Magnetic Circuit Analysis / Analysis of Eddy Current for Large Motor(1)

Accompanying the higher performance and lower cost of permanent magnets, permanent magnets are also coming into use in large machines such as motors for electric cars, motors for railroads, and wind-powered electric generators. With the expansion of usage applications, permanent magnets are also becoming larger and coming to be used at higher rotation rates, so the loss due to eddy currents from permanent magnets is becoming a problem. Therefore, the permanent magnet loss in magnetic circuits is simulated with magnetic field analysis and magnet shapes are proposed to reduce that loss.

The figure is an example that finds the eddy current loss for a 50x50x10 mm neodymium magnet in an AC magnetic field. This is the loss distribution when the strength of the AC magnetic field is ±12.3 kA/m (±154 Oe) and the frequency is 2000 Hz. Figure (a) shows the magnet 1/4 region when the magnets are not split and Figure (b) shows it when the magnets are split. This shows that the eddy current paths become finer with the split magnets. Splitting the magnets is effective for reducing magnet eddy current loss. Figure (c) shows the relationship between eddy current loss and the number of pieces the magnets are split into. The more pieces the magnets are split into, the less the eddy current loss. With the multiplier effect of the magnet shape aspect ratio increasing as the number of pieces the magnets are split into is increased, the proportion of loss suppressed becomes larger.

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Figure (a) Eddy current of the not divided magnets

Figure (b) Eddy current of the divided magnets

Figure (c) Relationship between eddy current loss and the number of parts the magnet are split into

Reference Literature:
Y. Aoyama and K. Matsuoka, "Analysis of Loss in Permanent Magnet under Alternating Magnetic Field", The Papers of Technical Meeting on Rotating Machinery, IEE Japan, RM-01-112 (2001) (in Japanese)
Y. Aoyama, K. Ohashi, K. Miyata, "Experiment and Analysis of Eddy Current Loss in Permanent Magnet under Alternating Magnetic Field", The Papers of Technical Meeting on Rotating Machinery, IEE Japan, RM-02-135, (2002)(in Japanese)

There is much development underway of large-size motors using rare earth magnets. As a result, heat generation due to eddy current in enlarged magnets is becoming a problem. The effect of countermeasures can be grasped quantitatively by analyzing the magnet eddy current using magnetic field simulation. Figure (a) is a model of analyzing a 4-pole, 36-slot IPM motor. Two permanent magnets are embedded per pole. The output is 140 kW. The eddy current loss occurring in these magnets was calculated for when the motor was driven with inverter current with much current distortion and the sinusoidal current in Figure (b). The eddy current loss was 20 W with sinusoidal drive and 710 W with inverter drive. Since sinusoidal drive causes almost no heating, this IPM motor is almost completely unaffected by slot ripple and the cause of the heat generation is the inverter current distortion.

As a countermeasure to eddy current, it is effective to split magnets. The relationship between the number of pieces magnets are split into and the eddy current loss is summarized in Figure (c). This made it clear that eddy current can be reduced by splitting magnets and insulating between the pieces. Figure (d) shows condition of eddy current in the magnets for inverter drive when the magnets are not split and when they are split into 6 pieces. This shows that the eddy current paths become finer with the split magnets.

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Figure (a) Analytic model for an IPM motor

Figure (b) Driving current waveforms

Figure (c) Relationship between eddy current loss and the number of parts the magnet are split into

Figure (d) Condition of eddy current in the magnets

Figure (e) is a model for an SPM motor. De-centered magnets are used to reduce the cogging torque. The loss from eddy current generated in the magnets was calculated for when the motor is driven with inverter current and the sinusoidal current shown in Figure (b). As Table 1 shows, for the sinusoidal drive, the loss was 1850 W and for inverter drive the loss was 2760 W. Unlike the IPM motor example, for the SPM motor, large heat generation is seen even when the motor is driven with sinusoidal current. This is because for the SPM motor, the impact of heat generated by slot ripple is large and this is overlaid with the heat generated by the drive current waveform. Figure (f) shows the eddy current that flows in the magnet when the motor is driven with the inverter.

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Figure (e) SPM motor

Figure (f) Eddy current in the magnets

Table 1

Reference Literature:
Y. Aoyama, K. Matsuoka and M. Kondo, "Analysis of Loss Reduction Effect by Divided Permanent Magnet of PMSM", 2001 Japan Industry Applications Society Conference, IEE Japan, 115 (2001) (in Japanese)
K. Miyata, Y. Aoyama, T. Yokoyama, K. Ohashi, M. Kondo, K. Matsuoka,"3-D Magnetic Field Analysis of Permanent Magnet Motor Considering Magnetizing, Demagnetizing and Eddy Current Loss", T.IEEE Japan, Vol.123-D, No.4 (2003) (in Japanese)