Permanent magnet motors (PMMs) have the potential to deliver performance improvements over standard induction motors (IMs), but safety concerns have limited the use of PMMs in oil and gas operations. That situation is about to change with the introduction of a new tool that improves safety, making PMMs a viable choice—and a better alternative— for cleaner, more efficient operations.
UNDERSTANDING THE OBSTACLES
Unlike the majority of conventional ESP systems that are powered by IMs, PMM-powered systems employ embedded permanent magnets in the rotor, Fig. 1. This eliminates induction losses, resulting in substantial efficiency gains.
Fig. 1. Unlike most conventional ESP systems powered by IMs, PMM-powered systems employ embedded permanent magnets in the rotor.
However, there are key differences in the way the two motors function. The use of a permanent magnet in a PMM means the magnetic field of the rotor is always on, even when the motor is not electrically energized. While the stator coils of both motors carry AC current to generate a rotating magnetic field and turn the rotors, a PMM functions as an AC generator when rotated forward or backward by the pump.
Unlike IMs, PMMs can generate lethal electrical charges (~rated voltage at rated rotation), if fluid flows through an ESP with enough force to rotate the shaft. However, PMMs can be used as safely as Ims, as long as field workers follow guidance and procedures designed to minimize risks.
Today, most IMs are operated with variable speed drives (VSDs), but a PMM must have a VSD for the precise speed control required to maintain synchronicity during startup and to manage varying loads during operations. Even though the market has generally standardized VSDs for ESP operation and optimization, not all VSDs can operate PMMs. Demonstrated performance with existing VSDs needed to be proven, to avoid additional costly surface upgrades.
In terms of performance, a PMM delivers a higher power density than an IM, which allows it to produce the same horsepower from a motor that is approximately 50% shorter or generate higher horsepower with the same size motor. PMM construction also allows for better tolerancing—a known limitation for IMs— which improves reliability and run life. PMMs also generate less heat, reducing thermal fatigue. As a result, the expected service life of a PMM is up to 30% more than that of an IM, which reduces costly ESP workovers and maintenance.
MEETING PERFORMANCE GOALS
One of the most persuasive reasons to switch from an IM to a PMM is that it improves efficiency while reducing the environmental impact of operations.
An IM typically is 78% to 84% efficient because of energy losses resulting primarily from rotor current induction. PMMs, which do not experience significant rotor current loss, have an efficiency rate of up to 93%. A PMM maintains higher and more consistent power and greater efficiency across a wider load range than an IM, so power loss can be reduced by up to 25%.
This combination of high energy density and a shorter motor allows the PMM to be placed deeper in slimhole and deviated wellbores, closer to the production zone to expand the operational flexibility of the ESPs and boost recovery rates through greater reservoir pressure drawdown, Fig. 2.
Fig. 2. Combining high energy density and a shorter motor allows the PMM to be placed deeper in slimhole and deviated wellbores, closer to the production zone, to expand ESP operational flexibility and boost recovery rates through greater reservoir pressure drawdown.
PROCEDURES AND PROCESSES MITIGATE RISK
Undeniably, PMMs have caused serious incidents, but proper training and procedures can minimize those risks.
API RP 11S9, “Permanent Magnet Motor Safety,” published in March 2023, provides recommended practices for handling, installing, troubleshooting and operating PMMs, as well as guidelines for removing them from service when used in subsurface artificial lift pumping systems. These guidelines were developed collectively by industry experts—including participation by Baker Hughes—who joined forces to produce recommendations for safe PMM installation and handling, based on in-depth knowledge of PMMs and extensive experience working with them in the field.
In addition to following API RP 11S9, PMM installation must strictly adhere to procedures that reduce risks for site workers. The procedures for installing a PMM are similar to those for installing an IM but include important differences that address the fact that the motor is always a potential power source. Key procedural differences include stricter adherence to existing best practices, such as installation (run in-hole) rate. It’s important to note that additional steps, such as grounding, confirmation that no voltage is present. Personal PP&E and isolation procedures are also required. These steps are particularly crucial during splicing or when in contact with the conductors. Isolation procedures also apply, i.e., disconnecting from downhole equipment while working on the VSD.
Finally, site signage should indicate that PMMs are in use, so no one mistakenly assumes that IM procedures are appropriate for installation, troubleshooting, maintenance or removal.
DRIVE OPTIMIZATION
Although most ESPs today use VSDs, not all VSDs can operate PMMs, so drive selection is important.
While it is possible to run a PMM with a scalar or V/Hz VSD, using more advanced methods, such as vector control, is recommended for optimal performance, efficiency and reliability. This method provides precise control over motor currents, ensuring better torque production, speed regulation, and overall motor performance.
Unlike conventional ESP VSD control, vector control decouples voltage from frequency, enabling independent control of magnetic flux and motor speed. Separately controlling magnetic flux unlocks the ability to consistently optimize voltage and current, ensuring peak performance, regardless of fluctuating operating loads.
SAFETY MODULE REDUCES OPERATIONAL RISK
One of the biggest concerns when using a PMM is the fact that the motor’s rotors are always magnetized. If the pump spins the motor in either direction, it can produce dangerous counter-electromotive force (aka back-EMF). Back-EMF-generated voltage can be produced when the pump drives the PMM during routine operations, including:
- Well intervention/ESP troubleshooting
- ESP installation and removal
Once back-EMF is produced, it can take up to an hour to stabilize during fallback conditions.
Historically, the industry has relied on administrative safety controls, such as check valves, diverter valves, y-tools, and barrier plugs, to isolate potential hazards. Although these tools are effective to some degree, they require human operators, which means they are subject to human error. They also offer little protection during normal operations and require additional work to set and pull after installation, and many of these tools experience wear, because they are operated in production fluid.
There is a better solution. A new module is available that can improve safety and reduce the likelihood of voltage generation at the surface to ensure safer operations, Fig 3. Designed to prevent voltage generation at surface for safer operations, the device functions like an extension of the motor, and because the module operates in motor oil, it is protected from the well environment.
Fig. 3. A PMM safety module.
The safety module operates using special clutches. When the motor rotates in the forward direction, sprags allow smooth engagement, permitting the pump to be driven. However, if the pump attempts to turn the motor in the same direction, the clutch disengages, preventing the transmission of torque. This effectively breaks the connection between the pump and the motor, allowing the safety module to act as a unidirectional barrier. When the pump attempts to turn the motor in the opposite direction, the clutch engages differently to transfer the pump’s torque to the equipment housing, preventing torque transfer to the motor.
The safety module prevents reverse rotation and assures power transmission is unidirectional, which makes it particularly useful in ESP PMM applications, in which the motor should remain stationary during non-operational phases to prevent potentially dangerous voltage generation from the PMM.
SAFETY MODULE TAKES THE FIELD
Baker Hughes has developed the safety module for our 400 series PMM and will extend the design to the rest of our PMM line. Developing the module for each motor includes a range of field trials, to ensure operational integrity and gather valuable performance data. Functional tests measure power output, performance under elevated temperature, backspin, endurance, and ability to hold torque.
Before being sent to the field, each safety module undergoes factory acceptance testing that includes torque testing to verify the rated torque from motor to seal (operational direction) and torque lock verification (counter operational direction) and a spin test that checked power draw during rotation.
Data from field trials in unconventional and conventional wells include 425 hp, 6,000 bpdm 9,500-ft set depths, and 235oF fluid temperatures. Trial results show consistent, reliable performance, with no recordable safety incidents.
CHANGING THE STATUS QUO
Reducing the risks associated with traditional PMMs opens the door for accelerated adoption. Positive results from installations across a range of operating environments in both conventional and unconventional wells in North America provide compelling evidence that PMMs, coupled with safety modules, can be applied safely in the oil and gas sector to deliver higher power density, lower power consumption, and reduced carbon emissions for cleaner, more efficient operations.
About the Authors
Joseph McManus
Baker Hughes
Joseph McManus is a seasoned expert with 20 years of experience in the oil and gas industry. Throughout his career, he has held diverse roles in sales, operations, engineering leadership, business development, and product management. He currently serves as a product manager at Baker Hughes, where he specializes in artificial lift systems, particularly electric submersible pumps (ESPs). Mr. McManus holds a Master of Science degree in chemical engineering and an MBA, both from the University of Tulsa.
Dana Meadows
Baker Hughes
Dana Meadows is the Global Portfolio director for Baker Hughes Artificial Lift Systems, based in Claremore, Okla. She began her career with Baker Hughes as an account manager for Drilling Services 14 years ago, and has progressed through sales, service delivery, AMO, and materials management roles during her tenure. Ms. Meadows holds a bachelor’s degree in marketing from Sam Houston State University.
