Page 41: of Offshore Engineer Magazine (May/Jun 2026)

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DC GRID SYSTEMS: THE EVOLUTION OF PROPULSION MOTORS

A SHIFT IN PROPULSION ARCHITECTURE There's another component in electrical propulsion sys-

A DC grid retains the use of standard AC generators tems undergoing major changes. That's with the typically and AC motors. However, it replaces both the AC pro- largest and most expensive electrical component, the pro- pulsion switchboards and propulsion Variable Frequency pulsion motor. The DC motor was dominant until per-

Drives (VFDs) with a single DC grid "switchboard". With haps the 1980's when separately excited synchronous mo- the main propulsion bus now DC, generators connect tors took over. Then about 20 years later, induction motors through rectifers located in the DC grid line-up which began to dominate. Nowadays, the permanent magnet also includes inverters feeding AC propulsion motors and synchronous motor (PMSM) is looking to take the lead.

an AC ship service switchboard. To meet the strictest underwater radiated noise (URN) re-

There are several advantages. The DC grid is composed quirements ever applied to a vessel of this class, Steerprop will of the same rectifers and inverters that are the front and supply Germany’s new Polarstern 2 icebreaking research ves- back halves of the VFDs in a standard diesel-electric. Yet, sel with PMSM units delivering 9 MW through a 15.7-foot large AC propulsion switchboards and their circuit break- diameter propeller. Not to be outdone, Kongsberg developed ers are no longer needed. The large phase-shifting trans- their incredible high-effciency, low-noise rim-drive thrusters formers from older systems can also be eliminated with the with the permanent magnets integrated right into the propel- potential for considerable volume and weight savings. The ler rim. ABB has designed and installed permanent magnet generators can operate at variable speed improving fuel ef- shaft generators that save up to 20% on footprint over other fciency and can also come online faster. There is no fre- AC machine types. Schottel, MAN/Ramme, DRS and many quency to synchronize; the rectifed generator voltage just others have also developed PMSM propulsion applications.

needs to match the DC bus voltage.

PERFORMANCE ADVANTAGES

CHALLENGES AND CONSIDERATIONS OF PMSMS

The primary disadvantage with this new approach is the PMSMs have considerable advantages when it comes to

DC short circuit current. While DC grids can achieve sig- power and torque density over induction motors. One can nifcantly lower arc fash energies for personnel safety, the typically expect a 20-50% savings in volume and weight brief peak in short circuit current typically exceeds that that can lead to installation or operating cost savings. of an AC propulsion bus. High-speed fuses and electronic They also generate less noise and vibration, thanks to their bus ties are needed with circuit breakers comparably too simpler design. Induction motors in large sizes typically slow for the much faster peak current. have effciencies in the 94-95% range. For the same size

Also, inverters supplying an AC ship service switch- and RPM, PM motors will typically achieve 97-98% or board can’t supply the same short circuit current as an AC an average of 3% in savings. Further, PMSMs effciency propulsion switchboard so coordination of downstream advantages increase at slower speeds where the fall off in circuit breakers can be a bit more challenging. effciency with induction motors is steeper.

MAY/JUNE 2026 OFFSHORE ENGINEER 41