A Constant Power Automatic Control System Based On RS485 Communication Between PLC And Converter For Oilfield Electric Workover

Oilfield electric workover plays an important role in petroleum exploitation work. There are many conventional workover being driven by diesel motors. But this kind of equipment always runs in low efficiency due to the low efficiency of the diesel motor and the gearing. Furthermore, this equipment has the problem of leaking, pollution, maintenance. Because AC frequency conversion speed regulating have the advantage of wide rang, convenience, safety of speed regulating and reliability and less power cost, more and more workover driven by AC frequency conversion speed regulating motor are applied in the oilfield. However, the load becomes lighter and lighter during work. Workover has to output high torque in first lifting, and then high speed is required to save time in the light load condition. Large power motors can meet the requirement that high torque and high speed, but after first lifting, when the load is light, the effective will be lower and the bulk of motor is very big. In the next place the operator have to be practical to regulate speed of the motor according to the varying load in order to run under the maximums output.In order to solve the problem foregoing, a novel constant power control system for oilfield electric workover based on RS485 communication between a Programmable Logic Controller and a converter is proposed in this paper. Constant power control is applied because the characteristic that the load is lighter and lighter is match to that of the motor when it run above normal frequency. The main circuit and the control circuit of this electric control system are redesigned. The program in Programmable Logic Controller controls the main motor through communication between Programmable Logic Controller-the core of this system and converter. This control system which can calculate the motor speed according to the condition of the load makes the induction motor run under the allowable maximal output and the operation more and more safe and reliable. At the same time, the efficiency of the workover system is higher than before because the motor runs faster when its load is light. Therefore, the safety, the reliability, the efficiency and the convenience of workover system dose not depend on the operator's diathesis and experience as before. Operators with different practices, diathesis and experience can make use of this workover system because of the system's manual function for speed regulating which meets the demand of humanized design.A series of experiment validate the feasibility and the superiority of this system. The constant power control system is already applied in LiaoHe oilfield and his system can be used in other lifting equipment whose operation mode is similar to workover that the motor have to output large torque in first lifting then require high speed to save time.