Machine Design

Performance under pressure

Electric motors powering undersea remotely operated vehicles (ROVs), such as the Quest ROV from Alstom GEC, Davis, Calif., must withstand pressures of 10,000 psi at depths to 20,000 ft below sea level.

This environment demands that motors be encapsulated in a very strong material with high thermal conductivity. That's where LNP Engineering Plastics of Exton, Pa., comes in.

Thermal conductivity of a motor's encasements is essential for preventing heat buildup and potential burn out. LNP's Konduit thermally conductive composites are said to provide significantly greater thermal conductivity than typical unfilled and reinforced thermoplastics.

In the Quest ROV, 10 Konduit-encased electric motors replace hydraulic thrusters.

"The motors must withstand approximately 5 tons of pressure," according to Griffith Neal, founder of the San Francisco-based Encap Technologies, an engineering consultancy contracted by Alstom to help develop the ROV's submersible motor. "Conventional plastics don't transfer the heat generated in the windings," he says. "In 7,500-W motors such as those in the ROV, the wire will melt if packaging material does not dissipate heat."

Konduit is claimed to be 10 to 50 times more thermally conductive than typical unfilled and reinforced thermoplastics. Another key benefit is its low coefficient of linear thermal expansion (CLTE). According to Neal, Konduit is the only material that matches the CLTE performance of metals over the molding temperature range. To successfully eliminate plastic micro-fissures caused by differential contraction on cooling, LNP tailored a resin with a specific CLTE performance. Matching the CLTEs eliminates cracking from thermal shock that can hinder plastic application development in larger parts.

Three other thermoplastic compounds from LNP are used in the Quest ROV including Verton long-fiber composites in the nozzle flanges that protect the motor, bearings, and impeller; Lubricomp-lubricated composites in hydrodynamic bearings; and Thermocomp-reinforced composites used in the bearing flanges.

Ten electric thrusters on each ROV are oriented in different planes to give full control of the vehicle's movement. Most ring motor parts are made from engineering thermoplastics. Both the magnets in the rotor and the windings in the stator are completely encapsulated in Konduit thermally conductive composites to both protect it from water and carry heat away from the windings.
TAGS: Hydraulics
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