§ 01 · Scope of a rebuild
Our scope is the alternator end — the end that produces electricity. The diesel side (engine, fuel system, cooling) is the marine-diesel mechanic's. We work alongside Northern Lights service and Cummins Eastern Power on the diesel side; the engine bay smells of two trades when this work is happening.
What we strip and replace, every rebuild: the AVR, the carbon brushes and brush ring, both bearings, the rotor seal, the slip ring (cleaned, undercut if needed), and every terminal in the output box. What we test, every rebuild: the rotor windings (insulation resistance > 100 MΩ at 500 V), the stator windings (same), the diodes in the rotating rectifier (each individually), and the slip-ring runout (≤ 0.04 mm).
§ 02 · The AVR
The Automatic Voltage Regulator is the part that ages and fails. The original AVR on a Northern Lights M99 is the Stamford SX440; on a Phasor it is the Marathon DVR2000. We replace both with current production from the same OEM — we do not retrofit a third-party AVR onto a working alternator unless the OEM has discontinued the part, in which case we use Marelli or Mecc Alte equivalents.
The replacement AVR is set up at the bench against a regulated 50 Hz / 60 Hz reference, with the underfrequency rolloff (UFRO) configured per the alternator's nameplate. We measure voltage rise from no-load to 100% in 5% steps and we record the curve. The regulation across the operating band sits at ±0.5%.
§ 03 · Bearings & brushes
Both bearings come out, both go back in new. We use the OEM bearing or the SKF cross-reference. We lubricate with the OEM-specified grease — not "marine grease" generic. We hand-rotate before energizing.
Brush rings come from the OEM. We never run brush rings beyond 6mm of remaining length; the wear is non-linear past that point and the slip-ring takes the damage.
§ 04 · The load-bank test (the part that proves it)
The rebuilt alternator is mated to its diesel and run on the bench under load. We use an Avtron K975 resistive load-bank (125 kW) and we step the load in 25% increments: 25%, 50%, 75%, 100%, with a one-hour soak at 100%. Voltage and frequency are logged with a Fluke 1750 power-quality analyzer at 1-second intervals. The test report is signed by Dom and bound into the rebuild documentation.
We also do a 110% overload for 30 minutes per IEEE 45.1, on units where the captain expects to occasionally see overload conditions (deck winches with heavy bags, large RSW pull-down events).
§ 05 · Cost & lead time
| Onan / Phasor / small Northern Lights, 5–25 kW | $3.4k–$6.2k, 4–7 working days |
|---|---|
| Northern Lights M-series, 25–80 kW | $5.8k–$11k, 7–12 working days |
| Larger sets, 80–125 kW | $10k–$18k, 10–18 working days |
The above are alternator-end rebuilds. Diesel work, governor work, and major overhauls are coordinated with the marine-diesel shop and quoted separately.
F/V Cape Cod Tradition's Northern Lights M99C13 came in to the bench in June 2025 with a slow voltage decay under load. Diagnosis: aged AVR, glazed slip-ring, one of two stator-winding insulation values low. We rebuilt the alternator end, witnessed the load-bank to 80 kW for an hour, and the unit has run cleanly through three trips since.
Cross-references: P-04 (switchboards) when the genset feeds a board being upgraded, P-08 (shipyard) for genset work coordinated with a refit.
Sources & further reading
- IEEE Standards Association. IEEE 45.1-2017 — Marine power systems on shipboard.
- Northern Lights. Marine generator product line.
- Phasor Marine. Phasor Marine generators.
- Cummins Onan. Onan marine generator service.
- Stamford-AvK. SX440 AVR documentation.
- Avtron Aerospace. Resistive load banks.
- NEMA. MG 1 — Motors and Generators.
- U.S. Coast Guard. Marine Engineering Publications, Vol. 3 — Vessel Inspection.