Acushnet Marine ElectricEST. 2008 · MA #ME-12740
Hull #3812F/V Cape Cod Tradition2025-06

An aged AVR, a glazed slip-ring, one low stator winding.

A Northern Lights M99C13 (80 kW) on a 71-foot groundfish dragger had been showing slow voltage decay under load for three weeks before the captain called us. The diagnosis was the predictable one: aged AVR, glazed slip-ring, and one stator-winding insulation reading just under threshold. We pulled the alternator end to the bench, rebuilt it, witnessed the load-bank, and put it back. Three trips of clean running since.

Length / hull
71 ft / steel
Builder / year
Eastern Shipyard, Panama City, FL / 1989
Plates worked
P-05
Days at the bench
9 working days
F/V Cape Cod Tradition, a 71-foot blue-hulled steel groundfish dragger, with a Northern Lights generator visible through the open engine-room hatch.
Hull #3812.a · Cape Cod Tradition at sunset, Pier 3

§ 01 · The vessel

F/V Cape Cod Tradition is a 71-foot steel groundfish dragger built at Eastern Shipbuilding in Panama City, Florida in 1989. She fishes haddock, cod, pollock, and flounder on Georges Bank and Stellwagen Bank; main is a Caterpillar 3406B (375 hp); ship's service is a Northern Lights M99C13 (80 kW, 60 Hz, 1800 rpm, 480/277 V wye).

§ 02 · Diagnosis

The captain reported a slow voltage sag at the panel under heavy load — particularly when the trawl winches and the RSW pull-down ran simultaneously. The voltage would dip from a nominal 480 to about 462 and stay there for ten or twelve seconds before recovering. The board's analog volt-meter agreed with the captain's read.

We isolated the sag to the alternator end (the diesel was holding 1800 rpm steady on the digital tachometer; the generator output was the variable). We measured the AVR's response time and it was lazy — the SX440 should respond inside 200 ms; this one was 800 ms and getting worse. Insulation resistance on the rotor was clean (320 MΩ); on the stator one of three windings read 84 MΩ — just below the 100 MΩ threshold for healthy.

Recommendation: alternator-end rebuild — AVR, brushes, brush ring, both bearings, slip-ring undercut, and a stator-winding heat-shrink (a varnish-and-bake re-impregnation) for the low-resistance winding.

§ 03 · The rebuild

The alternator was lifted off the diesel and trucked to the yard's rebuild bench. There:

  • The end-bell came off, the rotor was lifted clear.
  • Brush ring out, brushes out, slip-ring inspected. The slip-ring was glazed (a black sheen rather than copper) and showing 0.06 mm runout — just over the 0.04 mm spec. We took it to the lathe and cut 0.15 mm off, restoring round and bringing the runout to 0.02 mm.
  • The new Stamford SX440 AVR was installed and bench-set against a 60 Hz reference. UFRO configured to the alternator's nameplate (47 Hz knee).
  • The low-resistance stator winding was vacuum-pressure-impregnated by a sub-trade (we use Electric Motor Rewinding in Pawtucket); reading came back at 580 MΩ — healthy.
  • New bearings (SKF cross-reference to OEM), new brushes, new brush ring, new rotor seal. Hand-rotated, balanced, re-assembled.

§ 04 · The load-bank test

The rebuilt alternator was mated to a CAT 3406 driver on the bench (we lease a fixed-speed CAT for this work). The Avtron K975 resistive load bank was stepped: 0 → 25% → 50% → 75% → 100% (80 kW), with a 60-minute soak at 100%, then a 110% (88 kW) for 30 minutes per IEEE 45.1.

Voltage and frequency were logged at 1-second intervals on a Fluke 1750 power-quality analyzer. The voltage held within ±0.4% across the band; the frequency held to 60.00 ± 0.05 Hz; the AVR response on a 50% step load was 165 ms. The signed test report is in the boat's binder.

§ 05 · Outcome

The unit went back on the boat on day eight. Day nine was a sea trial: trawl winches and RSW pull-down simultaneously. Voltage held within 0.6% across both events. The captain texted the shop after the second trip in July with one word: "perfect."

A note on stator winding heat-shrinks

The varnish-and-bake re-impregnation on a stator winding is the kind of work the Maine generator-shop trade has been doing since the 1960s. It is not a permanent fix — it gets you another five to seven years out of a winding that would otherwise need replacing — but for a captain who plans to retire the genset in that window, it is the right call. We always tell the captain what they're getting.

Cross-references: Plate P-05 (generator rebuilds).

Sources & further reading

  1. Northern Lights. M99 series specifications.
  2. Stamford-AvK. SX440 AVR datasheet.
  3. Avtron Aerospace. Resistive load banks.
  4. Fluke. 1750 three-phase power-quality recorder.
  5. IEEE. IEEE 45.1-2017.
  6. NOAA. Northeast groundfish.
  7. SKF. Rolling bearing catalog.