Application of frequency converter in constant pressure water supply

Abstract: This paper analyzes the energy-saving principle of applying frequency converter to water pumps in the process of water supply treatment, and proposes a design scheme of using Shenou frequency converter as the frequency converter of water pumps. After using Shenou frequency converter, the water supply system runs stably.
Under normal circumstances, water pumps are driven by constant-speed AC motors, but the water consumption is variable. In order to ensure the normal water supply of the pool, workers have to frequently adjust the opening size of the baffle or valve on site to control the water pumping volume of the water pump, or waste the unused water in the pool. Doing so not only increases the labor intensity of workers, but also wastes a lot of electric energy on the loss of resistance of the water pump valve. Therefore, the water pump is required to operate in variable working conditions. If the frequency converter is used for speed regulation, the method of adjusting the motor speed instead of adjusting the baffle or valve can not only reduce the labor intensity of workers, but also achieve the purpose of saving electric energy, which is of great significance to improving the economic benefits of enterprises.

Energy-saving principle of water pump inverter control

1. Analysis of water pump characteristics

According to the relevant formula of the centrifugal pump characteristic curve:

P=K1Qh (1)

Q=K2n (2)

H=K3n2 (3)

From (1), (2), and (3), we can get:

P=k4n3 (4)

P/Q=K5n2 (5)

Where: K1, K2, K3, K4, K5 are constants:

P is the pump shaft power;

Q is the pump flow rate;

H is the pump head;

n is the pump speed.

(1) shows that under the same shaft power, if the pump flow rate is adjusted through the outlet valve, the pump head will change accordingly. The smaller the flow rate, the greater the head becomes. However, in actual pumping, since the head is basically unchanged, more surplus head is generated.

Formula (4) shows that the pump shaft power is proportional to the cube of the rotational speed. If we can reduce the rotational speed, we can reduce the pump shaft power. From (1), we can know that the flow rate or head can be freely adjusted.

Formula (5) shows that the energy consumption (i.e. power consumption) per cubic meter of water discharged per unit time is proportional to the cube of the rotation speed. This means that under the premise of achieving the actual water supply flow rate, the smaller the rotation speed, the smaller the power consumption.

2. Motor characteristics analysis

According to the working principle of AC motor:

n=60p.f(I-s)（6）

Where: f is the power frequency of the motor;

p is the number of magnetic pole pairs of the motor;

S is the slip rate.

Since the S and P of the motor are constants, it can be seen from equation (6) that the motor speed has a fixed positive proportional relationship with the power supply frequency.

3. Energy saving principle

From the analysis of motor characteristics, we know that by uniformly changing the motor power supply frequency F, we can smoothly change the motor speed, thereby changing the pump speed; combined with the analysis of pump characteristics, by reducing the motor speed, the motor input power will also decrease, and the pump shaft power will be reduced accordingly. This is the energy-saving principle of inverter control of water pumps.

In addition, the sharp twist when the water pump starts and the water hammer when it stops suddenly often cause the pipe to loosen or break, and in serious cases, the motor may be damaged. After the water pump adopts the inverter speed regulation, the soft start and soft stop of the pump can be realized according to the needs of the process, so that the sharp twist and water hammer phenomenon can be solved. Moreover, when the water consumption is not large, the speed of the pump can be reduced. This not only avoids the premature aging of the motor caused by the long-term operation of the water pump at full load, but also the soft start of the inverter greatly reduces the impact on the machine when the pump starts, and also has the effect of energy saving, and its power saving rate is generally around 15%-40%.

Design of frequency conversion control for water pump

The frequency converter uses Shenou frequency converter as the execution unit of the frequency-based speed regulation system. Shenou frequency converter is a high-performance space voltage vector control general frequency converter.

It has the following features:

32-bit DSP motor control dedicated microprocessor, high-precision frequency output.

The novel power accumulation function makes it more intuitive and convenient to observe the energy-saving effect.

Unique time accumulation function can display single power-on running time and cumulative running time.

Built-in RS-485 interface, computer network control. Multi-signal input, built-in simple PLC, automatic control is more convenient.

Parameter interactive function, easy to set.

The carrier frequency is adjustable and the machine operates silently.

The control methods are diversified and highly versatile.

Built-in PID adjustment function, simple closed-loop control.

Low speed rated torque output, stable operation.

The keyboard is easy to operate and can adjust and set relevant parameters online during operation

Built-in brake unit for quick stopping.

The operating status of the fan can be controlled, making scheduling and operation easier and more convenient.

Various inverter operating status parameters are displayed, providing a clear view of the control signal and load conditions.

The controlled object of the system is a 30KW water pump motor. The original industrial frequency starting unit of the motor is retained, and a frequency conversion control is added. The two are mechanically and electrically interlocked, which not only ensures the normal operation of the pump but also facilitates the maintenance of the system. The liquid level measurement transmitter is used to measure the liquid level of the water tank and convert the liquid level signal into a 4~20mA standard electrical signal to supply the SOB-V600 inverter.

When the water consumption of the factory increases, the water level of the pool will drop accordingly. The liquid level measurement transmitter transmits the liquid level drop in the form of an electrical signal to the SOB-V600 pool water supply inverter, which increases the output frequency of the SOC-V600 inverter accordingly, increases the speed of the water pump motor, and increases the water supply accordingly, thereby maintaining the relative stability of the pool liquid level. On the contrary, the inverter output frequency decreases to keep the pool water level from rising.

4. Conclusion

After the water pump uses SOB-V600 inverter control, it not only eliminates many tedious manual operations and reduces unsafe hidden dangers, but also keeps the system in an energy-saving state, prolongs the service life of the equipment, reduces the maintenance workload and maintenance frequency, and can also achieve soft start of the motor, reduce the starting voltage, reduce the high current impact on the power grid, and better adapt to production needs. In addition, Shenou inverter has rich internal control functions and can easily realize closed-loop automatic control with other control systems. Therefore, the use of inverters in the water supply process has great application value and promotion value.