功(gong)(gong)率是表征電(dian)信號特性的一個重要參數。在(zai)直流和低頻范圍，可(ke)以通過測(ce)量(liang)電(dian)壓和電(dian)流計算功(gong)(gong)率，功(gong)(gong)率的瞬時值可(ke)用下式(shi)表示(shi)：
對于周期信(xin)號，一個周期內的瞬時功率(lv)(lv)的平均值，稱(cheng)為有功功率(lv)(lv)。有功功率(lv)(lv)按(an)下式計算：
對于正弦電(dian)路，下式(shi)成立：
上式中，U、I分別為(wei)正(zheng)弦交(jiao)流(liu)(liu)電的(de)有效(xiao)值，φ為(wei)電壓與電流(liu)(liu)信號(hao)的(de)相位差。
在(zai)(zai)超高頻和微波(bo)(bo)(bo)頻段，有(you)TEM波(bo)(bo)(bo)和非TEM波(bo)(bo)(bo)之(zhi)分。在(zai)(zai)TEM波(bo)(bo)(bo)的同(tong)軸系統(tong)(tong)中(zhong)，電(dian)(dian)壓(ya)和電(dian)(dian)流雖有(you)確(que)切含意，但測(ce)量(liang)其絕對值很困難。在(zai)(zai)波(bo)(bo)(bo)導系統(tong)(tong)中(zhong)，因為存在(zai)(zai)不同(tong)的電(dian)(dian)磁模式，電(dian)(dian)壓(ya)和電(dian)(dian)流失去唯一(yi)性。在(zai)(zai)個(ge)頻段和各傳輸系統(tong)(tong)中(zhong)，功(gong)率(lv)是(shi)單值表征信號強度的重要方法。在(zai)(zai)射頻范圍直(zhi)接測(ce)量(liang)功(gong)率(lv)代替了電(dian)(dian)壓(ya)和電(dian)(dian)流的測(ce)量(liang)。
功率定義為單位(wei)時間(jian)內所(suo)做的功。基(ji)本單位(wei)為瓦（W），1W等于在(zai)1秒(miao)內做1焦耳(er)的功。常用的功率單位(wei)還(huan)有兆瓦（1MW=10^6W）、千瓦（1KW=10^3W）、毫(hao)瓦（1mW=10-3W）、微瓦（1μW=10-6W）、皮(pi)瓦（1Pw=10-12W）。
另一種常用的(de)(de)功(gong)(gong)率單位以分貝(bei)毫(hao)瓦（dBm）表示(shi)。它以1毫(hao)瓦為基準電(dian)平P0=1mW，實(shi)際(ji)功(gong)(gong)率值(zhi)P（mW）與(yu)P0比較后取對數。這是功(gong)(gong)率的(de)(de)絕對單位。也可用分貝(bei)瓦（dBW）作為功(gong)(gong)率單位，此時P0=1W，即1dBW=3dBm。
根據被測信(xin)號頻率分類
功(gong)率(lv)計(ji)可(ke)分為(wei)(wei)：直流功(gong)率(lv)計(ji)、工(gong)頻(pin)(pin)功(gong)率(lv)計(ji)、變頻(pin)(pin)功(gong)率(lv)計(ji)、射(she)頻(pin)(pin)功(gong)率(lv)計(ji)和微波功(gong)率(lv)計(ji)。由(you)于直流功(gong)率(lv)等于電(dian)壓(ya)和電(dian)流的(de)簡單乘積，實際測量(liang)(liang)中(zhong)，一般(ban)采用電(dian)壓(ya)表和電(dian)流表替代。工(gong)頻(pin)(pin)功(gong)率(lv)計(ji)是(shi)應用較普遍的(de)功(gong)率(lv)計(ji)，常說的(de)功(gong)率(lv)計(ji)一般(ban)都是(shi)指工(gong)頻(pin)(pin)功(gong)率(lv)計(ji)。變頻(pin)(pin)功(gong)率(lv)計(ji)是(shi)21世紀變頻(pin)(pin)調速技(ji)術(shu)高速發展的(de)產物(wu)。其測量(liang)(liang)對象為(wei)(wei)變頻(pin)(pin)電(dian)量(liang)(liang)，變頻(pin)(pin)電(dian)量(liang)(liang)是(shi)指用于傳輸功(gong)率(lv)的(de)，并且滿足下述條(tiao)件之一的(de)交流電(dian)量(liang)(liang)：
1、信號(hao)頻(pin)譜僅(jin)包含(han)一(yi)種頻(pin)率成分，而頻(pin)率不局(ju)限(xian)于工(gong)頻(pin)的交(jiao)流電信號(hao)。
2、信(xin)號頻譜包含兩種(zhong)或更(geng)多的被關(guan)注的頻率成分的電信(xin)號。
變頻電(dian)量包括電(dian)壓、電(dian)流(liu)以(yi)及電(dian)壓電(dian)流(liu)引出的有(you)功(gong)(gong)功(gong)(gong)率(lv)、無功(gong)(gong)功(gong)(gong)率(lv)、視(shi)在功(gong)(gong)率(lv)、有(you)功(gong)(gong)電(dian)能(neng)、無功(gong)(gong)電(dian)能(neng)等。
除了變(bian)頻器(qi)(qi)輸(shu)(shu)出的PWM波(bo)，二極管整(zheng)流的變(bian)頻器(qi)(qi)輸(shu)(shu)入的電(dian)流波(bo)形(xing)，直流斬波(bo)器(qi)(qi)輸(shu)(shu)出的電(dian)壓波(bo)形(xing)，變(bian)壓器(qi)(qi)空載的輸(shu)(shu)入電(dian)流波(bo)形(xing)等，均(jun)含有較(jiao)大的諧波(bo)，右圖(tu)中為(wei)常見(jian)變(bian)頻電(dian)量的波(bo)形(xing)及相(xiang)關頻譜圖(tu)。
由于變(bian)(bian)頻(pin)(pin)(pin)電量的頻(pin)(pin)(pin)率(lv)(lv)成分(fen)(fen)復雜(za)(za)，變(bian)(bian)頻(pin)(pin)(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)計的測(ce)量一般包括(kuo)基波有功(gong)(gong)(gong)(gong)功(gong)(gong)(gong)(gong)率(lv)(lv)（簡稱(cheng)基波功(gong)(gong)(gong)(gong)率(lv)(lv)）、諧(xie)波有功(gong)(gong)(gong)(gong)功(gong)(gong)(gong)(gong)率(lv)(lv)（簡稱(cheng)諧(xie)波功(gong)(gong)(gong)(gong)率(lv)(lv)）、總有功(gong)(gong)(gong)(gong)功(gong)(gong)(gong)(gong)率(lv)(lv)等，相比工頻(pin)(pin)(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)計而言，其(qi)功(gong)(gong)(gong)(gong)能較(jiao)多，技(ji)術(shu)較(jiao)復雜(za)(za)，一般稱(cheng)為(wei)變(bian)(bian)頻(pin)(pin)(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)分(fen)(fen)析儀(yi)或寬頻(pin)(pin)(pin)功(gong)(gong)(gong)(gong)率(lv)(lv)分(fen)(fen)析儀(yi)，部分(fen)(fen)高精度(du)功(gong)(gong)(gong)(gong)率(lv)(lv)分(fen)(fen)析儀(yi)也適用于變(bian)(bian)頻(pin)(pin)(pin)電量測(ce)量。
變頻(pin)功率分析儀(yi)可以作為工頻(pin)功率分析儀(yi)使用，除此之外，一般還需滿(man)足下述要求：
1、滿足必要的帶寬(kuan)要求(qiu)，并且采樣頻(pin)率應高于儀器帶寬(kuan)的兩(liang)倍。
2、要(yao)求分析儀在較寬的頻(pin)率范圍之內，精度均能滿足(zu)一定的要(yao)求。
3、具備傅里葉變換功能，可(ke)以(yi)分(fen)離信號的基波(bo)和諧波(bo)。
射頻或微波功率(lv)計按(an)照在測試(shi)系統中(zhong)的連接(jie)方(fang)式不同分類
有終(zhong)(zhong)端(duan)式(shi)和通過式(shi)兩種。終(zhong)(zhong)端(duan)式(shi)功(gong)率(lv)(lv)(lv)(lv)計(ji)(ji)把(ba)功(gong)率(lv)(lv)(lv)(lv)計(ji)(ji)探頭作為測(ce)試系(xi)統(tong)的(de)(de)終(zhong)(zhong)端(duan)負載，功(gong)率(lv)(lv)(lv)(lv)計(ji)(ji)吸(xi)收全部待測(ce)功(gong)率(lv)(lv)(lv)(lv)，由功(gong)率(lv)(lv)(lv)(lv)指示器直接(jie)讀取功(gong)率(lv)(lv)(lv)(lv)值。通過式(shi)功(gong)率(lv)(lv)(lv)(lv)計(ji)(ji)利用某(mou)種耦(ou)合裝置，如定(ding)向耦(ou)合器、耦(ou)合環、探針等從傳輸(shu)的(de)(de)功(gong)率(lv)(lv)(lv)(lv)中按一(yi)定(ding)的(de)(de)比(bi)例耦(ou)合出一(yi)部分功(gong)率(lv)(lv)(lv)(lv)，送入功(gong)率(lv)(lv)(lv)(lv)計(ji)(ji)度量(liang)，傳輸(shu)的(de)(de)總(zong)功(gong)率(lv)(lv)(lv)(lv)等于(yu)功(gong)率(lv)(lv)(lv)(lv)計(ji)(ji)指示值乘(cheng)以(yi)比(bi)例系(xi)數(shu)。
射頻或微波(bo)功(gong)率(lv)計(ji)按(an)的測量原(yuan)理分類(lei)
測熱(re)(re)(re)(re)電(dian)(dian)阻(zu)(zu)型功(gong)率(lv)計使用熱(re)(re)(re)(re)變(bian)電(dian)(dian)阻(zu)(zu)做(zuo)功(gong)率(lv)傳感元件。熱(re)(re)(re)(re)變(bian)電(dian)(dian)阻(zu)(zu)值(zhi)的溫(wen)度系數較(jiao)大。被測信號的功(gong)率(lv)被熱(re)(re)(re)(re)變(bian)電(dian)(dian)阻(zu)(zu)吸收后(hou)產生(sheng)熱(re)(re)(re)(re)量，使其自身溫(wen)度升高，電(dian)(dian)阻(zu)(zu)值(zhi)發生(sheng)顯(xian)著(zhu)變(bian)化(hua)，利用電(dian)(dian)阻(zu)(zu)電(dian)(dian)橋測量電(dian)(dian)阻(zu)(zu)值(zhi)的變(bian)化(hua)，顯(xian)示功(gong)率(lv)值(zhi)。
熱電(dian)偶型(xing)功率計(ji)(ji)熱電(dian)偶型(xing)功率計(ji)(ji)中的熱偶結直(zhi)接吸收(shou)高頻信(xin)號(hao)功率，結點溫度升高，產(chan)生溫差電(dian)勢，電(dian)勢的大小正比于吸收(shou)的高頻功率值。
量(liang)熱(re)式(shi)功(gong)率計典型的(de)熱(re)效應(ying)功(gong)率計，利用(yong)隔熱(re)負(fu)載(zai)吸(xi)收高頻信號功(gong)率，使負(fu)載(zai)的(de)溫度升高，再利用(yong)熱(re)電偶元(yuan)件測量(liang)負(fu)載(zai)的(de)溫度變化(hua)量(liang)，根據(ju)產生的(de)熱(re)量(liang)計算高頻功(gong)率值。
晶(jing)體檢(jian)波式功(gong)率(lv)(lv)計晶(jing)體二極管(guan)檢(jian)波器將高(gao)頻(pin)信(xin)號(hao)變換為低(di)頻(pin)或直(zhi)流電信(xin)號(hao)。適當選擇工(gong)作點，使檢(jian)波器輸出信(xin)號(hao)的幅(fu)度正(zheng)比于高(gao)頻(pin)信(xin)號(hao)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(lian)續(xu)波功率計和(he)脈沖峰值功率計。
以下(xia)是變(bian)頻功率分析儀的典型技術指標
帶(dai)寬：50kHz~100kHz；
采樣頻率(lv)：大于(yu)帶寬的2倍(bei)；
電(dian)壓(ya)、電(dian)流(liu)準確級(ji)(ji)(ji)：0.02級(ji)(ji)(ji)、0.05級(ji)(ji)(ji)、0.1級(ji)(ji)(ji)、0.2級(ji)(ji)(ji)、0.5級(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)；
準確級適(shi)用基(ji)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(ji)適用電流范圍：1%In~200%In；
準確級適用(yong)功率因數范圍(wei)：0.05~1。
以下是射頻功率計(ji)的典型(xing)技術(shu)指標
功率范圍
保證測(ce)量精度的可測(ce)功率值的范(fan)圍。功率計的功率范(fan)圍決定于功率探頭(tou)。
最大允許功率
探頭不被損(sun)壞(huai)的最(zui)大(da)輸入(ru)功(gong)率值(zhi)(zhi)，通常指平均(jun)功(gong)率。在測量大(da)功(gong)率峰值(zhi)(zhi)信號時，注意峰值(zhi)(zhi)電壓和(he)峰值(zhi)(zhi)功(gong)率不能超過一定(ding)值(zhi)(zhi)，否則會造(zao)成功(gong)率探頭燒毀。
頻率范圍
能保證(zheng)測量精(jing)度和(he)性能指標的被測信號的頻率(lv)范圍(wei)。
測量精度
指功(gong)率探(tan)頭校準修正后的(de)精度。不(bu)包括測試系統的(de)失配誤差。
穩定性
功(gong)率計的穩定(ding)性(xing)取決于功(gong)率探頭的穩定(ding)性(xing)和指示器的零漂(piao)及噪聲干擾。
響應時間
也稱功(gong)率(lv)傳(chuan)感元件的時間常(chang)數。通常(chang)指(zhi)功(gong)率(lv)指(zhi)示器(qi)上升到(dao)穩定值的64%所需的時間。
探頭的型號、阻抗
選(xuan)用(yong)功率(lv)計(ji)探(tan)頭時，功率(lv)探(tan)頭的(de)使用(yong)頻率(lv)、功率(lv)范圍必須與(yu)被測信(xin)號一致，探(tan)頭傳輸線的(de)結構和阻(zu)抗應與(yu)被測傳輸線相互匹配。
用于測(ce)(ce)(ce)量(liang)絕對(dui)(dui)光(guang)功(gong)率(lv)(lv)或(huo)通(tong)過一段(duan)光(guang)纖的(de)光(guang)功(gong)率(lv)(lv)相對(dui)(dui)損耗。在光(guang)纖系(xi)統中，測(ce)(ce)(ce)量(liang)光(guang)功(gong)率(lv)(lv)是最基本的(de)，非常像(xiang)電子學中的(de)萬用表。在光(guang)纖測(ce)(ce)(ce)量(liang)中，光(guang)功(gong)率(lv)(lv)計是重負荷常用表。通(tong)過測(ce)(ce)(ce)量(liang)發射(she)端機(ji)或(huo)光(guang)網絡的(de)絕對(dui)(dui)功(gong)率(lv)(lv)，一臺光(guang)功(gong)率(lv)(lv)計就能(neng)夠評(ping)價光(guang)端設備的(de)性能(neng)。用光(guang)功(gong)率(lv)(lv)計與穩定光(guang)源(yuan)組(zu)合(he)使(shi)用，則能(neng)夠測(ce)(ce)(ce)量(liang)連(lian)接損耗、檢(jian)驗連(lian)續(xu)性，并幫助評(ping)估(gu)光(guang)纖鏈(lian)路傳輸質(zhi)量(liang)。
變(bian)頻功率分析儀適用于電力推(tui)進、電機、風(feng)機、水泵、風(feng)力發電、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(ling)域(yu)的產品檢試(shi)驗、能效評測及電能質(zhi)量(liang)分析。
選(xuan)擇功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)表(biao)的(de)(de)量(liang)(liang)(liang)(liang)程就是選(xuan)擇功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)表(biao)中的(de)(de)電(dian)流(liu)(liu)量(liang)(liang)(liang)(liang)程和(he)電(dian)壓(ya)(ya)量(liang)(liang)(liang)(liang)程。使用(yong)時(shi)應(ying)使功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)表(biao)中的(de)(de)電(dian)流(liu)(liu)量(liang)(liang)(liang)(liang)程不(bu)小于(yu)(yu)負載(zai)電(dian)流(liu)(liu)，電(dian)壓(ya)(ya)量(liang)(liang)(liang)(liang)程不(bu)低于(yu)(yu)負載(zai)電(dian)壓(ya)(ya)，而不(bu)能(neng)僅從功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)量(liang)(liang)(liang)(liang)程來考慮。例如，兩只功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)表(biao)，量(liang)(liang)(liang)(liang)程分別(bie)是IA、300V和(he)2A、150V，由(you)計算可知其功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)量(liang)(liang)(liang)(liang)程均為300W，如果要測(ce)量(liang)(liang)(liang)(liang)一(yi)負載(zai)電(dian)壓(ya)(ya)為220V、電(dian)流(liu)(liu)為IA的(de)(de)負載(zai)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)時(shi)應(ying)逸用(yong)IA、300V的(de)(de)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)表(biao)，而2A、150V的(de)(de)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)表(biao)雖功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)量(liang)(liang)(liang)(liang)程也大于(yu)(yu)負載(zai)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)，但是由(you)于(yu)(yu)負載(zai)電(dian)壓(ya)(ya)高于(yu)(yu)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)表(biao)所能(neng)承受(shou)的(de)(de)電(dian)壓(ya)(ya)150V，故(gu)不(bu)能(neng)使用(yong)。所以，在測(ce)量(liang)(liang)(liang)(liang)功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)前(qian)要根據負載(zai)的(de)(de)額定(ding)電(dian)壓(ya)(ya)和(he)額定(ding)電(dian)流(liu)(liu)來選(xuan)擇功(gong)(gong)(gong)率(lv)(lv)(lv)(lv)表(biao)的(de)(de)量(liang)(liang)(liang)(liang)程。
電(dian)(dian)動系(xi)(xi)測(ce)量機構的(de)(de)(de)(de)轉動力矩方向和兩線(xian)(xian)(xian)(xian)圈(quan)中(zhong)的(de)(de)(de)(de)電(dian)(dian)流(liu)方向有(you)(you)關，為了防止(zhi)電(dian)(dian)動系(xi)(xi)功率表(biao)的(de)(de)(de)(de)指針(zhen)反偏，接線(xian)(xian)(xian)(xian)時功率表(biao)電(dian)(dian)流(liu)線(xian)(xian)(xian)(xian)圈(quan)標有(you)(you)“·”號的(de)(de)(de)(de)端(duan)(duan)(duan)(duan)鈕(niu)必須接到電(dian)(dian)源的(de)(de)(de)(de)正極端(duan)(duan)(duan)(duan)，而電(dian)(dian)流(liu)線(xian)(xian)(xian)(xian)圈(quan)的(de)(de)(de)(de)另一(yi)端(duan)(duan)(duan)(duan)則與負載相連，電(dian)(dian)流(liu)線(xian)(xian)(xian)(xian)圈(quan)以串聯形式接入電(dian)(dian)路(lu)中(zhong)。功率表(biao)電(dian)(dian)壓(ya)線(xian)(xian)(xian)(xian)圈(quan)標有(you)(you)“·”號的(de)(de)(de)(de)端(duan)(duan)(duan)(duan)鈕(niu)可以接到電(dian)(dian)源端(duan)(duan)(duan)(duan)鈕(niu)的(de)(de)(de)(de)任一(yi)端(duan)(duan)(duan)(duan)上(shang)，而另一(yi)電(dian)(dian)壓(ya)端(duan)(duan)(duan)(duan)鈕(niu)則跨接到負載的(de)(de)(de)(de)另一(yi)端(duan)(duan)(duan)(duan)。
當負載電(dian)(dian)(dian)阻遠遠大于電(dian)(dian)(dian)流(liu)線(xian)(xian)圈(quan)(quan)的(de)(de)電(dian)(dian)(dian)阻時(shi)，應采用電(dian)(dian)(dian)壓(ya)(ya)線(xian)(xian)圈(quan)(quan)前接(jie)法。這時(shi)電(dian)(dian)(dian)壓(ya)(ya)線(xian)(xian)圈(quan)(quan)的(de)(de)電(dian)(dian)(dian)壓(ya)(ya)是(shi)負載電(dian)(dian)(dian)壓(ya)(ya)和(he)(he)電(dian)(dian)(dian)流(liu)線(xian)(xian)圈(quan)(quan)電(dian)(dian)(dian)壓(ya)(ya)之和(he)(he)，功率表測(ce)量的(de)(de)是(shi)負載功率和(he)(he)電(dian)(dian)(dian)流(liu)線(xian)(xian)圈(quan)(quan)功率之和(he)(he)。如果(guo)負載電(dian)(dian)(dian)阻遠遠大于電(dian)(dian)(dian)流(liu)線(xian)(xian)圈(quan)(quan)的(de)(de)電(dian)(dian)(dian)阻，則可(ke)以略(lve)去(qu)電(dian)(dian)(dian)流(liu)線(xian)(xian)圈(quan)(quan)分(fen)壓(ya)(ya)所造成的(de)(de)影響，測(ce)量結果(guo)比較(jiao)接(jie)近負載的(de)(de)實際功率值。
當負載電(dian)(dian)(dian)阻(zu)遠遠小于(yu)電(dian)(dian)(dian)壓(ya)(ya)線圈(quan)電(dian)(dian)(dian)阻(zu)時(shi)，應采用電(dian)(dian)(dian)壓(ya)(ya)線圈(quan)后(hou)接法。這時(shi)電(dian)(dian)(dian)壓(ya)(ya)線圈(quan)兩端(duan)的電(dian)(dian)(dian)壓(ya)(ya)雖然等于(yu)負載電(dian)(dian)(dian)壓(ya)(ya)，但電(dian)(dian)(dian)流線圈(quan)中的電(dian)(dian)(dian)流卻等于(yu)負載電(dian)(dian)(dian)流與功率(lv)表電(dian)(dian)(dian)壓(ya)(ya)線圈(quan)中的電(dian)(dian)(dian)流之(zhi)和，測(ce)量時(shi)功率(lv)讀(du)數為負載功率(lv)與電(dian)(dian)(dian)壓(ya)(ya)線圈(quan)功率(lv)之(zhi)和。由于(yu)此時(shi)負載電(dian)(dian)(dian)阻(zu)遠小于(yu)電(dian)(dian)(dian)壓(ya)(ya)線圈(quan)電(dian)(dian)(dian)阻(zu)，所(suo)以電(dian)(dian)(dian)壓(ya)(ya)線圈(quan)分流作用大(da)大(da)減(jian)(jian)小，其(qi)對測(ce)量結(jie)果(guo)的影響也可以大(da)為減(jian)(jian)小。
如界(jie)被測(ce)負載(zai)本身功(gong)率(lv)較大，可以(yi)不考慮功(gong)率(lv)表本身的(de)功(gong)率(lv)對測(ce)量結果的(de)影響(xiang)，則兩種接法可以(yi)任意選(xuan)擇(ze)。但最好選(xuan)用電(dian)(dian)(dian)壓線(xian)圈前接法，因(yin)為功(gong)率(lv)表中電(dian)(dian)(dian)流(liu)線(xian)圈的(de)功(gong)率(lv)一般都小于電(dian)(dian)(dian)壓線(xian)圈支路的(de)功(gong)率(lv)。
一般(ban)安(an)裝式(shi)(shi)功(gong)率(lv)(lv)表(biao)為(wei)直讀(du)單(dan)量(liang)程(cheng)式(shi)(shi)，表(biao)上(shang)的示數(shu)即為(wei)功(gong)率(lv)(lv)數(shu)。但便攜式(shi)(shi)功(gong)率(lv)(lv)表(biao)一般(ban)為(wei)多量(liang)程(cheng)式(shi)(shi)，在表(biao)的標(biao)度(du)(du)尺上(shang)不(bu)直接標(biao)注示數(shu)，只標(biao)注分格。在選用不(bu)同的電流與電壓量(liang)程(cheng)時，每一分格都可以(yi)表(biao)示不(bu)同的功(gong)率(lv)(lv)數(shu)。在讀(du)數(shu)時，應先根(gen)據所選的電壓量(liang)程(cheng)U、電流量(liang)程(cheng)I以(yi)及標(biao)度(du)(du)尺滿(man)量(liang)程(cheng)時的格數(shu)&，求出每格瓦(wa)數(shu)（又(you)稱功(gong)率(lv)(lv)表(biao)常數(shu)）C，然后再乘(cheng)上(shang)指(zhi)針偏轉(zhuan)的格數(shu)夕，就可得到所測功(gong)率(lv)(lv)P
例題
例(li)：有一(yi)只電(dian)壓量程為250V，電(dian)流量程為3A，標(biao)度(du)尺分格數為75的功率表，現用它來(lai)測量負載(zai)的功率。
當指針偏轉(zhuan)50格時負載(zai)功率(lv)為多少？
解：先計(ji)算功(gong)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(ge)×50格(ge)=500W
對大多數從(cong)事(shi)電氣(qi)方面工(gong)作(zuo)(zuo)的(de)(de)人員來說，功(gong)率(lv)表(biao)(biao)的(de)(de)使(shi)用(yong)并(bing)非難事(shi)。但真正(zheng)做到正(zheng)確使(shi)用(yong)功(gong)率(lv)表(biao)(biao)，即在準確度一定的(de)(de)情況下確保(bao)測(ce)量的(de)(de)精度及(ji)儀(yi)表(biao)(biao)的(de)(de)使(shi)用(yong)壽命(ming)又并(bing)非易(yi)事(shi)。以單(dan)相電動系功(gong)率(lv)表(biao)(biao)為例，就功(gong)率(lv)表(biao)(biao)的(de)(de)使(shi)用(yong)及(ji)使(shi)用(yong)中應(ying)注意(yi)的(de)(de)問題作(zuo)(zuo)一介紹。
1、要遵守“發電機端(duan)守則”
由電(dian)(dian)(dian)(dian)動(dong)系功(gong)(gong)率(lv)(lv)表(biao)的(de)原理(li)可知，功(gong)(gong)率(lv)(lv)表(biao)的(de)轉矩與流(liu)過表(biao)內線(xian)圈(quan)(quan)(quan)的(de)電(dian)(dian)(dian)(dian)流(liu)方向(xiang)(xiang)有(you)關，一(yi)旦其中(zhong)一(yi)個線(xian)圈(quan)(quan)(quan)的(de)電(dian)(dian)(dian)(dian)流(liu)方向(xiang)(xiang)改變，轉矩方向(xiang)(xiang)也會改變。為此，在功(gong)(gong)率(lv)(lv)表(biao)兩個線(xian)圈(quan)(quan)(quan)對(dui)應于電(dian)(dian)(dian)(dian)流(liu)流(liu)進的(de)端(duan)鈕(niu)上，都(dou)注有(you)稱為發(fa)(fa)電(dian)(dian)(dian)(dian)機(ji)端(duan)的(de)“*”標志(zhi)。功(gong)(gong)率(lv)(lv)表(biao)在接(jie)線(xian)時，應使電(dian)(dian)(dian)(dian)流(liu)或電(dian)(dian)(dian)(dian)壓線(xian)圈(quan)(quan)(quan)帶“*”標志(zhi)的(de)端(duan)鈕(niu)接(jie)到(dao)電(dian)(dian)(dian)(dian)源同極性的(de)端(duan)子上，以保證兩線(xian)圈(quan)(quan)(quan)的(de)電(dian)(dian)(dian)(dian)流(liu)方向(xiang)(xiang)都(dou)從發(fa)(fa)電(dian)(dian)(dian)(dian)機(ji)端(duan)流(liu)入。這就是功(gong)(gong)率(lv)(lv)表(biao)接(jie)線(xian)的(de)“發(fa)(fa)電(dian)(dian)(dian)(dian)機(ji)端(duan)守(shou)則(ze)”。
2、合(he)理選擇電壓線圈的前、后接方式
盡管電(dian)(dian)壓線圈不(bu)論(lun)前(qian)接(jie)(jie)還是(shi)后接(jie)(jie)，功率表都能正偏，對(dui)(dui)于某些負載來說，測(ce)量的結果相差(cha)較(jiao)小(xiao)，這時(shi)兩(liang)種接(jie)(jie)法采用(yong)哪種均可。但對(dui)(dui)于那些電(dian)(dian)阻（或阻抗）過(guo)大或過(guo)小(xiao)的負載來說，兩(liang)種接(jie)(jie)法所(suo)得(de)結果相差(cha)較(jiao)大，有時(shi)甚至出現(xian)與理(li)論(lun)相矛盾(dun)的結果。
這種(zhong)方式(shi)的(de)接線(xian)，功(gong)(gong)率(lv)(lv)表(biao)電(dian)(dian)(dian)流(liu)線(xian)圈的(de)電(dian)(dian)(dian)流(liu)雖然等(deng)于(yu)負載電(dian)(dian)(dian)流(liu)，但功(gong)(gong)率(lv)(lv)表(biao)電(dian)(dian)(dian)壓(ya)支路兩(liang)端電(dian)(dian)(dian)壓(ya)卻等(deng)于(yu)負載電(dian)(dian)(dian)壓(ya)與功(gong)(gong)率(lv)(lv)表(biao)電(dian)(dian)(dian)流(liu)線(xian)圈的(de)電(dian)(dian)(dian)壓(ya)之(zhi)和，在功(gong)(gong)率(lv)(lv)表(biao)讀數中多了電(dian)(dian)(dian)流(liu)線(xian)圈的(de)功(gong)(gong)率(lv)(lv)消耗。這種(zhong)接線(xian)方式(shi)適用于(yu)負載電(dian)(dian)(dian)阻（或阻抗(kang)(kang)）遠比功(gong)(gong)率(lv)(lv)表(biao)電(dian)(dian)(dian)流(liu)線(xian)圈電(dian)(dian)(dian)阻（或阻抗(kang)(kang)）大得多的(de)情況，這樣才能保證功(gong)(gong)率(lv)(lv)表(biao)本身(shen)的(de)功(gong)(gong)率(lv)(lv)消耗對測量結(jie)果(guo)的(de)影響(xiang)比較小。
這(zhe)種(zhong)方式的(de)(de)(de)(de)接線(xian)，功(gong)(gong)率(lv)表(biao)電(dian)(dian)壓支(zhi)路(lu)兩端的(de)(de)(de)(de)電(dian)(dian)壓雖(sui)然等(deng)于(yu)(yu)負載(zai)(zai)電(dian)(dian)壓，但電(dian)(dian)流線(xian)圈(quan)的(de)(de)(de)(de)電(dian)(dian)流卻(que)等(deng)于(yu)(yu)負載(zai)(zai)電(dian)(dian)流與(yu)功(gong)(gong)率(lv)表(biao)電(dian)(dian)壓線(xian)圈(quan)支(zhi)路(lu)電(dian)(dian)流之和，功(gong)(gong)率(lv)表(biao)讀數中多了電(dian)(dian)壓支(zhi)路(lu)的(de)(de)(de)(de)功(gong)(gong)率(lv)消耗(hao)。因此，這(zhe)種(zhong)接線(xian)適(shi)用于(yu)(yu)負載(zai)(zai)電(dian)(dian)阻（或(huo)阻抗(kang)）遠(yuan)比功(gong)(gong)率(lv)表(biao)電(dian)(dian)壓支(zhi)路(lu)電(dian)(dian)阻（或(huo)阻抗(kang)）小得多的(de)(de)(de)(de)情(qing)況，這(zhe)樣才能保證功(gong)(gong)率(lv)表(biao)本(ben)身的(de)(de)(de)(de)功(gong)(gong)率(lv)消耗(hao)對測量結果的(de)(de)(de)(de)影響比較小。
