
Permanent Magnet motors using sensorless Vector Control commonly make 1.5 to 2 times the horsepower of induction motors, but what good is all that horsepower in the lab if you can't count on it in the field? For those with the resources to commit to retraining service personnel and troubleshooting installs, it is a viable option. At ElectromagnetiX, we have a solution that makes that power available without Vector Control.
After spending 14 years designing motors for aerospace, military, and residential applications I was blessed with the opportunity to be part of a team creating a permanent magnet ESP motor for a well-known service company. I had already witnessed the motor industry develop the brushless DC motor and watched that motor quickly overcome induction motors in all kinds of applications. The induction motor was no match for a permanent magnet motor, but how could we operate a permanent magnet motor a mile underground? The answer was a Permanent Magnet AC (PMAC) motor with sensorless drive.
Electrical submersible pumps do not allow for rotor position sensors due to the distance between the drive and the motor. In order to use a PMAC, it is necessary to pair it with a sensorless sinusoidal drive. These drives use a control scheme called Vector Control. The purpose of any drive is to switch polarity in the motor windings so that the magnetic field between the rotor and stator stays coupled, producing torque. The drive control has to know where the rotor is relative to the stator. Vector Control does this without a sensor by approximating the rotor position using the current draw of the motor. Now, imagine an artificial lift-ESP application where the current drawn by the motor is traveling through; a mile of cable, a transformer, and a sine wave filter, all of which shift the current and voltage around relative to time. In an ESP, Vector Control is attempting to approximate the rotor position and command the drive to output the appropriate voltages in a moment when the current it is observing is not the current of the motor in that instance. Worse yet, the voltage will shift in time, so it won’t reach the motor at the time the controller is expecting either! It is akin to flying an RC airplane in the dark.
When it works it is a thing of beauty. When Vector Control is working it drives the motor right on the edge of operation. This means it is great at optimizing power output as well as electrical power factor and efficiency. All of that makes the operation unstable in an ESP which requires a lot of work in the lab programming the controls to get the motor's full potential with some sense of reliability.
After our analysis of the strengths and weaknesses of the existing ESP offerings, we came away with three key insights:
Permanent magnet motor technology makes motors half the length of conventional ESP motors. This power density has advantages in many different kinds of wells.
Current permanent magnet ESP’s require special drives with pre-programmed vector controls in order to operate. Often, the complexity and cost of these drives keep well operators from taking advantage of permanent magnet motors.
Even after a well operator makes the commitment to permanent magnet, there are soft costs associated with field training and fixing troublesome installations.
The team at ElectromagnetiX has designed a new kind of permanent magnet ESP motor. We challenged ourselves to design the best motor for the surface equipment and personnel that were already in place. Our permanent magnet ESP motor is 2-pole, so it works at 60 hz just like the induction motors. It works with induction motor sine-wave filters as well as transformers and the controls on the VFD are all 60 hz based. Most importantly, stable operation is built into the motor design, meaning this motor can be ran on induction motor drives that use simple V/hz drive functions. The result is a motor with permanent magnet motor power without any added complexity on the operator.
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