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Synchronous motor pitch control shuts down turbines safely

10 March, 2014

Moog has developed a closed-loop pitch-speed control system for wind turbine blades that is based on synchronous motors and is claimed to deliver up to three times more torque from standstill than systems based on induction or synchronous motors, thus ensuring that the blades move a safely to a feathered position.

The self-sensing AC Pitch Axis Servo (PAS) technology can keep the blades under closed-loop speed control even if there is a failure in the motor position feedback. This avoids the high turbine loads that might otherwise result from non-synchronised pitch speeds in the turbine.

The technology uses a new interior permanent magnet synchronous motor that was designed specifically for the application. It is said to ensure maximum efficiency during pitch operations at low speeds, and a peak torque capability of up to 3.5 times the nominal pitch torque, even under grid fault conditions. The motion control concept provides a wide field-weakening operating area to control the feathering speed, even if the grid connection is lost.

Wind turbine pitch control systems have to fulfil two key roles: first, they have to control the turbine’s speed and power when the wind speed exceeds the turbine’s rated values; and, second, they serve as an actuator for the turbine’s braking system.

To stop a three-bladed turbine, all three blades have to be moved into the feathered position. To distribute and balance the loads during feathering, all three pitch axes have to move out of the wind synchronously.

Various open-loop control schemes are already available for the feathering drives if the pitch axis position feedback signal is lost, including hydraulic actuators, DC motor pitch actuators, and AC induction pitch actuators. But all three of these suffer from the same drawback, says Moog: the speed of each axis depends heavily on the load applied by the blade. The company claims that its new control scheme overcomes this disadvantage.

Moog says that its pitch control system will allow turbines to be shut down safely

”The solution was developed to prevent blades from sticking in the event of an encoder failure – the worst-case load scenario for a turbine,” explains Moog’s technology development manager, Dr Tobias Rösmann. “Simulations show an additional benefit: the solution also ensures a synchronised feathering speed.”

Compared to self-sensing control schemes for AC induction or synchronous motors, the new closed-loop speed control system provides up to three times the rated torque from standstill to ensure safe, high-performance movement into the feathering position.

Moog has carried out a case study on a 7MW turbine, working with Fraunhofer IWES in Kassel, Germany, to demonstrate the influence of pitch position incoherence on turbine structural loads during feathering runs.

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