Theory of Operation: In basic terms, our turbine generator is an electric motor, in our case a 3-phase induction motor, with a designed rotational speed of 1725 rpm. Induction motors are also known as “squirrel cage motors” and are used as furnace blowers, pump motors, washing machines, etc. Current flow in stationary windings induces a magnetic field into the rotor. This magnetic field causes the rotor to chase the fields created by the stator windings. There is no physical connection between the rotor and stator windings. There are no brushes or slip rings. When our turbine rotor blades cause the induction motor to rotate faster then the designed speed the motor begins to generate power back into the leads. Even though the motor is turning faster then the nameplate rpm it remains locked in frequency to the line power. As the torque applied to the motor shaft is increased the output current is also increased. More wind, more power.
Even though we are connected to incoming 1-phase power we use a 3-phase motor.Large 3-phase motors are much easier to find and less expensive to purchase (the third lead is coupled via capacitors as a "polyphaser" of sorts). The motor we have chosen is commonly used to drive overhead cranes in an industrial environment. It has a 15-1 low friction gearbox on the front and an electric brake on the rear. The nameplate specifies that it is “inverter duty” which to us means heavy-duty. The motor will probably be the largest single expense when gathering components but it is heart of the wind turbine.
The generator speed must be monitored electronically. We have designed a micro- controller based circuit and written the program that runs in it. It utilizes a hall-effect device attached to the rear of the motor to determine the speed at which the generator is turning. When power is applied to the generator controlling circuits the micro-controller releases the motor brake and looks at pulses coming down from the hall-device. As the wind turbine speeds up and the micro-controller determines that the generator is running at the correct speed it energizes the main relay connecting the generator windings to the power lines. If the wind speed decreases the main relay is de-energized and the generator is allowed to turn until the wind speed increases enough to allow the generator to begin producing power again. As a safety feature should the micro-controller determine that the generator is turning faster then our programmed upper speed limit the main relay is de-energized and the motor brake is applied. The controller will hold the circuits in this state until it is manually reset.
The turbine rotor blades have the task of applying torque to the shaft of the generator. The contoured shape of the blade surface (similar to an aircraft wing) forces the wind to travel farther over the top of the blade then the bottom creating a difference in air pressure between the upper and lower surfaces of the blade. It is this difference in pressure known as lift that causes the blades to rotate around center. There is a balance between the motor we’ve chosen and the size, pitch, and number of blades in the rotor.
The brake on this type of motor is very effective. The torque stored in the mass of the rotating blades could easily damage the connecting shaft and gearbox when the brake is applied. We have incorporated the use of a torque limiter in our design. It functions like a clutch in that it allows some slippage when it’s torque setting is exceeded.