(精品)《电机学》课后习题答案

5.24 三相异步电动机在运行时有一相断线,能否继续运行?当电机停转之后,能否再起

动?

5.25 怎样改变单相电容电动机的旋转方向?对罩极式电动机,如不改变其内部结构,它

的旋转方向能改变吗?

5.26 试画出三相笼型异步电动机由单相电网供电、当作单相电动机应用时的接线原理图。 5.27 感应调压器与自耦变压器相比,有何优缺点? 5.28 已知一台型号为JO2-82-4的三相异步电动机的额定功率为55kW,额定电压为380V,

额定功率因数为0.89,额定效率为91.5%,试求该电动机的额定电流

PN?3UNINcos?N?N ∴ IN?PN3UNcos?N?N?55?1033?380?0.89?91.500?102.62(A)

5.29 已知某异步电动机的额定频率为50Hz,额定转速为970r/min,问该电机的极数是多

少?额定转差率是多少? ∵nN?970rmin n1?1000rmin n1? s?n1?nn1?970?1000?0.3 100060fP ∴p?60?501000?3极数为6极。

5.30 一台50Hz三相绕线式异步电动机,定子绕组Y联接,在定子上加额定电压。当转

子开路时,其滑环上测得电压为72V,转子每相电阻R2?0.6?,每相漏抗

X2??4?。忽略定子漏阻抗压降,试求额定运行sN?0.04,时,

(1)转子电流的频率;

转子电流的频率f2?sNf1?0.04?50?2(Hz) (2)转子电流的大小;

滑环上测得电压为72V,这是线电压,相电压为72抗压降,说明转子上的电压为v2?72 ∴I2?''U2'R2'2)2?(X2?)S

3∵转子开路,且忽略定子漏阻''R?kkRX 2ei22??kekiX2? 3723?keU2keki(R222)?X2?S?(0.6)2?42ki(0.04'?2.68(A) ?2.68ki ∴I2?kiI2(3)转子每相电动势的大小;

∵转子开路时测得的转子感应电势为E2?723?41.57(V)此时转子不转,即

S?1,当SN?0.04时 E2s?SE2?0.04?41.57?1.66(V)

(4)电机总机械功率。

''21?s2s1?0.04 Pmec?m11?sR2I2?m2I2sR2?3?2.68?0.04?0.6?310.3(V)

5.31 已知一台三相异步电动机的数据为:UN?380V,定子?联接,50Hz,额定转速

???11.75?,??2.82?,X2nN?1426r/min,R1?2.865?,X1??7.71?,R2Rm忽略不计,Xm?202?。试求:

(1)极数;

解:∵nN?1426rmin ∴n1?1500rmin ∴P=2,即2P=4 (2)同步转速; n1?1500rmin

(3)额定负载时的转差率和转子频率; s?n1?nNn1?1426?1500?0.0493 f2?sf1?0.0493?50?2.467(Hz) 1500?。 (4)绘出T型等效电路并计算额定负载时的I1、P1、cos?1和I2' Z2?'R2S'o2.82 ?jX2??j11.75?57.2?j11.75?58.411.61?0.0493'XmgZ2'Xm?Z2 XmPZ?

'2?j202?58.411.61j202?57.2?j11.25101.6111796.8101.61?11796.8??53.3126.59o 57.2?j213.75221.2775o'Z?R1?jX1??XmPZ2?2.865?j7.71?47.67?j23.86?50.535?j31.57?59.5931.99&U1Zo

&?∴I13800o?59.593I1?11.04(A) o?6.337?31.99定子电流为31.99PU1I1cos?1?3?380?11.04?cos(?31.99o)?6162.7(W) 1?3cos?1?cos(?31.99o)?0.848(滞后)

o&?U&?I&E8.22569.62o?380?52.4537.63o 111Z1?380?6.337?31.99g =380?41.54?j32.02?338.46?j32.02?339.97?5.404

o&? ∴I2'&E1'Z2?5.405o?339.97?5.82?17.014(A) 58.411.61oUN?380V(定子?联接)5.32 已知JO2-92-4三相异步电动机的数据为:P2?75kW,,

???0.77?,??0.073?,X2nN?1480r/min,R1?0.088?,X1??0.404?,R2Rm?2.75?,Xm?26?,机械损耗pmec?1.1kW。试用T型、较准确Г型和简

化Г型三种等效电路计算额定负载时的定子电流、功率因数和效率,并对计算结果进

行分析比较。 T型等效电路: s? Z2?'n1?nn1'R2S?1480?1500?0.01333 1500'o0.073?jX2??0.01333?j0.77?5.475?j0.77?5.5298

o Zm?Rm?jXm?2.75?j26?26.14583.96(?)

ZmPZ?'2'ZmgZ2'Zm?Z2?26.14583.69?5.5298o2872.92o?5.16319.04o(?)

&3800oo1&&?UI?为了求 o?70.514?22.8(A)即I1?70.514(A) cos?1?0.9219(滞后)1Z5.38922.8功率,要计算I2,Im

'&?E&&&E12?U1?I1Z1?380?70.522.8?0.41377.71?380?5.312?j28.63'?386.37?4.249(A)'∴I2?386.37Z2

Im??386.375.529?69.9(A)E1Zm?386.3726.145?14.78(A)

222'2∴Pcu1?3I1R1?3?70.5?0.088?1312(W)Pcu2?3I2R2?3?69.9?0.073?1073(W) 2PFe?3ImRm?3?14.782?2.75?1802(W) Pmec?1100(W) Pad忽略。

P1312?1073?1802?11005287??1???(1?74101)?92.900 P?1?3UIcos?1111较准确P型电路

5.33 某三相异步电动机,PN?10kW,UN?380V(线电压),IN?19.8A,4极,Y联接,R1?0.5?。空载试验数据为:U1?380V(线电压),I0?5.4A,p0?0.425kW,机械损耗pmec?0.08kW。短路试验中的一点为:Uk?120V(线电压),Ik?18.1A,

?、X1?、Rm和pk?0.92kW。试计算出忽略空载附加损耗和认为X1??X2?时的参数R2Xm。

空载试验:Z0?U10I0?38035.4?40.63(?) 忽略Pad

2PFe?P0?m1I0R1?Pmec?425?3?5.42?0.5?80?301.26(W)

?80Rm?mFe?301.26?3.444(?)或者km?425?0.5 I23?5.423?5.4210P22X0?Z0?k0?40.632?4.8582?40.34(?)

短路试验:

kZk?UIk?120318.1?3.828(?) Rk?mPkI2?3?92018.12?0.936?)

1kXk?Zk2?Rk2?3.8282?0.9362?3.712(?)

'40.34 R2?(Rk?R1)X0?0XK?(0.936?0.5)?40.34?3.712?0.48?X

'X1??X2??X0?X0?XKX0'(R2?X02)?40.34?240.34?3.71240.34?(0.482?40.342)?1.9(?)

Xm?X0?X1??40.34?1.9?38.44(?)

5.34 一台三相异步电动机的输入功率为10.7kW,定子铜耗为450W,铁耗为200W,转

差率为s=0.029,,试计算电动机的电磁功率、转子铜耗及总机械功率。 Pem?P1?Pcu1?PFe?10700?450?200?10050(W)电磁功率 PCU2?SPem?0.029?10050?291.45W Pmec?(1?S)Pem?10050?291.45?9758.55W

5.35 一台JO2-52-6异步电动机,额定电压为380V,定子Δ联接,频率50Hz,额定功率

7.5kW,额定转速960r/min,额定负载时cos?1?0.824,定子铜耗474W,铁耗231W,机械损耗45W,附加损耗37.5W,试计算额定负载时, (1)转差率;

(2)转子电流的频率; (3)转子铜耗; (4)效率;

(5)定子电流。 (1) n1?60f3000??1000r

minp3n1?n1000?960??0.04 n11000 ∴S?(2)

f2?Sf1?0.04?50?2HZ

(3) Pmec?p2?pmec?pad=7500+45+37.5=7582.5W Pmec?(1?S)Pem

Tem?pem206.82?103?60??1317N.M?12??1500

PCU2?SPem?S?(4)

Pmec7582.5?0.04??316W 1?S1?0.04?P?P?=CU?PCU2?PFe?Pmec?Pad=474+316+213+45+37.5=1103.5W

P2P27500???87.17% P1P2??P7500?1103.5(5)PU1I1cos?1?3?380I1?0.824 1?3

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