The general performance curve of axial flow fan is shown in the figure:
The pressure curve has a hump, such as the working point in the right area of the hump, the fan working state is stable; If the working point is in the left region of the hump, the working state of the fan is difficult to be stable. At this time, the wind pressure and flow fluctuate. When the working point moves to the lower left, the flow and wind pressure have intense pulsation, and cause the whole fan to surge. The fan unit may be damaged by surge, so the fan is not allowed to operate under surge condition. In order to avoid the surge phenomenon of the fan at a small flow rate, frequency conversion transformation of the fan is the first choice, and when the fan speed change does not exceed 20%, the efficiency basically does not change, the use of frequency conversion speed regulation can make the fan in the small flow section effective operation, not only will not make the fan surge, but also expand the effective operation of the fan range.
The main ventilator is operated with power frequency, and the ventilation volume is generally adjusted by changing the Angle of guide vane and baffle plate during operation. Therefore, the ventilation efficiency is low, resulting in energy waste and increasing the production cost. In addition, due to the large design margin of the main ventilator, the main ventilator has been running under light load for a long time, and the energy waste is prominent.
When the main fan uses reactance starting, the starting time is long and the starting current is large, which has a great threat to the insulation of the motor, and even burns the motor in serious cases. The uniaxial torque phenomenon of high voltage motor in the process of starting makes the fan produce large mechanical vibration stress, which seriously affects the service life of the motor, fan and other machinery.
Considering the above reasons, it is better to use frequency converter to adjust the air volume of the main ventilator.
The high voltage frequency converter produced by Noker Electric takes high speed DSP as the control core, adopts no velocity vector control technology and series multilevel technology of power unit. It belongs to high – high voltage source type frequency converter, whose harmonic index is lower than IEE519-1992 harmonic national standard, with high input power factor and good output waveform quality. No need to use input harmonic filter, power factor compensation device and output filter; There is no harmonic caused by motor additional heating and torque ripple, noise, output dv/dt, common mode voltage and other problems, you can use ordinary asynchronous motor.
According to the actual situation of the user site, the bypass cabinet adopts the scheme of one tractor one operator frequency conversion automatic conversion. As shown in the picture below. In the bypass cabinet, there are two high voltage isolation switches and two vacuum contactors. To ensure that no power is sent back to the output end of the converter, KM3 and KM4 are interlocked electrically. When K1, K3, KM1 and KM3 are closed and KM4 is disconnected, the motor runs by frequency conversion; When KM1 and KM3 are disconnected and KM4 is closed, the power frequency of the motor runs. At this time, the frequency converter is isolated from the high voltage, which is convenient for repair, maintenance and debugging.
The bypass cabinet must be interlocked with the upper high voltage circuit breaker DL. When the DL is closed, do not operate the inverter output isolation switch to prevent arc-pulling and ensure the safety of operators and equipment.
The Medium Voltage Variable Speed Drives has been running stably since it was put into operation, the output frequency, voltage and current are stable, the fan runs stably, the measured power factor of the network side of the frequency converter is 0.976, the efficiency is higher than 96%, the total capacity of the network side current harmonic is less than 3%, and the output current harmonic is less than 4% when full load. The fan runs at a lower speed than the rated speed, which not only saves energy, reduces the maintenance cost, but also reduces the noise of the fan, and obtains good operation effect and economic benefit.
Post time: Apr-07-2023