# IEEE Electrification Magazine - March 2015 - 74

```The rotational speed of the rotor at which current starts
to flow in the batteries (i.e., the cut-in speed) has been measured by measuring the generator's induced EMF voltages
under no load and at different rotational speeds. This has
been measured to be at 210 revolutions per minute (r/min),
which is very close the theoretical value of 215 r/min for the

0.95

Efficiency

0.85
0.75
0.65
0.55
0.45
0.35
0.25
200 220 240 260 280 300 320 340 360
Rotational Speed (r/min)
Figure 11. The efficiency of the generator when connected to a 48-V
battery bank. (Source: rurerg.net.)

Temperature (°C)

95
85
75
65
55
45
35

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time (min)

Figure 12. Measuring the stator's temperature while operating at
rated power. (Source: rurerg.net.)

Figure 13. The current measurements with high harmonic distortion
when the generator is connected to a battery bank through a rectifier.
(Source: rurerg.net.)

74

I E E E E l e c t r i f i c ati o n M agaz ine / March 2015

2.4-m blade rotor used with this generator, and the small
deviation can be attributed to differences in the actual state
of charge of the battery bank. Observing the voltage
measurements under no load in Figure 10, the three phases
of the generator can be noted for their almost sinusoidal
induced voltage waveforms, and the phase difference is
measured at 119°, which is close to the theoretical value of
120° and implies a very good layout of the coils in the stator.
The efficiency of the generator has been measured while
connected to a 48-V battery (Figure 11) at varying rotational
speed and line currents by measuring the input mechanical
power using a torque meter and the output electrical power
using an oscilloscope and probes. Typically, the maximum
efficiency of a coreless axial flux permanent magnet generator is high, 0.88 in this case, and occurs for lower currents
and r/min, which correspond to low wind speeds, which are
more frequent in rural applications.
At the same time, the temperature rise in the stator
has been measured (Figure 12) with the generator producing rated power for 15 min. This provides an indication of
the cooling ability of the generator, which during operation in the field will be enhanced because of stronger and
cooler air flow. The stator temperature was found to stabilize at 85 °C, which is below the temperature of 100 °C at
which the vinylester resin will start to melt.
Observing the current measurements under load in
Figure 13, the three line currents of the generator phases can
be noted for their distorted waveforms. This is due to
harmonic distortion introduced in the system from the rectification process, where a three-phase uncontrolled bridge rectifier is used for lower cost and local availability, which increases the noise levels of the generator, especially for higher
currents, and produces its characteristic "humming" noise.
The operation of the generator under load has been measured by measuring the phase voltages and line currents at
different rotational speeds above cut-in r/min. When the actual power cable, in terms of length and conductor size, that
would be used in a typical installation to connect the generator to the battery bank has been included in the setup, the
power curve of the complete electrical system can be measured, as in Figure 14. This can then provide information on
the efficiency of the complete system, from the kinetic energy
of the wind to the electrical energy flowing into the batteries,
when combined with measurements made in the wind tunnel for the aerodynamic efficiency of the rotor blades.
Several wind tunnel tests have been conducted on the
rotor blades of locally manufactured small wind turbines to
determine the rotor's efficiency. Because of the size of the
wind tunnel, experiments have been conducted with a set
of 1.2-m-diameter rotor blades. Experimental results have
shown a maximum aerodynamic power coefficient of 0.38-
0.40 for a tip speed ratio of 5.5-6 for wind speeds ranging
from 8 to 11 m/s, as seen in Figure 15. At lower wind speeds
of 4-6 m/s, which are more frequent in rural applications,
the aerodynamic power coefficient had a lower value at
0.35, which is still close to the typical value of 0.4 for

```
http://www.rurerg.net http://www.rurerg.net http://www.rurerg.net

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