A vessel’s electrical system operates under conditions that land-based facilities rarely experience. Power generation capacity is limited, loads change constantly, and every major motor start might affect the stability of the entire onboard network.
Nothing places more stress on a marine electrical system like a propulsion and thruster motor.
Whether powering tunnel thrusters, azimuth drives, or dynamic positioning (DP) systems, these large motors demand extremely high starting current. Without proper control, a single motor start can create severe voltage dips, disrupt sensitive onboard systems, and place unnecessary mechanical stress on the propulsion drivetrain.
This is why soft starters have become a critical part of modern marine propulsion systems.
Unlike shore-based industrial facilities that are usually connected to large utility grids, ships operate with a finite onboard power supply generated by a limited number of diesel generators. Every large motor start draws directly from that available capacity.
When a large thruster motor starts direct-on-line (DOL), it can draw 6 to 8 times its full load current. For a 6.6 kV, 2 MW thruster motor, that represents a sudden and significant demand on the vessel’s electrical system.
That voltage dip does not remain isolated to the motor itself. It spreads across the ship’s electrical network almost immediately.
Figure 1: Comparison of direct-on-line starting versus soft starter-controlled motor starting on a marine electrical network.
Sensitive navigation and communication systems may log faults or reset. Variable speed drives can trip on undervoltage conditions. Lighting systems may flicker or dim. On offshore vessels, cruise ships, naval vessels, and DP-classed ships, even a brief power disturbance can affect critical operations.
Those phenomena require the electrical designer to increase the power capacity of the generation system.
This is exactly why major classification societies such as DNV, ABS, Bureau Veritas, and Lloyd’s Register impose strict limits on allowable voltage dips during motor starting.
Soft starters are one of the most effective engineering solutions for maintaining electrical stability while starting large marine motors.
Marine soft starters control the voltage applied to the motor during startup using thyristor-based power control technology.
Instead of applying full line voltage instantly, the soft starter gradually ramps voltage to the motor over a configurable acceleration period. This controlled ramp dramatically reduces motor inrush current and minimizes voltage disturbance across the vessel’s electrical network.
In most marine applications, starting current can typically be limited to approximately 2.5
to 3.5 times full load current, compared to the 6 to 8 times full load current commonly seen with direct-on-line starting.
|
Starting Method |
Typical Starting Current |
Voltage Dip Impact |
Mechanical Stress |
|
Direct on-line (DOL) |
6 to 8 x full load current |
High |
High |
|
Soft starter |
2.5 to 3.5 x full load current |
Reduced |
Reduced |
The result is a far more stable onboard electrical system.
The vessel’s generators have time to respond and stabilize. Voltage dips are reduced significantly, and other onboard systems continue operating normally during motor startup.
For low voltage marine systems such as pumps, compressors, cooling systems, ballast systems, and smaller thrusters, the RVS-DN provides reliable motor starting and stopping control in demanding onboard environments.
For larger propulsion motors and medium voltage thruster applications, the HRVS-DN delivers advanced starting control designed specifically for high-power marine applications.
Both platforms include generator-friendly starting functionality that helps reduce stress on the vessel’s power generation system during motor acceleration.
The benefits of a marine soft starter extend far beyond electrical stability.
Direct-on-line starting applies full starting torque almost instantly. That sudden torque shock places significant stress on the entire mechanical drivetrain, including couplings, gearboxes, shafts, bearings, and propeller systems.
Over time, repeated shock loading accelerates mechanical wear and contributes to:
• Coupling fatigue
• Gearbox wear
• Shaft stress concentration
• Bearing damage
• Increased maintenance intervals
A soft starter eliminates this sudden mechanical impact by gradually increasing motor torque throughout the acceleration ramp.
Figure 2: Sequential thruster starting reduces cumulative voltage dips during dynamic positioning operations.
For large marine thrusters, acceleration ramps are often configured between 10 and 30 seconds, allowing the drivetrain to load progressively and smoothly.
The result is reduced mechanical stress, improved equipment longevity, and lower maintenance requirements throughout the propulsion system.
Marine systems face another challenge during stopping: hydraulic shock.
When large pumps or thruster systems stop abruptly, the sudden change in fluid velocity can create water hammer pressure waves throughout the piping system. On vessels with long pipe runs, these pressure spikes can place severe stress on valves, joints, and pipework.
The soft stop function of a soft starter gradually reduces motor torque during shutdown, allowing fluid flow to decelerate smoothly.
This controlled stopping process helps minimize:
• Water hammer
• Hydraulic shock
• Pipe stress
• Valve wear
• Mechanical rebound within the drivetrain
For ballast systems, cooling water systems, fire suppression systems, and propulsion applications, controlled stopping can significantly improve system reliability.
Many offshore vessels, cruise ships, and heavy-lift vessels operate multiple thrusters simultaneously during manoeuvring or dynamic positioning operations.
Managing the startup of several large motors from a limited generator supply requires careful coordination.
Solcon-IGEL multi-motor starting solutions allow sequential motor starting with optimised timing between starts, giving generator voltage time to recover before the next thruster is brought online.
The synchronous multi-start/stop system supports up to nine motors from a single controller, helping reduce switchgear footprint in space-constrained engine rooms while maintaining precise control over motor starting profiles.
In marine applications, motor protection is not only about equipment longevity. It is also about operational continuity, safety, and compliance with classification society requirements.
Figure 3: Integrated marine motor protection architecture using soft starters and dedicated protection relays.
Solcon-IGEL marine soft starters include comprehensive protection features designed for demanding marine environments, including:
• Thermal overload protection
• Phase loss and phase imbalance protection
• Stall and locked rotor detection
• Under/overvoltage protection
• Under/over-frequency protection
• Shorted SCR detection
• Thermistor (PTC/RTD) motor temperature monitoring
• Fault recording with real-time event logging
Motor insulation protection (MIP6)
For applications requiring an additional independent protection layer, the MPS3000 / MIP6 integrates directly with the soft starter to support advanced motor protection and class compliance requirements.
Solcon-IGEL marine soft starters are available with approvals from major international marine classification societies, including:
• DNV
• ABS
• Bureau Veritas
• Lloyd’s Register
• Korean Register
• ClassNK
Products are also available with CE, UL, and ATEX approvals for offshore and hazardous-area applications.
Application-specific compliance documentation and technical support are available for shipyards, consultants, system integrators, and marine engineering teams.
Marine propulsion systems operate in some of the world’s most demanding electrical environments. Reliable motor starting is not only about protecting the motor itself, but also about maintaining stability across the entire vessel.
Solcon-IGEL soft starters are used worldwide across offshore vessels, cruise ships, heavy-lift carriers, naval applications, and specialized marine platforms where electrical reliability and propulsion continuity are critical.
Planning a marine propulsion or thruster upgrade?
Tell us your vessel type, thruster motor rating, supply voltage, and class requirements, and the Solcon-IGEL engineering team will recommend the right solution for your application.