功(gong)率是(shi)表征電信號特性(xing)的一個(ge)重要(yao)參(can)數。在(zai)直流(liu)和低頻范圍，可以通過測量電壓(ya)和電流(liu)計算(suan)功(gong)率，功(gong)率的瞬時(shi)值可用下式表示：
對(dui)于周期(qi)信號，一(yi)個(ge)周期(qi)內的(de)瞬時功率(lv)的(de)平(ping)均(jun)值，稱為有功功率(lv)。有功功率(lv)按(an)下式計算：
對于正弦(xian)電路(lu)，下式成立：
上(shang)式中(zhong)，U、I分別為正弦交流(liu)電的有效值，φ為電壓與電流(liu)信號的相位差。
在(zai)超高頻和微波頻段，有TEM波和非TEM波之(zhi)分(fen)。在(zai)TEM波的(de)同(tong)軸系(xi)(xi)統中，電(dian)壓和電(dian)流雖有確切含意，但(dan)測量(liang)其絕對值(zhi)很困難(nan)。在(zai)波導系(xi)(xi)統中，因為存在(zai)不同(tong)的(de)電(dian)磁模式，電(dian)壓和電(dian)流失去(qu)唯(wei)一性。在(zai)個頻段和各傳輸系(xi)(xi)統中，功率是單值(zhi)表(biao)征(zheng)信號強度(du)的(de)重要方法。在(zai)射頻范圍直(zhi)接(jie)測量(liang)功率代替(ti)了電(dian)壓和電(dian)流的(de)測量(liang)。
功率(lv)定(ding)義為(wei)單位時間內所做的功。基本單位為(wei)瓦(wa)（W），1W等于(yu)在(zai)1秒內做1焦(jiao)耳的功。常用的功率(lv)單位還有兆瓦(wa)（1MW=10^6W）、千(qian)瓦(wa)（1KW=10^3W）、毫(hao)瓦(wa)（1mW=10-3W）、微(wei)瓦(wa)（1μW=10-6W）、皮(pi)瓦(wa)（1Pw=10-12W）。
另一種常用的(de)功(gong)率(lv)單(dan)位(wei)以分貝毫瓦（dBm）表示(shi)。它以1毫瓦為(wei)基準電平P0=1mW，實際(ji)功(gong)率(lv)值(zhi)P（mW）與P0比(bi)較后取(qu)對數(shu)。這是功(gong)率(lv)的(de)絕對單(dan)位(wei)。也可用分貝瓦（dBW）作為(wei)功(gong)率(lv)單(dan)位(wei)，此時(shi)P0=1W，即(ji)1dBW=3dBm。
根據被測信號頻率分類
功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)可分(fen)為：直流(liu)功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)、工頻(pin)(pin)功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)、變(bian)頻(pin)(pin)功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)、射頻(pin)(pin)功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)和微波功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)。由于直流(liu)功(gong)(gong)率(lv)等于電(dian)壓(ya)和電(dian)流(liu)的簡單(dan)乘積，實際測量(liang)(liang)中，一般采用(yong)電(dian)壓(ya)表(biao)和電(dian)流(liu)表(biao)替(ti)代(dai)。工頻(pin)(pin)功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)是(shi)應用(yong)較普遍的功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)，常說(shuo)的功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)一般都是(shi)指工頻(pin)(pin)功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)。變(bian)頻(pin)(pin)功(gong)(gong)率(lv)計(ji)(ji)(ji)(ji)是(shi)21世(shi)紀變(bian)頻(pin)(pin)調速技術高速發展(zhan)的產(chan)物。其(qi)測量(liang)(liang)對(dui)象為變(bian)頻(pin)(pin)電(dian)量(liang)(liang)，變(bian)頻(pin)(pin)電(dian)量(liang)(liang)是(shi)指用(yong)于傳輸(shu)功(gong)(gong)率(lv)的，并(bing)且滿足下述(shu)條件之一的交流(liu)電(dian)量(liang)(liang)：
1、信(xin)號(hao)頻譜僅(jin)包含一種頻率成分(fen)，而頻率不局限于工頻的交流電信(xin)號(hao)。
2、信(xin)號(hao)頻譜包(bao)含兩種或(huo)更多的(de)被關注的(de)頻率成分的(de)電信(xin)號(hao)。
變頻電量包括電壓(ya)、電流以及電壓(ya)電流引出的有功(gong)(gong)功(gong)(gong)率、無(wu)功(gong)(gong)功(gong)(gong)率、視在(zai)功(gong)(gong)率、有功(gong)(gong)電能、無(wu)功(gong)(gong)電能等。
除(chu)了變(bian)(bian)頻(pin)器(qi)輸出(chu)的(de)PWM波(bo)，二(er)極管整流(liu)(liu)的(de)變(bian)(bian)頻(pin)器(qi)輸入的(de)電(dian)流(liu)(liu)波(bo)形，直(zhi)流(liu)(liu)斬(zhan)波(bo)器(qi)輸出(chu)的(de)電(dian)壓波(bo)形，變(bian)(bian)壓器(qi)空載(zai)的(de)輸入電(dian)流(liu)(liu)波(bo)形等，均含有(you)較大的(de)諧波(bo)，右圖中為(wei)常見(jian)變(bian)(bian)頻(pin)電(dian)量的(de)波(bo)形及相關頻(pin)譜圖。
由(you)于變頻電量的頻率(lv)(lv)成分(fen)復雜，變頻功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)計的測(ce)量一般包括基波(bo)有功(gong)(gong)(gong)(gong)(gong)功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)（簡稱(cheng)(cheng)基波(bo)功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)）、諧波(bo)有功(gong)(gong)(gong)(gong)(gong)功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)（簡稱(cheng)(cheng)諧波(bo)功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)）、總(zong)有功(gong)(gong)(gong)(gong)(gong)功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)等，相比工(gong)頻功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)計而(er)言，其功(gong)(gong)(gong)(gong)(gong)能較多，技術較復雜，一般稱(cheng)(cheng)為變頻功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)分(fen)析儀(yi)(yi)或寬(kuan)頻功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)分(fen)析儀(yi)(yi)，部分(fen)高精度功(gong)(gong)(gong)(gong)(gong)率(lv)(lv)分(fen)析儀(yi)(yi)也適用(yong)于變頻電量測(ce)量。
變頻功(gong)率(lv)分析儀(yi)可(ke)以作為工頻功(gong)率(lv)分析儀(yi)使(shi)用，除此之外，一(yi)般還需滿足下述(shu)要(yao)求(qiu)：
1、滿足(zu)必要(yao)的帶寬要(yao)求(qiu)，并且采樣頻率應高(gao)于儀器帶寬的兩倍(bei)。
2、要(yao)求分(fen)析儀在較寬(kuan)的(de)頻率范圍(wei)之內，精度均能滿足一定(ding)的(de)要(yao)求。
3、具(ju)備傅里葉(xie)變換功(gong)能，可以分離信號(hao)的基波和諧波。
射(she)頻或微波功率(lv)計按照在測試(shi)系統中的(de)連接(jie)方式不同(tong)分(fen)類
有終(zhong)(zhong)端式和通(tong)過式兩種(zhong)。終(zhong)(zhong)端式功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)計(ji)把功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)計(ji)探頭作(zuo)為測(ce)試系統的終(zhong)(zhong)端負載，功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)計(ji)吸收全部待測(ce)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)，由功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)指示器(qi)直接讀(du)取功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)值。通(tong)過式功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)計(ji)利用某種(zhong)耦合(he)裝置，如定(ding)(ding)向耦合(he)器(qi)、耦合(he)環、探針等(deng)從傳(chuan)輸的功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)中按一(yi)(yi)定(ding)(ding)的比例(li)耦合(he)出(chu)一(yi)(yi)部分功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)，送(song)入功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)計(ji)度(du)量，傳(chuan)輸的總功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)等(deng)于功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)(lv)計(ji)指示值乘以比例(li)系數。
射頻或微(wei)波(bo)功率計按的測量原理分類
測熱電(dian)阻型功率計使用熱變(bian)電(dian)阻做功率傳感元件。熱變(bian)電(dian)阻值(zhi)的(de)溫(wen)度(du)(du)系數較大。被(bei)測信號的(de)功率被(bei)熱變(bian)電(dian)阻吸收后產生熱量(liang)，使其自身溫(wen)度(du)(du)升高，電(dian)阻值(zhi)發生顯著變(bian)化，利(li)用電(dian)阻電(dian)橋測量(liang)電(dian)阻值(zhi)的(de)變(bian)化，顯示(shi)功率值(zhi)。
熱電偶(ou)型功率(lv)計(ji)熱電偶(ou)型功率(lv)計(ji)中的熱偶(ou)結直接吸(xi)收高(gao)頻(pin)信號(hao)功率(lv)，結點溫(wen)度(du)升高(gao)，產生溫(wen)差電勢(shi)，電勢(shi)的大(da)小(xiao)正比(bi)于吸(xi)收的高(gao)頻(pin)功率(lv)值。
量熱式功(gong)率計(ji)典型的熱效應功(gong)率計(ji)，利用(yong)隔熱負載(zai)吸(xi)收(shou)高(gao)頻信號功(gong)率，使(shi)負載(zai)的溫度升高(gao)，再利用(yong)熱電偶元件測量負載(zai)的溫度變(bian)化(hua)量，根據(ju)產生的熱量計(ji)算高(gao)頻功(gong)率值。
晶體(ti)檢(jian)波式功率(lv)(lv)計晶體(ti)二極(ji)管(guan)檢(jian)波器將高頻信號變換為低頻或直流電(dian)信號。適(shi)當選(xuan)擇工作(zuo)點，使檢(jian)波器輸出(chu)信號的幅度正比于高頻信號的功率(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(gong)率計和脈沖(chong)峰值功(gong)率計。
以(yi)下(xia)是變頻功率(lv)分析(xi)儀的(de)典(dian)型(xing)技(ji)術(shu)指標
帶寬：50kHz~100kHz；
采樣(yang)頻率：大于帶寬的2倍(bei)；
電壓、電流準確級(ji)(ji)：0.02級(ji)(ji)、0.05級(ji)(ji)、0.1級(ji)(ji)、0.2級(ji)(ji)、0.5級(ji)(ji)；
功率準(zhun)確級(ji)：0.05級(ji)、0.1級(ji)、0.2級(ji)、0.5級(ji)、1級(ji)；
準確級適用基波頻率范圍：DC，0.1Hz~400Hz；
準確級適用電壓范圍：0.75%Un~150%Un；
準確級適用電(dian)流范(fan)圍：1%In~200%In；
準確級適(shi)用功率因數范圍：0.05~1。
以(yi)下(xia)是射頻功率計的(de)典型技術(shu)指標
功率范圍
保證測量(liang)精(jing)度(du)的可測功率(lv)值的范圍(wei)。功率(lv)計的功率(lv)范圍(wei)決定(ding)于功率(lv)探頭。
最大允許功率
探(tan)頭(tou)不被損壞(huai)的最大(da)輸入(ru)功率(lv)(lv)(lv)值(zhi)(zhi)(zhi)，通常指平(ping)均(jun)功率(lv)(lv)(lv)。在測量大(da)功率(lv)(lv)(lv)峰值(zhi)(zhi)(zhi)信號時(shi)，注(zhu)意峰值(zhi)(zhi)(zhi)電壓和峰值(zhi)(zhi)(zhi)功率(lv)(lv)(lv)不能(neng)超過一(yi)定值(zhi)(zhi)(zhi)，否則會造成(cheng)功率(lv)(lv)(lv)探(tan)頭(tou)燒毀。
頻率范圍
能(neng)(neng)保證測(ce)量精度和性能(neng)(neng)指標的(de)(de)被測(ce)信號(hao)的(de)(de)頻(pin)率范圍(wei)。
測量精度
指功率(lv)探頭校準修正后的精(jing)度。不包括測(ce)試系統的失(shi)配誤(wu)差。
穩定性
功率(lv)計的(de)穩定性取(qu)決于功率(lv)探頭的(de)穩定性和(he)指示(shi)器的(de)零漂及噪聲干擾(rao)。
響應時間
也稱(cheng)功率傳感元件的(de)時(shi)間常數。通常指(zhi)功率指(zhi)示器上升到穩定值(zhi)的(de)64%所需的(de)時(shi)間。
探頭的型號、阻抗
選用功(gong)率計探頭時，功(gong)率探頭的(de)使用頻率、功(gong)率范圍必須與被(bei)測信號一致，探頭傳輸線(xian)的(de)結構和(he)阻抗應與被(bei)測傳輸線(xian)相互匹配。
用(yong)于測量(liang)絕對(dui)光功(gong)(gong)率(lv)(lv)或(huo)通過(guo)(guo)一(yi)段光纖(xian)的(de)(de)(de)光功(gong)(gong)率(lv)(lv)相(xiang)對(dui)損耗。在光纖(xian)系(xi)統中，測量(liang)光功(gong)(gong)率(lv)(lv)是(shi)最基(ji)本的(de)(de)(de)，非常像電子學中的(de)(de)(de)萬用(yong)表(biao)。在光纖(xian)測量(liang)中，光功(gong)(gong)率(lv)(lv)計是(shi)重負荷常用(yong)表(biao)。通過(guo)(guo)測量(liang)發(fa)射端機或(huo)光網絡的(de)(de)(de)絕對(dui)功(gong)(gong)率(lv)(lv)，一(yi)臺(tai)光功(gong)(gong)率(lv)(lv)計就能夠評(ping)價光端設備的(de)(de)(de)性能。用(yong)光功(gong)(gong)率(lv)(lv)計與穩定光源組合使用(yong)，則能夠測量(liang)連接損耗、檢驗連續性，并幫助評(ping)估光纖(xian)鏈路傳輸質量(liang)。
變頻功率分析儀適用于電(dian)(dian)力推進(jin)、電(dian)(dian)機(ji)(ji)、風機(ji)(ji)、水泵(beng)、風力發電(dian)(dian)、軌道(dao)交通、電(dian)(dian)動汽車(che)、變頻器、特種變壓器、熒光燈、LED照(zhao)明(ming)等領(ling)域的產品檢試驗、能效評測(ce)及(ji)電(dian)(dian)能質(zhi)量分析。
選(xuan)(xuan)擇(ze)功(gong)(gong)(gong)率(lv)(lv)表(biao)的量(liang)程(cheng)(cheng)(cheng)就(jiu)是(shi)選(xuan)(xuan)擇(ze)功(gong)(gong)(gong)率(lv)(lv)表(biao)中的電(dian)(dian)(dian)流量(liang)程(cheng)(cheng)(cheng)和(he)電(dian)(dian)(dian)壓(ya)(ya)(ya)量(liang)程(cheng)(cheng)(cheng)。使用時(shi)應(ying)使功(gong)(gong)(gong)率(lv)(lv)表(biao)中的電(dian)(dian)(dian)流量(liang)程(cheng)(cheng)(cheng)不(bu)小于(yu)(yu)負(fu)載(zai)電(dian)(dian)(dian)流，電(dian)(dian)(dian)壓(ya)(ya)(ya)量(liang)程(cheng)(cheng)(cheng)不(bu)低于(yu)(yu)負(fu)載(zai)電(dian)(dian)(dian)壓(ya)(ya)(ya)，而不(bu)能(neng)僅從功(gong)(gong)(gong)率(lv)(lv)量(liang)程(cheng)(cheng)(cheng)來考慮(lv)。例如(ru)，兩只功(gong)(gong)(gong)率(lv)(lv)表(biao)，量(liang)程(cheng)(cheng)(cheng)分別是(shi)IA、300V和(he)2A、150V，由(you)計算可知其功(gong)(gong)(gong)率(lv)(lv)量(liang)程(cheng)(cheng)(cheng)均為(wei)300W，如(ru)果要測量(liang)一負(fu)載(zai)電(dian)(dian)(dian)壓(ya)(ya)(ya)為(wei)220V、電(dian)(dian)(dian)流為(wei)IA的負(fu)載(zai)功(gong)(gong)(gong)率(lv)(lv)時(shi)應(ying)逸用IA、300V的功(gong)(gong)(gong)率(lv)(lv)表(biao)，而2A、150V的功(gong)(gong)(gong)率(lv)(lv)表(biao)雖(sui)功(gong)(gong)(gong)率(lv)(lv)量(liang)程(cheng)(cheng)(cheng)也大于(yu)(yu)負(fu)載(zai)功(gong)(gong)(gong)率(lv)(lv)，但是(shi)由(you)于(yu)(yu)負(fu)載(zai)電(dian)(dian)(dian)壓(ya)(ya)(ya)高于(yu)(yu)功(gong)(gong)(gong)率(lv)(lv)表(biao)所能(neng)承受的電(dian)(dian)(dian)壓(ya)(ya)(ya)150V，故不(bu)能(neng)使用。所以，在測量(liang)功(gong)(gong)(gong)率(lv)(lv)前(qian)要根據負(fu)載(zai)的額定(ding)電(dian)(dian)(dian)壓(ya)(ya)(ya)和(he)額定(ding)電(dian)(dian)(dian)流來選(xuan)(xuan)擇(ze)功(gong)(gong)(gong)率(lv)(lv)表(biao)的量(liang)程(cheng)(cheng)(cheng)。
電(dian)(dian)(dian)(dian)動(dong)系(xi)(xi)測量機構的(de)轉動(dong)力矩方(fang)(fang)向和兩(liang)線圈(quan)中的(de)電(dian)(dian)(dian)(dian)流(liu)方(fang)(fang)向有(you)關，為(wei)了防止電(dian)(dian)(dian)(dian)動(dong)系(xi)(xi)功率(lv)表的(de)指針反偏(pian)，接(jie)(jie)線時功率(lv)表電(dian)(dian)(dian)(dian)流(liu)線圈(quan)標有(you)“·”號的(de)端(duan)(duan)鈕(niu)必須接(jie)(jie)到電(dian)(dian)(dian)(dian)源的(de)正極端(duan)(duan)，而電(dian)(dian)(dian)(dian)流(liu)線圈(quan)的(de)另(ling)一端(duan)(duan)則與負載(zai)相連，電(dian)(dian)(dian)(dian)流(liu)線圈(quan)以串聯形(xing)式(shi)接(jie)(jie)入電(dian)(dian)(dian)(dian)路中。功率(lv)表電(dian)(dian)(dian)(dian)壓(ya)線圈(quan)標有(you)“·”號的(de)端(duan)(duan)鈕(niu)可(ke)以接(jie)(jie)到電(dian)(dian)(dian)(dian)源端(duan)(duan)鈕(niu)的(de)任(ren)一端(duan)(duan)上(shang)，而另(ling)一電(dian)(dian)(dian)(dian)壓(ya)端(duan)(duan)鈕(niu)則跨接(jie)(jie)到負載(zai)的(de)另(ling)一端(duan)(duan)。
當負(fu)(fu)(fu)載電(dian)(dian)(dian)阻(zu)遠遠大于(yu)電(dian)(dian)(dian)流線圈(quan)(quan)(quan)(quan)的電(dian)(dian)(dian)阻(zu)時，應(ying)采用電(dian)(dian)(dian)壓線圈(quan)(quan)(quan)(quan)前接(jie)法。這時電(dian)(dian)(dian)壓線圈(quan)(quan)(quan)(quan)的電(dian)(dian)(dian)壓是負(fu)(fu)(fu)載電(dian)(dian)(dian)壓和(he)(he)(he)電(dian)(dian)(dian)流線圈(quan)(quan)(quan)(quan)電(dian)(dian)(dian)壓之(zhi)和(he)(he)(he)，功(gong)率(lv)表(biao)測(ce)量的是負(fu)(fu)(fu)載功(gong)率(lv)和(he)(he)(he)電(dian)(dian)(dian)流線圈(quan)(quan)(quan)(quan)功(gong)率(lv)之(zhi)和(he)(he)(he)。如果負(fu)(fu)(fu)載電(dian)(dian)(dian)阻(zu)遠遠大于(yu)電(dian)(dian)(dian)流線圈(quan)(quan)(quan)(quan)的電(dian)(dian)(dian)阻(zu)，則可以(yi)略去(qu)電(dian)(dian)(dian)流線圈(quan)(quan)(quan)(quan)分壓所造(zao)成的影響，測(ce)量結果比(bi)較接(jie)近負(fu)(fu)(fu)載的實際功(gong)率(lv)值。
當(dang)負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)阻遠遠小(xiao)(xiao)于電(dian)(dian)(dian)(dian)壓線圈(quan)電(dian)(dian)(dian)(dian)阻時(shi)，應(ying)采用電(dian)(dian)(dian)(dian)壓線圈(quan)后接(jie)法。這(zhe)時(shi)電(dian)(dian)(dian)(dian)壓線圈(quan)兩端(duan)的(de)電(dian)(dian)(dian)(dian)壓雖然等(deng)于負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)壓，但電(dian)(dian)(dian)(dian)流(liu)線圈(quan)中(zhong)的(de)電(dian)(dian)(dian)(dian)流(liu)卻等(deng)于負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)流(liu)與功(gong)率表電(dian)(dian)(dian)(dian)壓線圈(quan)中(zhong)的(de)電(dian)(dian)(dian)(dian)流(liu)之和，測量時(shi)功(gong)率讀數為(wei)負(fu)載(zai)(zai)功(gong)率與電(dian)(dian)(dian)(dian)壓線圈(quan)功(gong)率之和。由于此時(shi)負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)阻遠小(xiao)(xiao)于電(dian)(dian)(dian)(dian)壓線圈(quan)電(dian)(dian)(dian)(dian)阻，所(suo)以(yi)電(dian)(dian)(dian)(dian)壓線圈(quan)分流(liu)作用大大減小(xiao)(xiao)，其(qi)對測量結果的(de)影(ying)響也(ye)可以(yi)大為(wei)減小(xiao)(xiao)。
如界被測(ce)負載(zai)本身功率(lv)(lv)較大，可(ke)以不考慮功率(lv)(lv)表(biao)本身的功率(lv)(lv)對(dui)測(ce)量結果的影響，則兩種接法可(ke)以任意選擇(ze)。但最好(hao)選用電(dian)壓線圈前接法，因為功率(lv)(lv)表(biao)中電(dian)流(liu)線圈的功率(lv)(lv)一般都小于電(dian)壓線圈支路的功率(lv)(lv)。
一般安(an)裝式功率(lv)表(biao)為(wei)直(zhi)讀單量(liang)程式，表(biao)上的(de)(de)(de)示數(shu)(shu)即(ji)為(wei)功率(lv)數(shu)(shu)。但便攜式功率(lv)表(biao)一般為(wei)多量(liang)程式，在(zai)表(biao)的(de)(de)(de)標(biao)度尺上不(bu)直(zhi)接標(biao)注示數(shu)(shu)，只標(biao)注分格(ge)(ge)。在(zai)選用不(bu)同的(de)(de)(de)電(dian)(dian)流(liu)與電(dian)(dian)壓(ya)量(liang)程時，每(mei)一分格(ge)(ge)都可以(yi)表(biao)示不(bu)同的(de)(de)(de)功率(lv)數(shu)(shu)。在(zai)讀數(shu)(shu)時，應先(xian)根據所(suo)選的(de)(de)(de)電(dian)(dian)壓(ya)量(liang)程U、電(dian)(dian)流(liu)量(liang)程I以(yi)及標(biao)度尺滿量(liang)程時的(de)(de)(de)格(ge)(ge)數(shu)(shu)&，求(qiu)出每(mei)格(ge)(ge)瓦數(shu)(shu)（又稱功率(lv)表(biao)常(chang)數(shu)(shu)）C，然后再乘上指針偏(pian)轉的(de)(de)(de)格(ge)(ge)數(shu)(shu)夕(xi)，就可得到所(suo)測(ce)功率(lv)P
例題
例：有一只電壓量程為250V，電流量程為3A，標度尺分格數為75的(de)功率(lv)表(biao)，現用(yong)它來測量負載的(de)功率(lv)。
當指針偏(pian)轉50格時負(fu)載功率為多少(shao)？
解：先計算功(gong)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(ge)×50格(ge)=500W
對大多數從事電(dian)氣方面工作的人員來說，功率(lv)表(biao)的使用(yong)(yong)(yong)并非(fei)難事。但真正(zheng)做到正(zheng)確使用(yong)(yong)(yong)功率(lv)表(biao)，即(ji)在準確度(du)一定的情況下確保測(ce)量的精度(du)及儀表(biao)的使用(yong)(yong)(yong)壽(shou)命又并非(fei)易事。以單相(xiang)電(dian)動(dong)系功率(lv)表(biao)為例，就功率(lv)表(biao)的使用(yong)(yong)(yong)及使用(yong)(yong)(yong)中應注(zhu)意的問題作一介紹。
1、要遵守(shou)“發電機端守(shou)則”
由(you)電(dian)(dian)動系功(gong)率(lv)表(biao)的(de)(de)(de)原理可知(zhi)，功(gong)率(lv)表(biao)的(de)(de)(de)轉(zhuan)矩與流(liu)過表(biao)內(nei)線(xian)(xian)(xian)(xian)圈(quan)(quan)的(de)(de)(de)電(dian)(dian)流(liu)方(fang)向有(you)關，一(yi)旦其(qi)中一(yi)個(ge)線(xian)(xian)(xian)(xian)圈(quan)(quan)的(de)(de)(de)電(dian)(dian)流(liu)方(fang)向改變，轉(zhuan)矩方(fang)向也會改變。為(wei)此，在功(gong)率(lv)表(biao)兩個(ge)線(xian)(xian)(xian)(xian)圈(quan)(quan)對(dui)應(ying)于電(dian)(dian)流(liu)流(liu)進的(de)(de)(de)端(duan)(duan)鈕上，都注有(you)稱為(wei)發(fa)(fa)電(dian)(dian)機端(duan)(duan)的(de)(de)(de)“*”標志(zhi)。功(gong)率(lv)表(biao)在接(jie)線(xian)(xian)(xian)(xian)時，應(ying)使電(dian)(dian)流(liu)或電(dian)(dian)壓(ya)線(xian)(xian)(xian)(xian)圈(quan)(quan)帶“*”標志(zhi)的(de)(de)(de)端(duan)(duan)鈕接(jie)到電(dian)(dian)源同極性的(de)(de)(de)端(duan)(duan)子上，以(yi)保證(zheng)兩線(xian)(xian)(xian)(xian)圈(quan)(quan)的(de)(de)(de)電(dian)(dian)流(liu)方(fang)向都從發(fa)(fa)電(dian)(dian)機端(duan)(duan)流(liu)入。這(zhe)就是功(gong)率(lv)表(biao)接(jie)線(xian)(xian)(xian)(xian)的(de)(de)(de)“發(fa)(fa)電(dian)(dian)機端(duan)(duan)守則”。
2、合(he)理(li)選擇電壓線圈的前、后接方式
盡管電壓線圈不論前接還(huan)是后接，功率表都能正偏，對(dui)于某(mou)些負載來說，測量的(de)結果相(xiang)差較小，這時(shi)兩(liang)種(zhong)(zhong)接法采用哪種(zhong)(zhong)均可。但(dan)對(dui)于那(nei)些電阻(zu)(zu)（或(huo)阻(zu)(zu)抗(kang)）過(guo)(guo)大(da)或(huo)過(guo)(guo)小的(de)負載來說，兩(liang)種(zhong)(zhong)接法所得結果相(xiang)差較大(da)，有時(shi)甚至出現與(yu)理(li)論相(xiang)矛盾的(de)結果。
這(zhe)種(zhong)方式的(de)(de)(de)接線(xian)，功(gong)率(lv)(lv)表(biao)電(dian)(dian)流(liu)(liu)(liu)線(xian)圈(quan)(quan)的(de)(de)(de)電(dian)(dian)流(liu)(liu)(liu)雖(sui)然等于負(fu)載(zai)電(dian)(dian)流(liu)(liu)(liu)，但功(gong)率(lv)(lv)表(biao)電(dian)(dian)壓(ya)支路兩端電(dian)(dian)壓(ya)卻(que)等于負(fu)載(zai)電(dian)(dian)壓(ya)與(yu)功(gong)率(lv)(lv)表(biao)電(dian)(dian)流(liu)(liu)(liu)線(xian)圈(quan)(quan)的(de)(de)(de)電(dian)(dian)壓(ya)之(zhi)和(he)，在(zai)功(gong)率(lv)(lv)表(biao)讀數中多了電(dian)(dian)流(liu)(liu)(liu)線(xian)圈(quan)(quan)的(de)(de)(de)功(gong)率(lv)(lv)消耗。這(zhe)種(zhong)接線(xian)方式適(shi)用于負(fu)載(zai)電(dian)(dian)阻(zu)（或阻(zu)抗）遠比功(gong)率(lv)(lv)表(biao)電(dian)(dian)流(liu)(liu)(liu)線(xian)圈(quan)(quan)電(dian)(dian)阻(zu)（或阻(zu)抗）大得多的(de)(de)(de)情況，這(zhe)樣才(cai)能保證功(gong)率(lv)(lv)表(biao)本(ben)身的(de)(de)(de)功(gong)率(lv)(lv)消耗對測量結果的(de)(de)(de)影響比較小(xiao)。
這(zhe)種(zhong)方式的接(jie)線(xian)(xian)，功(gong)率(lv)表電(dian)壓(ya)支(zhi)路兩端的電(dian)壓(ya)雖(sui)然等(deng)于負(fu)載電(dian)壓(ya)，但電(dian)流(liu)線(xian)(xian)圈的電(dian)流(liu)卻等(deng)于負(fu)載電(dian)流(liu)與(yu)功(gong)率(lv)表電(dian)壓(ya)線(xian)(xian)圈支(zhi)路電(dian)流(liu)之和，功(gong)率(lv)表讀數(shu)中多(duo)了電(dian)壓(ya)支(zhi)路的功(gong)率(lv)消(xiao)耗。因此，這(zhe)種(zhong)接(jie)線(xian)(xian)適用于負(fu)載電(dian)阻(zu)（或阻(zu)抗）遠比功(gong)率(lv)表電(dian)壓(ya)支(zhi)路電(dian)阻(zu)（或阻(zu)抗）小(xiao)得(de)多(duo)的情況，這(zhe)樣才能保證功(gong)率(lv)表本(ben)身的功(gong)率(lv)消(xiao)耗對測量結果(guo)的影響比較小(xiao)。
