1. Introduction Currently, beam-type pumping units are the most widely used and numerous type of pumping equipment in oilfields. However, traditional pumping units generally suffer from numerous problems such as large starting impact, high power consumption during operation, oversized motors for small loads, and low efficiency. Furthermore, the complex conditions of oil wells, with frequent occurrences of heavy oil, wax deposition, sand trapping, rod breakage, and motor burnout, coupled with the lack of reliable motor protection, result in high maintenance requirements. Therefore, there is an urgent need to upgrade existing pumping unit equipment. Variable frequency drives (VFDs) offer low-speed soft start, smooth and wide-range speed adjustment, and comprehensive motor protection functions, including short circuit, overload, overvoltage, undervoltage, and stall protection. They effectively protect the motor and mechanical equipment, providing advantages such as stable and reliable operation and improved efficiency, making them an ideal solution for pumping equipment upgrades. We have implemented VFD upgrades on nearly 40 wells in the Liaohe and Shengli oilfields, with very significant results. [b]2 Basic Principles of Frequency Converters[/b] According to motor theory, the speed formula is n=60f(1-s)/p, where P is the number of pole pairs of the motor, s is the slip, f is the frequency of the power supply, and n is the motor speed. From the above formula, it can be seen that the motor speed is approximately proportional to the frequency. Changing the frequency can smoothly adjust the motor speed. For frequency converters, the frequency adjustment range is very wide, adjustable arbitrarily between 0 and 400Hz. Therefore, the motor speed can be adjusted within a relatively wide range. Of course, after increasing the speed, the impact on the bearings and windings should also be considered to prevent excessive wear and overheating of the motor. Generally, the highest frequency is set at 50Hz or 65Hz. The frequency converters we use now are AC-DC-AC voltage source type frequency converters, and their circuit is shown in Figure 1: The power device is Siemens IGBT, and the core of the controller is the 87C196MC microcontroller. [b]3 Challenges of Pumping Unit Inverters[/b] As pumping equipment, its movement involves repeated up-and-down lifting, with one lifting stroke per cycle. Its power comes from two heavy steel sliders driven by an electric motor. When the sliders are lifted, they act like levers, sending the pumping rod into the well. When the sliders descend, the pumping rod is pulled out, carrying oil to the wellhead. Since the motor speed is constant, the load decreases during the slider descent, and the energy generated by the motor has nowhere to be released, inevitably entering a regenerative power generation state. This causes the main circuit bus voltage to rise. Frequent high-voltage surges can damage the main components of the inverter, including electrolytic capacitors and power modules. Therefore, a braking circuit is added to allow the regenerative voltage to be released in a timely manner, ensuring the equipment operates under a safe voltage. The regenerative braking circuit, commonly known as the brake circuit, is shown in the dashed box in Figure 1. It includes a comparator circuit, a brake module, and a brake resistor, etc. Its control block diagram is shown in Figure 2. We sample and amplify the voltage from the main circuit and compare it with the reference voltage. By adjusting the potentiometer, we make the braking unit conduct at 1.1Ue (Ue is the rated bus voltage) and disconnect at 1.05Ue, thus controlling the voltage of the main circuit and ensuring its operation within the allowable range. Since the braking module simply controls the voltage of the main circuit, its reliable operation is crucial. Therefore, the absorption circuit must be well-designed. Because the braking resistor leads are relatively long, their lead inductance and peak voltage are correspondingly increased. For this reason, an R, C, D absorption circuit is used. In Figure 2, R3 and C4 are used to absorb the peak voltage generated by the lead inductance. The added fast recovery diode provides a path for the reverse voltage generated across the braking resistor when the braking unit is cut off. Frequent operation of the braking unit will inevitably generate interference voltage, causing the module to conduct unnecessarily and damage it. Adding an anti-interference capacitor at the signal input is necessary to absorb the module's mis-conduction caused by interference signals. A non-inductive capacitor of about 0.01uF is generally suitable. Since the current limiting of the braking resistor is generally not large, a 100A rating is sufficient for the braking unit. Because the braking resistor operates once per stroke, its power must be sufficient to ensure long-term reliable use; a 10kW/80 heating wire is generally selected. [b]4 Field Application[/b] We conducted frequency conversion retrofits on early-stage and mid-to-late-stage wells. In the early-stage wells, due to the large oil reserves and sufficient fluid supply after recent extraction, we increased the speed of the frequency converter to 65Hz to improve efficiency. This increased the frequency by 1/3, correspondingly increasing the motor speed by 30%, and consequently increasing the oil production. The overall oil recovery rate can be 20% higher than under power frequency conditions, and the efficiency is increased by 1.2 times, which is very popular with oilfield workers. In mid-to-late stage wells, due to reduced well reserves and insufficient fluid supply, continued operation of the motor at the mains frequency would inevitably waste energy and cause unnecessary losses. Therefore, we adopt a method of reducing the speed and stroke, generally operating the frequency converter between 35 and 40 Hz. This reduces the motor speed by 30%. Combined with the generally lighter load on oil production equipment, the energy saving rate can reach approximately 25%, while also improving the power factor and reducing reactive power losses. The frequency converter also has soft start/soft stop functions, reducing mechanical impact on the sucker rod during motor startup. It effectively protects against heavy oil, wax deposits, sand jams, etc., protecting the motor and mechanical equipment, reducing maintenance, and preventing rod breakage. The frequency converter reliably protects against overvoltage, undervoltage, overload, short circuit, and motor stall, significantly extending motor life and reducing wear on mechanical equipment. In conclusion, the application of variable frequency drives (VFDs) in pumping units can improve efficiency, increase oil production, save energy, and protect motors and equipment, making its application prospects very broad. There are many oil pumping units in our country, and I believe that variable frequency drives can play a significant role in this field. [b]References[/b] [1] Variable Frequency Speed ​​Control System for Mine Hoists [2] User Manual for Fengguang Variable Frequency Drives About the Author Zhou Jiasheng Male Engineer Graduated from Shandong University of Technology in 1990 with a major in Applied Electronics. After graduation, he has been working at Shandong Fengguang Electronics Co., Ltd., dedicated to the development and manufacturing of variable frequency drives.