Entry & progression
Every convenience aboard is bought with distortion. Drives, converters and switch-mode supplies do beautiful things to the load and ugly things to the supply. The ETO is the only person aboard who sees both ends.
A variable frequency drive does one thing: it takes a fixed-frequency AC supply, rectifies it to DC, and inverts it back to AC at whatever frequency and voltage the motor needs. That is why a thruster can be feathered, why a chilled-water pump can run at 40 per cent instead of cycling on and off, and why a hybrid drive can exist at all.
The three stages, and where each one fails
| Stage | What it does | Typical failure |
|---|---|---|
| Rectifier | AC in → DC | Failed diode/thyristor; ripple on the DC link; blown input fuse |
| DC link | Stores and smooths | Capacitor ageing, bulging, loss of capacitance; holds charge after isolation |
| Inverter | DC → variable-frequency AC | IGBT failure, gate driver fault, output phase loss |
Symptoms map to stages. A drive that trips on undervoltage under load is usually telling you about its supply or its rectifier. A drive that runs but heats the motor and sings is usually telling you about its output. A drive that will not charge its DC link is telling you it has nothing to invert.
WARNING — The DC link is live long after the drive is isolated Every drive of consequence carries a discharge time on its cover — often several minutes. That plate is not a suggestion. Isolate, wait the stated time, then prove dead across the DC terminals before any part of you is inside the enclosure.
The drive is off. The lights are out. The bus is dead. The capacitors do not care.
Harmonics — the bill for all of it
A rectifier does not draw a sine wave. It draws current in pulses, at the peaks. Multiply that by every drive, every UPS, every switch-mode power supply and every LED driver on a yacht, and the current the generators see is no longer a sine wave at all. It is a distorted waveform full of harmonic components — the 5th, the 7th, the 11th, the 13th.
Harmonic current does no useful work. It just does damage:
- Transformers and alternators overheat — losses rise with frequency, so harmonic current heats far more than its RMS value suggests
- Neutral conductors overload — triplen harmonics add rather than cancel in the neutral, which may be sized for balanced load
- Voltage distortion appears on the bus — harmonic current across the source impedance distorts the voltage every other consumer sees
- Sensitive equipment misbehaves — AV, navigation and control gear does not fail cleanly, it fails intermittently
- Nuisance tripping — protection sees a current it was never characterised against
Mitigation is by design, not by hope: line reactors and DC chokes on the drives; multi-pulse rectifiers (12- or 18-pulse) that cancel the lower orders; passive tuned filters; active front ends that draw a near-sinusoidal current; and, above all, generation with enough short-circuit capacity that the distortion has somewhere to go.
Why the yacht is the worst case
A merchant ship has a few big, dumb, constant loads. A yacht has hundreds of small electronic ones, a heavy variable-speed HVAC plant, thrusters, stabilisers, and an owner who will notice a flickering downlight in the master suite before you notice the total harmonic distortion figure on the board.
The complaint arrives as 'the lights are flickering'. The cause is upstream of the light, upstream of the panel, and probably upstream of the deck. Learn to hear the real question inside the reported symptom — that is the whole trade.