排污泵是工厂中十分常见的一类设备，它负责将现场的积水、废液等及时排出，以便生产任务顺利进行。排污泵一般由三相交流电机驱动，它的工作要求是：
Sewage pump is a very common kind of equipment in the factory. It is responsible for timely discharge of water, waste liquid, etc. on site, so that the production task can be carried out smoothly. The sewage pump is generally driven by three-phase AC motor, and its working requirements are as follows:
1、当现场积水、废液等水位达到高液位时，排水泵自动运行，将废水排出；
1. When the water level of ponding and waste liquid on site reaches high level, the drainage pump will automatically operate to discharge the waste water;
2、当水位下降到低液位时，排水泵要及时停止，防止电机长时间空转受损。
2. When the water level drops to the low level, the drainage pump shall be stopped in time to prevent the motor from damage due to idling for a long time.
基于上述的工作要求，电机的启停必须都要求是自动完成的。其启动和停止的时机都是由水位控制的。可用浮球开关来实现这一功能。浮球开关里面封装了常开触点，其浮起时，常开点闭合，否则断开。
Based on the above requirements, the start and stop of the motor must be automatic. The timing of start and stop is controlled by water level. The float switch can be used to achieve this function. The floating ball switch is enclosed with a normally open contact. When floating, the normally open point is closed, otherwise it is open.