**5. Experimental Results**

Taking the well publicized small turbine design of Hugh Piggot as the starting point, we studied several generator and turbine parameters in order to optimize the design for the lower speed wind [10]. Generator measurements were performed on an electric motor driven dynamometer allowing simultaneous measurements of both mechanical and electri‐ cal power. The final generator was then placed in service on a turbine with adjustable pitch blades. The turbine power output was measured along with the wind speed for turbine optimization.

#### **5.1. Generator Optimization**

Initially a basic study of open circuit voltage was performed. Several coils with varying numbers of turns were prepared from 1mm diameter enamel coated magnet wire. In each case the coils were wrapped on a 20 x 40mm oval shaped core, slightly smaller than the ro‐ tor magnets. The thickness of the coil in the axial direction, which defines the thickness of the stator, was kept constant at 10mm. As the coils grow larger, the space between adjacent coils decreases, resulting in a maximum coil size of approximately 150 x 100mm. As can be seen in figure 6 the open circuit voltage increases linearly with the number of turns.

**Figure 6.** Individual coil open circuit voltage vs. number of turns at 50 rpm

Leveraging off of existing small turbine designs [1] the generator is to be a 3-phase, axial flux synchronous permanent magnet generator. We have selected a 12 pole design with 25 x 50mm, 11mm thick nickel plated NdFeB magnets. The generator is based around an auto‐ motive wheel bearing and disk brake, thereby defining the rotor diameters. The initial speci‐

> Wind Speed m/s 5 Power W 272 Balde Legnth m 2.25 Cp 0.29 Generator speed rpm 340 Generator Efficiency % 80 Voltage V 48 Current A 5.8 Poles 12 Phases 3 Rotor ID mm 125 Rotor OD mm 360

Taking the well publicized small turbine design of Hugh Piggot as the starting point, we studied several generator and turbine parameters in order to optimize the design for the lower speed wind [10]. Generator measurements were performed on an electric motor driven dynamometer allowing simultaneous measurements of both mechanical and electri‐ cal power. The final generator was then placed in service on a turbine with adjustable pitch blades. The turbine power output was measured along with the wind speed for

Initially a basic study of open circuit voltage was performed. Several coils with varying numbers of turns were prepared from 1mm diameter enamel coated magnet wire. In each case the coils were wrapped on a 20 x 40mm oval shaped core, slightly smaller than the ro‐ tor magnets. The thickness of the coil in the axial direction, which defines the thickness of the stator, was kept constant at 10mm. As the coils grow larger, the space between adjacent coils decreases, resulting in a maximum coil size of approximately 150 x 100mm. As can be

seen in figure 6 the open circuit voltage increases linearly with the number of turns.

fications for the turbine are listed in table 2.

274 Advances in Wind Power

**Table 2.** Wind turbine and generator initial specifications

**5. Experimental Results**

turbine optimization.

**5.1. Generator Optimization**

If the coils were allowed to grow larger eventually contradictory flux from adjacent magnet pairs could enter the larger coils reducing the net flux and thus the voltage. With the current design the largest coils possible for a given stator thickness will deliver the maximum power.

For maximum flux transfer through the coils of motors and generators the coils have cores of laminated soft iron, or other magnetically conductive materials in an electrically insulat‐ ing design (to reduce eddy currents). These soft iron cores provide a low resistance path for magnetic flux to pass through the coils. This however will also cause a significant "cogging" torque as the magnets tend to stick in positions over the cores [10]. High cogging torque will raise the turbine cut in speed, thus most low speed turbines are produced without magnetic materials in the cores, resulting in "core less" or "air core" coils. While the utility of this is appreciated, we decided to test both an air core coil and an identical coil with a core of steel baring epoxy. This epoxy was found to have very high electrical resistance, and significant magnetic susceptibility. The cores were tested on the generator dynamometer rotating at 125 rpm yielding the results in table 3. As only a single coil was installed, the resulting power extraction and efficiencies are very low. Both the electrical and mechanical power increase with the use of the epoxy in the coil's core as expected from the greater flux transfer. The efficiency of the epoxy core coil is also slightly higher than the air core coil. The cogging tor‐ que was measured to be significantly smaller than the rotor's bearing friction, thus the ep‐ oxy core coils were selected for the final configuration generator.


**Table 3.** Comparison between air core and metal filled epoxy core coils

Another important optimization was the stator axial thickness. A thicker stator will allow more turns of wire, increasing the output voltage, however it will also require a greater ro‐ tor spacing distance. As the rotors are spaced further apart, more flux from the magnets will tend to "short circuit" to the adjacent magnets, rather than traverse the stator to the magnet on the opposite stator [3]. This situation is shown in figure 7

**Figure 7.** Lateral flux short circuiting to adjacent magnets increases (right) with increased rotor separation distance.

The induced voltage per turn can be seen to drop rapidly as the rotors are spaced further apart in figure 8.

**Figure 8.** Voltage per turn versus rotor separation distance at 125 rpm

For a given rotor separation distance there is a maximum number of coil turns which will fit between the rotors. A margin of 2.5 mm is provided between the magnet surfaces and the stator to avoid physical contact, and allow air flow to cool the stator coils. Thus for a 10mm rotor separation, the stator is limited to a 5mm thickness which will allow about 50 turns per coil.

Coils of 5, 10 and 15mm thicknesses were prepared for 10, 15 and 20mm rotor separations respectively. These coils were then tested on the generator dynamometer at 125 rpm yield‐ ing the data of table 4.


**Table 4.** Open circuit voltage and coil parameters for various rotor separation distances.

Another important optimization was the stator axial thickness. A thicker stator will allow more turns of wire, increasing the output voltage, however it will also require a greater ro‐ tor spacing distance. As the rotors are spaced further apart, more flux from the magnets will tend to "short circuit" to the adjacent magnets, rather than traverse the stator to the magnet

**Figure 7.** Lateral flux short circuiting to adjacent magnets increases (right) with increased rotor separation distance.

The induced voltage per turn can be seen to drop rapidly as the rotors are spaced further

For a given rotor separation distance there is a maximum number of coil turns which will fit between the rotors. A margin of 2.5 mm is provided between the magnet surfaces and the stator to avoid physical contact, and allow air flow to cool the stator coils. Thus for a 10mm rotor separation, the stator is limited to a 5mm thickness which will allow about 50 turns per coil.

Coils of 5, 10 and 15mm thicknesses were prepared for 10, 15 and 20mm rotor separations respectively. These coils were then tested on the generator dynamometer at 125 rpm yield‐

on the opposite stator [3]. This situation is shown in figure 7

**Figure 8.** Voltage per turn versus rotor separation distance at 125 rpm

apart in figure 8.

276 Advances in Wind Power

ing the data of table 4.

As the rotors are moved closer together, the magnetic flux passing through the coil increases producing a higher open circuit voltage per coil turn. However the smaller separation dis‐ tance results in a smaller number of turns per coil. To achieve maximum open circuit volt‐ age, there is a compromise between number of turns and rotors separation distance. As shown in the Table 4 the 15mm rotor separation distance will give the maximum open cir‐ cuit voltage.

A generator was fabricated with the largest coils possible in a 10mm stator, and the cores were filled with metal bearing epoxy. The generator was then tested on the dynamometer with various loads. Figure 9 shows the results of electrical power measurements with the generator connected to various resistance loads.

**Figure 9.** Power versus rotational speed for various loads

The maximum power of the system was produced with the 6 ohm load which is approxi‐ mately equal to the internal resistance of the stator as the resistance per coil is 0.67ohm and there are 9 coils in series. Our initial design required approximately 270W of power produc‐ tion at 340 rpm. This power was above the capabilities of the relatively low power dyna‐ mometer, but falls within the range of power production predicted based on the square of the speed (black trend line) for the 6 ohm load.

## **5.1. Turbine Optimization**

The generator was then placed in service on the roof of the mechanical engineering building as shown in figure 10. The 2.25 meter long wooden blades were fabricated with a NACA 4412 profile commonly used for low speed turbines. During testing the turbine blades were set to a given pitch angle and the generator was connected to a fixed resistance load. Wind speed and electrical power production data was then continuously logged. After several weeks of testing the turbine would then be adjusted to a new angle of attack and/or the load resistance would be changed.

Due to inconsistencies in the wind, not all configurations were tested at the same speeds for the same durations. Overall trends, however, were readily apparent. During the period of field testing the maximum instantaneous wind speed recorded was 8m/s while the maxi‐ mum sustained wind speed was around 4 to 5m/s.

**Figure 10.** Wind turbine with optimized generator during turbine evaluation. Notice the anemometer in the back‐ ground to the left.

The data generated during testing at the 9 degree attack angle shown in figure 11 was typi‐ cal of the testing. The turbine's cut in speed is around 2m/s, and power output increases rap‐ idly with wind speed for all load resistances. Data for the 3 and 6 ohm loads show the 3 ohm load having slightly higher power output below 3 m/s and the 6 ohm load giving greater power above 3 m/s. Theoretically the 6 ohm load should give the greatest power extraction as the load is well matched to the generator. In general the 6 ohm load gave the best power extraction, and was selected for further analysis.

**Figure 11.** Electrical power versus wind speed at various loads for 9 degree angle of attack

**5.1. Turbine Optimization**

278 Advances in Wind Power

resistance would be changed.

ground to the left.

mum sustained wind speed was around 4 to 5m/s.

extraction, and was selected for further analysis.

The generator was then placed in service on the roof of the mechanical engineering building as shown in figure 10. The 2.25 meter long wooden blades were fabricated with a NACA 4412 profile commonly used for low speed turbines. During testing the turbine blades were set to a given pitch angle and the generator was connected to a fixed resistance load. Wind speed and electrical power production data was then continuously logged. After several weeks of testing the turbine would then be adjusted to a new angle of attack and/or the load

Due to inconsistencies in the wind, not all configurations were tested at the same speeds for the same durations. Overall trends, however, were readily apparent. During the period of field testing the maximum instantaneous wind speed recorded was 8m/s while the maxi‐

**Figure 10.** Wind turbine with optimized generator during turbine evaluation. Notice the anemometer in the back‐

The data generated during testing at the 9 degree attack angle shown in figure 11 was typi‐ cal of the testing. The turbine's cut in speed is around 2m/s, and power output increases rap‐ idly with wind speed for all load resistances. Data for the 3 and 6 ohm loads show the 3 ohm load having slightly higher power output below 3 m/s and the 6 ohm load giving greater power above 3 m/s. Theoretically the 6 ohm load should give the greatest power extraction as the load is well matched to the generator. In general the 6 ohm load gave the best power

**Figure 12.** Electrical power vs wind speed at various angles of attack for the 6 ohm load

The power production was not a strong function of angle of attack in the 7 to 11 degree range, but dropped significantly at 14 degrees. Based on extrapolation of the data to higher speeds, the 9 degree angle of attack is expected to give the highest power production in the 3.5 to 5 m/s wind speed range.

Taking the best fit curve to the 9 degree angle of attack blade pitch with the 6 ohm load (figure 13) we can calculate that the turbine should produce about 200W at a 4.2 m/s wind speed. Taking this with the known turbine blade length of 2.25 meters and an assumed gen‐ erator efficiency of 80% [7], we can use equation 1 to calculate the coefficient of performance to be 0.36, somewhat better than the assumed value of 0.29.

Additional measurements made on the turbine bearings indicated frictional losses account for 23W at 300 rpm. This is approximately 10% of the electrical power produced. The use of automotive bearings is perhaps not optimal from a friction stand point, thus with improve‐ ments in the bearings it may be possible to improve the turbine output by something on the order of perhaps 5% or so.

**Figure 13.** Electrical power versus wind speed at 9 degree attack angle with 6 ohm load

Looking back at figure 9 we can see that a 200W output should occur at approximately 300rpm with the 6 ohm load. Using this to calculate the TSR at a 4.2m/s wind speed we come up with a TRS of 17, close to our assumed value of 16, and significantly higher than the conventional value of 8.
