常用电机控制电路图讲义.ppt

、定子串电阻起动原理:电动机在起动时在三相定子绕组中串接电阻,使电动机定子绕组电压降低,起动结束后再将电阻短接。主电路:KM1实现串电阻起动,KM2实现全压运行。KM2KM1RL1L2L3QSFUFRMSB2SB1FRKM1KTKM1KM2KT图2-8(a)控制线路:1、基本原理:用时间继电器KT控制KM1、KM2切换。2、KM1、KM2允许同时吸合,但是电动机正常运行后,一般应该将KM1释放,以降低运行损耗。3、图2-8(a)为KM1不退出的控制线路。4、图2-8(b)为KM1退出而KT不退出的控制线路。5、图2-8(c)为KM1、KT都退出的控制线路SB2SB1FRKM1KTKM1KM2KT图2-8(a)起动完成后KM1不退出,不足之处:运行损耗大图2-8(b-1)KM1退出而KT不退出问题:KT延时触点切换是否可行??切换要求:起动过程平稳,减少冲击。对于主触点要求:KM2先闭合KM1后断开KM2KM1RL1L2L3QSFUFRMSB2SB1FRKTKM1KM2KTKTKT图2-8(b-1)KM1退出而KT不退出KT延时触点切换带来KM1、KM2线圈瞬时断电,切换过程带来冲击KT常开延时触点和KM常闭触点平稳切换!!SB2SB1FRKTKM1KM2KTKTKTSB2SB1FRKTKM2KM1KM2KTKT图2-8(b-2)KM1退出而KT不退出SB2SB1FRKTKM2KM1KM2KTKTSB2按下,KM1动作→电机降压起动;KT绕组上电开始计时,→KT延时时间到,KT延时闭合的常开触点闭合→KM2线圈上电,→→KM2主触点闭合→电机全压起动。→KM2延时断开的常闭触点断开→KM1线圈失电→KM主触点断开→降压起动回路断开。KM2KM1RL1L2L3QSFUFRM问题:如果要求切换时确保KM2先断开KM1后闭合,图2-8(b-1)是否可靠,为进一步增加可靠性应怎样做?方法:用KM1的常闭触点替代KT延时常开触点。图2-8(b-1)KM1退出而KT不退出KT延时触点切换带来KM1、KM2线圈瞬时断电,切换过程带来冲击SB2SB1FRKM1KM1KM2KTKTKTKM1KM2KTSB2SB1FRKTKTKT切换顺序比较SB2SB1FRKTKM2KM1KM2KTKTSB2SB1FRKTKM1KM2KTKTKTSB2SB1FRKM1KM1KM2KTKTKTKM2先通电,KM1后断电;KM1,KM2同时切换;KM1先断电,KM2后通电图2-8(b-2)KM1退出带来的自锁回路的改变,采用KA触点扩展采用KT瞬时动作触点SB2SB1FRKTKM1KM2KTKTKM2SB2SB1FRKTKM1KM2KTKAKAKM2自锁回路的转换SB2SB1FRKM1KTKM2KTKM2KM2KM1图2-8(c)退出KTSB2SB1FRKTKM2KM1KM2KM1KM2KT图2-8(b-3)KM1退出带来的自锁回路的改变,采用KM1、KM2触点切换