El pensar inmediato: la devoción ( Andacht )

There is a large number of papers published describing the roles of the firing angles on the generated power [148]. During the low speed operation, the initial rise of current is slow due to the high inductance value. One of the ways to aid in the fast increase of phase current is to advance the turn on angle into the decreasing inductance profile. However, when the turn on angle is placed further along the decreasing inductance profile, the dwell angle reduces.Also, the turn off angle has to be located near or along the minimum inductance region, thus reducing the generated power. The only energy being returned is the stored energy. During this time, energy will not be extracted from the prime mover due to zero torque. This section will analyse the effect of the turn on angle and turn off angle with other variables for the speed range 25rad/s to 500rad/s.

6.3.4.1

Turn on Angle

The following can be observed for variation of the turn on angle with turn off angle in Figure 6-6(a) when the SRG is subjected to a voltage level of 325V at a low speed of 35rad/s and in Figure 6-7(a) at a medium speed of 300rad/s. At a lower speed, the percentage of power generated increases as the turn on angle is placed in the decreasing inductance profile, whereas during the medium and high speed, a high percentage of power generated is obtained when the turn on angle is along the positive inductance region. Correspondingly, the optimal turn off angle for both the speed is between 300 to 400. Care must be taken to ensure that a continuous overlapping current profile is maintained to provide adequate overlap between the current with its adjacent phase. Larger turn off angle will reduce the overlap due to the longer conduction period and current being forced to return to zero at the minimum inductance region.

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Figure 6-6 (b) shows the variation of turn on angle with various reference currents. Although the percentage of power increases as the turn on angle is increased, the current profile may become discontinuous and does not overlap with the current in the adjacent phase. The reason is due to the reduced dwell angle as the turn on angle is varied while the turn off angle is fixed. The variation of the turn on angle with terminal voltage for the low and medium speed range is depicted in Figure 6-6(c) and Figure 6-7(b). The percentage of power generated does not show significant changes as the terminal voltage is varied at the higher speed range. However, for the lower speed range, the percentage of generated power decreases as the voltage is increased. The turn on angle is made prior to the alignment position for voltage level between 50V to 150V, whereas at higher voltage level, the turn on angle is made after the alignment position to achieve a high percentage of power generated. As mentioned earlier, the continuous or overlapping current profile must be maintained. At a higher speed range, the percentage of power generated decreases when the turn on angle is near the alignment position because the time for the current to rise and store the magnetic energy is reduced.

Figure 6-6: Graph of % power generated against turn on angle (degree) for low speed range between 25rad/s to 55rad/s. Each graph shows the effect of turn on angle with (a) turn off angle, (b) reference current, (c) voltage

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The variation of the turn on angle with speed is given in Figure 6-6(d) and Figure 6-7(c). It shows that there exists an optimal value of turn on angle at each speed range. The graph tends to converge to one value due to the constant turn off angle used. As the turn on angle moves along the decreasing inductance profile, the dwell angle reduces, therefore the percentage of power generated decreases. We can conclude that the optimal turn on angle for the low speed range is prior to or just after the alignment position whereas for the medium speed, the turn on angle has to be placed midway during the inductance profile, which is during the motoring region. The implication of having the turn on angle in the motoring region is the development of positive torque which may reduce the generating operation

6.3.4.2

Turn off Angle

Similar to the analysis on the implication of the turn on angle on various control variables, the variation of the turn off angle can also be categorized by the low and medium speed operation as depicted in Figure 6-8 and Figure 6-9. At lower speed, the variation of the turn off angle is shown in Figure 6-8(a). Even though the turn on angle is changed, the variation of the optimal turn off angle is minimal. Alternatively,

Figure 6-7: Graph of % power generated against turn on angle (degree) for medium speed range between 100rad/s to 500rad/s. Each graph illustrates the effect of turn on angle with (a) turn off angle, (b) terminal

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during the medium speed, the optimal turn off angle shows a slight variation according to the placement of turn on angle. The placement of the turn on angle along the inductance slope for both speeds differs; after alignment position for low speed and before alignment position for medium speed. Figure 6-8(b) shows the effect of reference current at various turn off angles during the low speed operation at a voltage level of 325V and a turn on angle of 180.The optimal value of turn off angle differs for each reference current level because of the current chopping mode operation. During the chopping mode, the current is regulated to follow the path on the energy conversion loop. The highest power generated occurs at reference current of 5A. On the contrary, the reference current has no effect during the medium speed operation as depicted in Figure 6-9(b).When the SRG is subjected to different levels of terminal voltage as in Figure 6-8(c) and Figure 6-9(c), the optimal turn off angle at both the low and medium speed operation has similar range of values. The percentage of power generated during the medium speed does not change when the terminal voltage level is varied as compared to the lower speed range. Likewise, minimal change in percentage of power generated is seen during the medium speed range as depicted in Figure 6-9(d). On the whole, it can be observed that although various variables have

Figure 6-8: Graph of % power generated against turn off angle (degree) for the low speed range between 25rad/s to 55rad/s. Each graph shows the effect of turn off angle with (a) turn on angle, (b) reference current, (c) terminal

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been changed, the optimal value of turn off angle remains in the same range. It can be said that turn off angle does not have considerable effects on maximizing the generated power during the low and medium speed range under the generating operation.

6.4

Conclusions

The purpose of this chapter was to investigate the variables affecting the performance of the machine. The variation of the control variables for a three phase 12/8 machine in terms of percentage of power generated has been discussed during both the low and medium speed range. Similar outcomes are obtained from the other machines but with different values of firing angles. In this context, the low and medium speed range has been defined to be in the range of 25rad/s to 100rad/s whereas the medium speed range is between 100rad/s to 500rad/s. The impact of the percentage of power generated is observed for the following variables:

• Reference current • Terminal voltage

• Firing angles – turn on and turn off angle

• Speed

Figure 6-9:Graph of % power generated against turn off angle(degree) for the medium speed range between 100rad/s to 500rad/s. Each graph shows the effect of turn off angle with (a) turn on angle, (b) reference current,

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The observations are categorized according to the low and medium speed range.