## 2 System Description
### Power Take-Off (PTO)  
The PTO system is realized as a winch and rope system, as illustrated in Figure 6. The generator can only produce power during upwards motion, and has to operate in motoring mode during downwards motion to maintain rope tension. The detailed control principle utilized by FO is described in our previous publications [10,11]. The generator is a high performance permanent magnet machine manufactured by Siemens(r) and is designed for industrial servo applications. It has high torque output and low inertia, and is suitable for direct-drive applications with low gear ratio or entirely without a gearbox. The generator is powered through a full bridge converter that gives full control of the generator torque with high precision and quick response, which in total gives a very powerful package with high performance that is well suited for the reciprocal wave motions. The electrical configuration of the generator and converter can be seen in Figure 7(b).  
Figure 6: Principal sketch of the WEC.  
Figure 7: Array design. (**a**) Array configuration; (**b**) Electrical configuration.  
As discussed earlier, the unidirectional production principle increases the peak-to-average ratio with a factor of two as the production becomes asymmetrical. The consequent reduction in power quality transfers to the generator and the converter, reducing the utilization factor on these components to 50%. Symmetrical production may be achieved by implementing a separate spring system that produces enough pre-tension to tense the rope and also pull the generator at full power during the downward motion. As this will be an expensive and powerful system, the costs must be carefully weighed against the increased generator and converter costs of the asymmetrical system. Our earlier prototype Bolt(r), shown in Figure 4 is designed with the symmetrical option based on a hydraulic spring system.  
When cost-optimizing the PTO, the rated production force turns out to be the design property with the highest cost impact, since it governs the strength and the physical size for most of the mechanical components. The rated power has the second highest cost impact and influences mostly the generator and the converter. There is also a high potential for cost savings on the inverter by extensive utilization of field weakening control, as described in our previous work [11]. The third driving cost parameter is the ability to handle mechanical inertia by the implemented control system. As the system has to follow the reciprocal motion of the waves, the dynamical drive train response is very important. Also, a high inertia will cause high power fluctuations due to change in kinetic energy. The inertia is dominated by the generator and can be reduced by moving towards more expensive direct drive generators with high torque and low speed. Thus, there will be a cost balance between good control and low inertia on the generator. Hence, it is important to keep in mind that the power rating of a PTO alone is not sufficient to give a clear picture of the PTO cost. However, to simplify further analysis, power rating is used as the key performance indicator in the following sections.