Wind power has been used for many years to pump water. In the past, most wind turbines were directly mechanically coupled to the water pump. In recent years the configuration of choice is as pictured above. A wind turbine generator converts wind energy to electrical energy which in turn drives an electric pump. The advantage of this system is that the wind turbine can be located in the best site for wind and the pump can be near the water well or tank.
This paper analyzes a water pumping system consisting of a wind turbine, a permanent magnet synchronous generator, an induction motor and a centrifugal water pump.
The generator and pump motor form a closed electrical system. The generator can operate at variable frequency as dictated by the wind. The pump motor must be capable of responding to that variable frequency electric power input. The paper concerns matching the output characteristic of the wind turbine to the pump's torque characteristic. The output power of the pump varies according to the cube of the generator's frequency.
The authors detail the operation of the system noting several conditions that require controls or protection. They compare the torque verses speed curves of the generator and the pump motor. The system can only operate at intersection points of the speed-torque curves. If for instance, the generator is operating at a high speed (frequency), there may not be enough power for the motor to support the load. The motor will operate more inefficiently and heat will build up in the motor's windings. This could cause the motor to overheat, breaking down its insulation and causing permanent damage. The authors conclude: --- this water pumping system can be optimized if the water pump is designed to be compatible with the wind turbine (the water pump and the loss characteristics should match the maximum power capture characteristic of the wind turbine).
---application of a capacitor at the terminal of the induction motor can change the volts/hertz at the terminal input of the motor, thus changing the torque-speed characteristics.
---The range of operation should be considered in choosing a capacitor value.
---The upper limit of system operating conditions must be considered for each component in the system.
This system of water pumping is effective, but there are non-trivial design considerations. If the components of the system are not properly matched and protected, early failure could result.