Why Your AVR Keeps Failing — Diagnostic Walkthrough
A working ETO's diagnostic walkthrough for repeating AVR faults on shipboard diesel generators: hunting at light load, kVAR imbalance, sensing CT errors and PCB capacitor drift.
Symptom 1: voltage hunts +/- 20 V at light load, settles under load
This is the textbook AVR-stability complaint, and four times out of five the cause is not the AVR itself. It is the sensing input to the AVR — a CT polarity reversal, a sensing PT fuse blown on one phase, or a sensing-loop wire that has been retorqued during a recent service and is now reading the wrong reference. The AVR is doing exactly what it was told to do; it was told the wrong thing.
Before you replace the AVR, prove the sensing signal. Open the AVR terminal cover, identify the sensing input pair (typically labelled V1, V2 or S1, S2 depending on maker), and measure the AC voltage across them with the generator running at light load. Compare against the maker-defined sensing range. If the value is within range, the AVR is being told the truth and the fault is internal to the AVR or in the field excitation loop. If the value is out of range, fix the sensing first and re-test before you touch anything else.
Symptom 2: reverse-power trip during paralleling
When an incoming alternator trips on reverse power within a few seconds of closing the breaker, the AVR is often blamed first because the operator sees a voltage swing on the synchroscope and concludes the AVR is mistuned. In practice, the more common cause is a governor droop setting that does not match the running set's droop, combined with a load-sensing CT on the wrong phase.
Confirm both governors are on the same droop setting (typically 3–5 %, fleet-dependent). Confirm the load-sharing CT on the incoming alternator is reading on the correct phase relative to the bus. A reversed CT on one set will cause that set to consume real power immediately on parallel — which trips the reverse-power relay on the other set. We see this most often after a maintenance period when a CT has been removed and reinstalled.
Symptom 3: kVAR sharing wildly unequal between parallel sets
Two diesel-generator sets running in parallel should share reactive power within roughly 10 % of the average kVAR demand, assuming both AVRs are set to the same kVAR-droop slope (typically 4 % at rated PF). If one set is carrying 80 % of the kVAR and the other is carrying 20 %, the fault is in the kVAR droop loop on one of the AVRs — not in the engine and not in the load.
Diagnose by swapping the AVRs between the two sets (if both are the same maker and model). If the imbalance follows the AVR, you have a drifted droop setting or a failed parallel-CT input on that AVR. If the imbalance stays with the set, the parallel-CT itself or its sensing-loop wiring is at fault. The cost difference between these two outcomes can be a four-figure AVR replacement versus a ten-dollar CT secondary repair.
Symptom 4: 'AVR keeps blowing' — repeated PCB failures on the same set
When a vessel calls and says 'the AVR is gone again', and it is the third unit in twelve months, the AVR is not the root cause. The AVR is a downstream casualty of something else — most commonly a voltage transient from a large consumer (bow thruster, cargo pump, mooring winch) on the same bus, or a poor neutral-earth bonding that lets common-mode noise inject directly into the sensing loop.
Treat repeated AVR failures as a power-quality complaint. Hook up a power-quality analyser (Hioki PW3198 or equivalent) at the alternator terminals for at least one full operating watch. Look for high THD-V, frequency excursions during heavy-consumer starts, and common-mode voltage between the alternator neutral and the hull. We have repaired this class of failure by installing surge suppression at the AVR sensing input, by re-bonding the alternator neutral to a single hull-ground reference, and by isolating the AVR power supply onto a clean source upstream of the noisy bus.
Symptom 5: AVR will not 'build up' on initial start
The classic 'no build-up' fault on a self-excited alternator (Stamford HCI/HCM, Mecc Alte, Marathon) is a loss of residual magnetism in the rotor. The textbook remedy — 'flash the field' with a 12 V battery — works, but the failure repeats unless you identify why the residual magnetism was lost in the first place.
Common causes: prolonged storage with the rotor inside an iron stator (e.g. after a long lay-up), repeated motoring of the alternator during paralleling tests, or a previous PCB failure that left a DC offset on the exciter winding. After flashing the field, run the alternator at no-load for thirty minutes and confirm the AVR holds voltage steadily; then load-step in 25 % increments and watch the response. If the build-up degrades again, you have an exciter rotating-diode failure (loose rotating rectifier, cracked diode mount) and the cure is a rotor pull, not another AVR.
Symptom 6: 'load-step under-shoot' — voltage dips heavily on large-motor start
When a bow thruster or a cargo pump starts and the bus voltage dips more than 15 % even though the AVR is reported as healthy, the AVR's transient response is being held back either by an undersized exciter (rare on properly specified vessels) or by an AVR gain that has been tuned down to mask a stability problem from years ago. Voltage dip is a tuning problem; voltage recovery time is an exciter problem.
Capture the event with a fast-sampling DSO (Yokogawa DLM3000 or equivalent) at the alternator terminals. Measure the depth of the dip (in volts and as a percentage), the recovery time to within 2 % of rated, and the AVR response output simultaneously. If the AVR output saturates immediately but recovery is slow, the exciter is current-limited. If the AVR output does not saturate, there is gain headroom left and the AVR is mistuned — re-tune it per the maker's procedure, in steps, and re-test the dip.
What an experienced ETO carries to an AVR job
A typical AVR-fault attendance is not solved with the AVR alone. The kit list for a single attendance is: a calibrated Fluke 87V multimeter, a four-channel power-quality analyser, a clamp-on AC/DC current probe rated for the exciter current (typically 30 A), a thermal camera for slip rings and rotating diodes, the maker's AVR commissioning manual on paper (yes, paper — battery-flat tablets do not help on the deck), and a known-good identical-spec AVR as a swap-test reference.
The point of carrying the spare is not to install it. The point is to swap it temporarily during the diagnostic, prove whether the fault follows the AVR or the set, and reinstall the original if the diagnosis says the fault is upstream. We have saved customers the cost of an AVR replacement by doing exactly this and identifying a CT secondary fault or a sensing-loop break that would have re-killed the new AVR within hours.
When to call for help
If the AVR has been replaced once with no improvement, do not replace it again. The probability that the second AVR is also defective is small; the probability that the upstream cause is still in place is large. Pause, capture the power-quality signature, and bring in someone with the right instruments and the right body of evidence.
We attend AVR diagnostic calls at all US ports under the standard wizard flow. If the vessel is in port and the AVR is intermittent, the right step is a power-quality capture during normal operation — not a replacement before the data is in hand.
FAQ
- Which AVR makers do we work with?
- Stamford (Cummins), Mecc Alte, Marathon, Leroy-Somer, Basler. Replacements are sourced through approved channels with marine declaration.
- How long does an AVR attendance typically take?
- Diagnostic attendance: four to six hours per set. Replacement and commissioning: a further three to four hours including load-step testing and parallel verification.
- Can we attend during cargo operations?
- Yes — diagnostics that do not require taking the set offline can be performed during normal operation. Replacement and commissioning typically require a planned shutdown of the affected set.
Request an AVR diagnostic attendance
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