Pump jacks are a common sight in oil fields across western North America. In the past, their most common driver has been a low-voltage, three-phase induction motor. The beam pump, or sucker rod pump, is driven with a cam action that has the effect of loading, then unloading the driver (motor). This loading and unloading occurs on every rotation of the cam.
Needing a motor that can go from unloaded to loaded has led the industry to rely on Design D motors. These motors operate with high slip, or the inherent characteristic of producing high torque, as they “catch” the load.
Traditionally, Design D motors have been controlled by pump-off control (POC). Variable frequency drives, with or without POC, are more common. An initial benefit of using VFDs is that users can choose a more traditional and common Design B motor. These motors are more efficient than the uncontrolled Design D motor.
Advancement in Variable Frequency Drive Technology
By connecting a variable frequency drive to a supervisory control and data acquisition (SCADA) system, the operator can remotely monitor the system. Software specifically designed for artificial lifting further enables this capability.
While today’s technology follows the same laws of physics as those designed a decade ago, advancement in the design and manufacturing of hardware and software enable them to have a higher power density and improved reliability.
The VFD’s output section, also referred to as an inverter, is typically configured with insulated gate bipolar transistors (IGBT) that are controlled by software using some type of pulse-width modulation (PWM) algorithm.
An IGBT-based inverter always creates common mode voltages (CMV) to the output circuit. CMV can cause bearing currents on the motor and can migrate throughout the rod pumping station, potentially causing disturbances in measurement and programmable logic controller (PLC) systems.
The most effective way to control CMV effects is through proper cabling practices, using VFD rated output cables, having a cable gland with a 360-degree shield, and connecting the drive, motor and cabling to ground.
The rectifier can be configured as a traditional six-pulse diode rectifier, a multi-pulse rectifier with matching transformer or an active front end (AFE), where rectifier diodes have been replaced with IGBTs. Diode-based rectifier units draw blocks of current from the supply side, creating harmonics.
Harmonic current will create additional heating and needs to be added to supply dimension. Harmonic current also causes harmonics in the voltage, distorting sinusoidal supply voltages. A stiff (high short-circuit capacity) supply network can withstand voltage distortion to higher load currents, so the relationship between short circuit current (Isc) and demand load current (Iload) is important.
Rod pump units can be fed from a local generator plant or utility via high line. Utilities typically require IEEE 519 compliance at the service entrance. IEEE 519 harmonic limits are set at the point of common coupling (PCC) to be met at continuous maximum load.
At a small rod pump station, the application motor’s VFD load may be the main load. To comply with the IEEE 519 standard, 18-pulse VFDs typically require a sizeable additional load. However, a VFD with an AFE complies on its own. An AFE also can feed energy back to the supply network, which reduces operational costs and negates the need for a braking chopper and matching resistor bank.
IER Services is proud to be a distributor of Baldor Electric Motors and products. As one of the world's largest electric motor manufacturers, Baldor specializes in speed variation, industrial process automation, power generation, power distribution and more.
They have products to suit your application requirements, from a fractional or 15,000 Hp motor, a variable frequency drive, mounted bearings or gearing, to a pulley or sheave.
We sell and repair Baldor electric motors, power transmission products and adjustable speed drives.
When it comes to the tools of your trade, you demand quality, value and availability. IER Services, your Columbus Baldor dealer, is proud to be able to deliver a high level of customer service. Call us for your Baldor electric motor repairs or replacements. We offer a wide range of Baldor motors and power transmission products.