Asynchronous Motor Direct Torque Control In The Application Of Variable Frequency Control Pumping Unit

At present, the commonly used oil pumping units in the oil field in our country are working in under load or light load condition, caused by imbalanced matching. It not only makes the motor run inefficiently and the waste energy, but also makes the power factors low and pollutes the grid. The energy saving of oil pumping unit is the goal of this research work, and we take the asynchronous motor as our object to control. According to the direct torque control(DTC) and sensorless control theory, the thesis presents a new pumping unit variable frequency scheme based on the unit stroke of the energy.The thesis summarizes the pumping unit saving energy technology, DTC technology and sensorless control technology development and the present situation. Then it divided into two important parts. The first part focuses on the direct torque control theory starting with the traditional DTC and sensorless theory. The advantages and disadvantages of traditional DTC are discussed further. Then in order to overcome the disadvantages, such as torque and flux ripple, unfixed switching frequency and flux identification error at lower speed range, the thesis presents a modified flux identification method, space vector module and fuzzy speed regulator to improve the DTC performance. Final simulation results show that modified DTC identify the flux more accurately, reduces torque and flux ripples under constant witching frequency.The second part of the thesis firstly studies the working principle of the pumping unit, discusses the reasons for the low efficiency of the pumping unit, and then the energy losses of each part in the pumping unit are analyzed in details. After that an assumption which relates the motor output energy changed with oil production in the unit stroke time is presented. Working conditions on site verifies the assumption. The new scheme is presented based on the assumption, DTC and sensorless control theory. Closed-loop control is used in the scheme which could automatic change the motor frequency according to the site conditions to satisfy the oil production in unit stroke. The presented method is simple and can save the energy. Furthermore the other hardware equipments such as displacement sensors and load sensors are not needed for this method. So it is convenient to upgrade the existing pumping by simply replacing the traditional converter unit with the proposed unit converter without adding any extra cost. Finally simulation results proves that the energy saving is satisfactory and the proposed system structure is adequate for practical realization.