［ Development of Rotating Machinery-Based System Products ］

Development for Practical HighPerformance Ferrite Magnet
MotorsKeywords

Keisuke Matsuo,
Daiki Matsuhashi,
Hitoshi Fujihara,
Isamu Takeda

Ferrite magnet motor, Efﬁciency, Irreversible demagnetization analysis, Magnetizing after rotor assembly

Abstract

Rare-earth magnet parts are indispensable for making compact and energy
efﬁcient PM motors. There are, however, problems such as price ﬂuctuation andthe concern regarding stable supply as a rare-earth material. As such, it prompted
R&D activities on the rare-earth-less or rare-earth-free motors.
As a candidate for a rare-earth-less motor, we developed an Interior Permanent
Magnet Synchronous Motor (IPMSM) with ferrite magnets that are available at a
relatively low price under steady supply. At this time, we performed a motor efﬁciency measurement and analysis of irreversible demagnetization and magnetization of the IPMSM with ferrite magnets (F-IPMSM). We further evaluated theperformance of the F-IPMSM with due consideration to the productivity and thenmade a prospect for practical use. In the future, we will continue to work on improving torque performance and efﬁciency, and will aim to commercialize theseproducts.

1 Preface
Recently, against the background of environmental concerns and growing demands to addresssaving energy, Permanent Magnet Synchronous
Motors (PMSMs) are actively used in various industrial ﬁelds because it realizes a compact design andhigh efﬁciency. The rare-earth magnets used in thePMSMs, however, have inherent problems such as
price ﬂuctuation and the concern with the stablesupply of rare-earth materials. The industrial market, therefore, calls for the development of alternative new technologies. As an alternative for the rareearth magnet, we have developed the Interior
Permanent Magnet Synchronous Motor (IPMSM)
with ferrite magnets that are available at a relatively
low price under stable supply, and we could realize
the rotor structure of the IPMSM with ferrite magnets (F-IPMSM) which could achieve the same performance as the IPMSM with rare-earth magnets(R-IPMSM)(1).
This paper introduces various performance evaluations performed for practical use of the F-IPMSM.

2 Evaluation of the Prototype ModelFig. 1 shows the rotor structure of the F-IPMSM

d-axisq-axisFerrite magnet C
SeparatedCore bridge

Ferrite magnet B

Ferrite magnet A: Direction of magnetization
d-axis : Center axis of the magnetic poleq-axis : Axis crossing at right angles with the magnetic poleFig. 1

Rotor Structure of the F-IPMSM

Ferrite magnet A is arranged in a spoke state. In order to reduce
the leakage of magnetic ﬂux, the core on the outer diameter sideof the ferrite magnet A is separated and the between ferrite magnet A and B is joined through a narrow core bridge.

and Table 1 shows the motor speciﬁcations. The
F-IPMSM proposed comes with the ferrite magnet A
arranged in a spoke state to secure a large magnet
surface and with the ferrite magnet B to reduce leakage of the magnetic ﬂux. Further, in order to increase
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Table 1 Motor Specifications
The motor speciﬁcations are shown below.

85%

Specifications

Number of poles

6

Number of slots

36

Stator outer diameter

φ220mmStack length

108mm

Base speed

1000min−1

Maximum speed

4000min−1

90%Torque (P.U.)

Items

10kWResidual flux density (Ferrite magnet)

0.45T

93%

0Maximum speed (at the limit of rotor strength) 7200min−1
Output power

92%

1000

2000

3000

4000

Rotational speed (min－1)(a) R-IPMSM
10085%

1.290%

Average torque (P.U.)

1.0
0.892%

Efficiency (%)

F-IPMSM

Torque (P.U.)

R-IPMSM93%

0.6
F-IPMSM(without ferrite magnet C)

0.4

00

1000

2000

3000

4000

Rotational speed (min－1)
0.2(b) F-IPMSM

0.00Fig. 210203040506070
Current phase angle (deg)8090

Torque Characteristics (Analysis Result)

By generating reluctance torque effectively, the F-IPMSM could
generate about 90% torque performance of that of the R-IPMSM.
If the ferrite magnet C is not arranged, reluctance torque decreased and the resultant torque performance is reduced toapproximately 80%.

the saliency ratio by decreasing the d-axis inductance, the ferrite magnet C is arranged. These arrangements realized the effective generation of the reluctance torque. While the residual ﬂux density of theferrite magnet is one third of that of the rare-earthmagnet, the F-IPMSM could generate about 90%torque under the same conditions of current andsize of conventional R-IPMSM as indicated in Fig. 2.

2.1 Evaluation of Efﬁciency
Fig. 3 shows the efﬁciency map of the R-IPMSM
and the F-IPMSM. According to this ﬁgure, it could
conﬁrm that the maximum efﬁciency of both IPMSMs
is about equal. As indicated above, the torque of theF-IPMSM is smaller than that of the R-IPMSM under
the same current condition. As a result, the currentof the F-IPMSM tends to increase during low-speed

12MEIDEN REVIEW Series No.167 2016 No.2

Fig. 3

Efficiency Map (Measured Result)

The F-IPMSM could obtain a maximum efﬁciency that is almost
equivalent to the level of the R-IPMSM. High efﬁciency area will
be shifted to the high speed region.

and the large-torque region and efﬁciency tends todecrease due to an increase in the copper loss.
During the high-speed region, however, it showsthat the efﬁciency of the F-IPMSM is higher thanthat of the R-IPMSM because the ﬁeld-weakening
current is small.

2.2 Analysis of Irreversible Demagnetization
Compared with the rare-earth magnet, thecoercivity of the ferrite magnet is as low as around
one third. For this reason, evaluation of irreversible
demagnetization is indispensable. Fig. 4 shows theresult of analysis of demagnetizing ﬁeld distribution.
As shown in Fig. 4, a large demagnetizing ﬁeld isobserved on the corner of the ferrite magnet A. Asindicated by Expression (1) below, the inﬂuence ofdemagnetization is evaluated based on the decreasing rate of the induced voltage and the result isshown in Fig. 5.
E −Eb(1)δe ＝ a×100 (%)
Eawhere,δe : Decreasing rate of induced voltage
Ea : Induced voltage before demagnetizing
Eb : Induced voltage after demagnetizing
According to Fig. 5, we could conﬁrm thatdemagnetization is negligibly small under the condition of the rated current. In the event that the motor
current exceeds the rated level (inverterʼs limit current), however, we could see that there is a decreasein the induced voltage of approximately 0.5% in
during a low temperature operation period.

2.3 Analysis of Magnetization

0Decreasing rate of induced voltage (%)

In manufacturing the prototype model in Fig. 1,we put the magnetized ferrite magnets into the rotor
core. For the mass production for practical use of
the F-IPMSM, however, setting up a manufacturing

process for magnetizing after the rotor assembly isrequired. In order to verify that ferrite magnets arecompletely magnetized after the rotor assembly, weperformed an analysis of magnetization.
Fig. 6 shows the result of the analysis of magnetization for the prototype model in Fig. 1. In thiscase, the required condition for complete magnetization is deﬁned as when the intensity of the magnetic ﬁeld generated in the direction of the magnetization orientation of each magnet is more than threetimes the coercivity. As shown in Fig. 6 (b), we seethat the inner diameter side of the ferrite magnet A
and both ends of the ferrite magnet B do not satisfy
the required condition.

400 (kA/m)

Ferrite magnet C

1.0
Maximum current carried(Inverter’s limit current)
0.5
Rated current carried

0.0－40Ferrite magnet A－20

Ferrite magnet B

When a maximum current is carried, a large demagnetizing ﬁeld
is observed on the corner on the outer diameter side of the ferrite
magnet A.

Fig. 5Ferrite magnet C

Ferrite magnet B

Ferrite magnet A(a) Magnetization analysis model (Present model)

Fig. 680100

Decreasing Rate of Induced Voltage

When the rated current is carried, there is almost no inﬂuence ofdemagnetization. When the maximum current is carried, however,demagnetization occurs at a low temperature.

Generated magnetic
field (A/m)

Demagnetizing Field Distribution (Analysis Result)

2.5E＋06

Generated magnetic
field (A/m)

Fig. 40204060
Magnet temperature (℃)

2.5E＋06

2.0E＋06
1.5E＋06
1.0E＋06
5.0E＋05
0.0E＋00

2.0E＋06
1.5E＋06
1.0E＋06
5.0E＋05
0.0E＋00

Ferrite magnet A
Position

Ferrite magnet C
Ferrite magnet B
Position(b) Result of magnetization analysis

Result of Magnetization Analysis (Present Model)

The arrow mark indicates the direction of magnetization orientation. The generated magnetic ﬁeld is expressed by the value in this direction. The dotted line indicates the required magnetic ﬁeld. The ferrite magnet A and B are, therefore, known to involve some sections
where the magnetization level is insufﬁcient.

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Generated magnetic
field (A/m)
Ferrite magnet A
Ferrite magnet B(a) Magnetization analysis model (Improved model)

Fig. 7

Generated magnetic
field (A/m)

Ferrite magnet C

2.5E＋06
2.0E＋06

Ferrite magnet A

1.5E＋06
1.0E＋06
5.0E＋05
0.0E＋00

Position

2.5E＋06
2.0E＋06
1.5E＋06
1.0E＋06
5.0E＋05
0.0E＋00

Ferrite magnet C
Ferrite magnet B
Position(b) Result of magnetization analysis

Result of Magnetization Analysis (Improved Model)

Magnetization was accomplished even on the inner diameter side where the magnetic ﬂux was generally difﬁcult to ﬂow. All of the magnets are able to be magnetized completely.

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3 Investigation into Improvements

4 Postscript

For achieving magnetizing completely after therotor assembly, we reviewed the arrangement of theferrite magnets. As mentioned above, as it is difﬁcultfor the magnetic ﬂux to ﬂow into the inner diameterside of the ferrite magnet A in Fig. 1, the width of themagnet was shortened and the angle of the magnet
arrangement was changed for the magnetic ﬂux
ﬂowing smoothly into the inner diameter side of theferrite magnet A. In addition, the ﬂux barriers wereprovided at the inner diameter side of the ferrite
magnet A and both ends of the ferrite magnet B for
more magnetic ﬂux to ﬂow into the ferrite magnet Aand B. In addition, all of the ferrite magnet A, B, andC were designed to have the same size as an
approach toward the goal of mass production forpractical use.
Fig. 7 shows the result of analysis of magnetization for an improved model. Since the generated
magnetic ﬁeld in the ferrite magnet A, B, and C
exceeds the magnetic ﬁeld of the required condition, it is concluded that magnetizing after the rotor
assembly is achieved for an improved model.

This paper introduced the performance evaluation of the IPMSM with ferrite magnets and ourstudy result for future commercialization. We are
one step closer to the ﬁnal development stage on
the F-IPMSM with due consideration to the productivity while obtaining the equivalent performance of
the R-IPMSM.
In the future, we will work on torque performance and efﬁciency improvements and will innovate the manufacturing processes. Further, we willcontinue the evaluation work on the prototype andwork hard to accelerate the study of F-IPMSM application for practical use.

MEIDEN REVIEW Series No.167 2016 No.2

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《Reference》(1) Ota, Matsuhashi, Okitsu, Matsuo: “Development of High-Performance Ferrite Magnet Motor,” Meiden Jiho No.341, 2013/No.4,
pp.61-64 (in Japanese)