Selection and Precautions for Matching Frequency Converter with Backward-Curved Double-inlet Centrifugal Fan (Part I)
Advantages of Frequency Regulation for Backward-Curved Double-inlet Centrifugal Fans
Energy saving
The output power of a backward-curved double-inlet centrifugal fan is proportional to the cube of its rotating speed, while its air volume is directly proportional to rotating speed. Therefore, frequency conversion speed regulation is an energy-saving preferred scheme for fans and blowers.
Reduced starting current and less impact on power grid
Special inverters dedicated to backward-curved double-inlet centrifugal fans adopt V/F control mode. During startup, voltage and frequency rise gradually from zero, resulting in relatively low starting current and minor adverse impact on the power supply grid.
Inverter Selection
Backward-curved double-inlet centrifugal fans and Roots blowers feature different torque characteristics, so different types of inverters shall be selected correspondingly. Centrifugal fans present quadratic torque characteristics, so fan & water pump dedicated inverters are applicable for backward-curved double-inlet centrifugal fans. In contrast, Roots blowers feature constant-torque characteristics, for which general-purpose inverters can be adopted.
The conventional principle for inverter capacity selection is to select a current rating higher by one gear than the rated current of the backward-curved double-inlet centrifugal fan. This selection method reserves a certain safety margin for fans used in boiler systems, yet such margin is often redundant in practical operation. In most working conditions, the backward-curved double-inlet centrifugal fan seldom runs at full load. To cut equipment investment cost, users may select an inverter whose rated current is slightly higher than the actual maximum operating current of the fan. Some manufacturers even adopt inverters with marginally lower current specifications. Regardless of the selection scheme adopted, the inverter must satisfy the actual operating requirements of the centrifugal fan. If the actual operating parameters of the fan cannot be confirmed, a conservative selection scheme is recommended to guarantee safe operation of the boiler unit.
In addition, a braking resistor must be configured for the inverter. When the backward-curved double-inlet centrifugal fan decelerates, regenerative voltage will be generated. Excessively fast deceleration may trigger an overvoltage fault on the input side of the inverter.
Control Circuit
The attached schematic shows a typical main control circuit for the frequency converter of backward-curved double-inlet centrifugal fan, which is essentially a switching circuit for switching between power-frequency operation and variable-frequency operation. Contactor K3 is designed to prevent reverse power feedback when the motor runs under power-frequency mode.
Furthermore, the motor shall be stopped via the stop command of the inverter under variable-frequency operation. It is not allowed to implement free coasting stop for the inverter by disconnecting contactors K2 and K3 separately. Residual kinetic energy of the fan impeller may feed back electric energy to the power grid in this case. Simultaneous disconnection of K2 and K3 may cause permanent damage to the inverter.
