Study On The Beam Unit Dragging System Based On Variable Frequency Control

Oil extraction with rod is the main means of raw oil exploitation in our country. Statistically, rod-pumped wells occupy 94 percent of the whole oil wells, and the output is 87 percent of the raw oil's whole output. However, at present there are some problems in oil extraction system with rod. a). System efficiency is very low, therefore, it is very serious that big horse hauls small cart; b). Load feature of the beam unit is periodical and alterative load with impact, sometime there is a problem of generate electricity; c). It raises the oil extraction of cost due to large energy loss; d). The problem of half load or light load is still serious.Base on the series of problems in beam unit dragging system above mentioned, some of them can be solved by adopting high-slipping motor, which makes beam unit dragging system saving energy in all. But because the slipping increasing of this kind of motor is realized by increasing the rotor resistor, it raises the motor's slipping loss and cuts down the motor's efficiency. The softer the feature of motor is, the lower motor's efficiency is. Therefore on one hand driving beam unit adopting high-slipping motor can save energy, on the other hand it can increase the factors of energy loss, Therefore, saving energy in all is very limited. In additions, it can't adjust automatically the stroke's frequency in term of the status of oil well.Currently, variable frequency technology is widely used in oil fields. But at present variable frequency control sets mainly the given frequency on the ground of the status of oil well, and makes the beam unit extract oil by the given frequency. Consequently, the output feature of motor is still hard, and can't adjust the output torque of motor according to the load feature of beam unit. Sometime there is the problem of generate electricity.The beam unit is considered as controlled object in this paper, and the fundamental principle of variable frequency, the operating principle and theload feature of the beam unit are expatiated and analyzed in detail. Based on the series of problems of the practical beam unit dragging system, an excellent energy conservation method is proposed in this paper, i.e. the slip frequency is kept constant in variable frequency system. And a digital SPWM generator is adopted to put this control method into practice. The main feature of this method is that voltage U is independent to frequency /, which can be controlled separately within limits. So it can change the ratio of U/f according to the variable load torque. Accordingly, this method can restrain the generate electricity of the beam unit, soften its feature and realize the stepless speed regulation, so that optimal control is brought into use and large quantity of energy is economized. The results of experiments and simulation show that the proposed method is valid and feasible. Meanwhile, some conclusions are drawn, and they have a good importance to instruct further research and fabrication in this field later.