1．电动机在起动时在三相定子电路中串接电阻，使电动机定子绕组的电压降低，待起动结束后将电阻短接，电动机在额定电压下正常运行。这种起动方式不受电动机接线形式的影响，设备简单，因而在中小型生产机械设备中应用较广。

但起动电阻一般采用板式电阻或铸铁电阻，电阻功率大，能通过较大电流，但能量损耗较大。

2．工作过程：

合上QS，按下SB2，KM1线圈得电，KM1主触点闭合，电动机M串电阻减压起动，KM1辅助常开触点闭合，实现自锁，KT线圈得电。

a图KT延时时间到，KT常开触点闭合，KM2线圈得电并自锁，KM2主触点闭合，电动机M全压运转。

b图KT延时时间到，KT常开触点闭合，KM2线圈得电并自锁，KM2主触点闭合,电动机M全压运转。KM2辅助常闭触点断开，KM1、KT线圈失电，

按下SB1，KM2线圈断电，KM2主触点、辅助触点断开，电动机M停止。

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