Preliminary Study on Adjustable Field NdFeB Permanent
Magnet Motor Based on Permeability Modulation Technique
Toshihiko Noguchi1, Kiyohiro Iwama2 and Masahiro Aoyama3
Shizuoka University, Hamamatsu, Japan

1,2,3

E-mail: iwama.kiyohiro.14@shizuoka.ac.jp
Abstract. This paper describes an adjustable field permanent magnet (PM) motor based on apermeability modulation technique. The proposed motor has leakage magnetic flux paths
between the rotor magnetic poles. The permeability modulation is a technique for controllingthe amount of the leakage magnetic flux by manipulating the permeability of the magnetic paths.
This paper proposes two adjustable field PM motor models based on the permeability modulation
technique. One is a model using electromagnetic steel sheets and different kind of metalmaterials in the rotor, and the other is a model using only the electromagnetic steel sheets in therotor. The paper presents some computer simulation results and makes a comparison between
the two models.

1. Introduction
Generally, the permanent magnet (PM) motors have achieved higher power density than any other
different types of motors, taking a great advantage of high-energy density PM such as NdFeB. It isdifficult, however, to design a particular PM motor for low-speed-high-torque applications and highspeed-low-torque applications at the same time because of the constant magnetic field caused by the PM.
Therefore, a field weakening technique has been employed by a negative injecting d-axis current to themotor, but the significant copper loss increase due to the d-axis current injection can cause serious
degradation of the efficiency.
In order to solve the problems described above, adjustable magnetic field PM motors are intensively
discussed in recent years [1]-[4]. The references [1] and [3] describe techniques to adjustable operationof an air gap flux by using static magnetic field generated by a field winding. However, there are someproblems about power losses of a DC/DC converter for the field windings, and other problems on higher
power losses by a field winding also remain. On the other hand, the reference [4] reports a different
adjustable magnetic flux technique using inverse magnetic poles generated by a second-order spaceharmonic component. The technique makes it possible to control the magnetic flux freely particularly ina low speed range. Furthermore, almost all the adjustable magnetic flux PM motors are hard to generate
reluctance torque, so they have an essential demerit of low power density, compared with standard interior
permanent magnet (IPM) motors.
For the above, the writers have proposed the adjustable field PM motor based on permeability
modulation technique by using the magnetic saturation of the soft magnetic material positively. The
motor is the model inserted the soft magnet material which is dissimilar metal between the rotor magnetic
poles of the institute of Electrical Engineers of Japan D-model. This model makes it possible to realizes

the adjustable field PM motor with the inverse saliency. However, this model has problem that output
density is low because this model uses the ferrite magnet having low energy density. Two motor models
are proposed in this paper as the adjustable field PM motor based on the permeability modulation using
NdFeB. One is the motor model using different kind of metal and the other is the motor model using only
the electromagnetic steel sheets. The point using the permeability modulation in both motor models iscommon. This paper describes the magnetic circuit design, the performance evaluation of adjustable field
and the fundamental driving characteristics.
2. Principle of adjustable field based on permeability modulation
Fig. 1 shows a basic principle of the permeability modulation used for adjusting the magnetic flux.
As can be seen in the figure, the main magnetic flux is horizontally penetrating the soft magnetic metal,
which is adjusted by the different magnetic flux for permeability modulation penetrating the soft magnetic
metal vertically. By intensifying the magnetic flux for the permeability modulation, the soft magnetic
metal is magnetically saturated, and its permeability is lowered, resulting in the decrease of the main
magnetic flux horizontally flowing into the soft magnetic metal. Therefore, it is possible to control themain magnetic flux amount by using the different magnetic flux for the permeability modulation.
Fig. 2 illustrates an application of the permeability modulation described above, which makes it
possible to control the leakage magnetic flux between the PM poles. The figure shows cross sections of
the 4-pole IPM rotor, where the soft magnetic metal is inserted between the N poles and the S poles ofthe embedded PMs in the rotor. As already described, the permeability modulation of the soft magnetic
metal is carried out by using the differently allocated special winding in the stator in addition to thestandard three-phase windings. As shown in Fig. 2(a), when the soft magnet metal is not magneticallysaturated, short circuits are caused between the PM poles of the rotor. Therefore, the main magnetic fluxof the PMs hardly interlinks to the stator windings, which is equivalent to the field weakening operation.
On the other hand, since the reluctance between the PM poles is increased due to the magnetic saturationof the soft magnetic metal, most part of the PM flux interlinks to the stator windings, which corresponds
to the intensified field operation as illustrated in Fig. 2(b). As described above, the proposed strategy
achieves the adjustable magnetic flux operation by means of permeability variation caused by themagnetic saturation of the soft magnetic metal placed between the PM poles of the rotor. Many of theadjustable magnetic flux approaches give the motor some kinds of electromagnetic energy to weaken the
PM flux of the rotor as presented [1]-[4], but the proposed strategy does not energize the rotor for thefield weakening operation, which is completely different from any other approaches in the past. The
magnetic saturation of the soft magnetic metal is controlled by a radial direction magnetic flux, which isa DC magnetic flux and is generated by a zero-phase current. Therefore, in order to have a magnetic flux
path for the permeability modulation, a three-dimensional magnetic circuit design of the motor is required.
Magnetic flux for permeability modulation
Permanent magnet magnetic flux

Leakage magnetic flux

Rotor

Rotor

Soft magnetic metal(a) Without magnetic saturation.
(b) With magnetic saturation.
Fig. 1. Permeability modulation principle.

Soft magnetic metal(a) Without magnetic saturation. (b) With magnetic saturation.
Fig. 2. Principle of proposed motor.

3. Magnetic circuit design of proposed motor
3.1. Magnetic circuit of model using different kind of metal materials
All the following electromagnetic analyses are conducted with JMAG-Designer 18.0 TM. Fig. 3 showsa quarter motor model using different kind of metal materials. And Table 1 shows the main motor
specifications in this model. A soft ferrite core is inserted between the magnetic poles in this model. Thesaturation magnetic flux density of this soft ferrite core is 0.5 T. As shown in Fig. 3(b), the rotors of the
proposed motor is divided into 2 parts and has special winding to make magnetic flux 3-dimensionally.
The proposed motor makes it possible to realize adjustable field with low magnetomotive force (m.m.f.)
for the permeability modulation and reduce harmonics by dividing the rotor. In addition, the change ofthe inductance decreases, and the winding coefficients increase by adopting distributed winding.
3.2. Magnetic circuit of model using only electromagnetic steel sheets
Fig. 4 shows the quarter motor model using only the electromagnetic steel sheets. The main motor
specifications in this model is the same in Table 1. This model is consisted of the electromagnetic steel
sheets only. This motor configuration can widen adjustable field control width and increase machine
strength because the saturation magnetic flux density of leakage magnetic paths is higher than modelusing different kind of metal materials. And the leakage magnetic paths of the V-shape make it easy topenetrate the magnetic flux for the permeability modulation and decrease m.m.f. for permeability
modulation.
Permeability modulation coilElectromagnetic
steel sheet
Magnetic steel sheet

Permeability modulation coil

(35JN300)

Electromagnetic
steel sheet
Magnetic steel sheet(35JN300)

y
z

z

x

x
z

Flux barrier
y
x

x
z

y
Soft magnet conposit(HB1)

Magnet(N39UH)

Soft magnetic material(MB1H)(a) 3D quarter model.
(b) Cross section.
Fig. 3. Proposed motor modelusing different kind of metal materials.
Table 1.

ySoft magnet conposit(HB1)

Magnet(N39UH)(a) 3D quarter model.
(b) Cross section.
Fig. 4. Proposed motor modelusing only electromagnetic steel sheet.

Specifications of proposed motor.

DC Voltage

270 V

Phase current

50 Arms

Number of poles and slots

8 poles, 48 slots

Number of turns

8

Stator diameterϕ160 mm

Rotor diameter

ϕ100 mmStack length

62 mm

Max m.m.f. for permeability modulation

1500 AT

Armature winding resistance25 mΩ

4. Magnetomotive force source for permeability modulation
Fig. 5 shows 3 motor drive circuits as system for realizing adjustable field PM motor based on
permeability modulation. The characteristics of those circuits is written below down.
4.1. Method using DC/DC converter
Fig. 5(a) shows the method using DC/DC converter. This method introduced in [1] and [3] has good
controllability because it can control the field magnetic flux independently of the d-axis and the q-axis
currents. It also has good performance in terms of the power source voltage utility because the separate
DC/DC converter can apply its voltage to the zero-phase winding regardless of the main inverter DC bus
voltage. However, additional power semiconductor devices are definitely indispensable for the DC/DC
converter circuit, which leads to the extra power losses such as the switching loss and the conduction loss.

4.2. Method using time and/or space harmonics
Fig. 5(b) shows the method using time and/or space harmonics. This method introduced in [4] has good
power supply voltage utility. However, it is impossible to control the m.m.f. for the permeability
modulation freely regardless of the operating points of the motor. The method is based on use of spaceharmonics generated by the concentrated windings of the motor, and it requires diode rectifiers on therotor to obtain the zero-phase winding excitation. The space harmonic magnetic flux induces the e.m.f.
proportional to the rotating speed, so the zero-phase current is almost proportional to the rotating speed,which implies that this method is not applicable to the proposed adjustable magnetic flux PM motor.
4.3. Method using zero-phase current
Fig. 5(c) shows the method using zero-phase current. This motor drive circuit configuration using thethree-phase-four-wire system does not require extra power semiconductor devices at all, and makes it
possible to control the zero-phase current independently of the d-axis and the q-axis currents. In addition,
the potential level at the neutral point of the motor does not vary because the zero-phase current is DC.
The zero-phase winding can be wound separately from the three-phase windings, so the number of turns
can be determined regardless of the three-phase windings, which implies that the superimposed zerophase current onto the three-phase currents can be reduced without sacrificing the m.m.f.
DC

DC

AC

AC
Rotor

DC

AC
DC

AC

DC
StatorDC(a) Method using DC/DC converter.

(b) Method using time and/or space harmonics.
Fig. 5. Motor drive circuits.

(c) Method using zero-phase current.

5. Comparison of performance evaluation of adjustable field in two motor models
5.1. Relative permeability distribution in leakage magnetic paths
Fig. 6 shows the relative permeability distributions when the m.m.f. of 1500 AT is given to thepermeability modulation winding. The motor model in Fig. 6(a) is used the soft ferrite and the model inFig. 6(b) is used the electromagnetic steel sheet in the leakage magnetic paths. The initial relative
permeability of the soft ferrite is 1600 and the maximum relative permeability of the electromagnetic
steel sheet is 18000. As can be seen in Fig. 6, it is confirmed that relative permeability in leakage
magnetic paths decreases by the m.m.f. for the permeability modulation. Therefore, it is possible to vary
the permeability of the soft magnetic metal placed between the PM poles by using the m.m.f. of thepermeability modulation winding.
Leakage magnetic path

Relative
permeability
1000
800
600
400
200
0

(a) Model using different kind of metal.
(b) Model using only electromagnetic steel sheet.
Fig. 6. Relative permeability distributions.

5.2. No-load electromotive force
Fig. 7(a) shows no-load e.m.f. waveforms of the model using different kind of magnetic metals at therotating speed of 3000 r/min under the conditions of 0 AT and 1500 AT m.m.f. of permeability
modulation winding. And Fig. 7(b) shows FFT result of the no-load e.m.f. As can be seen in the Fig. 7,
it is confirmed that this model makes it possible to control 26.9% of the fundamental component of the
no-load e.m.f. Fig. 8 shows no-load e.m.f. waveforms and FFT result of the model using only theelectromagnetic steel sheets on the same condition as Fig. 7. As can be seen in the Fig. 8, it is confirmedthat this model makes it possible to control 61.9 % of the fundamental component of the no-load e.m.f.
Therefore, the model using only the electromagnetic steel sheets has better performance of the adjustable
field than the model using different kind of metal materials. The saturation magnetic flux density of theelectromagnetic steel sheet which 3 times has bigger than the soft ferrite contribute great performance of
the adjustable field. This paper describes basic characteristics of the model using only theelectromagnetic steel sheets after this, because this model has superiority as the adjustable field PM motor.
5.3. Electromagnetic torqueFig. 9(a) shows electromagnetic torque waveforms under the condition of the armature current 50 Arms,
and their FFT analysis results are indicated in Fig. 9(b). As can be seen in these figures, it is confirmedthat the model using only the electromagnetic steel sheet can control 43.2 % of the average electromagnet
torque. However, the ratio of the controllable average electromagnetic torque is lower than the ratio of
the controllable fundamental component of the no-load e.m.f. because of the magnetic saturation in theleakage magnetic paths by the magnetic flux in the q-axis.

0
-20
-40
0

60

120

180

240

300

360

40
30
20

20
0
-20

10

-40

0

-60

0 1 2 3 4 5 6 7 8 9 10 11 12

0

60

20

15

15

5
0

0 AT
1500 AT
0

60

120

180

240

300

30
20

120

180

240

300

360

0

0 1 2 3 4 5 6 7 8 9 10 11 12Oder(a) Waveforms of back e.m.f.
(b) FFT results of back e.m.f.
Fig. 8. No-load e.m.f. of model only electromagnetic steel sheets.

20

10

40

10

Torque (Nm)Torque (Nm)(a) Waveforms of back e.m.f.
(b) FFT results of back e.m.f.
Fig. 7. No-load e.m.f. of model using different kind of metal materials.

0 AT
1500 AT

50

Electrical angle (deg)OderElectrical angle (deg)

60

0 AT
1500 AT

40

Back e.m.f. (V)

20

-60

0 AT
1500 AT

50

Back e.m.f. (V)Back e.m.f. (V)

40

60

60

0 AT
1500 AT

Back e.m.f. (V)

60

360

Electrical angle (deg)

0 AT
1500 AT10
50

0 1 2 3 4 5 6 7 8 9 10 11 12Oder(a) Waveforms of magnet torque.
(b) FFT results of magnet torque.
Fig. 9. Magnet torque with 50 Arms armature current in q-axis.

5.4. N-T characteristics with q-axis current

Fig. 10 shows the rotating speed and torque characteristics (N-T characteristics) with maximum armature
current of 50 Arms in q-axis only. The motor drive circuit introduced in Fig. 5(a) is used in this analysis
to predict the performance only for the motor hardware. As can be seen in this figure, the model using
only the electromagnetic steel sheet makes it possible to realize high-torque and low-rotating speed whenthe m.m.f. of 1500 AT is given to permeability modulation winding and low-torque and high-rotating
speed when the m.m.f. of 0 AT is given to permeability modulation winding.

18

0 AT
1500 AT16

Torque (Nm)

14
12
10
8
6
4
2
0

0

3000

6000

9000

12000

15000

Rotating speed (r/min)Fig. 10. N-T characteristic with q-axis current only.

6. Conclusion
Two adjustable field neodymium PM motor based on the permeability modulation technique werecompared in this paper. One is the model using different kind of metal materials inserted the soft ferrite
core between the magnetic poles. The other is the model using only the electromagnetic steel sheet thatcomposes the rotor core of only the magnetic steel sheet. It was shown from the result of the no-load
e.m.f. that the model using only the electromagnetic steel sheet can control 61.9 % of the field magnetic
flux whereas the model using different kind of metal materials the dissimilar metal can control 26.9 %.
And it is confirmed from the N-T characteristics that the proposed motor makes it possible to balancehigh-torque-low-speed and low-torque-high-speed.
The comparison of adjustable field based on permeability modulation technique and field weakening
control, and verification experiment are our future work.
References[1]
T. Mizuno, K. Nagayama, T. Ashikaga, and T. Kobayashi: “Bashic Principles and Characteristics
of Hybrid Excitation Type Synchronous Machine”, IEEJ Trans. on IA., Vol. 115, No. 11, pp.
1402-1411 (1995)[2]
J.A. Tapia, F. Leonardi, and T.A. Lipo: “Consequent-pole Permanent- Magnet Machine with
Extended Field-Weakening Capability”, IEEE Trans. on IA., Vol. 39, No. 39, pp. 1704-1709(2003)[3]
T. Ogawa, T. Takahashi, M. Takemoto, S. Ogasawara, H. Arita, and A. Daikoku: “TheExamination of Slot and Pole Number Combinations of Consequent-Pole-Type Ferrite
PMAxial Gap Motor with DC Field Windings,” IEEJ Trans. on IA., Vol. 138, No. 8, pp. 659668(2018)[4]
M. Aoyama, K. Nakajima, and T. Noguchi: “Proposal and Preliminaly Experimental Verificatioinof Electrically Reversal Magnetic Pole Type Variable Magnetic Flux PM Motor”, IEEJ Trans.
on IA., vol 137, No. 9, pp. 725-736 (2017)[5]
T.C. Julio：“3D cross coupling effect for flux control in magnetic circuit with Permanent
Magnet”，International Symposium on Power Electronics, Electrical Drives, Automation andMotion, pp. 742-745 (2014)