Intelligent power control circuit design of C51 microcontroller for oil pumping unit

In order to achieve the goal of the oil pumping unit automatically performing Δ/Y bidirectional switching and intermittent timing control in real time according to load requirements, the control circuit shown in Figure 1 was designed using embedded control technology.

The hardware system of the intelligent power controller is based on the experimental board provided by New Hualong Electronics Company for C8051F206, and is formed by adding an output circuit and a keyboard display circuit. Directly using the C8051F206MCU, JTAG interface, crystal oscillator, reset and power circuit on the experimental board greatly reduces the hardware design workload.

(1) Output circuit

The controlled object of this system is the switch of the motor. In order to meet the star-angle switching and start/stop requirements, three AC contactors (C1 ~ C3) need to be controlled according to a certain combination relationship. The circuit principle and control truth table are shown in Figure 2 (a) and Figure 2 (b) shown. Taking into account the need for on-site anti-interference, a photoelectric isolation circuit is used after the output ports P0.1, P0.3 and P0.5 (Figure 2(c)), in which the working contacts of the sensitive relay J for driving are connected in series. In the motor control loop, to control the action of the corresponding AC contactor.

(2) Design of display and keyboard circuits

The display uses a 4-bit LED serial communication method, and the interface circuit consists of three 74HC595 serial shift registers, five triode power amplifiers 9014 and four LED digital tubes (as shown in Figure 3). Serial data is input from port P0.2, shift pulse is generated by P0.4, and latch pulse is generated by P0.0.

The keys use a 1×5 non-coded keyboard. One end of the five keys is connected to the output terminals Q3, Q4, Q5, Q6, and Q7 of the serial shift register U1 respectively. The common terminal is directly connected to P0.6. By continuously detecting the computer Pin P0.6 to get whether a button is pressed.

(3) Design of analog input circuit

The design of the pumping unit power switching control program adopts mixed programming of C51 and MCS51 assembly language. The program flow is shown in Figure 4.

After the instrument is powered on and initialized, it will first send out a bell alarm to indicate that the motor is about to start; then the motor will start in a triangle pattern to meet the requirements for heavy-load starting of the pumping unit; after starting, the system will make the motor operate in a triangle or star pattern based on the real-time power consumption of the motor. Or automatically switch between the three states of overload shutdown, that is, when the real-time power consumption is less than the set delta-to-star switching power, the motor works in star mode, and when the power consumption is greater than the set star-to-delta switching power, the motor works in delta mode. , it will stop when it is greater than the set overload power; if intermittent control parameters are set in the system, the system will also time the cumulative time of startup and shutdown to achieve timing control. The above process is carried out over and over again until manual intervention stops the machine.

In order to prevent external interference from affecting system stability, the system design adopts technologies such as median filtering, automatic zero calibration, software redundancy, error traps and watchdogs.

The energy-saving control instrument for pumping units developed in this article has been industrially tested in oil fields such as Jiangsu and Changqing. The results show that the reliability of the system is very good, and it has been working normally for about 2 years under the conditions of -30~50℃. The energy-saving effect of the pumping unit is satisfactory, and the power-saving rate generally reaches more than 15% (measured under the condition of 20% load rate). Practice has shown that the production cost of energy-saving controllers designed with C8051F206 is very low, easy to be accepted by on-site users, and fully demonstrates good market prospects.