电工计算公式

电工计算公式
INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/123.bmp" \* MERGEFORMATINET 一电力变压器额定视在功率Sn=200KVA，空载损耗Po=0.4KW，额定电流时的短路损耗PK=2.2KW,测得该变压器输出有功功率P2＝140KW时，二次则功率因数 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/129.bmp" \* MERGEFORMATINET 2=0.8。求变压器此时的负载率??和工作效率 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/130.bmp" \* MERGEFORMATINET 。
　解：因P2=?×Sn× INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/129.bmp" \* MERGEFORMATINET 2×100%
?=P2÷(Sn× INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/129.bmp" \* MERGEFORMATINET 2)×100%
???? =140÷(200×0.8）×100%=87.5%
??? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/130.bmp" \* MERGEFORMATINET =（P2/P1)×100%
??? P1=P2+P0+???PK
????? =140+0.4+(0.875)2×2.2
????? =142.1(KW)
　所以
INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/130.bmp" \* MERGEFORMATINET =（140×142.08）×100%＝98.5%
　答：此时变压器的负载率和工作效率分别是87.5%和98.5%。
　
　有一三线对称负荷，接在电压为380V的三相对称电源上，每相负荷电阻R＝16 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET ，感抗XL=12 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET 。试计算当负荷接成星形和三角形时 的相电流、线电流各是多少？
　解;负荷接成星形时，每相负荷两端的电压，即相电压为　U入Ph= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/132.bmp" \* MERGEFORMATINET = INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/133.bmp" \* MERGEFORMATINET =220(V)
负荷阻抗为　Z= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/134.bmp" \* MERGEFORMATINET = INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/135.bmp" \* MERGEFORMATINET =20( INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET )
每相电流（或线电流）为
　I入Ph=I入P－P= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/136.bmp" \* MERGEFORMATINET = INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/137.bmp" \* MERGEFORMATINET =11(A)
负荷接成三角形时，每相负荷两端的电压为电源线电压，即 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/138.bmp" \* MERGEFORMATINET ＝ INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/139.bmp" \* MERGEFORMATINET ＝380V
流过每相负荷的电流为
INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/140.bmp" \* MERGEFORMATINET
流过每相的线电流为
INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/141.bmp" \* MERGEFORMATINET
　某厂全年的电能消耗量有功为1300万kwh，无功为1000万kvar。求该厂平均功率因数。
　解：已知P=1300kwh,Q=1000kvar
则 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/142.bmp" \* MERGEFORMATINET
　答：平均功率因数为0.79。
INCLUDEPICTURE "http://521314

https://www.wendangku.net/doc/621150434.html,/images54/124.bmp" \* MERGEFORMATINET INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/125.bmp" \* MERGEFORMATINET INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/126.bmp" \* MERGEFORMATINET INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/127.bmp" \* MERGEFORMATINET INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/128.bmp" \* MERGEFORMATINET


计算：　
? 一个2.4H的电感器，在多大频率时具有1500 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET 的电感？
?解：感抗　XL＝ INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/143.bmp" \* MERGEFORMATINET ?????? 则 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/144.bmp" \* MERGEFORMATINET
??????? =99.5(HZ)
?答：在99.5HZ时具有1500 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET 的感抗。
? 某企业使用100kvA变压器一台(10/0.4kv),在低压侧应配置多大变比的电流互感器？
?解：按题意有
???? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/145.bmp" \* MERGEFORMATINET
?答：可配置150/5的电流互感器。
? 一台变压器从电网输入的功率为150kw，变压器本身的损耗为20kw。试求变压器的效率？
?解：输入功率 Pi=150kw
???? 输出功率 PO=150-20=130(KW)
???? 变压器的效率
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/146.bmp" \* MERGEFORMATINET
?答：变压器的效率为86.6%
? 某用户装有250kvA变压器一台，月用电量85000kwh，力率按0.85计算，试计算该户变压器利率是多少？
? 解：按题意变压器利用率
?? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/147.bmp" \* MERGEFORMATINET
? 答：该用户变压器利用率为56%。
? 一台变压器从电网输入的功率为100kw,变压器本身的损耗为8kw。试求变压器的利用率为多少？
? 解：输入功率为? P1=100kw
????? 输出功率为? P2=100-8=92kw
????? 变压器的利用率为
????? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/148.bmp" \* MERGEFORMATINET
? 答：变压器的利用率为92%。
? 有320kvA,10/0.4kv变压器一台，月用电量15MWh,无功电量是12Mvarh,试求平均功率因数及变压器利用率？
? 解：已知 Sn=320kva,WP=15Mva
????? WQ=12Mvarh,一个月以30天计，
????? 日平均有功负荷为
??? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/149.bmp" \* MERGEFORMATINET
?日平均无功为
?? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/150.bmp" \* MERGEFORMATINET
?? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/151.bmp" \* MERGEFORMATINET
?? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/152.bmp" \* MERGEFORMATINET
变压器利用率为
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/153.bmp" \* MERGEFORMATINET
? 答：平均功率因数为0.78；变压器利用率为83%。
? 一条380v线路，导线为LJ-25型，电阻为0.92 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET /km，电抗

为0.352 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET /km,功率因数为0.8，输送平均有功功率为30KW，线路长度为400m,试求线路电压损失率 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/154.bmp" \* MERGEFORMATINET 。
? 解;400m导线总电阻和总电抗分别为
???? R=0.92×0.4 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/155.bmp" \* MERGEFORMATINET 0.37( INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET )
???? X=0.352×0.4 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/155.bmp" \* MERGEFORMATINET 0.14( INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET )
???? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/156.bmp" \* MERGEFORMATINET
? 导线上的电压损失 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/157.bmp" \* MERGEFORMATINET
? 线路上的电压损失
INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/158.bmp" \* MERGEFORMATINET
? 答：线路电损失率为9.8%

? 有两台100kvA变压器并列运行，第一台变压器的短路电压为4%，第二台变压器的短路电压为5%。求两台变压器并列运行时负载分配的情况？
? 解：由题意可知
????? S1n=S2n=100kva
????? U1%=4%? U2k%=5%
? 第一台变压器分担的负荷
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/159.bmp" \* MERGEFORMATINET
? 第二台变压器分担的负荷
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/160.bmp" \* MERGEFORMATINET
? 答：第一台变压器因短路电压小而过负荷；第二台变压器则因短路电压大却负荷不足。
? 某厂年用电量609.5万kwh,求该厂最大负荷约为多少？(最大负荷年利用小时数Tmax=5300h)
? 解：已知A=609.5×104kwh,
???????? Tmax=5300h
? 则Pmax= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/161.bmp" \* MERGEFORMATINET =1150(kw)
? 答：该厂最大负荷约为1150kw。
? 某单位月有功电量500000kwh,无功电量400000kvarh,月利用小时为500h,问月平均功率因数 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/162.bmp" \* MERGEFORMATINET 为多少？若将功率因数提高到 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/163.bmp" \* MERGEFORMATINET 2=0.9时，需补偿多少无功功率QC？
? 解：补偿前月平均功率因数
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/164.bmp" \* MERGEFORMATINET
? 补偿后月平均功率因数 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/163.bmp" \* MERGEFORMATINET 2=0.9
? 则需补偿的无功容量为
??? QC=PP(tg INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/165.bmp" \* MERGEFORMATINET 1-tg INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/165.bmp" \* MERGEFORMATINET 2)
??? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/166.bmp" \* MERGEFORMATINET
??? =1000(0.802-0.484)=318(kvar)
? 答： INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/162.bmp" \* MERGEFORMATINET 为0.78，需补偿318kvar。
? 有一台三角形连接的三相电动机，接

于线电压为380v的电源上，电动机的额定功率为2.7kw,效率 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/130.bmp" \* MERGEFORMATINET 为0.8，功率因数为0.83。试求电动机的相电流Iph和线电流Ip-p?
? 解;已知线电压Up-p=380v,电动机输出功率Pou=2.74kw,功率因数 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/129.bmp" \* MERGEFORMATINET =0.83，电动机效率 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/130.bmp" \* MERGEFORMATINET =0.8。则电动机输出功率为
? Pou=3Up-pIp-p INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/129.bmp" \* MERGEFORMATINET INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/130.bmp" \* MERGEFORMATINET
? 线电流 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/167.bmp" \* MERGEFORMATINET
???? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/168.bmp" \* MERGEFORMATINET
? 由于在三角形接线的负载中，线电流Ip-p= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/169.bmp" \* MERGEFORMATINET Iph,则相电流
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/170.bmp" \* MERGEFORMATINET
? 答：电动机的相电流为3.62A，线电流为6.27A。
? 某工厂最大负荷月的平均有功功率为400kw,功率因数=0.6，要将功率因数提高到0.9时，问需要装设电容器组的总容量应该是多少？
? 解：根据公式
???? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/171.bmp" \* MERGEFORMATINET
? 式中：P为最大负荷大的平均有功功率(kw), INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/172.bmp" \* MERGEFORMATINET 1、 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/172.bmp" \* MERGEFORMATINET 2为补偿前后的功率因数值，则
??? INCLU


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x= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/169.bmp" \* MERGEFORMATINET UI INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/163.bmp" \* MERGEFORMATINET
????? =1.732×0.38×200×0.9
????? =118.47(kw)
? 则日负荷率Kd= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/176.bmp" \* MERGEFORMATINET ×100%=84.4(%)
? 答：该厂的晚负荷率为84.4%。
? 一个化工厂某月用电72万kwh，最大负荷为2000kw。求月负荷率K。
? 解;月平均负荷
? Pav= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/177.bmp" \* MERGEFORMATINET =1000(kw)
? K INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/178.bmp" \* MERGEFORMATINET =50(%)
? 答;月负荷率为50%。
? 一机械厂某月用电为36万kwh，月最大负荷为900kw，求月负荷率K。
? 解：月平均负荷
????? Pav= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/179.bmp" \* MERGE

FORMATINET =500kw
????? 月负荷率
????? K= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/180.bmp" \* MERGEFORMATINET =55.5%
? 答;月负荷率为55.5%。
? 某用户有功功率为1.1kw，供电电压为220v，工作电流为10A。试求该户功率因数是多少？
? 解;按题意和功率计算公式，得
??? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/181.bmp" \* MERGEFORMATINET
? 答：该用户的功率因数为0.5。
? 已知某10kv高压供电工业用户，TA变比为50/5，TV变比为10000/100，有功表起码为165kwh，止码为236kwh。试求该用户有功计费电量w为多少？
? 解：该用户计费倍率为
???? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/182.bmp" \* MERGEFORMATINET =50/5×10000/100=1000
? 该用户有功计费电量
?? W= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/182.bmp" \* MERGEFORMATINET (236-165)
??? =1000×(236-165)=71000(kwh)
? 答：该用户有功计费电量为71000kwh。
? 某工厂有一台315kva的三相变压器，原有负荷为210kw，平均功率因数为0.7，试问此变压器能否满足供电需要？现在生产发展负荷增到280kw，问是否要增加变压器容量？若不增加变压器容量，可采取什么办法？
解：根据题意，视在功率
??? S1=P/COS INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/165.bmp" \* MERGEFORMATINET =210/0.7=300(KVA)
??? 此时变压器能够满足供电需要。
??? 当负荷增到280kw时
??? S2=P/COS INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/165.bmp" \* MERGEFORMATINET =280/0.7=400kva>315kva
??? 则原变压器已不能够满足正常供电需要。
??? 若采取措施将平均功率因数由0.7提高到0.9，则
??? S=280/0.9=311<315(kva)
?? 此时变压器可满足需要，不必加容量。
答：1、原有负荷为210kw,功率因数为0.7时，S1为300kva,变压器能满足要求；
??? 2、当负荷为280kw,功率因数仍为0.7时，S2为400kva，变压器不能满足要求；
??? 3、当负荷仍为280kw,将功率因数提高为0.9时，变压器可不增加容量即可满足负荷要求。
? 一台容量为1000kva的变压器，24h的有功用电量为15360kwh,功率因数为0.85。试求24h的平均负荷为
?? P= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/183.bmp" \* MERGEFORMATINET =640(kw)
?? 平均使用容量为
?? S= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/184.bmp" \* MERGEFORMATINET =753(kva)
?? 则变压器的利用率为
?? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/185.bmp" \* MERGEFORMATINET
? 答：变压器的利用率为75%。
? 某用户申请用电1000kva,电源电压为10kv,用电点距供电线路最近处约6km,采用50mm2的钢芯铝绞线。计算供电电压是否合格。(线路 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/186.bmp" \* MERGEFORMATINET 数RO=0.211 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINET /km,XO=0.4 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/131.bmp" \* MERGEFORMATINE

T /km)
? 解;额定电压Un=10kv,Sn=1000kva,
???? L=6km,则线路阻抗为
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/187.bmp" \* MERGEFORMATINET
? 1000kva变压器的一次额定电流为
? In= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/188.bmp" \* MERGEFORMATINET =57.74(A)
? 则电压降为
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/189.bmp" \* MERGEFORMATINET U=IZ=57.74×2.71=156.48(V)
? 电压降值占10kv的比例为
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/189.bmp" \* MERGEFORMATINET U%= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/190.bmp" \* MERGEFORMATINET ×100%=1.56(%)
? 答：根据规定，10kv电压降合格标准为±7%，故供电电压合格。
? 一台三相变压器的电压为6000V，负荷电流为20A，功率因数为0.866。试求其有功功率、无功功率和视在功率。
? 解：三相变压器的有功功率为
P= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/169.bmp" \* MERGEFORMATINET UI INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/129.bmp" \* MERGEFORMATINET = INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/169.bmp" \* MERGEFORMATINET ×6000×20×0.866=180(kw)
? 无功功率
? Q= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/169.bmp" \* MERGEFORMATINET U1I1 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/191.bmp" \* MERGEFORMATINET
? = INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/169.bmp" \* MERGEFORMATINET ×6000×20× INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/192.bmp" \* MERGEFORMATINET =103.8kvar
? 视在功率
? S= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/169.bmp" \* MERGEFORMATINET U1I1= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/169.bmp" \* MERGEFORMATINET ×6000×20=207.8(kva)
? 答：P为180kw。Q为103.8kvar。
???? S为207.8kva。

? 一台10kv、1800kva变压器，年负荷最大利用小时为5000h,按电流密度选用多大截面的铝芯电缆比较合适？(铝导线经济电流密度为1.54A/mm2)。
? 解：已知Un=10kv,Sn=1800kva,T=5000h,
? 则变压器额定电流为
? In= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/193.bmp" \* MERGEFORMATINET =104(A)
? 导线截面为
? S= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/194.bmp" \* MERGEFORMATINET 67.5(mm2)
? 答：可选用50-70mm2的铝芯电缆。
? 一条电压为35kv的输电线路，输送最大功率为6300kw,功率因数为0.8，经济电流密度按1.15A/mm2来考虑，使用钢芯铝绞线，其长度为2km。试按经济电流密度求导线截面？
? 解：已知P=6300kw, INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/163.bmp" \* MERGEFORMATINET =0.8,
????? L=2km ,J=1.15A/mm2,
????? 则视在电流值为
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/195.bmp" \* MERGEFORMATINET
???? 导线截面为
???? S= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images54/196.bmp" \* MERGEFORMATINET 113(mm2)
? 据此应选用LGJ-95-120型导线。
? 答：导线截面约为113mm2。
? 为什么高压负荷开关要与熔断器配合

使用？
? 答：高压负荷开关在10kv系统和简易的配电室中被广泛采用。它虽有灭弧装置，但灭弧能力较小，因此高压负荷开关只能用来切断或接通正常的负荷电流，不能用来切断故障电流。为了保证设备和系统的安全运行，高压负荷开关应与熔断器配合使用，由熔断器起过载和短路保护作用。
? 通常高压熔断器装在高压负荷开关后面，这样当更换高压熔断器时，只拉开负荷开关，停电后再进行更换是比较安全的。

简易电工计算公式
一、欧姆定律公式(单相)
?? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/01.bmp" \* MERGEFORMATINET
?? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/03.bmp" \* MERGEFORMATINET
二、欧姆定律公式(三相)
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/02.bmp" \* MERGEFORMATINET
例：在三相交流电路中，已知KW=1(KW)
??? V=380V，求A。
解：三角形符号公式中，KW与KV、 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/169.bmp" \* MERGEFORMATINET 、 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/129.bmp" \* MERGEFORMATINET 是除数关系，故得下式
?? A=W÷(KV× INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/169.bmp" \* MERGEFORMATINET × INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/129.bmp" \* MERGEFORMATINET )
代入得：A=1÷(0.38×1.732×0.8)
??????? =1÷0.562=1.9 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/155.bmp" \* MERGEFORMATINET 2(安培)
?(如果没有明确力率，可按0.8计算)
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/04.bmp" \* MERGEFORMATINET
? 例：有台100kva变压器，其高压侧额定电压为10.5kv，低压侧为0.4kv，问高、低压侧的额定电流各是多少？
? 解：根据公式A=KVA÷(KV× INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/169.bmp" \* MERGEFORMATINET )
? 代入得：A高=100÷(10.5×1.732)=5.5
???????? A低=100÷(0.4×1.732)=144.5
? 答：高压侧额定5.5电流A；
????? 低压侧额定电流144.5A。
? 例：某厂有功电度抄见为100度，无功电度76度，问该厂设备力率是多少？
? 解：三角形公式中，a与 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/05.bmp" \* MERGEFORMATINET 的关系是除数关系，故得下式： INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/129.bmp" \* MERGEFORMATINET =a÷( INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/05.bmp" \* MERGEFORMATINET )
? 代入得： INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/129.bmp" \* MERGEFORMATINET =100÷( INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/08.bmp" \* MERGEFORMATINET )
????????????? =100÷ INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/09.bmp" \* MERGEFORMATINET =0.8
? 答：力率为0.8。
?? 如果几个并联电阻的阻值相同，也可以用并联个数的倒数和电阻值相乘求出。
?? 如:3个400欧电阻并联，总阻值是
???? R总= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/07.bmp" \* MERGEFORMATINET

×400=133.3(欧)
? 电容并联后，总电容等于各个电容之和，电压不变。
? 电容器串联后，总电容减少，总电容倒数等于各个电容器的电容量倒数之和，电压增加等于各电容电压之和。
? 例如：两个10微法电容器串联的总电容。
?????? C总= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/10.bmp" \* MERGEFORMATINET ×10=5(PF)
变压器并列运行条件：
? 1、变压比相等地，比差不允许超过0.5%(即400V的0.5%=2V)。
? 2、短路电压(即阻抗百分数，不应超过10%)大体相同。
? 3、联接级别必须相同。
? 4、容量比不宜超过3：1。
变压器的Y、 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/189.bmp" \* MERGEFORMATINET 接法及其关系：
? A、火线与零线之间的电压叫相电压；流过一相线圈的电流叫相电流，流过导线的电流叫线电流。
? B、在Y接时? 线电压= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/169.bmp" \* MERGEFORMATINET ×相电压；
???????????? 线电流=相电流；
???????????? 相电压=线电压÷ INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/169.bmp" \* MERGEFORMATINET 。
? C、在 INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/189.bmp" \* MERGEFORMATINET 接时? 线电压=相电压；
???????????? 相电流=线电流÷ INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/169.bmp" \* MERGEFORMATINET
???????????? 线电流= INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/169.bmp" \* MERGEFORMATINET ×相电流。
　C、“起动、断續、停止”按钮接线立法
?? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/13.bmp" \* MERGEFORMATINET
? D、顺逆按钮的接线方法：
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/14.bmp" \* MERGEFORMATINET
?注：按顺按钮时接触器吸合，同时要切断逆按钮的磁吸线圈回路，避免误动逆按钮造成损坏电动机。
电动机断相运行起动装置：
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/15.bmp" \* MERGEFORMATINET
?? J--可用36V138mA通用继电器。
? QC--交流接触器的线圈。
? QC1--交流接触器常开触点。
?? J1--继电器的常闭触点。
?电流--沿着导线作一定向的移动。
?形成电流需具备两个条件：一是迫使电子运动的能力；二是电子运动的通路。
?电压--推动电子流动的能力称为电压。
?电压又称为电位差：当导线两端的电子具有不


按钮的接线方法：
? A、按钮与磁吸线圈接线简图
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/11.bmp" \* MERGEFORMATINET
? B、用两只按钮分别在两地方控制电动机的接线方法
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/12.bmp" \* MERGEFORMATINET


?使用两个交流接触器作Y INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/189.bmp" \* MERGEFORMATINET 起动器接线图如下： INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/16.bmp" \* MERGEFORMATINET
? IN

CLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/17.bmp" \* MERGEFORMATINET
??? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/18.bmp" \* MERGEFORMATINET
?1、在电感、电容、电阻串联正弦交流电路中，画出其阻抗三角形图。
?答：如图1-1所示。
?2、在电阻、电感、电容串联电路中，画出其功率三角形图。
?? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/19.bmp" \* MERGEFORMATINET
?3、画出利用三相电压互感器进行核定相位(即定相)的接线图。
?答：见图1-3
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/20.bmp" \* MERGEFORMATINET
　
?4、画出电流互感器不完全星形接线图，并在图中标明A、B、C三相二次侧电流方向和互感器极性。
?解：如图1-4所示
同的位能时，就出现“电位差”。
? 于是电子移动，形成电流。
?电压降--电流通过电阻时所引起的电压降低称为电压降。
　
　
　
? INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/21.bmp" \* MERGEFORMATINET
　
　
?5、画出双绕组变压器差动保护原理接线图。
?答：如图1-5??
　
INCLUDEPICTURE "https://www.wendangku.net/doc/621150434.html,/images55/22.bmp" \* MERGEFORMATINET





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