1. Question raised: the function of the water tower is to pump the groundwater into the water tower at the top of the tower for storage through the water pump, so as to ensure that the water supply system has a certain pressure. As shown in Figure 1-1.

  

2. Solution to the problem: the water pump is usually driven by cage three-phase AC asynchronous motor. Therefore, the control of water tower is actually the control of three-phase asynchronous motor. Methods include manual control and automatic control.

Firstly, the simplest control method of water tower – manual control is introduced.

3. Control schematic diagram Figure 1-2 (a) and (b) are the water pump control schematic diagram controlled by knife switch (or iron shell switch) and low-voltage circuit breaker respectively. Since manual operation is required to turn on and off the power supply of knife switch and low-voltage circuit breaker, this circuit is called manual control of water tower water supply, (c) is the physical drawing of control elements and AC motor,

  

4. Description of control principle and characteristics

(1) The schematic diagram only shows the logical relationship between electrical components. If the knife switch or circuit breaker is in the closing position, the cage three-phase AC asynchronous motor will be connected to the three-phase power supply, and the motor will start and run; Operate the switch to turn it off, and the water pump will stop running. Instead of drawing the real object, the device name is indicated by the symbol specified in the national standard.

(2) The power supply is connected between electrical components through wires to control the operation of three-phase AC asynchronous motor. The size, color, number of poles and other parameters of the same electrical component shall be selected according to different control objects (three-phase AC asynchronous motor) and applications.

(3) As long as the water pump rotates in one direction, the motor operates in one direction, but the rotation direction of the motor shall be consistent with that required by the water pump.

(4) To achieve a control goal (such as the one-way rotation of the water pump motor), different electrical components and control circuits can be selected, but some auxiliary functions are different. A diagram has the function of short circuit protection, which can prevent damage to electrical appliances such as power supply due to phase-to-phase short circuit or short circuit to ground of motor or wire. In addition to the short-circuit protection function, figure B also has the overload protection function, which can prevent the motor from being damaged due to water pump jamming or overload caused by undervoltage. If leakage protection function is required, circuit breaker with leakage protection function shall be used.

(5) Figure a self starting will occur after power grid voltage loss, which may cause adverse consequences. The circuit breaker with voltage loss protection is used in Figure B. In case of voltage loss and undervoltage, the circuit will not start automatically to avoid personal injury caused by motor or load.

(6) The start and stop of the pump motor must be operated manually. The possibility of water tower overflow and water supply system shutdown due to failure to shut down and start up in time cannot be ruled out.

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