山东建筑大学电工学期末考试试卷 2

4 页Р···········································································································装订线··································································································Р得分Р阅卷人Р七、(20分)Р三相异步电动机的额定数据如下:f = 50Hz, PN = 4kW,nN = 1440 r/min,UN = 220 / 380V, △/Y接法, cosφN = 0.82,ηN = 84.5%,Ist / IN = 7,Tmax / TN = 4.4,Tst / TN = 2.2。Р(1)试求:转差率SN、IN、TN、Ist、Tst、Tmax及f2N;Р(2)若采用Y-△起动,电动机正常工作时定子绕组应如何连接?此时电源线电压为多少?Р(3)若采用Y-△起动,起动电流和起动转矩各为多少?当TL = 80%TN和TL = 20%TN时,能否起动起来?Р得分Р阅卷人Р八、(10分)Р试画出三相笼型电动机的既可以正转又可以反转的控制线路,要求有短路、过载、失压保护。Р29 画出即能长期工作又能点动的三相笼式异步电动机的继电器-接触器控制电路。Р解:图示电路中,按下按钮SB1,则电动机实现连续运行。按下按钮SB2,则电动机实现点动控制。Р既能点动又能长动的控制SB3РSB2РKMРSB1РKMРFRРKMРFRРQРFUР3 ~Р电路Р30画出能分别在两地控制同一台电机起停的控制电路。Р解:能分别在两地控制同一台电机起停的控制电路如下图所示。РMР3~РSB2РQРFU1РKMРFRРSB3РFRРKMРKMРFU2РSB1РSB4