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Application of medium voltage frequency converter in water supply
date: 2019-05-25
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Abstract: Variable frequency speed regulation is widely used for its excellent speed regulation and braking performance, high efficiency, high power factor and energy saving effect. The use of SINCR medium voltage frequency converters in the water supply industry can effectively extend the life of the motor and reduce the impact on the waterway.

1.Introduction

The industrial pump house of the water plant has been in full load after the pump is turned on, and the water consumption varies with production. Therefore, it is necessary to frequently start and stop the pump or use a baffle, a valve, etc. to adjust the water supply. This will inevitably cause mechanical shock to the pumps and motors, reducing their service life and wasting energy. Adopting frequency conversion speed regulation not only saves energy, prolongs the service life of the equipment, but also satisfies the requirements well.

2, frequency conversion speed introduction

An AC motor frequency conversion speed control system consists of a variable frequency speed control drive, an AC motor and a controller. The rotation speed n has the following relationship with the power supply frequency f, the slip ratio s, and the motor pole pair number p:

The frequency conversion speed changes the motor speed by changing the power frequency f. Generally, the main magnetic flux is kept unchanged, so that the torque can be kept constant within the speed regulation range. According to the formula: U1≈E1=4.44fN1kΦ to keep the main magnetic flux Φ unchanged, while changing the f speed regulation, it is also necessary The voltage U1 is changed and the U1/f ratio is kept constant. At present, the variable voltage frequency conversion technology has been replaced by more advanced vector control, which is to divide the stator current of the asynchronous motor into a current component (excitation current) that generates a magnetic field and a current component (torque current) that generates a torque perpendicular thereto. And control it. In this control mode, it is necessary to simultaneously control the amplitude and phase of the stator current of the asynchronous motor, that is, to control the stator current vector, so it is called a vector control mode. For the water pump, since the flow rate is proportional to the primary speed of the rotational speed, the torque is proportional to the square of the rotational speed, and the power of the pump is proportional to the cube of the rotational speed. The speed and power are represented by n and N respectively, and the foot mark “0” indicates the rated working condition parameter. When the flow rate is reduced from the rated value Q0 to Q, the power consumption of the motor with speed regulation is compared with the rated power:

When the flow rate is reduced from 100% to 70%, the speed is reduced to 70%, and the power consumption of the motor is reduced to 34.3%, which is to save 65.7%. The power saving effect is also very significant, taking into account factors such as the difference between the power consumption and the rated power consumption when the valve is adjusted, and the decrease in the speed of the motor caused by the decrease in the speed of the motor.

3. Technical requirements

The frequency converter used is a well-known domestic brand. The operation of the variable frequency pump should meet the original post operation rules, and the water supply quantity can be smoothly adjusted within a certain range; it is carried out on the basis of the original power distribution system. The variable frequency pump can be switched at random frequency and power frequency, and meet the production requirements in time; the variable frequency pump can start or stop in time to accurately control the water level; in the case of frequency conversion, it can automatically close the work according to the pipeline pressure, or can be operated by the duty personnel. Manual adjustment of the duty room; operation and chain are completed by PLC; safety maintenance mode, the equipment is in a safe state during the maintenance process; accurate display of the frequency conversion / power frequency working state of the water pump and voltage, current, speed, frequency and other electrical data; protection.

4. Operation status after transformation

The rated power of the motor is 850kW, and the annual electricity consumption is about 7 million degrees. After the frequency conversion transformation, the annual power consumption of the pump is (the power consumption during the power frequency operation, the power consumption during the D frequency conversion operation)*24h * 365 days, ie (850-750) * 24 * 365 = 876,000 (KW · h) can save about 800,000 degrees of energy per year. The energy saving effect is quite remarkable (power 750KW is the running average of the retrofit operation so far). The maximum starting current when the motor is directly started is 7 times of the rated current; the star angle is 4.5 times; the motor soft starter is also 2.5 times. Observing the load curve of the inverter starting, it can be found that there is basically no impact when starting, the current rises from zero and does not exceed the rated current.

The use of the frequency converter can change the motor speed change along the acceleration and deceleration characteristic curve of the pump, and no stress load acts on the bearing, which prolongs the life of the bearing. The water supply device reduces the number of unnecessary start and stop pumps, and the electric meter substation water supply and pressure reduction station reduces unnecessary measurement insulation. Timely adjustment of water output according to the water consumption of production provides a reliable guarantee for stable production. Before the transformation of the frequency conversion system, the pressure of the industrial water pipe network is generally around 0.65Mpa, and the fluctuation is relatively large. The frequency conversion system adopts pressure closed-loop control to ensure that the water pressure of the industrial water pipe network is 0.60MPa. Because the speed regulation of the frequency converter is smooth and the control precision is high, the pressure fluctuation range of the pipe network is small, which can meet the water supply requirements, ensure the quality of the water supply, and prolong the service life of the pipe network and its valves.

5, the conclusion

The application of frequency conversion speed regulation technology in industrial pump room not only optimizes the process, but also saves electricity and reduces consumption.

Application of medium voltage frequency converter in water supply


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