Artificial Lift Techbook 2019

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34 | April 2019 | ARTIFICIAL LIFT: TECHNOLOGY Controlling PMMs for an ESP application is not trivial. Borets developed a proprietary algorithm for vector-loop control of a PMM. "I would point out that variable-speed drives that can control PMMs are not all equal," Simmons said. There are three types of perma- nent magnet control algorithm used by the industry. The first is a very simplistic back electromotive force (EMF) method, which doesn't yield optimum efficiency. It mea- sures rotor flux on the idle phase current that's applied to the stator, he noted. The second is a scalar control method. While it is a little more complex than back EMF, it basically works to maintain a constant ratio of voltage and frequency. It can optimize motor control but only for a static downhole load. With any kind of variable load, this method won't achieve optimal efficiency. The third method is vector-loop control, which works mathemat- ically through an algorithm run- ning on a high-speed processor in the drive. It resolves two vector components or components of the current, which can be considered to be one as the magnetizing current and the other as a torque current, Simmons explained. "That allows the drive to opti- mize the amount of current that is applied to the stator through con- tinuously changing load conditions. It is superior in how it controls the permanent magnet rotor and results in maximum system energy efficiency," he said. "In the past few months at our facility in Tulsa, we've doing a lot of testing of PMMs with drives from manufacturers that just make variable-speed drives. We test them with the same size motor for consistent comparison. We see differences in the performance and efficiency numbers." One of the obstacles to wider adoption of PMMs has been that these require a compatible drive to operate. "If an operator has a field full of drives that can only control induction motors, and they then want to start using PMMs, it will require a different drive to operate it. They're like, hang on I've got all these other drives that I want to make use of. They typically don't want to swap their drives for new ones without a compelling justification. I get that. That justification is there without question for PMMs for ESP systems on new wells," he said. Borets also has a new ESP system in field testing. The company did an alpha test a couple of years ago. The new system can manage flow rates over a wide operating range—4,400 bbl/d to 250 bbl/d—and incor- porates highly wear-resistant pump stages. "One of the reasons we're able to achieve a wider operating range is because of the high-speed PMM the system utilizes," Simmons explained. Conventional induction motors operate at 3,600 rpm or 60 Hz. The high-speed PMM operates up to The Borets team deploys a Permanent Magnet Motor ESP system in the Permian Basin. (Photo courtesy of Borets)

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