Page 42: of Offshore Engineer Magazine (May/Jun 2026)

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TECH FEATURE MARINE ELECTRICAL SYSTEMS

To achieve the low output speed commonly required issues could be of signifcant concern.

of propulsion motors, a higher quantity of motor poles is Though any motor might be seriously damaged by over- needed. For instance, while a four-pole machine utilizing heating, such as with an undetected loss of the cooling medi- 60Hz AC power will output about 1800 RPM, a 16-pole um, PM motors have the unique weakness that such an event motor will output roughly 450 RPM. So, another advan- could demagnetize the expensive permanent magnets. Finally, tage for PMSMs is their relative ease in increasing the pole a safety caution is in order about the incredible strength of count vs. an induction machine. The latter suffer an extra these magnets. At very infrequent intervals during installa- effciency penalty with higher pole counts. If one adds a re- tion or later maintenance, the rotor containing these magnets duction gear to the lower pole induction motor, that adds may be partially or fully exposed to personnel. It is important its own ineffciency to the drive train. Either way, one can to avoid approaching these motors with magnetic materials, expect to suffer roughly an additional 3% effciency loss for instance having steel tools, keys and similar items in one's for the induction propulsion motor. pockets, or if you have a pacemaker or surgically implanted metal. One must also avoid rotating the rotor relative to the

TRADE-OFFS AND PRACTICAL stator or voltage will be generated on exposed conductors.

CONSIDERATIONS

All things of course have trade-offs and there are chal- MAKING INFORMED DECISIONS IN A

COMPLEX LANDSCAPE lenges with PMSMs. The biggest one may be cost. The typical magnets used are a neodymium-iron-boron type It is hoped that the discussion here on key decision and they aren't cheap. Along with the rare earth neodym- points for a diesel-electric system will make the reader a ium, dysprosium and terbium are often added for heat little more comfortable in tackling such issues. There are resistance. The term rare earth is a bit of a misnomer, at obviously advantages and disadvantages to such complex least compared to the truly rare platinum group minerals. propulsion systems. But with the proper amount of time

Nevertheless, they can be toxic to process. China possesses and effort, an informed decision can be made that will by far the large reserves in the world and so supply chain beneft those making such investments.

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