by Charles Tahara & Ben Haberkorn
May 31, 2016

Pump jacks, also known as beam pumps, are used to mechanically pump oil when well pressure is not high enough to force it to the surface. These machines operate with a weight and a counterweight in the same way a teeter-totter does. On one side is a metal sucker rod that can extend roughly 3 kilometers into the earth. On the other side is a large counterweight to offset the weight of the sucker rod head and oil. At the end of the sucker rod, a one-way valve traps the oil and forces it to rise through tubing as the counterweight descends and the sucker rod rises.

The right variable frequency drive (VFD) can help oil and gas solutions such as pump jacks become more efficient and boost performance while overcoming industry challenges.

Drives & Pump Jacks

The National Electrical Manufacturers Association (NEMA) categorizes induction motor designs into four groups. Drives can be used to make NEMA B motors (which are less expensive and offer less slip) act as NEMA D motors (which are more expensive and offer greater slip). This is accomplished by varying the driving frequency so that the motor can deliver torque across a wide range of speeds.

To avoid damaging the sucker rod on the downstroke, drives use dynamic/resistive braking or an innovative regenerative energy management feature to dissipate or store the energy during the descent of the sucker rod.

Any downtime is costly for pump jack operators, yet several common complications with this application can lead to downtime. Some examples and their remedies include:

  • Sucker rods can break from cyclic fatigue and deteriorating operating conditions in the well. If the sucker rod breaks, the oil column will not be lifted, and the counterweight will be too large for the motor to lift. This can lead to motor burnouts and large replacement costs. To remedy this, torque limits can be set in the VFD to stop the motor and prevent unnecessary wear.
  • Valves can become worn or filled with sediment. This can be caused by wells beginning to run dry or by large amounts of sand in the oil reservoir. In these situations, the valve will act as a viscous damper, which can lead to high inefficiencies. This situation can be detected by the drive and remedied by setting trip points for motor and drive operation.
  • The oil level inside the well can drop to the level of the one-way valves. This is caused either by pumping the oil too rapidly or by an empty reservoir. This situation leads to destructive vibrations and a sharp drop in efficiency. The drive’s control system can be used to detect the condition and stop popping.

VFD Features

Globally, VFDs are being used more and more in commercial, industrial and original equipment manufacturer (OEM) applications, whether to meet energy efficiency standards, 
to expand the functionality of the mechanical device, or for 
the advanced protection they offer motors and systems. For 
oil and gas users, one manufacturer’s VFD provides the following features:

  • regenerative energy management
  • integrated brake chopper
  • cold-weather/preheat mode

This VFD is a general-purpose drive that incorporates an energy-control algorithm, extensive onboard industrial communication protocols and built-in harmonic reduction to help users reduce the cost of using a VFD. The modular design, using the latest generation of semiconductor technology, is intended to provide greater reliability and reduce maintenance time and costs. Enhanced graphical displays and communication capabilities are also embedded, providing users with detailed system data to simplify installation, commissioning and maintenance.

This VFD uses a regenerative feature to store the power that is generated by the motor when it is being offset by the counterweight, then reapplies it to the motor on the downstroke to do the work. The regenerative feature is a circuit that recognizes the rise in bus voltage. Through this operation, it stores that power and uses it on the application’s motoring side to dissipate the power and keep the motor operating. The drive frequency dynamically changes to discharge the added 
voltage rise on the direct current (DC) bus. The DC bus caps are used to store that energy for a short period of time as long as it does not exceed the voltage 
trip levels.

A more common VFD feature is the brake chopper, a circuit that recognizes when the motor is acting like a generator and is charging the DC bus. That circuit diverts power to the brake resistor when the motor begins to deliver power in excess of a determined DC bus voltage level. Brake resistors are large and generate heat to dissipate the power delivered by the motor to the DC bus. For this reason, braking resistors are not kept in the same enclosure as 
the drive.

The regenerative feature in the drive will absorb the voltage regenerated on the non-motoring side of the curve. In most cases, the drive will be set up to travel between a minimum of 40 hertz (Hz) and a maximum of 60 Hz, with 47 Hz as an average. The overvoltage controller often will kick in with the motoring and regenerating of the pump jack. While the overvoltage controller 
is enabled, the drive will allow dynamic changes in the frequency to allow for 
the voltage and power control loops 
to adjust.

If the operator is experiencing issues with current trips on starting because of the motor’s hard starting torque requirements, the drive can compensate for this with a torque boost through a volts-per-hertz (V/Hz) optimization. When enabled, the drive gives an additional voltage boost when starting to provide more starting torque. This helps to start motor rotation, and, after it is moving, the drive will continue by operating in the set V/Hz curve.

The brake chopper size is fixed by the drive rating. The brake resistor is selected on the basis of the VFD rating, the magnitude of the energy to be dissipated and the braking duty cycle. Using this technology, the drive can dissipate any excess energy that the DC bus is unable to contain. This is an important feature so the energy does 
not get dissipated as heat by the mechanical system.

When the pump jack sits in a field, there is potential either for the temperature to drop and cause condensation or for other starting issues to occur as a result of the drive temperature. One VFD has built-in features that can warm up the drive 
and motor in these cases. The drive sends a small amount of voltage when a run command is given and the temperature falls below minus 20 C. For a set amount of time, this is to have some current draw in the insulated-gate bipolar transistors (IGBTs) to warm the heat sink. If the heat sink goes above minus 20 C, the drive will begin to run as normal.

The other option is to use internal space heaters to prevent condensation. One VFD has a built-in preheat 
function that can enable a small 
amount of voltage when the drive is stopped to allow for current to create heat in the enclosure and motor to prevent condensation.

This feature has to be enabled and can either follow the drive’s heat sink temperature or leverage an external thermal couple for initiation.

Improved Efficiency

By pairing advanced VFDs with pump jack applications, operators can realize performance and efficiency gains from industry-leading energy efficiency algorithms, high short-circuit current ratings and a more robust design. These solutions afford higher processing 
power and precise motor control in a small footprint.