Apparently, Britain leads the way in wind turbine blade testing, and manufacturers across Europe are queuing up to book in their latest designs for static and fatigue testing. The dynamic performance of modern drives ensures that these tests are conducted safely and accuratelyThe new wind turbine blade testing facility at the New and Renewable Energy Centre (NaREC) in Northumberland was opened earlier this year. It is the only facility of its kind in the world that is capable of testing blades of 70m length (and more), and has its own adjacent deep water dock to receive these enormous components from specialist manufacturers across Europe.The tests involve loading the blade uniformly to simulate the effects of wind loading on its structure. This is achieved by fitting up to seven specified load points along the blade, which are connected via steel cables to winches. The winches impose precise tensions on these cables, moving through a series of set points under the control of variable speed drives. The loading of each is calculated using finite element techniques, and each drive has to reach its set point target simultaneously, irrespective of load and the amount of cable take-up.The ac drives chosen for this application are Unidrive SPs from Control Techniques (CT). NaREC's choice was influenced by CT's experience of building drives into test rigs for the Formula 1 and automotive development sectors, as well as the adaptability of the SP platform (network independence, multiple position feedback protocols, customer-programmable application modules and so on).As well as modules for DeviceNet communications, all of the drives are fitted with an SM application module, each programmed via CT's own servo programming tool, SyPTPro. Each drive monitors the 4-20mA analogue signals from its associated cable tension load cell and transmits these via DeviceNet to a Rockwell FlexLogic PLC. Test programme engineer, Rod Morgan stresses the importance of the drives' internal intelligence to this application: Even though the winches move very slowly, the dynamic performance of the drives is critical. For each set point, each drive, running in speed control, calculates the required movement profile - speed, cable take-up and torque - simultaneously communicating with all the other drives, so that each reaches its target at the same time. The full test sequence for ten or more set points up to 150% of the blade's design-rated bend is repeated for the windward and leeward sides, as well as its leading and trailing edges. For most blades, the static test will be repeated again after the fatigue test to monitor the stress changes brought about by the tests. The 0.35-0.7Hz fatigue test is based on an hydraulically actuated test rig, capable of 250kN, developed by NaREC's laboratory in the USA.On the civil engineering side, it is interesting to note that the huge reinforced concrete turbine blade test are fitted with purpose built adapter plates that take about twelve weeks to design, manufacture and install. These are scrapped following the completion of tests for each blade - which probably helps to explain why the cost of taking a blade through a complete test cycle is in the region of £70,000.