Error in airgap

The size of airgap is the biggest single factor to influence the flux density in the VHM magnetic circuit. Although reasonable care was taken to set the airgap with bronze feeler gauges, human error must be accounted for. More significantly, the gauges are only used to measure and alter the clearance between the rotor teeth and the magnet surface. The mounting of the magnets onto the stator, namely with glue, has resulted in some of the magnets not sitting true to the steel. Figure 4.57 shows how a thin layer of glue may have formed between the magnet and stator, constituting an increase in the overall size of airgap.

Figure 4.57: Source of error in airgap

Measuring the airgap with feeler gauges revealed a variation of the airgap size between 1.1-0.9 mm across stator the faces, possibly implying that the translator and stator were not truly parallel or that the magnets were not mounted flat.

The combination of these two airgap inaccuracies means that the overall error in the airgap size is likely to be in the range of 0.8-1.2 mm. The resulting effect on the per phase self inductance is shown Figure 4.58.

Figure 4.58: The effect of alternative airgaps on effective inductance map

The effect of using these values in model E for two different loading conditions is shown in Figure 4.59 and Figure 4.60, with the numerical values displayed in Table 4-8.

Figure 4.59: Effect of varying airgap on predicted voltage, C=150^,, R =630Q (experiment a)

on on

Figure 4.60: Effect of varying airgap on predicted voltage, C=100^, R=29Q (experiment b)

Table 4-8: Comparison of experimental results for large airgap model E i

C=150|F, R=360Q

C=100|F, R=29Q

exp.

(a)

0.8 mm airgap

1.2 mm airgap

exp. (b)

0.8 mm airgap

1.2 mm airgap

Value

%

Value

%

Value

%

Value

%

Max

504

659

31

494

1.98

51.6

61.8

19

52.6

1.96

Min

-476

-655

38

-499

4.83

-51.2

-62.7

22

-53.3

4.14

RMS

207

241

17

184

11

26.24

32.7

240

27.5

4.94

The error is significantly reduced for the 1.2 mm airgap. Although greater confidence can be placed in the models when examining these results, it is inaccurate to assume that the prototype had an effective airgap of this value. It is the author's opinion that the three dimensional effect, which will also be affected by the airgap, is still most significant, as indicated by the reasonable open circuit prediction.

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