6.2.1
Firing Angles
Since the operation of the SRG is during the decreasing inductance slope, the placement of the turn on angle should be made along the increasing inductance region to allow the phase current to build up. The turn off angle on the other hand can be placed anywhere before the poles starts to separate, which is before the minimum inductance region. The peak current for the generating operation occurs after both the firing angles are turned off depending on the back EMF and speed. Therefore, there is a possibility of the firing angles yielding the same output power [20]. Hence, the optimal firing angles are selected in terms of the highest percentage of generated power. Some possible factors which may affect the selection of optimal firing angles are stated below:
• If the turn on angle is placed too low in the motoring region, the machine will develop positive torque, decreasing the efficiency of the generating operation. • If the turn on angle is placed along the decreasing inductance profile, the dwell
angle will reduce, resulting in a non-overlapping current profile.
• As for the turn off angle, it cannot be placed too near the minimum inductance region as this will result in the generating current tailing off into the excitation stage of the next phase. The current will increase continuously instead of returning to zero during each cycle [148, 149].
Based on the factors above, the optimal firing angles have to be determined heuristically to avoid any overlapping parameters. The range of the firing angles to be determined is narrowed according to the conditions above by starting from the mid motoring region and into the decreasing inductance region. For each turn on angle, the range of turn off angle for the 12/8 machine will be from 250 to 400. By doing this, the range of the firing angles on the inductance profile will be covered. As mentioned earlier, the angles which give the highest percentage of generated power is selected.
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6.2.2
Terminal Voltage
The terminal voltage is the voltage across the battery which is connected directly to the converter. According to equation (6.2), an increase in the terminal voltage will increase the phase current provided that the back EMF is lower than the terminal voltage. Since the turn on angle will be along the increasing inductance profile, the initial increase in current will be slow. In certain application, a buck boost converter is added to aid in the increase of phase current [61]. This however, requires additional circuitry, increasing costs and reduces the available space within the existing circuit. As the voltage is proportional to the current, any increase in voltage will increase the current. However, a high voltage may reduce the generating current; thus, a suitable value of voltage to be used has to be determined. This can be achieved using the heuristic method. A different level of voltage is simulated for each of the turn on angle groups mentioned in subsection 6.2.1.
6.2.3
Speed
The velocity of the wind will turn the blades of the turbine hence turning the shaft, which is directly connected to the rotor. Thus, the speed of the rotor is influenced by the wind velocity. According to equation (6.2) the wind speed has a major impact on the back EMF. A low wind speed results in the back EMF being lower than the terminal voltage whereas a high wind speed increases the back EMF to be more than the terminal voltage. Even though the speed is not a controlled variable, it influences the selection of parameters such as the firing angles, voltage and number of rotor poles. Therefore, the optimal parameters will be determined at various speed ranges.
6.2.4
Phase Inductance
The phase inductance can be obtained from the magnetization curve profile according to the following equation:
( , ) ( , ) i L i i λ θ θ = (6.6)
Both the inductance, L and flux linkage, λ varies as a function of current, i and rotor position, θ. The location of the firing angles on the inductance profile influences the shape of the phase current. From equation (6.6), the phase current is inversely
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proportional to the inductance value, which is represented as the inductance profile curve. The inductance is also proportional to the reluctance, hence at the aligned position, inductance is maximum whilst reluctance is minimum.
6.2.5
Reference Current
During motoring operation, the closed loop current control is employed to shape the phase current. It was established that the generating operation of the machine is said to mirror the operation of the motoring mode. But, the generating excitation is made whilst the inductance is increasing hence the initial current will be low. On the contrary, the excitation for motoring operation is during the minimum inductance profile with high starting current. Due to this reason, the reference current for motoring is required for protection from the initial high current whilst for generating operation, for overcurrent protection after the switches are turned off.
6.2.6
Phase Resistance
The resistance of the phase winding is designed to minimise the copper loss, hence the value is made as low as possible. Due to this reason, most of the literatures neglect the resistive voltage drop in the calculation as it is insignificant as compared to the terminal voltage. Nevertheless, the effect of resistance is pronounced when the phase current is high.
The following section describes the procedures to determine the optimal parameters of the machine.