The trailing suction hopper dredger (TSHD) Seine is over 30 years old so failures can inevitably occur or components may exceed their ‘mechanical lifespan’. This includes relays which have been switched on and off so often that they cease to function. Not only does this result in lots of extra work onboard for the engineers, it may also affect the dredging process.
After consultation with Piet Brouwer electrotechnology service manager Piet Roskam, the decision was taken to replace the old relays with soft starters. These are modern star-delta switches that limit the current input of electric motors. In these soft starters the electronics ensure that the voltage is limited (similar to the function of a dimmer for a lamp at home). With fast electronic switches (triacs or thyristors) only part of the sine is passed on. As a result the engine is only provided with voltage and, therefore, energy for a short time period, while the energy is lower than for a full sine.
When switched on, the voltage is approximately 30% of 400V = 120V. This is comparable to a star-delta switch, which puts 230V on the motor windings. Due to the low voltage, the input current is also substantially less: it has been reduced to approximately 1.5 x I nominal.
The difference with a star-delta switch is that the voltage is now slowly increased to 400V on a timeframe that is adjustable from three to twenty seconds. This ensures a reliable prevention of both mechanical loads and short-term voltage drops. Due to the continuous control of the feed, the motor is adapted to the load behaviour of the equipment. Mechanical tools are started up in a controlled way, their behaviour is positively affected and their lifespan is extended. Additionally the generator is spared as well, with no high input currents and fewer voltage drops. In short: the soft input/output spares all connected components, both in front of and behind the soft starter, ensuring a smooth production process.
Based on these data, a service team on site in Turkey replaced several old relays with new soft starters: two for the general hydraulic pumps of 92kW, two for the hydraulic winches of 132kW and two for the jet pumps of 400kW. The relays were disassembled onboard and replaced by new soft starters that were integrated in the existing system, complete with new control current and the former starter conditions. The system still functions in the same, familiar way for the crew, only in a much more reliable configuration.