Phase processing and optimization

The algorithms of auto-phase correction can be divided into two parts:

a. Extraction of the DC bus current information

Fig. 5-7 shows the DC bus current processing process in one electric cycle. It includes six steps and each step can be further divided into two regions (1 and 2) for discussion.

In region 1:

Step 1.Step 1: Detect the occurrence of a commutation signal change.

Step 2.Wait until the optimized phase commutation signal is obtained.

Step 3.Do commutation.

Step 4.Move previous maximum current information (in REG1 and REG2) to temporary registers (REG1_TEMP and REG2_TEMP).

Step 5.Reset flag to 0.

Step 6.Obtain the first new maximum current information recorded in REG1.

In region 2:

Step 1.Wait until a 30° rotation of correct commutation.

Step 2.Set flag to 1.

Step 3.Obtain the second new maximum current information recorded in REG2.

Step 4.Call the ‘phase corrector’ to do phase correction.

b. Phase processing and optimization

REG1_TEMP and REG2_TEMP save the previous maximum DC bus current of regions 1 and 2, respectively. A flowchart of auto-phase correction called the ‘phase corrector’ is depicted in Fig. 5-8. At the end of region 2 (before the start of the next

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region 1) the phase corrector will be called to find the shifted angle using the values of REG1_TEMP and REG2_TEMP. If REG1_TEMP is smaller than REG2_TEMP, the status of the commutation signal is phase lagging; otherwise, the status is phase leading. A synchronous phase is found when the result is neither of these two cases. If any leading or lagging status is found, the next commutation point will be corrected by phase compensation. The shifted angle is continuously corrected until a synchronous status is found. It is clear that the operations used in the phase corrector compare only the values of REG1_TEMP and REG2_TEMP, which is a simple arithmetic operation.

Fig. 5-8. Flowchart of the phase corrector

It can be seen that the phase corrector will correct the phase angle dynamically by detecting the DC bus status. After obtaining the phase shift information, the minimum torque ripple can be determined easily. To implement the phase corrector, need only an A/D convertor and two general purpose timers. In a real application, most low cost MCUs

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can perform these tasks well. In this way, the use of simple and low cost MCUs can optimize the best performance of sensorless driving.

5.3 Experimental results

Experiments were conducted to verify and compare the proposed method with that in unexcited phase method on chapter 4. From unexcited phase method, it is clear that when commutation is done at the correct phase point, the two freewheeling currents of the unexcited phase are equal during each full electric cycle. However, due to the impact of the winding inductance characteristic, a large torque ripple will be induced at high speeds, although in the correct commutation phase. The induced large torque will reduce the efficiency and generate noise and vibration. Note that the freewheeling current characteristics cannot be clearly caught when the PWM is set nearly at the full duty cycle (duty over 98%). To observe the freewheeling current conducting phenomenon of the PWM switched between low and high speed situations, the experiment set the PWM duty at 30% and 100%, giving the same load, to compare their differences. The waveforms were recorded using an oscilloscope (Figs. 5-9 – 5-12). In the following figures, channel 1 is colored yellow and denotes the phase-B phase current, channel 2 is colored blue and denotes the sensorless commutation signal, channel 3 is colored pink and denotes the phase-B voltage, and channel 4 is colored green and denotes the DC bus current. A general purpose BLDCM with 10 poles, rated power 80 W, rated voltage 24 V, rated torque 2.6 Kg-cm, winding resistance 210 mΩ, winding inductor 300 uH and rated speed of 3000 rpm is used as the experimental target.

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Fig. 5-9. Results of using the unexcited phase method in the 30% PWM duty condition

Fig. 5-10. Results of using the proposed method in the 30% PWM duty condition

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Fig. 5-11. Results of using the unexcited phase method in the 100% PWM duty condition

Fig. 5-12. Results of using the proposed method in the 100% PWM duty condition

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We compared the freewheeling current characteristic in 30% and 100% PWM duty conditions. For clarity, the scale of the phase current has been magnified. In Fig. 5-9, the unexcited phase method was applied, as presented in chapter 4. It can be seen that the two freewheeling currents of the two unexcited phases were equal during each full electric cycle. The running speed was 1626 r/min and the DC bus current ripple was 1.8 A. In Fig.

5-10, the proposed method was applied. It can be seen that the two freewheeling currents of the two unexcited phases were not equal during each full electric cycle, because the phase angle was automatically advanced to force the system current flat-topped waveform by calling the phase corrector. However, the running speed was 1646 r/min, increased by 20 r/min, and the DC bus current ripple was 1.2 A, reduced by 33%. These greatly improved results can be explained as follows: in the 100% duty condition, there is no PWM switching signal; thus, the unexcited phase freewheeling current will not exist, and the freewheeling characteristic does not show up. However, proposed method does not have this problem. Comparing Figs. 5-11 and 5-12, the current ripple was reduced by 17% from 1.8 to 1.4 A, and the speed was increased by 18 r/min from 3390 to 3408 r/min.

It can be seen that extracting the DC bus information to correct the phase angle to do phase compensation is a very effective approach. To bolster the argument further, that expanded the experiment and increased PWM duty in 10% increments. The experimental results are summarized in table 5-2, including increased running speed, reduced current ripple, and advanced degrees. Using proposed method, the running speed was steadily increased and the current ripple steadily reduced. The increased running speed is the difference between the proposed method and unexcited phase method, which can be obtained from (5.12). The percentage of the reduced current ripple can be obtained using (5.13). The phase corrector helps perform the auto-phase advanced, which is the main contributor to the increased speed and reduced current ripple. The advanced degree is

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calculated using (5.14). Obviously, in terms of reduced torque ripple and increased running speed, the proposed method is better than the unexcited phase method in the same PWM duty condition. This is because the proposed method not only performs auto- correction but also reduces torque ripple through phase correction.

_ _ . Degree_ctrlU is the control degree of the unexcited phase method.

Table 5-2 Comparison of the proposed method and unexcited phase method PWM duty

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5.4 Conclusion

Experimental results show that the experimental waveform approximated to the theoretical waveform, which proves the correctness of the proposed method. The effectiveness not only auto-calibrates to obtain the correct commutation point but also optimizes the DC bus ripple current induced from winding inductance characteristics when running at a high speed. The proposed method is simple and can be implemented through simple arithmetic operation. Of course, this method wants to emphasize that this method could be embedded into the Hall sensor driver system, which would provide the benefit of reduced torque ripple and overcome the problems of Hall sensor misplacement and aging.

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