The Principle of the Water Pump Control Box

The water pump control box is an electrical device used to control the start, stop, operation, and protection of water pumps, 

and it is widely applied in fields such as water supply and drainage, fire protection, heating, ventilation, and air conditioning. 

Its core function is to automatically or manually control the operation of the water pump according to signals such as water

 level and pressure, and provide protections against overload, short circuit, etc. The following are its working principle, composition,

 and key points:

 I. Core Functions of the Water Pump Control Box

1. **Start and Stop Control**: Control the start and stop of the water pump motor through manual/automatic modes.

2. **Status Monitoring**: Provide real-time feedback on the operation status of the water pump (such as running/stopping,

 fault alarm).

3. **Protection Functions**: Overload protection, phase loss protection, short circuit protection, water level/pressure over-limit 

protection.

4. **Multi-pump Linkage**: Support the switching between the main and standby pumps, and the alternate operation of multiple

 pumps (such as one in use and one standby, two in use and one standby).

 II. Main Components

1. Main Circuit Part

- **Circuit Breaker (QF)**:

  The main power switch provides short circuit and overload protection.

YouTube：https://youtube.com/shorts/Ld3jN3yMJq8 

- **Contactor (KM)**:

  Controls the on-off of the motor power supply, has an arc extinguishing function, and can be operated frequently.

- **Thermal Relay (FR)**:

  Monitors the motor current and disconnects the circuit in case of overload (the operating current is generally set at 1.1 times

 the rated current of the motor).

- **Motor**:

  The power source of the water pump, which can be single-phase or three-phase, with a power ranging from 0.5kW to several

 hundred kW.

2. Control Circuit Part

- **Control Switch**:

  Manual/automatic switching knob, start and stop buttons.

- **Sensors/Signal Sources**:

  - **Water Level Control**: Float switches, hydrostatic level sensors (used for controlling the water level in pools and water tanks).

  - **Pressure Control**: Electric contact pressure gauges, pressure transmitters (used for constant pressure water supply in pipelines).

  - **Temperature Control**: Temperature sensors (used for cooling water pumps).

- **Relay/PLC**:

  The core of logical control, triggering the action of the contactor according to the sensor signals.

- **Indicator Lights/Alarms**:

  Display the power, operation, and fault status, and give audible and visual alarms in case of faults.

 III. Typical Control Principles and Workflow

1. Water Level Control (Taking Pool Drainage as an Example)

- **Automatic Mode**:

  ① When the water level in the pool reaches the **high water level**, the float switch closes, and the PLC/relay outputs a signal → 

the contactor KM closes → the water pump starts to drain water.

  ② When the water level drops to the **low water level**, the float switch opens → the contactor KM opens → the water pump stops.

- **Manual Mode**: Directly press the start/stop button to control the water pump, which is not affected by the water level signal.

2. Pressure Control (Constant Pressure Water Supply System)

- **Automatic Mode**:

  ① When the pipeline pressure is lower than the **lower limit value**, the pressure switch closes → the water pump starts to increase

 the pressure.

  ② When the pressure reaches the **upper limit value**, the pressure switch opens → the water pump stops.

- **Advanced Solution**:

  Equipped with a variable frequency drive (VFD) to achieve stepless speed regulation, adjust the speed of the water pump in real-

time according to the pressure, which is energy-saving and makes the water pressure more stable.

3. Switching between the Main and Standby Pumps (Taking One in Use and One Standby as an Example)

- **Logic**: Automatically switch to the standby pump when the main pump fails to avoid system paralysis.

- **Control Process**:

  ① When the main pump is running, the thermal relay monitors the current. If there is an overload, the FR opens → the contactor KM1 

of the main pump opens, and at the same time, the contactor KM2 of the standby pump is triggered to close.

  ② After the fault is eliminated, the status of the main and standby pumps needs to be manually reset or automatically reset through the PLC.

 IV. Core Protection Functions

V. Common Types and Application Scenarios

1. Ordinary Water Level Control B