Rectifiers & Inverters

2 February 2026 3 min read Electrical, Latest Articles

6-Pulse, 12-Pulse, AFE — Harmonics, Heat, and System Instability

Introduction — DC systems destabilise AC ships

Rectifiers and inverters sit at the boundary between AC and DC systems. They are essential for:

battery charging
UPS systems
propulsion drives
thrusters
automation power

They are also one of the largest sources of harmonic pollution onboard.

Many “mysterious” voltage problems start here.

Rectifier basics — what pulse number actually means

6-pulse rectifiers

simplest and most common
high harmonic distortion
inject 5th, 7th, 11th harmonics

12-pulse rectifiers

phase-shifted transformer feeds
significantly reduced harmonics
larger footprint and cost

Active Front End (AFE)

actively controls current waveform
near-sinusoidal current
regenerative capability
high complexity and control dependence

ETO reality:

Pulse number is not theory — it is heat, voltage distortion, and transformer stress.

🔧 Regulatory anchors (explicit)

IEC 60092-201 / 401

Require:

electrical installations to avoid harmful interference
equipment compatibility within ship systems

IEC 61800 (drives) & IEC 62040 (UPS)

Referenced by Class for:

harmonic limits
EMC behaviour
thermal performance

SOLAS Chapter II-1, Regulation 45

Harmonic-induced heating and instability are considered fire and shock hazards.

Harmonics — how rectifiers quietly damage ships

Harmonics cause:

transformer overheating
neutral conductor overload
protection mis-operation
voltage waveform distortion
nuisance alarms and trips

They rarely cause immediate failure — instead they reduce margin everywhere.

🔻 Real-World Case: Offshore Vessel — UPS Rectifier Harmonic Overload

An offshore support vessel experienced repeated unexplained transformer overheating.

Root cause:

large 6-pulse UPS rectifier installed
transformer not derated for harmonics
insulation life reduced drastically
replacement required within 3 years

The UPS never failed.
The system around it did.

Inverters — stability depends on control power

Inverters are highly sensitive to:

DC voltage stability
control power integrity
cooling effectiveness
software configuration

When inverters fail, they often:

shut down cleanly
remove critical loads
destabilise PMS logic

In DP vessels, inverter failure can mean loss of position in seconds.

Active Front End (AFE): promise and risk

AFE systems:

reduce harmonics
improve efficiency
allow regeneration

But they:

rely heavily on control software
require clean reference signals
can trip unpredictably if parameters drift

AFE is powerful — and unforgiving.

Professional ETO mindset

A professional ETO never asks:

“Is the rectifier working?”

They ask:

What is it doing to the rest of the system?
Where is the heat going?
What margins has it silently removed?

Rectifiers and inverters don’t break ships directly — they weaken them until something else does.

Knowledge to Carry Forward

AC ships increasingly depend on DC interfaces. Every rectifier and inverter reshapes current, heat, and stability.

If harmonic content, cooling, and control integrity are not actively managed, these devices will quietly shorten the life — and resilience — of the entire power system.