Hermetically Sealed High-Speed Permanent Magnet Motors
Permanent magnet (PM) motors have just recently been introduced into downhole equipment. PM motors have proven their superior performance and cost effectiveness in numerous industries. They have been used in everything from high precision, high reliability applications in the aerospace industry to low cost, high volume applications in the automotive industry.
More and more industries are adopting PM motors over induction motors due to the added value they offer, including higher energy density, higher efficiency (lower heat generation), higher speed capabilities and ease of fabrication. PM motors also offer the ability to integrate them into rotating tools to optimize the complete system, enabling applications that would otherwise not be feasible.
Upwing Energy’s parent company, Calnetix Technologies, is the world leader in the system level integration of PM machines in a wide range of industries worldwide. Applying Calnetix’s knowhow, Upwing is integrating PM motors into its downhole rotating equipment. In doing so, we are able to leverage the unique capabilities of PM machines for our Subsurface Compressor Systems (SCS) and Magnetic Drive Systems (MDS). Some of the unique features and key values that Upwing’s hermetically sealed high-speed PM motors provide are discussed below.
High-speed operation is a must for compressible fluids, especially in a restricted diameter environment. High speed also brings about unique value for pump systems. For PM motors, speed is directly proportional to horsepower (HP). PM machines provide constant torque throughout an operating speed range, therefore increasing the speed directly increases the HP. For example, if a motor with a fixed geometry delivers 200HP at 3,000rpm, the same physical size PM motor will deliver 400HP at 6,000rpm. Or, a 6,000 rpm 200HP motor will be half the length of a 3,000 rpm 200 HP motor. High speed also has significant value for pump systems; as the pump speed doubles for electric submersible pump (ESP) applications, only one-third of the pump stages are necessary to perform the same amount of work. This significantly reduces the overall pump size, cost and handling complexity.
Larger Air/Fluid Gaps
Conventional induction motors, which dominate the downhole rotating equipment, achieve 85 percent efficiency with an approximately 1 mm air gap. PM machines achieve efficiencies over 90 percent with an approximately 13mm air gap. This is due to the high magnetic field created by permanent magnets vs. energizing coils. This ability to operate with large clearances between the rotor and stator allows for larger air/fluid gaps for PM machines, reducing fluid drag losses and enabling the capability of canning or sealing the motor from the environment in production fluid flooded applications.
Lower Heat Dissipation
Magnetic losses are generated when magnetic field variations cause losses in magnetically and/or electrically conductive materials. The high strength magnets positioned on the rotor that provide the magnetic field (vs. coils that need to be energized) in a PM machine serve to minimize any field variations to which the components on the rotor may be exposed. Thus, there is very minimal rotor heating on a PM machine, which is one of the reasons why PM machines have such high efficiencies. This factor alone provides an approximately 10 percent efficiency increase in favor of PM motors over induction motors for a majority of applications.
But, existing ESP topologies cannot take full advantage of PM motor benefits by replacing an induction rotor with a PM rotor. The limiting factor of delivering HP in ESP applications is thermal management (i.e. the removal of heat from the motor). In conventional ESP motors, which operate in liquid filled chambers that have a balanced pressure with the environment, the friction losses generate more heat than the motor electromagnetic losses. This heat must be removed from the motor and put into the pumped fluid, where the only path is through the housing over the stator. Therefore, replacing an induction motor rotor with a PM rotor will not have significant value because the same amount of friction losses are generated with the PM rotor, and the same poor conduction path for heat to the production fluid is present. In order to take full advantage of PM motors, specifically for downhole applications, a system level approach is required.
Isolation from Harsh Environments
In order to deliver the full potential of PM motors, Upwing Energy has developed a unique motor and hydraulic architecture for both the SCS and MDS.
For the SCS, the complete motor section is hermetically sealed to isolate the motor from the outside environment. The motor operates in an inert gas at atmospheric pressure with large gaps, and therefore, does not generate significant friction losses even at 50,000rpm. The torque is transferred from the motor to the compressor via a magnetic coupling. The heat, which is mostly generated in the stator component of the motor, is cooled through the flow of the gas over the surface of the sealed chamber.
For the MDS, the PM motor assemblies are completely canned. The stator and rotor are independently hermetically sealed and operate with a large clearance. The PM motor is integrated such that the separate rotor can be retracted along with the pump section independent of the stator, as the stator is part of the permanent completion. This flow through architecture not only provides cost effective rotor retrievability without the need to affect any electrical connections or the permanent completion downhole, but also provides direct access to the production fluid as a coolant medium to the maximum motor surface area to increase the heat dissipation. Upwing MDS PM motors are on the order of five times the energy density of conventional induction motors due to their open architecture that is able to dissipate more heat and to run to higher speeds (over 6,000rpm).
Combining all the unique features of PM motors along with Upwing’s system level integration ensures the most energy dense, compact and efficient hydraulic system that maximizes reliability, availability and retrievability of downhole rotating equipment.