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Importance of Using VFD for Fans and Pumps

1. Necessary for application. Generally, the designed flow allowance of fans and pumps with throttling and bypass control is 60%~90%, the head allowance is 70%~90%. The energy-saving effect becomes more obvious after using the VFD speed regulation control.

2. According to the fluid mechanics, the flow rate Q is proportional to the rotation speed z, the pressure head H is proportional to the square of the rotation speed n, the shaft power P is proportional to the cube of rotation speed n. When the pump efficiency is constant, if the flow to be regulated decreases, the rotation speed n decreases proportionally, the shaft output power P decreases in cubic relation. Namely, the power consumption of pump motor is cubic to the rotation speed. For example, the power of a pump motor is 55 kW, when its speed drops to 4/5 of the original speed, its power consumption is 28.16 kW, corresponding power saving is 48.8%. When its speed drops to 1/2 of the original speed, its power consumption is 6. 875kW, corresponding power saving is 87. 5%.

3. Power factor compensation energy saving. Reactive power not only increases line loss and equipment heating, but also reduces power factor, thus resulting in the decrease of active power in power grid, reducing the equipment efficiency and energy waste. Active power can be expressed as:
                        P=Scosφ
In the formula, P refers to the active power, cosφ refers to the power factor. The larger power factor cosφ, the greater active power P is. Power factor of the motor of ordinary pump equipment is about 0.6-0.7, after using the variable frequency drive speed regulation, cosφ≈1 due to the function of filter capacitor inside the VFD. As a result, the reactive power loss decreases and the active power in power grid increases.

4. Generally, the motor is started directly or by star triangle, corresponding starting current is about 4-7 times of rated current, which causes a serious impact on mechanical and electrical equipment and power supply network. Moreover, it will also lead to excessive requirements to the power grid capacity. The large current and vibration generated during start-up have a great impact on throttle baffles and valves, thus reducing the service life of equipment and pipelines. With the VFD speed regulation control system, soft start function of VFD enables the the starting current to start from zero, and the maximum current would not exceed the rated current, thus greatly reducing the impact of start-up process on power grid and the requirement to power supply capacity, extending the service life of equipment and valves, saving the equipment maintenance costs.

5. VFD has the advantages of no contact with process medium, no corrosion and no erosion, it can greatly reduce the cost of throttle or bypass valves when transporting the corrosive and viscous fluids.


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