功(gong)率是表(biao)征電(dian)信號(hao)特性的(de)一個重要參數。在直(zhi)流(liu)和低頻范圍，可(ke)(ke)以通過測量(liang)電(dian)壓和電(dian)流(liu)計算功(gong)率，功(gong)率的(de)瞬(shun)時值可(ke)(ke)用下式表(biao)示(shi)：
對于周(zhou)期信號，一(yi)個(ge)周(zhou)期內的瞬(shun)時(shi)功(gong)率的平(ping)均值(zhi)，稱為有功(gong)功(gong)率。有功(gong)功(gong)率按下式計(ji)算：
對于正(zheng)弦電(dian)路，下(xia)式成立：
上(shang)式(shi)中，U、I分別為正弦交流電(dian)(dian)的有效(xiao)值，φ為電(dian)(dian)壓(ya)與電(dian)(dian)流信(xin)號的相位差。
在(zai)超高頻和微波(bo)頻段，有TEM波(bo)和非TEM波(bo)之分。在(zai)TEM波(bo)的(de)同(tong)軸(zhou)系(xi)統中，電(dian)(dian)(dian)壓(ya)和電(dian)(dian)(dian)流雖(sui)有確切含意，但(dan)測(ce)量(liang)其絕對(dui)值很(hen)困(kun)難。在(zai)波(bo)導系(xi)統中，因(yin)為存在(zai)不同(tong)的(de)電(dian)(dian)(dian)磁模式，電(dian)(dian)(dian)壓(ya)和電(dian)(dian)(dian)流失去(qu)唯一性(xing)。在(zai)個(ge)頻段和各傳輸系(xi)統中，功(gong)率是單值表征信號強度的(de)重(zhong)要方法。在(zai)射(she)頻范圍直接測(ce)量(liang)功(gong)率代(dai)替(ti)了電(dian)(dian)(dian)壓(ya)和電(dian)(dian)(dian)流的(de)測(ce)量(liang)。
功(gong)(gong)率定義(yi)為(wei)單位時間內(nei)所做的功(gong)(gong)。基本單位為(wei)瓦(wa)（W），1W等于在1秒內(nei)做1焦耳的功(gong)(gong)。常(chang)用的功(gong)(gong)率單位還有兆瓦(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）。
另一種常用的功(gong)率單位以(yi)分貝毫瓦(wa)（dBm）表示。它以(yi)1毫瓦(wa)為基準(zhun)電(dian)平P0=1mW，實際功(gong)率值P（mW）與P0比較后取對數。這是功(gong)率的絕對單位。也可用分貝瓦(wa)（dBW）作為功(gong)率單位，此時(shi)P0=1W，即(ji)1dBW=3dBm。
根據被測信號(hao)頻率分類
功率計可分為：直流功率計、工頻(pin)(pin)功率計、變(bian)頻(pin)(pin)功率計、射頻(pin)(pin)功率計和微波功率計。由于直流功率等于電(dian)壓(ya)和電(dian)流的(de)(de)(de)(de)簡單(dan)乘積，實(shi)際測量(liang)(liang)中，一(yi)般采(cai)用電(dian)壓(ya)表和電(dian)流表替(ti)代。工頻(pin)(pin)功率計是(shi)應用較普遍的(de)(de)(de)(de)功率計，常(chang)說的(de)(de)(de)(de)功率計一(yi)般都是(shi)指工頻(pin)(pin)功率計。變(bian)頻(pin)(pin)功率計是(shi)21世紀變(bian)頻(pin)(pin)調速(su)技術高速(su)發展的(de)(de)(de)(de)產物。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)指用于傳輸功率的(de)(de)(de)(de)，并且(qie)滿足下(xia)述條件之一(yi)的(de)(de)(de)(de)交流電(dian)量(liang)(liang)：
1、信號頻(pin)譜僅包含一種頻(pin)率(lv)成分(fen)，而頻(pin)率(lv)不(bu)局限于工(gong)頻(pin)的交流(liu)電信號。
2、信(xin)(xin)號(hao)頻(pin)譜包含兩種(zhong)或更多的(de)(de)被(bei)關注的(de)(de)頻(pin)率成分的(de)(de)電信(xin)(xin)號(hao)。
變頻電(dian)量包括電(dian)壓(ya)(ya)、電(dian)流以(yi)及(ji)電(dian)壓(ya)(ya)電(dian)流引出的有功功率、無(wu)功功率、視在(zai)功率、有功電(dian)能、無(wu)功電(dian)能等。
除了變(bian)(bian)頻(pin)器(qi)輸(shu)出的(de)(de)(de)PWM波(bo)(bo)，二極(ji)管整流的(de)(de)(de)變(bian)(bian)頻(pin)器(qi)輸(shu)入的(de)(de)(de)電流波(bo)(bo)形，直流斬波(bo)(bo)器(qi)輸(shu)出的(de)(de)(de)電壓(ya)波(bo)(bo)形，變(bian)(bian)壓(ya)器(qi)空載的(de)(de)(de)輸(shu)入電流波(bo)(bo)形等，均含有較大的(de)(de)(de)諧波(bo)(bo)，右圖中為常見變(bian)(bian)頻(pin)電量(liang)的(de)(de)(de)波(bo)(bo)形及相關頻(pin)譜圖。
由于變(bian)頻(pin)(pin)電(dian)(dian)量的頻(pin)(pin)率(lv)(lv)成分(fen)復(fu)雜(za)(za)，變(bian)頻(pin)(pin)功(gong)(gong)(gong)率(lv)(lv)計的測(ce)量一般(ban)包括基(ji)(ji)波(bo)(bo)有功(gong)(gong)(gong)功(gong)(gong)(gong)率(lv)(lv)（簡稱(cheng)(cheng)基(ji)(ji)波(bo)(bo)功(gong)(gong)(gong)率(lv)(lv)）、諧(xie)波(bo)(bo)有功(gong)(gong)(gong)功(gong)(gong)(gong)率(lv)(lv)（簡稱(cheng)(cheng)諧(xie)波(bo)(bo)功(gong)(gong)(gong)率(lv)(lv)）、總有功(gong)(gong)(gong)功(gong)(gong)(gong)率(lv)(lv)等，相(xiang)比工(gong)頻(pin)(pin)功(gong)(gong)(gong)率(lv)(lv)計而言，其功(gong)(gong)(gong)能(neng)較多，技術較復(fu)雜(za)(za)，一般(ban)稱(cheng)(cheng)為變(bian)頻(pin)(pin)功(gong)(gong)(gong)率(lv)(lv)分(fen)析(xi)儀(yi)或寬頻(pin)(pin)功(gong)(gong)(gong)率(lv)(lv)分(fen)析(xi)儀(yi)，部分(fen)高精(jing)度功(gong)(gong)(gong)率(lv)(lv)分(fen)析(xi)儀(yi)也適(shi)用于變(bian)頻(pin)(pin)電(dian)(dian)量測(ce)量。
變頻功(gong)率(lv)(lv)分析(xi)儀(yi)可以作(zuo)為工頻功(gong)率(lv)(lv)分析(xi)儀(yi)使用，除此(ci)之外，一般還需滿足(zu)下述要求(qiu)：
1、滿足必要(yao)的帶寬要(yao)求，并(bing)且采樣頻率應高于儀(yi)器(qi)帶寬的兩倍。
2、要求分析儀在較寬的頻率范圍(wei)之內，精度均能滿足(zu)一定的要求。
3、具備傅(fu)里葉(xie)變換(huan)功能，可以分離信號的(de)基波和諧(xie)波。
射頻或微波(bo)功(gong)率(lv)計(ji)按照在(zai)測試系統(tong)中的連接(jie)方式(shi)不同(tong)分類
有終(zhong)(zhong)端式(shi)(shi)和通(tong)過式(shi)(shi)兩種。終(zhong)(zhong)端式(shi)(shi)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計把功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計探頭作為測(ce)試系統的(de)終(zhong)(zhong)端負載，功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計吸收全部待(dai)測(ce)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)，由功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)指示(shi)器直接讀取(qu)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)值。通(tong)過式(shi)(shi)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計利用某種耦(ou)合(he)裝置，如定向耦(ou)合(he)器、耦(ou)合(he)環、探針等(deng)從傳輸的(de)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)中按(an)一定的(de)比(bi)例耦(ou)合(he)出(chu)一部分功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)，送(song)入功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計度(du)量，傳輸的(de)總(zong)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)等(deng)于功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)計指示(shi)值乘以比(bi)例系數。
射頻(pin)或微波功率計按的(de)測量原理分類
測熱(re)電(dian)(dian)阻(zu)型功(gong)率(lv)計使用(yong)(yong)熱(re)變電(dian)(dian)阻(zu)做功(gong)率(lv)傳感元件。熱(re)變電(dian)(dian)阻(zu)值的(de)溫(wen)度系數較大。被測信號的(de)功(gong)率(lv)被熱(re)變電(dian)(dian)阻(zu)吸收后產生熱(re)量，使其自身溫(wen)度升高，電(dian)(dian)阻(zu)值發生顯著變化，利用(yong)(yong)電(dian)(dian)阻(zu)電(dian)(dian)橋(qiao)測量電(dian)(dian)阻(zu)值的(de)變化，顯示功(gong)率(lv)值。
熱(re)電偶(ou)型功(gong)(gong)率計(ji)熱(re)電偶(ou)型功(gong)(gong)率計(ji)中的(de)熱(re)偶(ou)結直接吸(xi)收高(gao)頻信號功(gong)(gong)率，結點溫度升(sheng)高(gao)，產生溫差電勢，電勢的(de)大小正(zheng)比于(yu)吸(xi)收的(de)高(gao)頻功(gong)(gong)率值。
量(liang)熱(re)式功(gong)率(lv)(lv)計(ji)(ji)典型的(de)熱(re)效應功(gong)率(lv)(lv)計(ji)(ji)，利(li)用隔熱(re)負(fu)載(zai)吸收高頻信號功(gong)率(lv)(lv)，使負(fu)載(zai)的(de)溫度升(sheng)高，再利(li)用熱(re)電(dian)偶(ou)元(yuan)件測量(liang)負(fu)載(zai)的(de)溫度變化量(liang)，根據產(chan)生的(de)熱(re)量(liang)計(ji)(ji)算高頻功(gong)率(lv)(lv)值。
晶(jing)體檢波(bo)式功率計晶(jing)體二(er)極管檢波(bo)器(qi)將高(gao)頻信(xin)號(hao)變換為低頻或(huo)直流(liu)電信(xin)號(hao)。適當選擇工作(zuo)點，使檢波(bo)器(qi)輸出信(xin)號(hao)的幅度正比于高(gao)頻信(xin)號(hao)的功率。
射頻或微波功率計按被測信號連續性分類有(you)連續波功率(lv)計和脈沖(chong)峰(feng)值功率(lv)計。
以下是變頻功率(lv)分析儀的典型技術指標
帶寬：50kHz~100kHz；
采樣(yang)頻率：大于帶寬的2倍；
電壓、電流準(zhun)確(que)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(que)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(ji)波頻率范圍：DC，0.1Hz~400Hz；
準(zhun)確級(ji)適(shi)用電壓范圍：0.75%Un~150%Un；
準(zhun)確級適(shi)用(yong)電(dian)流范圍(wei)：1%In~200%In；
準確級適用功率因數(shu)范圍(wei)：0.05~1。
以下是射頻功率計的典型技術指(zhi)標
功率范圍
保(bao)證測量精度的可測功(gong)率(lv)值的范(fan)圍。功(gong)率(lv)計的功(gong)率(lv)范(fan)圍決(jue)定于(yu)功(gong)率(lv)探頭。
最大允許功率
探頭(tou)不(bu)被損壞(huai)的(de)最大(da)輸入(ru)功(gong)率(lv)值(zhi)，通(tong)常指平均功(gong)率(lv)。在測量大(da)功(gong)率(lv)峰值(zhi)信(xin)號(hao)時(shi)，注意(yi)峰值(zhi)電壓和峰值(zhi)功(gong)率(lv)不(bu)能(neng)超過(guo)一定(ding)值(zhi)，否則會造(zao)成(cheng)功(gong)率(lv)探頭(tou)燒毀(hui)。
頻率范圍
能保證(zheng)測量(liang)精度和性能指標的被測信(xin)號的頻率(lv)范圍。
測量精度
指功率(lv)探頭校(xiao)準修正(zheng)后的精(jing)度。不包括測(ce)試系統的失配誤差。
穩定性
功(gong)(gong)率計(ji)的穩(wen)(wen)定(ding)性(xing)取決于(yu)功(gong)(gong)率探頭(tou)的穩(wen)(wen)定(ding)性(xing)和指示(shi)器的零漂及噪(zao)聲干擾。
響應時間
也稱功率傳感元件的(de)(de)時間(jian)常數。通(tong)常指功率指示器(qi)上升到穩定值的(de)(de)64%所需的(de)(de)時間(jian)。
探頭的型號、阻抗
選(xuan)用(yong)功(gong)率(lv)計探(tan)頭(tou)時，功(gong)率(lv)探(tan)頭(tou)的使用(yong)頻率(lv)、功(gong)率(lv)范(fan)圍必須與被測信號一致，探(tan)頭(tou)傳輸線的結構和阻(zu)抗應與被測傳輸線相互匹配。
用(yong)于測量(liang)絕對(dui)光(guang)(guang)(guang)(guang)功率(lv)或通(tong)過一段光(guang)(guang)(guang)(guang)纖(xian)的(de)光(guang)(guang)(guang)(guang)功率(lv)相對(dui)損耗。在(zai)光(guang)(guang)(guang)(guang)纖(xian)系統中(zhong)，測量(liang)光(guang)(guang)(guang)(guang)功率(lv)是最基本的(de)，非常(chang)像電子學中(zhong)的(de)萬用(yong)表。在(zai)光(guang)(guang)(guang)(guang)纖(xian)測量(liang)中(zhong)，光(guang)(guang)(guang)(guang)功率(lv)計是重負荷常(chang)用(yong)表。通(tong)過測量(liang)發射端機或光(guang)(guang)(guang)(guang)網絡的(de)絕對(dui)功率(lv)，一臺光(guang)(guang)(guang)(guang)功率(lv)計就能(neng)(neng)夠評價光(guang)(guang)(guang)(guang)端設備的(de)性能(neng)(neng)。用(yong)光(guang)(guang)(guang)(guang)功率(lv)計與穩定光(guang)(guang)(guang)(guang)源組合使用(yong)，則能(neng)(neng)夠測量(liang)連接損耗、檢驗連續(xu)性，并幫助評估光(guang)(guang)(guang)(guang)纖(xian)鏈路(lu)傳輸質量(liang)。
變(bian)頻功率分析(xi)(xi)儀適用于電(dian)(dian)(dian)力推進、電(dian)(dian)(dian)機、風機、水泵、風力發電(dian)(dian)(dian)、軌道交(jiao)通、電(dian)(dian)(dian)動(dong)汽車、變(bian)頻器(qi)(qi)、特(te)種(zhong)變(bian)壓器(qi)(qi)、熒(ying)光燈、LED照明等領域(yu)的(de)產品檢試驗、能效評測及電(dian)(dian)(dian)能質量分析(xi)(xi)。
選(xuan)擇功(gong)(gong)(gong)(gong)(gong)率(lv)表(biao)(biao)的(de)(de)量(liang)(liang)程(cheng)(cheng)就是(shi)選(xuan)擇功(gong)(gong)(gong)(gong)(gong)率(lv)表(biao)(biao)中的(de)(de)電(dian)(dian)(dian)(dian)流(liu)量(liang)(liang)程(cheng)(cheng)和(he)(he)電(dian)(dian)(dian)(dian)壓(ya)(ya)量(liang)(liang)程(cheng)(cheng)。使用(yong)時(shi)應(ying)使功(gong)(gong)(gong)(gong)(gong)率(lv)表(biao)(biao)中的(de)(de)電(dian)(dian)(dian)(dian)流(liu)量(liang)(liang)程(cheng)(cheng)不(bu)小(xiao)于(yu)負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)流(liu)，電(dian)(dian)(dian)(dian)壓(ya)(ya)量(liang)(liang)程(cheng)(cheng)不(bu)低于(yu)負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)壓(ya)(ya)，而不(bu)能僅從功(gong)(gong)(gong)(gong)(gong)率(lv)量(liang)(liang)程(cheng)(cheng)來(lai)考慮。例如(ru)，兩只(zhi)功(gong)(gong)(gong)(gong)(gong)率(lv)表(biao)(biao)，量(liang)(liang)程(cheng)(cheng)分別是(shi)IA、300V和(he)(he)2A、150V，由計算可知其功(gong)(gong)(gong)(gong)(gong)率(lv)量(liang)(liang)程(cheng)(cheng)均為(wei)300W，如(ru)果要(yao)測量(liang)(liang)一(yi)負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)壓(ya)(ya)為(wei)220V、電(dian)(dian)(dian)(dian)流(liu)為(wei)IA的(de)(de)負(fu)載(zai)(zai)功(gong)(gong)(gong)(gong)(gong)率(lv)時(shi)應(ying)逸用(yong)IA、300V的(de)(de)功(gong)(gong)(gong)(gong)(gong)率(lv)表(biao)(biao)，而2A、150V的(de)(de)功(gong)(gong)(gong)(gong)(gong)率(lv)表(biao)(biao)雖功(gong)(gong)(gong)(gong)(gong)率(lv)量(liang)(liang)程(cheng)(cheng)也大于(yu)負(fu)載(zai)(zai)功(gong)(gong)(gong)(gong)(gong)率(lv)，但(dan)是(shi)由于(yu)負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)壓(ya)(ya)高于(yu)功(gong)(gong)(gong)(gong)(gong)率(lv)表(biao)(biao)所能承受(shou)的(de)(de)電(dian)(dian)(dian)(dian)壓(ya)(ya)150V，故不(bu)能使用(yong)。所以，在測量(liang)(liang)功(gong)(gong)(gong)(gong)(gong)率(lv)前要(yao)根據負(fu)載(zai)(zai)的(de)(de)額定(ding)電(dian)(dian)(dian)(dian)壓(ya)(ya)和(he)(he)額定(ding)電(dian)(dian)(dian)(dian)流(liu)來(lai)選(xuan)擇功(gong)(gong)(gong)(gong)(gong)率(lv)表(biao)(biao)的(de)(de)量(liang)(liang)程(cheng)(cheng)。
電(dian)(dian)(dian)動(dong)系(xi)測量機構的(de)(de)轉動(dong)力矩方(fang)向和兩線(xian)(xian)(xian)圈(quan)中的(de)(de)電(dian)(dian)(dian)流方(fang)向有(you)關，為(wei)了(le)防止電(dian)(dian)(dian)動(dong)系(xi)功(gong)率表的(de)(de)指針反偏，接(jie)線(xian)(xian)(xian)時功(gong)率表電(dian)(dian)(dian)流線(xian)(xian)(xian)圈(quan)標有(you)“·”號(hao)的(de)(de)端(duan)鈕(niu)(niu)(niu)必須(xu)接(jie)到電(dian)(dian)(dian)源的(de)(de)正極端(duan)，而電(dian)(dian)(dian)流線(xian)(xian)(xian)圈(quan)的(de)(de)另(ling)一端(duan)則與負載相連，電(dian)(dian)(dian)流線(xian)(xian)(xian)圈(quan)以(yi)串(chuan)聯(lian)形式接(jie)入(ru)電(dian)(dian)(dian)路中。功(gong)率表電(dian)(dian)(dian)壓線(xian)(xian)(xian)圈(quan)標有(you)“·”號(hao)的(de)(de)端(duan)鈕(niu)(niu)(niu)可以(yi)接(jie)到電(dian)(dian)(dian)源端(duan)鈕(niu)(niu)(niu)的(de)(de)任(ren)一端(duan)上(shang)，而另(ling)一電(dian)(dian)(dian)壓端(duan)鈕(niu)(niu)(niu)則跨接(jie)到負載的(de)(de)另(ling)一端(duan)。
當(dang)負載電(dian)(dian)阻(zu)遠遠大于電(dian)(dian)流(liu)線(xian)(xian)圈的電(dian)(dian)阻(zu)時，應采用(yong)電(dian)(dian)壓(ya)線(xian)(xian)圈前接(jie)(jie)法。這(zhe)時電(dian)(dian)壓(ya)線(xian)(xian)圈的電(dian)(dian)壓(ya)是負載電(dian)(dian)壓(ya)和電(dian)(dian)流(liu)線(xian)(xian)圈電(dian)(dian)壓(ya)之(zhi)和，功率表測(ce)量的是負載功率和電(dian)(dian)流(liu)線(xian)(xian)圈功率之(zhi)和。如果負載電(dian)(dian)阻(zu)遠遠大于電(dian)(dian)流(liu)線(xian)(xian)圈的電(dian)(dian)阻(zu)，則可以略去電(dian)(dian)流(liu)線(xian)(xian)圈分壓(ya)所造(zao)成(cheng)的影響，測(ce)量結(jie)果比較接(jie)(jie)近負載的實際功率值。
當負(fu)載電(dian)(dian)阻(zu)遠遠小于電(dian)(dian)壓(ya)(ya)線(xian)(xian)圈電(dian)(dian)阻(zu)時(shi)，應采用(yong)電(dian)(dian)壓(ya)(ya)線(xian)(xian)圈后(hou)接法。這時(shi)電(dian)(dian)壓(ya)(ya)線(xian)(xian)圈兩(liang)端的(de)(de)(de)電(dian)(dian)壓(ya)(ya)雖然等于負(fu)載電(dian)(dian)壓(ya)(ya)，但電(dian)(dian)流線(xian)(xian)圈中的(de)(de)(de)電(dian)(dian)流卻(que)等于負(fu)載電(dian)(dian)流與功率(lv)表電(dian)(dian)壓(ya)(ya)線(xian)(xian)圈中的(de)(de)(de)電(dian)(dian)流之和，測量(liang)時(shi)功率(lv)讀數為(wei)負(fu)載功率(lv)與電(dian)(dian)壓(ya)(ya)線(xian)(xian)圈功率(lv)之和。由于此時(shi)負(fu)載電(dian)(dian)阻(zu)遠小于電(dian)(dian)壓(ya)(ya)線(xian)(xian)圈電(dian)(dian)阻(zu)，所以電(dian)(dian)壓(ya)(ya)線(xian)(xian)圈分(fen)流作用(yong)大大減小，其對測量(liang)結(jie)果的(de)(de)(de)影(ying)響也可以大為(wei)減小。
如界被測(ce)(ce)負載本(ben)身功率較大，可以不考慮功率表本(ben)身的功率對測(ce)(ce)量結(jie)果的影響(xiang)，則兩種接(jie)法可以任意(yi)選擇(ze)。但最好選用電(dian)(dian)壓線(xian)圈(quan)前(qian)接(jie)法，因(yin)為功率表中電(dian)(dian)流(liu)線(xian)圈(quan)的功率一般都(dou)小于電(dian)(dian)壓線(xian)圈(quan)支路(lu)的功率。
一般(ban)安(an)裝(zhuang)式功(gong)(gong)率表(biao)(biao)為(wei)直讀(du)單量(liang)程(cheng)式，表(biao)(biao)上(shang)的(de)(de)示(shi)數(shu)(shu)(shu)即為(wei)功(gong)(gong)率數(shu)(shu)(shu)。但(dan)便(bian)攜式功(gong)(gong)率表(biao)(biao)一般(ban)為(wei)多量(liang)程(cheng)式，在(zai)表(biao)(biao)的(de)(de)標(biao)(biao)度(du)尺上(shang)不(bu)直接標(biao)(biao)注(zhu)示(shi)數(shu)(shu)(shu)，只標(biao)(biao)注(zhu)分格(ge)。在(zai)選(xuan)用不(bu)同(tong)的(de)(de)電(dian)流(liu)與電(dian)壓(ya)量(liang)程(cheng)時(shi)，每一分格(ge)都可以表(biao)(biao)示(shi)不(bu)同(tong)的(de)(de)功(gong)(gong)率數(shu)(shu)(shu)。在(zai)讀(du)數(shu)(shu)(shu)時(shi)，應先根據(ju)所(suo)選(xuan)的(de)(de)電(dian)壓(ya)量(liang)程(cheng)U、電(dian)流(liu)量(liang)程(cheng)I以及標(biao)(biao)度(du)尺滿量(liang)程(cheng)時(shi)的(de)(de)格(ge)數(shu)(shu)(shu)&，求出每格(ge)瓦數(shu)(shu)(shu)（又稱(cheng)功(gong)(gong)率表(biao)(biao)常數(shu)(shu)(shu)）C，然后再乘上(shang)指針偏轉的(de)(de)格(ge)數(shu)(shu)(shu)夕(xi)，就可得到所(suo)測功(gong)(gong)率P
例題
例：有一只電(dian)(dian)壓量(liang)程(cheng)為250V，電(dian)(dian)流(liu)量(liang)程(cheng)為3A，標度尺分(fen)格數(shu)為75的功(gong)率表，現用它來測量(liang)負(fu)載的功(gong)率。
當指針(zhen)偏轉50格(ge)時負載功率為多少？
解：先計算(suan)功率表常數C
C=UI/a，=250V×3A/75格(ge)=10W/格(ge)
故被測功率為
P=C色=10W/格×50格=500W
對大多數從事電氣(qi)方面工(gong)作的(de)人員(yuan)來說(shuo)，功(gong)率(lv)(lv)表的(de)使用(yong)并非(fei)難事。但(dan)真(zhen)正做到正確(que)使用(yong)功(gong)率(lv)(lv)表，即在準確(que)度一定的(de)情(qing)況下(xia)確(que)保測(ce)量的(de)精度及儀表的(de)使用(yong)壽命又(you)并非(fei)易事。以單相電動系功(gong)率(lv)(lv)表為例，就功(gong)率(lv)(lv)表的(de)使用(yong)及使用(yong)中應(ying)注意(yi)的(de)問題作一介(jie)紹(shao)。
1、要遵守“發電機端守則”
由(you)電(dian)(dian)動系功率(lv)表(biao)(biao)的(de)原(yuan)理可知，功率(lv)表(biao)(biao)的(de)轉矩與流(liu)(liu)(liu)過表(biao)(biao)內線(xian)圈的(de)電(dian)(dian)流(liu)(liu)(liu)方向有關(guan)，一旦其中(zhong)一個(ge)線(xian)圈的(de)電(dian)(dian)流(liu)(liu)(liu)方向改變，轉矩方向也會改變。為此，在(zai)功率(lv)表(biao)(biao)兩(liang)個(ge)線(xian)圈對應于電(dian)(dian)流(liu)(liu)(liu)流(liu)(liu)(liu)進的(de)端(duan)(duan)鈕上，都(dou)注有稱(cheng)為發電(dian)(dian)機(ji)(ji)(ji)端(duan)(duan)的(de)“*”標志(zhi)。功率(lv)表(biao)(biao)在(zai)接(jie)線(xian)時，應使電(dian)(dian)流(liu)(liu)(liu)或電(dian)(dian)壓線(xian)圈帶“*”標志(zhi)的(de)端(duan)(duan)鈕接(jie)到電(dian)(dian)源同極性(xing)的(de)端(duan)(duan)子上，以保證(zheng)兩(liang)線(xian)圈的(de)電(dian)(dian)流(liu)(liu)(liu)方向都(dou)從(cong)發電(dian)(dian)機(ji)(ji)(ji)端(duan)(duan)流(liu)(liu)(liu)入。這就是功率(lv)表(biao)(biao)接(jie)線(xian)的(de)“發電(dian)(dian)機(ji)(ji)(ji)端(duan)(duan)守則”。
2、合(he)理(li)選擇電壓線圈(quan)的前、后接(jie)方式
盡(jin)管電(dian)壓(ya)線(xian)圈(quan)不論前接還(huan)是后接，功率表(biao)都能(neng)正偏，對于(yu)某些(xie)負載(zai)來說，測量的結果(guo)相(xiang)差較小(xiao)，這時兩(liang)種(zhong)接法采用哪(na)種(zhong)均可。但對于(yu)那(nei)些(xie)電(dian)阻(zu)（或(huo)阻(zu)抗）過大或(huo)過小(xiao)的負載(zai)來說，兩(liang)種(zhong)接法所得(de)結果(guo)相(xiang)差較大，有時甚(shen)至出現與理論相(xiang)矛盾的結果(guo)。
這種方式的(de)接線，功(gong)(gong)率(lv)(lv)表(biao)(biao)電(dian)(dian)(dian)流(liu)線圈(quan)(quan)的(de)電(dian)(dian)(dian)流(liu)雖然等于負載(zai)電(dian)(dian)(dian)流(liu)，但功(gong)(gong)率(lv)(lv)表(biao)(biao)電(dian)(dian)(dian)壓(ya)(ya)(ya)支(zhi)路兩(liang)端電(dian)(dian)(dian)壓(ya)(ya)(ya)卻(que)等于負載(zai)電(dian)(dian)(dian)壓(ya)(ya)(ya)與功(gong)(gong)率(lv)(lv)表(biao)(biao)電(dian)(dian)(dian)流(liu)線圈(quan)(quan)的(de)電(dian)(dian)(dian)壓(ya)(ya)(ya)之和，在功(gong)(gong)率(lv)(lv)表(biao)(biao)讀數中多了電(dian)(dian)(dian)流(liu)線圈(quan)(quan)的(de)功(gong)(gong)率(lv)(lv)消(xiao)耗。這種接線方式適(shi)用于負載(zai)電(dian)(dian)(dian)阻(zu)（或阻(zu)抗(kang)）遠比功(gong)(gong)率(lv)(lv)表(biao)(biao)電(dian)(dian)(dian)流(liu)線圈(quan)(quan)電(dian)(dian)(dian)阻(zu)（或阻(zu)抗(kang)）大得多的(de)情況，這樣才能保證功(gong)(gong)率(lv)(lv)表(biao)(biao)本身(shen)的(de)功(gong)(gong)率(lv)(lv)消(xiao)耗對測量結果的(de)影響比較小(xiao)。
這種方式的(de)(de)(de)接線，功(gong)(gong)率表電(dian)(dian)壓支(zhi)路(lu)(lu)兩(liang)端(duan)的(de)(de)(de)電(dian)(dian)壓雖然(ran)等于(yu)負(fu)載電(dian)(dian)壓，但電(dian)(dian)流(liu)線圈(quan)的(de)(de)(de)電(dian)(dian)流(liu)卻等于(yu)負(fu)載電(dian)(dian)流(liu)與功(gong)(gong)率表電(dian)(dian)壓線圈(quan)支(zhi)路(lu)(lu)電(dian)(dian)流(liu)之(zhi)和，功(gong)(gong)率表讀(du)數中多(duo)了電(dian)(dian)壓支(zhi)路(lu)(lu)的(de)(de)(de)功(gong)(gong)率消耗。因此，這種接線適(shi)用于(yu)負(fu)載電(dian)(dian)阻(zu)(zu)（或(huo)阻(zu)(zu)抗(kang)）遠比功(gong)(gong)率表電(dian)(dian)壓支(zhi)路(lu)(lu)電(dian)(dian)阻(zu)(zu)（或(huo)阻(zu)(zu)抗(kang)）小得多(duo)的(de)(de)(de)情況，這樣(yang)才能(neng)保證功(gong)(gong)率表本身的(de)(de)(de)功(gong)(gong)率消耗對測(ce)量結果的(de)(de)(de)影響比較小。
