Page 27: of Marine News Magazine (July 2026)

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ni? cantly lower arc ? ash energies for personnel safety, the of propulsion motors, a higher quantity of motor poles is brief peak in short circuit current typically exceeds that needed. For instance, while a four-pole machine utilizing of an AC propulsion bus. High-speed fuses and electronic 60Hz AC power will output about 1800 RPM, a 16-pole bus ties are needed with circuit breakers comparably too motor will output roughly 450 RPM. So, another advan- slow for the much faster peak current. tage for PMSMs is their relative ease in increasing the pole

Also, inverters supplying an AC ship service switch- count vs. an induction machine. The latter suffer an extra board can’t supply the same short circuit current as an AC ef? ciency penalty with higher pole counts. If one adds a re- propulsion switchboard so coordination of downstream duction gear to the lower pole induction motor, that adds circuit breakers can be a bit more challenging. its own inef? ciency to the drive train. Either way, one can expect to suffer roughly an additional 3% ef? ciency loss

The Evolution of Propulsion Motors for the induction propulsion motor.

There’s another component in electrical propulsion sys- tems undergoing major changes. That’s with the typically Trade-Offs and Practical Considerations largest and most expensive electrical component, the pro- All things of course have trade-offs and there are challenges pulsion motor. The DC motor was dominant until perhaps with PMSMs. The biggest one may be cost. The typical mag- the 1980’s when separately excited synchronous motors nets used are a neodymium-iron-boron type and they aren’t took over. Then about 20 years later, induction motors be- cheap. Along with the rare earth neodymium, dysprosium gan to dominate. Nowadays, the permanent magnet syn- and terbium are often added for heat resistance. The term rare chronous motor (PMSM) is looking to take the lead. earth is a bit of a misnomer, at least compared to the truly rare

To meet the strictest underwater radiated noise (URN) platinum group minerals. Nevertheless, they can be toxic to requirements ever applied to a vessel of this class, Steerprop process. China possesses by far the large reserves in the world will supply Germany’s new Polarstern 2 icebreaking re- and so supply chain issues could be of signi? cant concern.

search vessel with PMSM units delivering 9 MW through Though any motor might be seriously damaged by a 15.7-foot diameter propeller. Not to be outdone, Kongs- overheating, such as with an undetected loss of the cool- berg developed their incredible high-ef? ciency, low-noise ing medium, PM motors have the unique weakness that rim-drive thrusters with the permanent magnets integrat- such an event could demagnetize the expensive permanent ed right into the propeller rim. ABB has designed and in- magnets. Finally, a safety caution is in order about the in- stalled permanent magnet shaft generators that save up to credible strength of these magnets. At very infrequent in- 20% on footprint over other AC machine types. Schottel, tervals during installation or later maintenance, the rotor

MAN/Ramme, DRS and many others have also developed containing these magnets may be partially or fully exposed

PMSM propulsion applications. to personnel. It is important to avoid approaching these motors with magnetic materials, for instance having steel

Performance Advantages of PMSMs tools, keys and similar items in one’s pockets, or if you

PMSMs have considerable advantages when it comes to have a pacemaker or surgically implanted metal. One must power and torque density over induction motors. One can also avoid rotating the rotor relative to the stator or voltage typically expect a 20-50% savings in volume and weight will be generated on exposed conductors.

that can lead to installation or operating cost savings.

They also generate less noise and vibration, thanks to their Making Informed Decisions in a Complex Landscape simpler design. Induction motors in large sizes typically It is hoped that the discussion here on key decision have ef? ciencies in the 94-95% range. For the same size points for a diesel-electric system will make the reader a and RPM, PM motors will typically achieve 97-98% or little more comfortable in tackling such issues. There are an average of 3% in savings. Further, PMSMs ef? ciency obviously advantages and disadvantages to such complex advantages increase at slower speeds where the fall off in propulsion systems. But with the proper amount of time ef? ciency with induction motors is steeper. and effort, an informed decision can be made that will

To achieve the low output speed commonly required bene? t those making such investments.

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