I. Introduction of mine and axial-flow fan

Xingcun Coal Mining locates in Qufu city Shandong province. Capacity is 1200000t/year. Ventilation mode is central paralleling. Auxiliary mine intake and main mine return. There are two aixal-flow fans, one works and the other is spare.

Install an impeller behind of another but rotating direction of impellers is opposite. We call this contra-rotating axial-flow fan. Two levels of impellers are contraposition and they are driven by same model motors. Watching from inlet, first level impeller rotates clockwise and the second level impeller rotates anticlockwise. Entering the first impeller, air gets energy and then air is discharged through the second impeller. Second impeller also plays role of fixed blade, getting speed flow in circle direction, increases flow energy, thereby reaching high efficiency and high pressure.

Fans produced by Xiangtan Pingan is BD-II-8-NO.27. One works and the other is spare. Two motors are 450kW, 10kV, 744r/min, 33.9A. Motor produced by Nanyang is asynchronous YBF630M-8. Air flow is controlled by blade angle and damper.

II. Reform Necessity

Axial-flow fan is backbone of breath system in mine. It works in 24 hours continuously. There are some problems in operation:

1. Serious waste

Fan model is designed with big redundancy, but it works in light load mostly. Damper adjustment causes serious energy waste and cost rising.

2. Difficult start-up and serious mechanical damage

Time of direct start is long and start current is big, which threatens motor insulation. Single shaft torque makes fan cause big vibration force, affecting lifetime of motor, fan and other equipments.

3. Lowautomatization level

Damper adjustment has no function of instant auto-adjustment. In the case of fault, such as flow short circuit, it will affect production badly.

VFD is a new power inverter and it has applied in industry maturely. It not only starts motor easily with lots of protections but also saves energy. So it is very necessary to utilize VFD to improve safety and automatization.

III. VFD technology

   In order to overtake above problems, we decide to reform with VFD. We choose HARSVERT-A 10/070 VFD at last.

   HARSVERT-A series VFD adopts technology of cell-cascade. It is high-medium voltage source VFD. 10kV input and 10kV outut. VFD is composed of transformer, power cells and controller. It adopts IGBT to realize digital control.

   1. Power cell structure: it is AC-DC-AC single phase inverter circuit. Every cell is same and can be replaced by each other.

   2. Input: transformer supplies power to each cell. Transformer secondary is divided into 3 groups. According to voltage and power cell level, 48 pulse constitutes rectifier mode, which can improve current waveform, make power factor near to 1 without compensator.

   3. Output: output U and V series connect in star to supply power to motor. PWM constitution gets ladder sine PWM waveform. This waveform is good sine waveform and in small dv/dt without damage on cable and motor insulation. It can prolong output cable length without filter. Meanwhile, haromonic is reduced, blade and bearing vibration is eliminated.

   4. Controller: controller is composed of DSP chip, HMI and PLC.

IV. Effect

1. No impact on power grid. Because VFD realizes soft start, it avoids current impact and reduces impact on power grid. It can start/stop at any time, indicating its soft starter function.

2. Air flow is controlled by variable speed. Adjusting range is 0~100%. So it can adjust air flow as per production requirement.

3. During operation, if it needs unit switch, personnel can click “start 1#” or “start 2#” in HMI.

4. After reform, fan does not work in full load for long time. Energy saving rate is 30% or more.

5. After reform with VFD, during operation, two motors are required to keep same frequency, thereby speed of two motors is equal. That one runs in low speed and the other runs in high speed should be avoided, otherwise air resistance creates, affecting fan normal operation.

V. Result

VFD was installed on 30th May, 2008 and started to run on 3rd July. With upper PC and HMI, operation data and alarm information are shown. Two levels impeller can start at the same time and can rotate to high speed in short time to reach required flow. Short start-up time ensures safety. VFD has been running stably since it was installed. Output frequency, voltage and current meet requirement. Current harmonic in full load is less than 3%. Fan runs in speed less than the rated, which not only saves energy but also reduces maintenance cost and noise.

VFD implements soft start and saves energy. Speed can be controlled as per air flow demand. The effect is ideal. In variable frequency state, fan efficiency is 80% and energy saving rate is considerable, about 30% or more.