Group SD0706

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Prototype Wind Generation System

Members

Electrical Engineering

James Sanden

Lindi Hruby

Mechanical Engineering

Chris Schaff

Joey Specht

David Olien

Advisor

Rajesh Kavasseri

Project Description

This project included the research, design and construction of a small scale wind generator that is to produce a maximum of 1kW of power. This small scale wind turbine has been mounted on the roof of the ECE building with cooperation with 3 students from the ME department who supplied us with a tower and nacelle design.

Requirements

1. Generator must produce up to 1kW of power

Given the wind speeds in our location an output of greater than 1kW is not likely. We are required to build a wind powered generator that produces a reasonable amount of power given the wind speeds at our location. Given the style of blades chosen by the ME department a wind speed of approximately 18 mph would be required to produce 1kW. With an average wind speed of around 12 mph at our site we can expect a reasonably lower output than 1kW depending on the efficiency of our generator.

2. Design must include a rectified output.

Since the output of our generator will vary in frequency and magnitude along with varying wind speeds a rectifier is required to convert the output into useable power. The output from the rectifier with smoothing capacitors should be a relatively smooth DC signal.

3. There must be a way to brake the rotor.

Since the turbine will need to be lowered when not in use due to design constraints placed on the ME department, we will need to have an easy way to brake the rotor when it is spinning so that it can safely be lowered to the ground. This brake may also be used to stop the rotor in high wind speeds to protect it from over speed.

Generator Design

For this project we chose to build our own generator. We used a 2hp Baldor induction motor and converted it into a permanent magnet AC generator. To do this we machined down the original rotor and affixed 4 NdFeB (Neodymium Iron Boride) magnets of alternating polarity around the rotor. The magnets are custom made to fit the exact dimensions of our rotor so we can keep the air gap unchanged to minimize losses.

Theory Behind the Motor Conversion

The concept behind the motor conversion is that by machining out and placing high powered magnets in alternating polarity around the rotor a strong magnetic field will be produced. When the shaft is turned it will create a rotating magnetic field that will induce a current in the windings of the stator.

This whole concept is based on the principle of Faraday’s Law which explains that the induced emf in the stator windings is the result of the changing magnetic flux with respect to time through a perpendicular surface area created by some conductor. In our case the conductor was our four pole stator and the changing magnetic flux is created by the rotation of the magnets placed around the rotor.

Where B is the Magnetic Field perpendicular to the loop created by some conductor, and ΦB is the Magnetic Flux which is equal to:

                              ΦB = B*S

And S = Surface Area covered by the loop

Below is a look at the internals of our motor:

Below: This is the original rotor from our 2Hp motor.

Above: This sketch shows the original design of how the magnets were placed around the outside of the rotor. The magnets are 1/8" thick so the rotor was machined down by 1/4" in diameter. The original idea was to have a 12-pole machine so that the operating speed would be lower but we found out later on in the project that this wasn't practical and disobeyed Faraday's Law. Our final design involved the use of only 4 magnets for 4 poles.

Above: This shows our original design for the rotor. In this picture all 12 magnets are applied in alternating polarity. Our final design used only 4 magnets, removing 2 magnets from each pole.

Bottom: This shows the stator windings in which the current and voltage is induced to give us our output power.

Below is a chart of the open-circuit output that we saw from the generator after removing some of the magnets so that we had only 4 poles.

The following images may provide a better idea of how this whole thing works. It is a simulation run using Finite Elements Method Magnetics software for a 4 pole induction motor. The first plot shows the magnetic field density in the core of the machine and the second shows the induced emf over 1/2 of a rotation (Over 2 poles).

Major Design Issues

A major problem that we had during the conversion process is that we tried to add 12 poles to our rotor when our stator is only wound for 4. This essentially caused most of the magnetic flux to cancel and we were left with a generator that didn't generate. When we tested the generator with 12 poles we really saw almost complete cancelations in the windings. We were reading only a few millivolts at 1800rpm as opposed to the few hundred volts it is producing now. If attempting to do a motor conversion yourself this is something that needs to be taken into consideration from day 1. If the stator is wound for 4 poles and you do not want to have to rewind it the rotor must also have 4 poles.

However, if you are willing and capable of taking on the task of rewinding the stator for a greater number of poles just be certain that the number of poles you wind for matches the number of poles you have on your new rotor otherwise you will get cancellations in your stator windings and therefore very minimal power.

Rectifier Design

The output of the generator varies in frequency and magnitude according to the wind speed. We needed to rectify this signal to DC so that it can be used for things such as charging batteries. We used a 3 phase bridge rectifier capable of handling the high current and output voltage of the generator along with a 10uF capacitor in parallel and a 1.2mH inductor in parallel.

Circuit Diagram:

Simulation Output:

Picture of our Rectifier:

The capacitor size may need to be increased since we still saw rippling in the end voltage of the rectifier.

Brake Design

The idea for braking the rotor involves a switch to short two phases of the generator output together. This acts to establish a magnetic field in opposition to the rotational momentum of the rotor and in effect along with turning the turbine yaw out of the wind slows it down so that the turbine can be more safely lowered to the ground.

Diagram of Switching Mechanism:

It is required that the rectifier be isolated from the generator before the shorting switch is engaged due to the high currents that are associated with shorting 2 phases together.