Application overview
In marine applications, Shaft Electrical Machines (SEM’s) are connected to the vessels propulsion engine and are often required to operate in a multi-function role. They operate as an alternator providing the vessels primary electrical power supply, or as an electric motor providing a short term power boost alongside the propulsion engine, or as an independent method of propulsion, when the engine is out of commission. This article sets out to give an overview of the different modes of operation and the various methods of control.
Fig.1 – A typical ship’s propulsion system layout
Power Take Out (PTO)
The SEM is operating as an alternator, driven from the main propulsion engine, providing the primary power
supply for the vessel electrical systems.
Fig. 2 – Illustration showing PTO mode
Power Take In (PTI)
Here, the SEM operates as a synchronous motor, providing a power boost, alongside the main engine to
increase vessel speed, or allowing the main engine to reduce power, thereby lowering fuel consumption and
wear.
Fig. 3 – Illustration showing PTI mode
Power Take Home (PTH)
Like PTI, here the SEM operates as a synchronous motor. However, this time it provides 100% of the vessels propulsion power. This could be because the main engine has failed or requires critical maintenance or this can be a normal mode of operation, e.g. the main engine requires routine maintenance or has been stopped when the vessel is entering port. Here, SEM needs to have a self-starting capability to run up from zero speed.
Normally, this mode is required few times during the lifespan of the SEM. Therefore, an alternator is adopted
for use as a motor and so will not have the robust rotor design and excitation system needed to produce sufficient torque for starting from zero speed. Consequently, a method of soft-starting has to be applied to the SEM during start up and with AvK products, some modifications are also necessary.
Fig. 4 – Illustration showing PTH mode
Soft-starting methods (PTH mode only)
There are different methods to start the SEM in the PTH mode. The following explains five well-known solutions:
1 Pony motor start
The SEM shaft is connected and driven to speed by a small electric motor, which then disengages once
synchronous speed is achieved.
Fig. 5 – Line diagram showing Pony motor start method
2 Auto-Transformer start
Here, an auto-transformer reduces the voltage supplied to the SEM, thereby restricting the inrush current, whilst still ensuring that sufficient breakaway torque is available to
turn the shaft.
The SEM starts as an asynchronous motor and therefore, requires some modifications to the excitation system of the machine.
Fig. 6 – Line diagram showing Auto-transformer start method
3 Excitation controlled start (single propeller)
Here the inrush current is limited by using a Stator Current limiting feature on the AVR of the auxiliary alternators.
Again, the machine starts asynchronously and so requires some excitation system modifications.
Fig. 7 – Line diagram showing Excitation start method for single SEM
4 Excitation controlled start (twin propeller)
This is similar to 3). However, here the vessel has twin propellers, twin engines and two SEM’s. The Stator
Current limiting capability is present on each SEM, allowing one (operating in PTO mode) to start the second (operating in PTI/PTH mode).
With two identical SEM’s, it is possible to swap the PTO and PTI/PTH modes from one to the other.
Fig. 8 – Line diagram showing Excitation start method for twin SEM
5 VFD start
Here the SEM is controlled through a VFD which ramps up the machine shaft, ensuring the current is limited but the correct amount of torque is delivered. The machine can be started as a synchronous motor. This system can also be applied to twin propeller vessels.
Fig. 9 – Line diagram showing VFD start method
Cummins Generator Technologies have suitable products for all the modes of operation and control methods detailed above, not only for the shaft electrical machines, but also for the auxiliary alternators, as well.
For more information please contact the Applications team