(dian)(dian)流表替代。工(gong)頻(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)是(shi)應用較(jiao)普遍的(de)功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)，常(chang)說的(de)功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)一般都是(shi)指(zhi)工(gong)頻(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)。變(bian)頻(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計(ji)是(shi)21世紀(ji)變(bian)頻(pin)調速技術高(gao)速發展的(de)產物(wu)。其測(ce)量(liang)對(dui)象為變(bian)頻(pin)電(dian)(dian)量(liang)，變(bian)頻(pin)電(dian)(dian)量(liang)是(shi)指(zhi)用于傳輸功(gong)(gong)(gong)(gong)率(lv)(lv)(lv)(lv)的(de)，并且滿足(zu)下述(shu)條(tiao)件(jian)之一的(de)交流電(dian)(dian)量(liang)：
1、信號頻(pin)(pin)譜僅(jin)包含一種(zhong)頻(pin)(pin)率成(cheng)分，而頻(pin)(pin)率不局限于工頻(pin)(pin)的交流電信號。
2、信號頻譜包含兩種或(huo)更多的被關注的頻率成分的電信號。
變頻(pin)電(dian)量包括電(dian)壓、電(dian)流以及電(dian)壓電(dian)流引出的有(you)(you)功(gong)(gong)(gong)(gong)功(gong)(gong)(gong)(gong)率、無(wu)(wu)功(gong)(gong)(gong)(gong)功(gong)(gong)(gong)(gong)率、視(shi)在功(gong)(gong)(gong)(gong)率、有(you)(you)功(gong)(gong)(gong)(gong)電(dian)能、無(wu)(wu)功(gong)(gong)(gong)(gong)電(dian)能等。
除了(le)變(bian)頻(pin)器(qi)輸(shu)出的(de)(de)(de)PWM波(bo)，二極管整流的(de)(de)(de)變(bian)頻(pin)器(qi)輸(shu)入(ru)的(de)(de)(de)電流波(bo)形，直流斬(zhan)波(bo)器(qi)輸(shu)出的(de)(de)(de)電壓波(bo)形，變(bian)壓器(qi)空(kong)載的(de)(de)(de)輸(shu)入(ru)電流波(bo)形等，均含有(you)較大的(de)(de)(de)諧(xie)波(bo)，右圖中為常見(jian)變(bian)頻(pin)電量的(de)(de)(de)波(bo)形及相關(guan)頻(pin)譜圖。
由于變(bian)頻(pin)(pin)(pin)電(dian)量的頻(pin)(pin)(pin)率(lv)成分復(fu)雜，變(bian)頻(pin)(pin)(pin)功(gong)(gong)率(lv)計(ji)的測(ce)量一般(ban)(ban)包括基波(bo)有(you)(you)功(gong)(gong)功(gong)(gong)率(lv)（簡稱基波(bo)功(gong)(gong)率(lv)）、諧波(bo)有(you)(you)功(gong)(gong)功(gong)(gong)率(lv)（簡稱諧波(bo)功(gong)(gong)率(lv)）、總有(you)(you)功(gong)(gong)功(gong)(gong)率(lv)等，相比工(gong)頻(pin)(pin)(pin)功(gong)(gong)率(lv)計(ji)而言，其功(gong)(gong)能較多，技(ji)術(shu)較復(fu)雜，一般(ban)(ban)稱為變(bian)頻(pin)(pin)(pin)功(gong)(gong)率(lv)分析儀(yi)或(huo)寬(kuan)頻(pin)(pin)(pin)功(gong)(gong)率(lv)分析儀(yi)，部(bu)分高精(jing)度(du)功(gong)(gong)率(lv)分析儀(yi)也適用于變(bian)頻(pin)(pin)(pin)電(dian)量測(ce)量。
變頻(pin)功率(lv)(lv)分(fen)析儀可以作為工(gong)頻(pin)功率(lv)(lv)分(fen)析儀使用，除此之(zhi)外，一般還需滿足下述要求：
1、滿足必(bi)要(yao)的(de)(de)帶寬要(yao)求，并(bing)且采樣頻率應(ying)高于(yu)儀器帶寬的(de)(de)兩倍。
2、要求分析(xi)儀在(zai)較寬(kuan)的(de)頻率(lv)范(fan)圍(wei)之內(nei)，精度均能滿(man)足一(yi)定的(de)要求。
3、具備傅(fu)里葉變換功能，可以(yi)分離信(xin)號(hao)的基波和諧(xie)波。
射頻或微(wei)波功率計(ji)按照在測試系統中的連接方式不(bu)同(tong)分類
有終端式和通過式兩種(zhong)。終端式功(gong)(gong)(gong)率(lv)(lv)(lv)計把功(gong)(gong)(gong)率(lv)(lv)(lv)計探頭作為測試系統(tong)的(de)(de)(de)終端負載，功(gong)(gong)(gong)率(lv)(lv)(lv)計吸收全部待測功(gong)(gong)(gong)率(lv)(lv)(lv)，由功(gong)(gong)(gong)率(lv)(lv)(lv)指(zhi)示器(qi)(qi)直(zhi)接讀取功(gong)(gong)(gong)率(lv)(lv)(lv)值。通過式功(gong)(gong)(gong)率(lv)(lv)(lv)計利(li)用某種(zhong)耦(ou)(ou)合(he)(he)裝置(zhi)，如定向(xiang)耦(ou)(ou)合(he)(he)器(qi)(qi)、耦(ou)(ou)合(he)(he)環、探針(zhen)等從(cong)傳輸的(de)(de)(de)功(gong)(gong)(gong)率(lv)(lv)(lv)中按一定的(de)(de)(de)比(bi)例(li)耦(ou)(ou)合(he)(he)出一部分功(gong)(gong)(gong)率(lv)(lv)(lv)，送(song)入功(gong)(gong)(gong)率(lv)(lv)(lv)計度量，傳輸的(de)(de)(de)總功(gong)(gong)(gong)率(lv)(lv)(lv)等于功(gong)(gong)(gong)率(lv)(lv)(lv)計指(zhi)示值乘(cheng)以比(bi)例(li)系數。
射(she)頻或微波功率計按的測量原(yuan)理分類
測熱(re)電阻(zu)型功率計使用(yong)熱(re)變(bian)(bian)電阻(zu)做功率傳感元件。熱(re)變(bian)(bian)電阻(zu)值的溫(wen)度(du)系數較大(da)。被測信號(hao)的功率被熱(re)變(bian)(bian)電阻(zu)吸收(shou)后(hou)產生熱(re)量，使其自身溫(wen)度(du)升高(gao)，電阻(zu)值發生顯著變(bian)(bian)化(hua)，利用(yong)電阻(zu)電橋測量電阻(zu)值的變(bian)(bian)化(hua)，顯示功率值。
熱(re)(re)電偶(ou)型(xing)功(gong)(gong)(gong)率計熱(re)(re)電偶(ou)型(xing)功(gong)(gong)(gong)率計中的熱(re)(re)偶(ou)結直接吸收(shou)高(gao)頻(pin)(pin)信號功(gong)(gong)(gong)率，結點溫(wen)度升高(gao)，產生(sheng)溫(wen)差電勢(shi)，電勢(shi)的大小正比于吸收(shou)的高(gao)頻(pin)(pin)功(gong)(gong)(gong)率值。
量熱式(shi)功(gong)(gong)(gong)率計(ji)(ji)典型的熱效應功(gong)(gong)(gong)率計(ji)(ji)，利用(yong)隔(ge)熱負載(zai)(zai)吸收高(gao)頻(pin)信號功(gong)(gong)(gong)率，使負載(zai)(zai)的溫(wen)(wen)度升高(gao)，再利用(yong)熱電偶元(yuan)件測量負載(zai)(zai)的溫(wen)(wen)度變(bian)化量，根據產生的熱量計(ji)(ji)算高(gao)頻(pin)功(gong)(gong)(gong)率值(zhi)。
晶體檢波式功(gong)率(lv)(lv)計(ji)晶體二極管檢波器(qi)將高頻(pin)信(xin)號變換為低頻(pin)或直流電信(xin)號。適當選擇(ze)工作點，使檢波器(qi)輸(shu)出信(xin)號的幅度正(zheng)比于高頻(pin)信(xin)號的功(gong)率(lv)(lv)。
射頻或微波功率計按被測信號連續性分類有連續波(bo)功率計(ji)和脈沖峰值功率計(ji)。
以下是變頻(pin)功(gong)率分析儀的典型技(ji)術指(zhi)標
帶寬：50kHz~100kHz；
采樣頻率：大于帶(dai)寬的2倍；
電壓、電流準確級(ji)(ji)(ji)(ji)：0.02級(ji)(ji)(ji)(ji)、0.05級(ji)(ji)(ji)(ji)、0.1級(ji)(ji)(ji)(ji)、0.2級(ji)(ji)(ji)(ji)、0.5級(ji)(ji)(ji)(ji)；
功率準確(que)級(ji)：0.05級(ji)、0.1級(ji)、0.2級(ji)、0.5級(ji)、1級(ji)；
準確(que)級(ji)適用基波頻率(lv)范圍：DC，0.1Hz~400Hz；
準確級適用電(dian)壓范(fan)圍(wei)：0.75%Un~150%Un；
準(zhun)確級適用電流范(fan)圍：1%In~200%In；
準確級適(shi)用(yong)功率因數范圍：0.05~1。
以下是(shi)射頻功率(lv)計的典型技術指標
功率范圍
保證測量(liang)精度的可測功率值的范圍。功率計(ji)的功率范圍決定于功率探(tan)頭。
最大允許功率
探頭(tou)不被損壞的最(zui)大輸入(ru)功(gong)率(lv)值(zhi)(zhi)(zhi)，通常指平均功(gong)率(lv)。在測量大功(gong)率(lv)峰值(zhi)(zhi)(zhi)信(xin)號時，注意峰值(zhi)(zhi)(zhi)電壓和峰值(zhi)(zhi)(zhi)功(gong)率(lv)不能超過(guo)一定值(zhi)(zhi)(zhi)，否則會(hui)造(zao)成功(gong)率(lv)探頭(tou)燒毀(hui)。
頻率范圍
能保證測量精度和性能指標的被測信號的頻率范圍(wei)。
測量精度
指功率探頭校(xiao)準修正后的精度。不包(bao)括(kuo)測(ce)試系統(tong)的失配誤差。
穩定性
功(gong)率計的(de)(de)穩定(ding)性取決于功(gong)率探頭的(de)(de)穩定(ding)性和指示器的(de)(de)零(ling)漂及噪聲(sheng)干擾。
響應時間
也稱功率傳感元件的(de)時(shi)間常數(shu)。通常指功率指示器(qi)上(shang)升到穩定值的(de)64%所需的(de)時(shi)間。
探頭的型號、阻抗
選(xuan)用(yong)功(gong)率計探(tan)頭(tou)時，功(gong)率探(tan)頭(tou)的使用(yong)頻率、功(gong)率范圍(wei)必(bi)須與(yu)被測信號(hao)一致，探(tan)頭(tou)傳輸線的結構和(he)阻抗應與(yu)被測傳輸線相互匹配。
用(yong)于測量絕(jue)對光(guang)功(gong)率(lv)或通過一段光(guang)纖(xian)的(de)光(guang)功(gong)率(lv)相(xiang)對損耗。在(zai)光(guang)纖(xian)系統中，測量光(guang)功(gong)率(lv)是最(zui)基本的(de)，非(fei)常像(xiang)電子學中的(de)萬用(yong)表(biao)。在(zai)光(guang)纖(xian)測量中，光(guang)功(gong)率(lv)計是重負荷常用(yong)表(biao)。通過測量發射端(duan)機或光(guang)網絡的(de)絕(jue)對功(gong)率(lv)，一臺光(guang)功(gong)率(lv)計就(jiu)能(neng)夠評價光(guang)端(duan)設備的(de)性(xing)(xing)能(neng)。用(yong)光(guang)功(gong)率(lv)計與穩定光(guang)源(yuan)組合使用(yong)，則能(neng)夠測量連接損耗、檢驗連續性(xing)(xing)，并幫助(zhu)評估光(guang)纖(xian)鏈(lian)路傳輸質量。
變頻(pin)功率分(fen)析儀(yi)適用于電(dian)力推進、電(dian)機(ji)(ji)、風(feng)機(ji)(ji)、水泵、風(feng)力發電(dian)、軌道交通、電(dian)動汽車、變頻(pin)器(qi)、特種變壓器(qi)、熒光燈(deng)、LED照明等(deng)領(ling)域的產(chan)品檢(jian)試驗、能效評測(ce)及電(dian)能質量(liang)分(fen)析。
選(xuan)擇功(gong)(gong)(gong)(gong)率(lv)表的(de)(de)(de)量(liang)(liang)(liang)(liang)程(cheng)(cheng)就(jiu)是(shi)選(xuan)擇功(gong)(gong)(gong)(gong)率(lv)表中的(de)(de)(de)電(dian)(dian)(dian)流(liu)量(liang)(liang)(liang)(liang)程(cheng)(cheng)和(he)電(dian)(dian)(dian)壓(ya)(ya)量(liang)(liang)(liang)(liang)程(cheng)(cheng)。使(shi)用(yong)時(shi)應使(shi)功(gong)(gong)(gong)(gong)率(lv)表中的(de)(de)(de)電(dian)(dian)(dian)流(liu)量(liang)(liang)(liang)(liang)程(cheng)(cheng)不小于(yu)(yu)(yu)負(fu)載(zai)(zai)電(dian)(dian)(dian)流(liu)，電(dian)(dian)(dian)壓(ya)(ya)量(liang)(liang)(liang)(liang)程(cheng)(cheng)不低于(yu)(yu)(yu)負(fu)載(zai)(zai)電(dian)(dian)(dian)壓(ya)(ya)，而不能(neng)僅(jin)從功(gong)(gong)(gong)(gong)率(lv)量(liang)(liang)(liang)(liang)程(cheng)(cheng)來(lai)考(kao)慮(lv)。例(li)如，兩只功(gong)(gong)(gong)(gong)率(lv)表，量(liang)(liang)(liang)(liang)程(cheng)(cheng)分別(bie)是(shi)IA、300V和(he)2A、150V，由計算可知(zhi)其功(gong)(gong)(gong)(gong)率(lv)量(liang)(liang)(liang)(liang)程(cheng)(cheng)均為(wei)300W，如果要(yao)(yao)測(ce)(ce)量(liang)(liang)(liang)(liang)一(yi)負(fu)載(zai)(zai)電(dian)(dian)(dian)壓(ya)(ya)為(wei)220V、電(dian)(dian)(dian)流(liu)為(wei)IA的(de)(de)(de)負(fu)載(zai)(zai)功(gong)(gong)(gong)(gong)率(lv)時(shi)應逸用(yong)IA、300V的(de)(de)(de)功(gong)(gong)(gong)(gong)率(lv)表，而2A、150V的(de)(de)(de)功(gong)(gong)(gong)(gong)率(lv)表雖功(gong)(gong)(gong)(gong)率(lv)量(liang)(liang)(liang)(liang)程(cheng)(cheng)也大于(yu)(yu)(yu)負(fu)載(zai)(zai)功(gong)(gong)(gong)(gong)率(lv)，但是(shi)由于(yu)(yu)(yu)負(fu)載(zai)(zai)電(dian)(dian)(dian)壓(ya)(ya)高于(yu)(yu)(yu)功(gong)(gong)(gong)(gong)率(lv)表所(suo)能(neng)承(cheng)受的(de)(de)(de)電(dian)(dian)(dian)壓(ya)(ya)150V，故(gu)不能(neng)使(shi)用(yong)。所(suo)以，在測(ce)(ce)量(liang)(liang)(liang)(liang)功(gong)(gong)(gong)(gong)率(lv)前(qian)要(yao)(yao)根據負(fu)載(zai)(zai)的(de)(de)(de)額定(ding)電(dian)(dian)(dian)壓(ya)(ya)和(he)額定(ding)電(dian)(dian)(dian)流(liu)來(lai)選(xuan)擇功(gong)(gong)(gong)(gong)率(lv)表的(de)(de)(de)量(liang)(liang)(liang)(liang)程(cheng)(cheng)。
電(dian)(dian)動系測量機構的(de)(de)(de)轉(zhuan)動力(li)矩方(fang)向和兩線(xian)(xian)(xian)圈(quan)中(zhong)的(de)(de)(de)電(dian)(dian)流(liu)方(fang)向有(you)關，為了防止(zhi)電(dian)(dian)動系功率表(biao)的(de)(de)(de)指針反偏，接線(xian)(xian)(xian)時(shi)功率表(biao)電(dian)(dian)流(liu)線(xian)(xian)(xian)圈(quan)標有(you)“·”號(hao)(hao)的(de)(de)(de)端(duan)鈕(niu)(niu)必(bi)須接到(dao)電(dian)(dian)源的(de)(de)(de)正極(ji)端(duan)，而(er)電(dian)(dian)流(liu)線(xian)(xian)(xian)圈(quan)的(de)(de)(de)另(ling)(ling)(ling)一(yi)端(duan)則與(yu)負載相連，電(dian)(dian)流(liu)線(xian)(xian)(xian)圈(quan)以串聯形式(shi)接入(ru)電(dian)(dian)路中(zhong)。功率表(biao)電(dian)(dian)壓(ya)線(xian)(xian)(xian)圈(quan)標有(you)“·”號(hao)(hao)的(de)(de)(de)端(duan)鈕(niu)(niu)可以接到(dao)電(dian)(dian)源端(duan)鈕(niu)(niu)的(de)(de)(de)任一(yi)端(duan)上，而(er)另(ling)(ling)(ling)一(yi)電(dian)(dian)壓(ya)端(duan)鈕(niu)(niu)則跨接到(dao)負載的(de)(de)(de)另(ling)(ling)(ling)一(yi)端(duan)。
當負載(zai)電(dian)(dian)(dian)阻遠(yuan)遠(yuan)大(da)于電(dian)(dian)(dian)流(liu)線(xian)圈(quan)的(de)電(dian)(dian)(dian)阻時，應采(cai)用(yong)電(dian)(dian)(dian)壓(ya)線(xian)圈(quan)前接(jie)法(fa)。這時電(dian)(dian)(dian)壓(ya)線(xian)圈(quan)的(de)電(dian)(dian)(dian)壓(ya)是負載(zai)電(dian)(dian)(dian)壓(ya)和(he)電(dian)(dian)(dian)流(liu)線(xian)圈(quan)電(dian)(dian)(dian)壓(ya)之和(he)，功(gong)率表測量的(de)是負載(zai)功(gong)率和(he)電(dian)(dian)(dian)流(liu)線(xian)圈(quan)功(gong)率之和(he)。如(ru)果(guo)負載(zai)電(dian)(dian)(dian)阻遠(yuan)遠(yuan)大(da)于電(dian)(dian)(dian)流(liu)線(xian)圈(quan)的(de)電(dian)(dian)(dian)阻，則可以略去電(dian)(dian)(dian)流(liu)線(xian)圈(quan)分(fen)壓(ya)所造成的(de)影響，測量結(jie)果(guo)比較接(jie)近負載(zai)的(de)實際功(gong)率值。
當負載(zai)電(dian)(dian)(dian)(dian)阻遠(yuan)遠(yuan)小(xiao)于電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)線(xian)(xian)圈電(dian)(dian)(dian)(dian)阻時(shi)(shi)，應采用電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)線(xian)(xian)圈后接法(fa)。這時(shi)(shi)電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)線(xian)(xian)圈兩端(duan)的(de)(de)(de)電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)雖然等于負載(zai)電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)，但電(dian)(dian)(dian)(dian)流(liu)(liu)線(xian)(xian)圈中的(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)卻等于負載(zai)電(dian)(dian)(dian)(dian)流(liu)(liu)與功(gong)(gong)率表電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)線(xian)(xian)圈中的(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)之(zhi)和，測量時(shi)(shi)功(gong)(gong)率讀數為負載(zai)功(gong)(gong)率與電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)線(xian)(xian)圈功(gong)(gong)率之(zhi)和。由于此時(shi)(shi)負載(zai)電(dian)(dian)(dian)(dian)阻遠(yuan)小(xiao)于電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)線(xian)(xian)圈電(dian)(dian)(dian)(dian)阻，所(suo)以電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)線(xian)(xian)圈分流(liu)(liu)作用大(da)大(da)減(jian)小(xiao)，其對測量結果的(de)(de)(de)影響也可以大(da)為減(jian)小(xiao)。
如界被(bei)測(ce)負載本身(shen)功(gong)率(lv)(lv)(lv)(lv)較(jiao)大，可以不考慮功(gong)率(lv)(lv)(lv)(lv)表本身(shen)的功(gong)率(lv)(lv)(lv)(lv)對測(ce)量結果的影響，則兩種接(jie)法可以任(ren)意選(xuan)(xuan)擇。但最好選(xuan)(xuan)用電壓線(xian)(xian)圈(quan)前(qian)接(jie)法，因為功(gong)率(lv)(lv)(lv)(lv)表中電流線(xian)(xian)圈(quan)的功(gong)率(lv)(lv)(lv)(lv)一般(ban)都小于電壓線(xian)(xian)圈(quan)支路的功(gong)率(lv)(lv)(lv)(lv)。
一般安裝(zhuang)式(shi)功(gong)率(lv)(lv)表為(wei)(wei)直讀單(dan)量程(cheng)式(shi)，表上的(de)示(shi)數(shu)(shu)即為(wei)(wei)功(gong)率(lv)(lv)數(shu)(shu)。但便攜式(shi)功(gong)率(lv)(lv)表一般為(wei)(wei)多量程(cheng)式(shi)，在(zai)(zai)表的(de)標度(du)尺上不(bu)(bu)直接標注(zhu)示(shi)數(shu)(shu)，只標注(zhu)分格(ge)。在(zai)(zai)選(xuan)用不(bu)(bu)同的(de)電流與電壓(ya)量程(cheng)時，每一分格(ge)都可以表示(shi)不(bu)(bu)同的(de)功(gong)率(lv)(lv)數(shu)(shu)。在(zai)(zai)讀數(shu)(shu)時，應(ying)先根據所(suo)選(xuan)的(de)電壓(ya)量程(cheng)U、電流量程(cheng)I以及標度(du)尺滿量程(cheng)時的(de)格(ge)數(shu)(shu)&，求出(chu)每格(ge)瓦數(shu)(shu)（又稱功(gong)率(lv)(lv)表常數(shu)(shu)）C，然后再乘上指針(zhen)偏轉(zhuan)的(de)格(ge)數(shu)(shu)夕，就可得到所(suo)測(ce)功(gong)率(lv)(lv)P
例題
例：有一(yi)只電(dian)壓(ya)量程(cheng)(cheng)為(wei)(wei)250V，電(dian)流量程(cheng)(cheng)為(wei)(wei)3A，標度尺分(fen)格(ge)數為(wei)(wei)75的功(gong)率表(biao)，現用它來測量負(fu)載的功(gong)率。
當指針(zhen)偏(pian)轉50格時負載功(gong)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(se)=10W/格×50格=500W
對大多數從(cong)事電氣(qi)方面工作(zuo)的人員來說，功(gong)率表的使用(yong)并非難事。但(dan)真(zhen)正做到正確(que)(que)使用(yong)功(gong)率表，即在準(zhun)確(que)(que)度(du)一定的情況下確(que)(que)保(bao)測量的精度(du)及儀表的使用(yong)壽命又(you)并非易事。以單相電動(dong)系功(gong)率表為例，就(jiu)功(gong)率表的使用(yong)及使用(yong)中(zhong)應(ying)注意的問(wen)題作(zuo)一介紹。
1、要遵守“發電機端守則”
由電(dian)(dian)(dian)動系(xi)功(gong)率表(biao)的原理可知，功(gong)率表(biao)的轉矩(ju)與流(liu)過表(biao)內線(xian)圈(quan)的電(dian)(dian)(dian)流(liu)方向(xiang)(xiang)(xiang)有關，一(yi)旦(dan)其中一(yi)個(ge)線(xian)圈(quan)的電(dian)(dian)(dian)流(liu)方向(xiang)(xiang)(xiang)改變，轉矩(ju)方向(xiang)(xiang)(xiang)也會(hui)改變。為此(ci)，在功(gong)率表(biao)兩個(ge)線(xian)圈(quan)對應于電(dian)(dian)(dian)流(liu)流(liu)進的端(duan)鈕上，都注有稱為發電(dian)(dian)(dian)機端(duan)的“*”標(biao)志(zhi)。功(gong)率表(biao)在接(jie)線(xian)時，應使電(dian)(dian)(dian)流(liu)或電(dian)(dian)(dian)壓線(xian)圈(quan)帶(dai)“*”標(biao)志(zhi)的端(duan)鈕接(jie)到電(dian)(dian)(dian)源同極性的端(duan)子(zi)上，以(yi)保證兩線(xian)圈(quan)的電(dian)(dian)(dian)流(liu)方向(xiang)(xiang)(xiang)都從(cong)發電(dian)(dian)(dian)機端(duan)流(liu)入。這就是功(gong)率表(biao)接(jie)線(xian)的“發電(dian)(dian)(dian)機端(duan)守則”。
2、合(he)理選擇電(dian)壓線圈的前、后(hou)接方(fang)式
盡(jin)管電壓線圈不論(lun)前(qian)接還是后(hou)接，功率(lv)表都能正偏，對于(yu)(yu)某些負載來(lai)說(shuo)，測量的結果相差較小(xiao)，這時(shi)兩(liang)種接法采用哪(na)種均可。但對于(yu)(yu)那些電阻（或阻抗(kang)）過大或過小(xiao)的負載來(lai)說(shuo)，兩(liang)種接法所得結果相差較大，有(you)時(shi)甚至出(chu)現與理(li)論(lun)相矛盾的結果。
這種方(fang)式的(de)接(jie)線，功率(lv)(lv)表(biao)電(dian)(dian)流線圈的(de)電(dian)(dian)流雖然等于(yu)負(fu)(fu)載(zai)電(dian)(dian)流，但功率(lv)(lv)表(biao)電(dian)(dian)壓支路兩端電(dian)(dian)壓卻(que)等于(yu)負(fu)(fu)載(zai)電(dian)(dian)壓與功率(lv)(lv)表(biao)電(dian)(dian)流線圈的(de)電(dian)(dian)壓之和，在功率(lv)(lv)表(biao)讀數中多了(le)電(dian)(dian)流線圈的(de)功率(lv)(lv)消(xiao)耗(hao)。這種接(jie)線方(fang)式適用于(yu)負(fu)(fu)載(zai)電(dian)(dian)阻（或阻抗(kang)(kang)）遠比功率(lv)(lv)表(biao)電(dian)(dian)流線圈電(dian)(dian)阻（或阻抗(kang)(kang)）大得多的(de)情(qing)況，這樣才能保(bao)證功率(lv)(lv)表(biao)本身的(de)功率(lv)(lv)消(xiao)耗(hao)對測(ce)量結果的(de)影響(xiang)比較小。
這種方式的(de)(de)(de)(de)接線，功(gong)(gong)率(lv)表(biao)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)支(zhi)路兩端的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)雖(sui)然(ran)等(deng)于(yu)(yu)負載電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)，但電(dian)(dian)(dian)(dian)(dian)流線圈的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流卻等(deng)于(yu)(yu)負載電(dian)(dian)(dian)(dian)(dian)流與功(gong)(gong)率(lv)表(biao)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)線圈支(zhi)路電(dian)(dian)(dian)(dian)(dian)流之和，功(gong)(gong)率(lv)表(biao)讀數中多了電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)支(zhi)路的(de)(de)(de)(de)功(gong)(gong)率(lv)消耗(hao)。因此，這種接線適用于(yu)(yu)負載電(dian)(dian)(dian)(dian)(dian)阻(zu)（或阻(zu)抗）遠比功(gong)(gong)率(lv)表(biao)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)支(zhi)路電(dian)(dian)(dian)(dian)(dian)阻(zu)（或阻(zu)抗）小(xiao)得多的(de)(de)(de)(de)情況，這樣才能保證功(gong)(gong)率(lv)表(biao)本(ben)身(shen)的(de)(de)(de)(de)功(gong)(gong)率(lv)消耗(hao)對(dui)測量結果的(de)(de)(de)(de)影響比較小(xiao)。
