Energy efficiency is the name of the shipping game these days. In addition to lowering operational costs, increased efficiency often translates to fewer emissions and can mean better compliance with environmental legislation. Over the last few decades, shaft generator systems have become a more common option to help ship owners and operators achieve profitability in a sea of fierce competition and keep those waters cleaner for future generations.
Shaft generators on board ships are driven by the main engine to supply power to the mains. The mains have to be supplied with constant voltage and frequency by the shaft generator whilst the speed of the main engine changes, i.e. when the vessel travels at different speeds or if the propeller speed strongly varies in heavy seas.
On ships with fixed pitch propellers, the speed is set via the propeller speed. If using controllable pitch propellers, the shaft speed and the propeller pitch are adjusted simultaneously in order to achieve optimum propeller efficiency in this so-called combinator mode. Even with this type of propeller, it is thus economical to use shaft generator systems with frequency converter for variable speed in order to permit combinator mode from pier to pier.
Operation Principles
PTO
Shaft generator systems with frequency converters supply a three-phase current of constant voltage and frequency to the mains at variable main engine speeds. The useful speed range of the shaft generator can be defined from the ship’s operational requirements. For example, it is possible to select a shaft generator speed range of 60% to 100% of the main engine speed with a constant system output.
All shaft generator systems have one thing in common: The active power required for the mains is generated by the main engine. However, the shaft generator can be arranged and driven by the main engine in various configurations.
- The shaft generator is installed in the shaft line between the low-speed main diesel engine and the propeller. This configuration has a large air gap between the stator and the rotor, and without additional bearings has proven very successful as the most frequently used configuration. It is particularly simple and sturdy and requires little maintenance. Contrary to other arrangements, torsional vibration problems relating to the design and operation are not anticipated with this configuration.
- The shaft generator is operated via a power take-off by a reduction gear which is arranged between the medium-speed main diesel engine and the propeller. This is the usual and appropriate configuration for medium-speed main diesel engines.
- The shaft generator is operated via a tunnel gear unit installed in the shaft line between the low-speed main diesel engine and the propeller.
PTO/PTI/PTH
For additional shaft motor operation the diode rectifier is replaced by a PWM rectifier similar to the PWM inverter and the system control is adapted accordingly. With this configuration motor operation is provided as follows:
- Booster propulsion operation parallel to the main engine to enlarge the propulsion power and the ship’s speed
- Take home respectively take away propulsion operation as emergency propulsion drive without main engine in case of a malfunction of the main engine. For this mode the main engine has to be declutched from the propeller shaft
- The “Power Take In” mode is also possible to be fed by the main switchboard, which is supplied by auxiliary generator(s) and independent from a WHRS. Summarized this allows a maximum degree of freedom in power distribution between different units by using only one shaft generator converter.
PTO/PTI/PTH + Hybrid
For shore connection with 50 Hz or 60 Hz the frequency converter of the shaft generator system can be used to adapt the shore frequency to the ship’s mains frequency. For that purpose the connection between shaft generator and converter is switched off and the converter is supplied (via cable reel, circuit breaker and transformer) from the shore. With this configuration the ship is full flexible for shore connection worldwide.
PTO/PTI/PTH + Thruster
One of the latest features is the integration of the shaft generator converter with thruster. In conjunction with our partners, our shaft generator system further enhances overall operational efficiency and cost savings for our customers.
For more details please see below file: