Rare-Earth-Less Motor with Field Poles

Excited by Space Harmonics

― Theory of Self-Excitation and Magnetic Circuit Design ―

Masahiro Aoyama Toshihiko Noguchi

Department of Environment and Energy System, Department of Electrical and Electronic Engineering,
Graduate School of Science and Technology, Graduate School of Engineering,
Shizuoka University Shizuoka University
3-5-1 Johoku, Naka-Ku, Hamamatsu, 3-5-1 Johoku, Naka-Ku, Hamamatsu,
Shizuoka 432-8561, Japan Shizuoka 432-8561, Japan
Electric Drive Vehicle Design Department ttnogut@ipc.shizuoka.ac.jp
Suzuki Motor Corporation
aoyamam@hhq.suzuki.co.jp

Abstract— This paper describes a synchronous motor in extremely expensive rare-earth metals such as Dy and Tb must
which space harmonics power is utilized for field magnetization be added to the Nd-Fe-B magnet.
instead of permanent magnets. The stator has a concentrated
winding structure, and the rotor has two sorts of windings, i.e., A variety of rare-earth-less and rare-earth-free motors are
an induction pole (I-pole) winding that retrieves mainly the third recently proposed due to remarkable rise of Nd-Fe-B magnet
space harmonics and an excitation pole (E-pole) winding for the market prices and a global maldistribution problem of the
field magnetization. The both coils are connected via a diode magnet material such as Dy and Tb. Wound-field motors that
rectifying circuit. By utilizing the E-pole torque magnetized with replace the permanent magnets with electromagnets are
self-excitation, permanent magnet volume of the proposed motor intensively investigated both in industries and in academia as
is reduced by 81.4 %, compared with the benchmark IPM motor. post IPM motors [1][2]. For example, a separate excitation
wound-field synchronous motor is proposed in [1]. This motor
Keywords—component; synchronous motor, self-excitation, is capable to utilize armature reaction torque by wound-field
induced current, field current, electromagnets torque, and the field magnetization control allows high
efficiency operation. An external chopper circuit is, however,
I. INTRODUCTION indispensable for the wound-field winding. Furthermore, it is
In recent years, hybrid vehicles (HEVs) and electric rather difficult to transfer the field magnetization power from
vehicles (EVs) driven by shared power with an internal- the primary to the secondary, and an extra copper loss in the
combustion engine or only by electric power are focused on wound-field winding is also a serious problem. Therefore, a
because of environmental concerns such as global warming, self-excitation technique proposed in [3][4] is reevaluated by
exhaustion of fossil fuels, and air pollution problems. An the authors to solve the problems regarding the separate
electric machine is one of the key parts in the HEVs and the excitation wound-field motors. The classic self-excitation
EVs from the viewpoint of dynamic and fuel consumption based synchronous motor has a stator with distributed windings
performances. Traction motors for the HEVs have and a salient pole rotor with a single winding connected via a
significantly unique features, compared with industry applied half-bridge rectifier. The second space harmonics linking to
motors. Wide adjustable speed drive range, high maximum the rotor winding is utilized for the field magnetization.
torque, high power density without sacrificing its efficiency are However, low linkage of the space harmonics to the rotor
demanded to improve the total system efficiency. An IPM winding makes it difficult to improve the motor efficiency
(Interior Permanent Magnet) motor is applied to the HEVs further. In addition, it is rather difficult to retrieve the space
owing to its highly improved efficiency and specific power per harmonics power because the single rotor winding plays both
physical volume. Permanent magnets used for the IPM motor roles of an induction coil and an electromagnet coil at the same
are very expensive because Nd-Fe-B magnets are generally time.
adopted to realize high energy density and to improve fuel In this paper, the problems of the classical self-excited
efficiency at low load operation for street use. Moreover, the synchronous motor are solved, and a new configuration and a
traction motors are usually installed on the chassis where new operation mechanism of the self-excitation are proposed,
special countermeasures must be taken for environmental and focusing on the space harmonics power of the motor. In
thermal issues. In order to restrain demagnetization caused by general, the self-excited motors do not have sufficient field-
temperature rise of the permanent magnets for example, magnetization power in the low speed range due to low

978-1-4799-0224-8/13/$31.00 ©2013 IEEE 7337

 Fig. 2. Rotor winding connection diagram of full-bridge rectifier.

(a) SynRM. (b) Model 1 (without sub-poles).

Fig. 1. Cross section diagram of SynRM and model 1.

TABLE I SPECIFICATION OF MOTOR.

Number of poles 12

Number of slots 18

Stator outer diameter 200 mm

Rotor diameter 138.6 mm

Axial length of core 54 mm

Air gap length 0.7 mm

Maximum current 273 Apk

Fig. 3. Torque characteristics at 1000 r/min and 273Apk.
Stator winding resistance 32.1 mΩ / phase

Number of coil-turn 48

Winding connection 6 parallel

I-pole winding resistance 12.1 mΩ / pole

E-pole winding resistance 26.9 mΩ / pole

Fig. 4. Classical rotor winding connection diagram.
Thickness of core steel plate 0.35 mm
273 Apk under MTPA (Maximum Torque Per Ampere) control
induced voltage in the rotor winding, so a hybrid field calculated by an FEM based magnetic field analysis. As
magnetization of less rare-earth magnet and self-excitation type shown in Fig. 3, increase of the torque of Model 1 can be
wound-filed to reinforce the field magnetization regardless of confirmed in comparison with the conventional SynRM shown
the operation speed is also discussed. in Fig. 1(a) and the classic self-excited motor that has a simple
winding construction as shown in Fig. 4. This is because
Model 1 can add the E-coil torque generated by the self-
II. BRUSHING UP OF THE SELF-EXCITATION TECHNOLOGY excitation to the conventional reluctance torque.
A. Full-Bridge Rectifiier and Double Rotor Windings
B. Sub-Poles in Space Harmonics Magnetic Path
Figure 1(a) shows a SynRM (Synchronous Reluctance
Motor) with a concentrated winding structure in the stator. Assuming a combination between the rotor pole counts and
Figure 1 (b) shows a motor (Model 1) where the wound-field the stator slot counts is 2 to 3, a U-phase self inductance Lu can
coils are added to the rotor salient poles of the normal SynRM. be given as
Conventional common motors dissipate space harmonics LU (θ ) = LS 0 + LS cos 2θ , (1)
power caused by the stator with the concentrated structure,
whereas the proposed motor positively utilizes the space where LS0 is a constant part and LS is an amplitude of the
harmonics power for the field magnetization. Each of I-pole is periodical variation of the self inductance.
a special pole exclusively used for the magnetizing energy Hence, U-phase magnetic flux φS-U is derived as
generation from the third space harmonics. On the other hand,
each E-pole is a salient pole on the rotor for the field excitation, LU (θ )IU (t ) {LS 0 + LS cos 2θ }I S cos(ωt + β )
which uses the retrieved third space harmonics power. Every φS −U = = , (2)
NS NS
I-pole and E-pole is connected in series via a diode rectifying
circuit as shown in Fig. 2, where p means a pole number. where IU(t) is a U-phase armature current, NS is the number of
Specifications of the motor shown in Fig. 1 are listed in Table I. armature winding turn, and β is a current phase. Similarly, V-
Figure 3 shows output torque characteristics at 1000 r/min and

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phase magnetic flux φS-V and W-phase magnetic flux φS-W can
be given as
⎧ ⎛ 2 ⎞⎫ ⎛ 2 ⎞
⎨ LS 0 + LS cos⎜ 2θ − π ⎟ ⎬ I S cos⎜ ωt + β − π ⎟
⎩ ⎝ 3 ⎠⎭ ⎝ 3 ⎠
φS −V = , and (3)
NS

⎧ ⎛ 2 ⎞⎫ ⎛ 2 ⎞
⎨ LS 0 + LS cos⎜ 2θ + π ⎟ ⎬ I S cos⎜ ωt + β + π ⎟
⎩ ⎝ 3 ⎠⎭ ⎝ 3 ⎠
φS −W = . (4)
NS

Therefore, a three-phase armature flux is calculated as follows:

3 1
LS I S cos(ωt + β − 2θ ) .
Fig. 5. Wound-field salient pole motor simplified model.
φS −UVW = φS −U + φS −V + φS −W = (5)
2 NS

An α-axis magnetic flux φS-α and a β-axis magnetic flux φS-β on

the stationary orthogonal reference frame are obtained that
converted two-phase static coordinates from three-phase static
coordinates are given as: I-coil

2 4
φS −α = φS −U + φS −V cos π + φS −W cos π
3 3
, E-coil
I
= S
NS
⎡1
⎣
{ }
1 ⎤
⎢ 2 LS 0 3 cos(ωt + β ) + 3 sin (ωt + β ) − 4 LS cos(ωt + β − 2θ )⎥
⎦
and (6) (a) Model 1 (without sub-poles)

2 4
φ S − β = φ S −V sin π + φ S −W sin π
3 3
. (7)
3 IS ⎡ 1 ⎤
=
2 NS ⎢ LS 0 sin (ωt + β ) + 2 LS sin (ωt + β + 2θ )⎥
⎣ ⎦
Applying a rotational coordinate transform to the above
equation by using a d-axis phase θd and a q-axis phase θq
I-coil E-coil
expressed in Eq. (8), a d-axis magnetic flux φS-d and a q-axis
magnetic flux φS-q can be obtained as Eqs. (9) and (10).
(b) Model 2 (with sub-poles)
π
θ d = ωt , θ q = ωt − , (8) Fig. 6. Third space harmonics vector and flux lines.
2
addition, Eq. (10) shows that the field magnetization energy for
φS − d
I
= Sd
NS
⎡1
{ }
1 ⎤
⎢ 2 LS 0 3 cos(ωt + β ) + 3 sin (ωt + β ) − 4 LS cos(− ωt + β )⎥ ,
⎣ ⎦
the self-excitation is mainly obtained from the third space
harmonics.
and (9) Figure 6(a) shows a magnetic flux vector and flux lines of
3 I Sq ⎡ 1 ⎤ the third space harmonics simulated by the magnetic field
φS − q =
2 NS ⎢ LS 0 sin (ωt + β ) + 2 LS sin(3ωt + β )⎥ . (10) analysis. As shown in the figure, the third space harmonics
⎣ ⎦ magnetic flux mainly flows through a space between the rotor
Figure 5 depicts relationship among the reference frames and a salient poles, of which direction corresponds to the q-axis, and
simplified model of the wound-field salient pole motor. As can leaks to the salient poles on the d-axis. Therefore, the third
be seen in the figure, the rotor salient pole is aligned with the space harmonics power can efficiently be retrieved by placing
d-axis and the q-axis is supposed to be between the two salient sub-poles on the q-axis as shown in Fig. 6(b). Since Model 1
poles. An induced voltage in the rotor winding is caused by has both of the induction coil and the excitation coil on the
the magnetic flux linkages of the d-axis and the q-axis as same salient pole, i.e., the d-axis, it is rather difficult to retrieve
expressed on Eqs. (9) and (10). For simplification of the the third space harmonics power efficiently. Model 2, however,
calculation, a cross-linkage magnetic flux and a leakage has the induction coil and the excitation coil separately on the
magnetic flux are ignored. Since the dq reference frame rotates q-axis and the d-axis, respectively, which results in effective
synchronously at the speed of an angular frequency ω, the rotor retrieval of the third space harmonics power.
winding and the d-axis and the q-axis magnetic fluxes cause
magnetic coupling of the frequency that is higher than the C. Effect of Sub-Poles on Torque Characteristics
synchronous speed. Therefore, Eqs. (9) and (10) show that the Figure 7 shows a cross section diagram of Model 2 that has
space harmonics magnetic path is established on the q-axis. In the I-poles on the q-axis. The I-poles are designed to be

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 Fig. 9. Cross section diagram of Model 2 (with sub-poles).
Fig. 7. Torque characteristics at 1000 r/min under MTPA control.

Fig. 10. Mechanical configuration of Model 2.

Fig. 8. Induced current waveforms in forward direction.
magnetically independent of a main magnetic flux path to
prevent reluctance torque degradation and to concentrate on where Pp is a pole-pair number, Ld and Lq is the d-axis and the
retrieval of the third space harmonics power. Figure 8 shows a q-axis self-inductances, Md is a mutual inductance between the
mechanical configuration of the Model 2 motor, where the I- stator d-axis and the E-pole winding, Id and Iq are the d-axis
poles are fixed by two endplates from the axial direction. and the q-axis armature currents, and Ird is a field current. As
expressed in the above equation, the output torque is
Figure 9 shows comparison of torque characteristics composed of two terms, i.e., a reluctance torque and a
among the three motors described above at 1000 r/min and electromagnet torque. The former is basically independent of
273 Apk under MTPA control calculated by the magnetic field the operation angular frequency ω. The field current to
analysis. As shown in the figure, the output torque of Model 2 generate the electromagnet torque, however, is proportional to
is improved by 19.5 %, compared with Model 1. Furthermore, ω because the q-axis magnetic flux in Eq. (10) links to the I-
the torque ripple of Model 2 is remarkably reduced from that pole winding and time derivative of Eq. (10) gives an induced
of the normal SynRM. Figure 10 shows an induced current of voltage in the I-pole winding. In addition, the second term of
the series connected I-pole winding in forward direction of the Eq. (11), i.e., the electromagnet torque, is proportional to the
diode rectifying circuit. As can be seen in the figure, the field current of E-pole winding; thus it is essential to feed the
induced current of Model 2 is much higher than that of Model current from the I-pole winding.
1, which demonstrates effectiveness of the sub-poles and the
separated two windings, i.e., I-pole and E-pole windings. The
third space harmonics magnetic flux path is established on the III. CONFIGURATION OF PROPOSED MOTOR
q-axis as expressed in Eq. (10) and illustrated in Fig. 6(b), and
the harmonics power can effectively be retrieved by the sub- A. Magnetic Shielding by Minimum Permanent Magnets
poles placed on the q-axis, resulting in the higher output The field current of Model 2 is remarkably improved by
torque. The increased induced current of the I-pole winding splitting the rotor winding into I-pole and E-pole windings.
gives higher field current to the E-pole winding. However, the torque density of Model 2 in the steady state is
still lower than that of the IPM motor. This limitation of the
The output torque of the proposed motor is expressed as torque density improvement is caused by the fact that only the
rotor salient pole produces the electromagnet force in the
{( ) }
Tdq = Pp Ld − Lq I d I q + M d (ωt )I rd (ω, t ) , (11) circumferential direction that contributes torque generation as
shown in Fig. 11. On the other hand, the conventional IPM

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 Fig. 11. Magnetic flux vector of Model 2 in CCW rotation.

Fig. 14. Cross section diagram of benchmark IPM motor.

proposed motor rotates in the CCW direction. As can be seen

in the figure, the armature reaction torque is generated by the
I-pole as well as the torque generated by the E-pole.

B. Benchmark IPM Motor

A benchmark IPM motor is designed to compare the
torque density with the proposed motor. The IPM motor has a
concentrated winding structure in the stator and V-shaped Nd-
Fe-B permanent magnets in the rotor as shown in Fig. 14. All
of the benchmark, Model 1, Model 2, and the proposed motors
are designed on the same basis of the core size and the stator
Fig. 12. Cross section diagram of proposed motor. specifications, e.g., iron core materials, gap length and so forth.
The only difference among these motors is a magnetic circuit
of the rotor. Physical dimensions of one permanent magnet
piece are 12 mm by 5.5 mm for the benchmark IPM motor and
7.5 mm by 3.5 mm for the proposed one, respectively. The
benchmark is designed to utilize efficiently the reluctance
torque as well as the permanent magnet torque by precisely
adjusting the magnetic pole opening angle, flux barrier shape
and magnet quantity.

IV. TORQUE CHARACTERISTICS OF PROPOSED MOTOR

Figure 15 shows an output torque comparison between the
benchmark and the proposed motor. As shown in the figure,
Fig. 13. Magnetic flux vector of proposed motor in CCW rotation.
the torque density at 1000 r/min of the proposed motor is
motor can generate higher electromagnetic force in the 8.2 % lower than that of the benchmark, while the proposed
circumferential direction by strong armature reaction motor surpasses the benchmark at 2000 r/min. It can be
composed by the armature magnetic flux and the rotor confirmed that the additional torque generated by the self-
magnets located in the d-axis. Therefore, it is possible to excitation using the space harmonics power is significant to
generate extra torque with the I-poles without spoiling an satisfy the same level of the total output torque as the
inherent I-pole role, i.e., retrieval of the third space harmonics benchmark. This additional torque by the I-pole greatly
power. This operation can be realized by the armature contributes to increase the total output torque regardless of the
reaction torque in the I-pole by means of magnetic shielding reduced permanent magnet torque although the permanent
with small amount of permanent magnets. The armature magnet volume of the proposed motor is reduced by 81.4 %,
reaction torque is available by facing the permanent magnet compared with the benchmark. Moreover, another superior
flux toward the armature magnetic flux. Figure 12 shows a point of the proposed motor can be found in the torque ripple
configuration of the proposed motor capable to generate the characteristic. The torque ripple of proposed motor is 18.2 %
armature reaction torque in the I-poles. Small pieces of the under MTPA control, which is remarkably improved from
permanent magnets are inserted between the I-poles and the Model 2 (43.2 %) and is comparable with the benchmark IPM
rotor core and its magnetic flux prevent the increase in d-axis motor (12.8 %). Figure 16 shows the induced current
inductance. In addition, that magnets can generate higher waveforms. It can be seen that the induced current of
electromagnetic force in the circumferential direction by proposed motor has a slow falling edge. This waveform
armature reaction with the same principle of IPM conventional improvement reduces the field current ripple and enlarges the
motor. Figure 13 shows a magnetic flux vector when the E-coil torque. Figure 17 shows current phase-average torque

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 Fig. 15. Torque characteristics of proposed motor. Fig. 17. Current phase-torque characteristics of proposed motor.

Fig. 16. Induced current characteristics of proposed motor. Fig. 18. Current phase-torque characteristics of benchmark and proposed
motors.
characteristics of the benchmark and the proposed motors at
retrieved and the total output torque can effectively be
1000 r/min. Since the self-excited electromagnet torque has
enhanced. In addition, the output torque characteristic can
the maximum value at the current phase of 90 deg, the MTPA
dramatically be improved by inserting small permanent
control point of the proposed motor slightly advances with
magnets into the I-poles. The future work is to setup an actual
respect to the current phase, compared with that of the
prototype machine and to verify the analysis results through
benchmark. Although the magnet torque of the proposed
experimental tests.
motor is largely decreased due to the small size of the
permanent magnet, the total output torque is comparable with
the benchmark, owing to the electromagnet torque generated REFERENCES
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V. CONCLUSION Japanese).
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This paper has proposed a new rare-earth-less motor which IEEJ Transactions on Industry Applications, vol.78, no. 842, pp. 1430-
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