DOL, Soft Starters & VFDs — Why Starting Philosophy Shapes Blackout Risk
Introduction — starting a motor is when ships lose control
Most electrical disturbances on ships occur during motor starting, not steady operation. Starting draws high current, collapses voltage, stresses generators, and exposes weaknesses in PMS coordination.
Starting philosophy determines whether:
- a motor starts cleanly
- voltage remains stable
- generators stay online
- protection remains dormant
Poor starting decisions have caused blackouts, loss of propulsion, and DP incidents — even when all equipment was technically healthy.
Direct-On-Line (DOL) starting — simple, brutal, unforgiving
DOL starting applies full line voltage instantly. Starting current is typically 6–8 times rated current, with correspondingly high torque and electrical stress.
On ships with limited short-circuit capacity or minimal spinning reserve, DOL starts can:
- cause voltage collapse
- stall other motors
- trip generators on undervoltage or frequency
- destabilise PMS logic
DOL is reliable only when system stiffness exists. Many ships no longer have that margin.
Soft starters — electrical mercy with thermal consequences
Soft starters reduce inrush current by controlling voltage during startup. This protects the electrical system but increases starting time and motor heating.
On ships, this trade-off matters. Long acceleration times can overheat motors driving high-inertia loads such as pumps or fans. If cooling is marginal, soft starting becomes destructive rather than protective.
Soft starters protect the system — not the motor — unless correctly applied.
Variable Frequency Drives (VFDs) — control at the cost of complexity
VFDs control both voltage and frequency, allowing smooth starts, speed control, and reduced mechanical stress. They also introduce:
- harmonic distortion
- sensitivity to control power
- dependence on DC link stability
- EMC and earthing challenges
VFDs reduce starting stress but increase system interdependence. When a VFD trips, the load disappears instantly — sometimes destabilising the entire power system.
🔧 Regulatory context
IEC 60092-201 requires electrical installations to avoid harmful interference and instability.
SOLAS II-1 Reg 45 treats voltage collapse and electrical fire risk as safety issues, not efficiency issues.
Starting methods that repeatedly destabilise power violate the intent of these regulations, even if equipment is certified.
🔻 Real-World Case: DP Blackout During Thruster Load Change — MV Skandi Aker (2013)
The offshore construction vessel MV Skandi Aker suffered a dynamic positioning blackout during operations in 2013, following a sudden load increase on its thruster system.
What actually happened (documented investigation findings)
- The vessel was operating in DP with reduced online generation to improve fuel efficiency
- A large thruster load change occurred during manoeuvring
- Generator load increased faster than voltage and frequency recovery capability
- Undervoltage and frequency protection activated
- Generators tripped sequentially
- The vessel suffered a total blackout, resulting in loss of position
No single component failed mechanically or electrically.
The thrusters were functional.
The generators were functional.
The protection relays operated exactly as designed.
The failure was systemic — not equipment-based
The investigation concluded that:
- available spinning reserve was insufficient
- starting/load philosophy did not match the operational risk
- protection settings prioritised equipment protection over continuity
- energy optimisation reduced fault tolerance below safe limits
In other words:
Nothing was broken.
The power philosophy was wrong for the operational context.
How professionals choose starting methods
A professional ETO does not ask:
- “What starter is cheapest?”
They ask:
- What happens to voltage when this motor starts?
- How does PMS respond in the first two seconds?
- Which failure mode is safer — delayed start or voltage collapse?
- What is the ship doing when this motor starts?
Starting decisions are navigation decisions, not electrical ones.
Knowledge to Carry Forward
Motor starting is one of the most dangerous electrical moments on a ship. DOL, soft starters, and VFDs each move risk between the motor, the generator, and the system.
Choosing the wrong method does not damage equipment — it removes time. And at sea, loss of time is often loss of control.
Tags
ETO, Motor Starters, DOL Starting, Soft Starters, VFD Marine, Ship Blackout Risk, IEC 60092, Marine Power Stability