Proposal and Operation Characteristics of Novel ConsequentPole Permanent Magnet Motor
Toshihiko Noguchi
Shizuoka Universitynoguchi.toshihiko@shizuoka.ac.jp
Kazuhiro Murakami
Shizuoka University
murakami.kazuhiro.14a@shizuoka.ac.jp
Akihisa Hattori
DENSO CORPORATION
akihisa.hattori.j6p@jp.denso.com
Yoichi Kaneko
DENSO CORPORATION
yoichi.kaneko.j6y@jp.denso.com
Abstract. A novel consequent-pole permanent magnet (PM) motor is proposed in the paper,
which has an image pole pair for every pole pair of N and S. The three-phase synchronous
impedances of the motor are balanced owing to the rotor configuration. In addition, it ispossible to carry out field weakening with less d-axis current, which leads to a highly efficient
operation in a high-speed range. This paper describes a basic principle and its operation
characteristics of the newly proposed consequent-pole PM motor.

1. Introduction
Research and development aiming at operation range enlargement of the permanent magnet (PM)motor are intensively promoted in recent years. The PM motor can be designed to generate a higher
torque in a low-speed range by increasing the electromotive force (e.m.f.) with high-energy-density
PMs such as NdFeB and SmCo. In principle, however, it is impossible to design the PM motor toachieve both the high-speed operation and the high-torque generation at low-speeds at the same time;hence field weakening techniques are widely introduced to suppress the e.m.f. in the high-speed
operation range[1][2][3][4][5]. Field weakening techniques are carried out by flowing a negative d-axis
current to counteract the PM magnetic flux and suppress the e.m.f. generation. The copper loss is
produced by the negative d-axis current lead to degrade efficiency of the PM motors. Therefore, it isdesired that a novel configuration of the PM motor is capable of operating field weakening techniqueswith less d-axis currents. In order to make it possible to reduce the d-axis current, a novel geometry of
the PM motor that can increase the d-axis inductance is indispensable because more counter magnetic
flux against the PM magnetic flux can be easily generated with the less d-axis current.

U2

V2
W2

U3SW1N

V1(N)

(S)

(S)

(N)
N

U1

Facing image pole

U1

Facing magnet

U2

Facing image pole

U3

Facing magnet

U4S

StatorRotor

Three-phasewindings

Image‐pole

W1W2W3W4 V4V3V2V1U4PM(a) Proposed motor.
(b) Winding connection of proposed motor.
Fig. 1. Proposed motor model.

In this paper, a novel consequent pole PM motor is proposed, which form a unique alignment of thePMs rather different from a conventional consequent-pole PM motor. The proposed motor has animage pole pair per one pair of N and S PM poles on the rotor. There are three important features
about the proposed motor. The first, three-phase synchronous impedance is balanced. The second, thee.m.f. of the winding on the real PM pole sides offset the e.m.f. of the winding on the image pole sides
in field weakening operation. The third, the d-axis inductance is increased. Therefore, the field
weakening operation is efficiently carried out with the less d-axis current. The above advantages
make it possible to expand the whole operation range and to improve the operation efficiency. In thepaper, basic operation characteristics of the proposed consequent pole PM motor are discussed,compared with a standard surface PM (SPM) motor and a conventional consequent pole PM motor.
2. Configuration and Operating Principle
Figure 1 shows the proposed consequent pole PM motor model. The proposed motor is composed ofequivalent eight-pole (four real PM poles and four image poles) and 12 slot concentrated winging
structure, which make it possible to downsize the motor. The real PM pole pairs are mounted on therotor and the image pole pairs are formed between the real PM pole pairs. This configuration is rather
different from conventional consequent pole PM motors, and this unique configuration gives thefollowing advantages. Focusing on the U-phase winding , for example, two tooth of the twelve arealways facing with the real PM pole pairs, while the other two tooth are always on the image polepairs, i.e., iron core parts. This feature is same in the V-phase and the W-phase, which cannot beexpected in the conventional consequent pole PM motors. Therefore, the magnetic circuit of the rotor
iron core is necessarily imbalance, but the impedance of the stator windings can be balanced if the twowindings on the real PM pole sides and the other two windings on the image pole sides are connected
in series. The three-phase balanced windings do not affect on the motor current control in spite of theimbalance rotor magnetic circuit and give the freedom of magnetic circuit design of the image polepair parts on the rotor.
Field weakening operation of the proposed motor is different from a conventional PM motor, which
decreases the PM magnetic flux by producing a demagnetizing field for the PM. Field weakening
operation of the proposed motor is performed by offsetting the e.m.f. of the winding on the real PMpole sides and the winding on the image pole sides.
The proposed motor has a high inductance value because it has lower magnetic flux due to the half
amount of the PM and the higher amount of the rotor iron than standard SPM motors. The e.m.f. ofthe motor is expressed as (1)V0 = ω( Ψa + Ld id ) + ( Lqiq )
2

2.

(1)

Therefore, the electromagnetic torque of the motor is lower in a low-speed range, but the efficient
high-speed operation can be expected because it requires reduced negative d-axis current owing to thelower magnetic flux of the field PM and the higher inductance value of the stator windings.
3. Operation Characteristics of Proposed Motor
3.1. Comparison of No-Load Electromotive Force
The proposed motor compared with a standard surface PM (SPM) motor and a conventional
consequent pole PM motor by using an electromagnetic analysis software JMAG-Designer 18.0TM.
Fig. 1 shows the proposed motor model, Fig. 2 shows SPM motor and a conventional consequent polePM motor model, of which detailed specifications are listed in TABLE I. As for these motors, themotor size is equivalent, but the rotor structure is different. The proposed motor and the conventional
consequent pole PM motor is composed of equivalent eight-pole (four real PM poles and four imagepoles), but the SPM motor is composed of eight real PM pole. Thus the PM of the SPM motor is
double of the consequent pole PM motors.
Three-phasewindings

Three-phasewindings
(S)

NS

N(S)

N(S)

NS

N(S)

StatorRotorSNNPM

StatorImage‐poleRotor

NS

PM(a) Consequent pole PM motor.
(b) SPM motor.
Fig. 2. Comparison motor models.
TABLE I. Motor specifications.

Proposed PMmotor

Consequentpole PMmotor

SPM motor

Stator diameter

80 mm

←

←

Rotor diameter

42.85 mm←←Stack length

37 mm

←

←

Air gap length

1.045 mm←←

Number of poles8(PM:4,Image pole:4)

8(PM:4,Imagepole:4)

8

Number of slots

12

←

←

Number of turns

16

←

←

PM volume7.672 cc

7.672 cc15.344 cc

Armature
windingconnection

4-series
starconnection←←

PM type

NMX-43SH←←886746

Voltage (V)Voltage (V)Figure 3 shows the no-load e.m.f. waveforms of the U-phase and their FFT analysis results when thethree models rotated at 600 r/min. The e.m.f. amplitude of the proposed motor is half of the SPM
motor because the PM volume is 50 % of the SPM motor's. Therefore, the proposed motor can extend
the high-speed operation range. The proposed motor is lower e.m.f. amplitude and has a lot of
fundamental component in the e.m.f. waveform, compared with the conventional consequent pole PMmotor. The proposed motor is suitable for the high-speed operation with field weakening control.

20-2-4-8060120180240

5432Proposed motor
Consequent pole motor
SPM motor-6Proposed motor
Consequent pole motor
SPM motor

1300

0360

0 1 2 3 4 5 6 7 8 9 10 11 12Order(b) FFT result.

Electrical angle (deg)(a) Back e.m.f. waveform

Fig. 3. Comparison of back e.m.f. and FFT analysis results.

550500450400350300250200150100500q-axis inductance (μH)

d-axis inductance (μH)

3.2. Comparison of Inductance
Figure 4 shows variations of the d-axis and the q-axis inductances of the three models when thecurrent phase angle β is changed under the constant current amplitude (20 A). As the current phaseangle β increase, the d-axis and q-axis inductance is increased because the amounts of both axis
magnetic flux are reduced.
The d-axis inductance of the proposed motor is highest among these motors, and it is larger than theSPM motor by approximately 45 %. The q-axis inductance of the conventional consequent pole PM
motor is highest among these motors. The minimum d-axis and q-axis inductance is SPM motor. Anytypes of the consequent pole PM motor have high inductance value because they are a high proportionof iron in the rotor cores and the reluctance of the magnetic circuit is small. Since the proposed motor
has the largest d-axis inductance, the back e.m.f. can be reduced with less negative d-axis current
during field weakening control.

Proposed motor
Consequent pole motor
SPM motor

015304560

Current phase β (deg)7590550500450400350300250200150100500Proposed motor
Consequent pole motor
SPM motor

015304560

Current phase β (deg)(a) d-axis inductance.
(b) q-axis inductance.
Fig. 4. Comparison of inductance.

7590

3.3. Harmonics Comparison of Magnetic Flux
Harmonic components of the magnetic flux has been compared among the three motors. Only the daxis current is supplied to the motor, and the current amplitude is 20 A . The electromagnetic analysis
has been conducted under an assumption of the ideal iron rotor using the PMs replaced with air.
Figure 5 shows the magnetic flux linkages of the U-phase and their FFT analysis results of the threemotors. The FFT result of the SPM motor includes only the fundamental component, because thepermeance variation is hardly caused. In the conventional consequent pole PM motor, the permeancevariations is caused by the image poles and PMs on the rotor surface: thus resulting in the even-orderharmonic components of the magnetic flux linkage. However, the proposed motor is not affected by
the permeance variation because one U-phase winding is facing with the real PM pole while the otherU-phase winding is on the image pole side: hence the even-order harmonic components are not seen in
the magnetic flux linkage to the U-phase winding.
0.010

Proposed motor
Consequent pole motor
SPM motor

0.008

Magnetic flux linkage (Wb)

Magnetic flux linkage (Wb)

0.012

0.004
0.000
-0.004-0.008-0.012

060

120

180

240

Electrical angle (deg)300360

Proposed motor
Consequent pole motor
SPM motor

0.008
0.006
0.004
0.002
0.000

012

3456789 10 11 12Order(a) U-phase magnetic flux linkage waveform.
(b) FFT result.
Fig. 5. Comparison of U-phase magnetic flux linkage.

3.4. Comparison of N-T Characteristics
The N-T characteristics of the three motors at maximum output power are indicated in Fig. 6. The
test conditions are 12-V DC bus voltage, the maximum current amplitude of 20 A, and the maximum
magnetomotive force (m.m.f.) of 1280 AT.
The SPM motor can deliver high torque in low-speed range due to the no-load e.m.f. is highest
among these motors. On the other hand, the proposed motor can operate widely in high-speed range.
The proposed motor can expand the high-speed range by field weakening control due to the motor has
a low-coefficient of the e.m.f. and the large d-axis inductance of the proposed motor cause the countere.m.f., which is high enough to cancel the PMs' e.m.f. The N-T characteristic of the conventional
consequent pole PM motor is equivalent to that of the SPM motor in high-speed range. This isbecause the e.m.f. includes even-order harmonic components when the field weakening control isapplied to the conventional consequent pole PM motor.
As described above, it has been found that the proposed motor has the largest high-speed operation
range with the least negative d-axis current; thus the field weakening operation can efficiently beachieved among the three motors.
4. Conclusion
A novel consequent pole PM motor has been proposed, and its operating characteristics have also
been investigated in the paper. The proposed motor has a unique rotor configuration where the real
PM pole pairs and the image pole pairs are placed every other pole pair. The image pole pairs areconstituted on the iron core parts next to the real surface PMs on the rotor. One of the most importantfeatures of the proposed motor is high-speed operation capability with less field weakening current

(negative d-axis current); thus efficient expansion of the high-speed operation range is possible
without increasing copper loss.
In the future work, the magnetic circuit of the proposed motor must be investigated because theamount of effective magnetic flux is low, which causes a lower torque in a low-speed range. In order
to increase the amount of effective magnetic flux, it is required to investigate the proposed motor withflux barrier.
4.0
Proposed motor
Consequent pole motor
SPM motor

3.5

Torque (Nm)

3.0
2.5
2.0
1.5
1.0
0.5
0.001000

2000

3000

4000

5000

Speed (r/min)Fig. 6. Comparison of N-T Characteristics.

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1704-1709, 2003.
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