This article has been accepted for inclusion in a future issue of this journal.

Content is final as presented, with the exception of pagination.

IEEE TRANSACTIONS ON MAGNETICS 1

Consequent-Pole Hybrid Brushless Wound-Rotor

Synchronous Machine
Asif Hussain 1, Shahid Atiq2 , and Byung-il Kwon 1

1 Department of Electronic Systems Engineering, Hanyang University, Ansan 15588, South Korea
2 Department of Electrical Engineering, Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan

This paper presents a novel consequent-pole hybrid brushless wound-rotor synchronous machine (CPH-BL-WRSM). The proposed
machine employs permanent magnets (PMs) on the alternate poles of the salient pole rotor, which has considerably less magnet
usage compared to the regular (PM) assisted synchronous machine. The proposed CPH-BL-WRSM offered the advantages of high
starting torque, high torque density, and better performance under full-load conditions. The BL operation for the proposed machine
is achieved by generating a composite magneto motive force (MMF) with a fundamental component and sub-harmonic component.
The sub-harmonic component of the stator MMF is used for BL excitation of the rotor. The finite-element method was used to analyze
the performance of the CPH-BL-WRSM in comparison with the performance of the existing BL-WRSM and regular PM-assisted
BL-WRSM. Finally, a prototype of the CPH-BL-WRSM was manufactured and the experiment was conducted to demonstrate the
performance of the proposed CPH-BL-WRSM.
Index Terms— Brushless (BL) excitation, consequent pole, harmonic winding, sub-harmonic component, synchronous machine.

I. I NTRODUCTION

S EVERAL brushless (BL) topologies for wound-rotor syn-

chronous machines (WRSMs) have been presented in
recent years to replace the brushes and slip ring assembly in
conventional WRSMs. In [1]–[3], a third-harmonic component
of the stator magnetomotive force (MMF) was utilized to
induce a voltage in the harmonic winding placed on the rotor.
This induced voltage was then rectified, and the current was
supplied to the field winding of the machine for BL operation. Fig. 1. Single-inverter fed BL-WRSM topology [5].
A similar kind of idea was utilized in [4], [5]. However, in
these BL topologies, the sub-harmonic component of the stator
MMF was generated and used to provide the rotor flux for the However, the presence of the PM on each pole increased the
BL excitation of the WRSM. cost of the machine.
The BL operation of the WRSM enables these machines The consequent-pole PM machines can be a cost-effective
to perform as an alternative to the permanent-magnet syn- solution, which utilize less magnet volume to exhibit similar
chronous machines (PMSMs) due to their low cost and performance compared to the regular PM-assisted synchronous
competitive performance compared with PMSMs [6]. How- machine [9]–[12]. In [10], a consequent-pole hybrid excited
ever, the drawback of the BL WRSMs is their low start- vernier machine is presented for achieving high torque density
ing torque due to the absence of a constant excitation at low speed and excellent flux weakening performance at high
source on their rotor. Although BL-WRSMs achieve the speed. However, a dc field winding is used to enable the
rated torque at the base speed, low starting torque under the flux regulation, which requires an external dc supply for field
base speed makes these machines less suitable for variable- excitation.
speed applications. In [7], a partitioned-stator flux-switching In this paper, a novel consequent-pole hybrid BL-WRSM
permanent-magnet (PM) machine using mechanical flux (CPH-BL-WRSM) is proposed to overcome the problem
adjusters has been presented and analyzed for hybrid electric of low starting torque and to provide high torque density
vehicles. and better performance compared with those of the existing
A PM-assisted BL-WRSM (PMa-BL-WRSM) was pre- BL-WRSM presented in [5] and PMa-BL-WRSM. The BL
sented in [8] to overcome the problem of low starting torque operation of the proposed machine is achieved by utilizing
and to provide better performance under full-load conditions. the BL topology shown in Fig. 1. The performance of the
proposed CPH-BL-WRSM is analyzed and compared with the
Manuscript received March 16, 2018; revised April 17, 2018; accepted performance of the existing BL-WRSM and regular PMa-BL-
May 10, 2018. Corresponding author: B. Kwon (e-mail: bikwon@ WRSM by using 2-D finite-element analysis (FEA). Finally,
hanyang.ac.kr). the performance of the proposed machine is verified through
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org. an experimental test, which demonstrates good agreement with
Digital Object Identifier 10.1109/TMAG.2018.2837690 the simulation results.
0018-9464 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
 This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

2 IEEE TRANSACTIONS ON MAGNETICS

TABLE I
M ACHINE D ESIGN PARAMETERS

Fig. 2. Machine topologies. (a) Stator with double-layer distributed winding.

(b) BL wound rotor. (c) PM-assisted BL rotor. (d) Consequent-pole hybrid
BL rotor.

II. M ACHINE T OPOLOGIES AND O PERATING

P RINCIPLE OF CPH-BL-WRSM
A. Machine Topologies
Fig. 2 demonstrates the machine topologies of existing
BL-WRSM, PMa-BL-WRSM, and the proposed CPH-BL-
WRSM. The stator configuration of all three machines is the Fig. 3. Diagram of the flux lines of the proposed CPH-BL-WRSM.
same: 48-slots with eight-pole double-layer distributed wind-
ing. The stator winding is divided into two sets of windings, sinusoidal currents to the stator windings. The sub-harmonic
ABC and XYZ, as shown in Fig. 2(a). The outer dimensions component of the stator MMF induces the voltage in the
of the rotor are the same for the three machines. The rotor for harmonic winding placed on the rotor, which is then recti-
the BL-WRSM is only wound type, which has an eight-pole fied to supply the dc current to the field windings for BL
field and four-pole harmonic winding as shown in Fig. 2(b) [5]. operation [5]. Due to the BL excitation, initially there will be
However, PMa-BL-WRSM and the proposed CPH-BL-WRSM no induced voltage in the harmonic winding, which results in
have hybrid rotor with field winding, harmonic winding, and low starting torque.
the PM inserted in the rotor pole face. The number of turns Fig. 3 shows the flux line plot of the proposed CPH-BL-
for field and harmonic winding and the PM dimensions for the WRSM with windings and PMs on the alternate poles of the
PMa-BL-WRSM are kept the same as PM-assisted machine rotor. The PMs are magnetized in such a direction that the
studied in [8]. The configuration of the rotor for the PMa- flux generated by the PMs strengthens the flux generated by
BL-WRSM is shown in Fig. 2(c). Fig. 2(d) shows the rotor the field windings. Moreover, the PMs help the machine with
for the proposed CPH-BL-WRSM. The only difference in the smooth startup without any external aid. This phenomenon
rotor of the proposed machine is that the PMs are placed on is the same in both the PMa-BL-WRSM and the proposed
the alternate rotor poles. To improve the performance in terms CP-BL-WRSM, but the proposed machine with consequent-
of average torque and torque ripple, the magnet pole span poles utilizes lesser magnet volume compared to the PMa-BL-
for the CPH-BL-WRSM was optimized. The detailed design WRSM in which PMs are inserted on each pole of the rotor.
parameters for all three machines are listed in Table I. Initially, the proposed machine starts as the consequent-pole
PMSM because there is no induced voltage in the harmonic
B. Operating Principle winding. Therefore, at starting, the excitation flux is provided
The single-inverter fed BL excitation topology applied to the only by the PMs for the torque production. With the increase
existing BL-WRSM, PMa-BL-WRSM, and CPH-BL-WRSM in speed, voltage is induced in the harmonic winding which is
is shown in Fig. 1. In this BL topology, the sub-harmonic then rectified and supplied to the rotor field winding. At this
component of the stator MMF is generated and utilized to stage, the machine operates as the CPH-BL-WRSM with the
excite the rotor field winding. The sub-harmonic component flux from both the PMs and rotor field winding.
is generated by dividing the stator windings into two sets of Fig. 4 shows the flux line plots predicted by FEA of the
series-connected windings: winding ABC and winding XYZ. BL-WRSM, regular PMa-BL-WRSM, and the proposed CPH-
The number of turns of winding ABC is double than that of BL-WRSM at the rated load conditions. All three FEA models
winding XYZ, and an inverter is used to supply three-phase have the same dimension, except for the rotor type. The
 This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

HUSSAIN et al.: CONSEQUENT-POLE HYBRID BRUSHLESS WOUND-ROTOR SYNCHRONOUS MACHINE 3

Fig. 6. Induced rotor currents. (a) Harmonic current. (b) Field current.

Fig. 4. Flux line plot predicted by FEA. (a) Existing BL-WRSM. (b) Regular
PMa-BL-WRSM. (c) Proposed CPH-BL-WRSM.

Fig. 5. Back EMF comparison.

peak-to-peak flux linkage in the stator winding of the proposed

CPH-BL-WRSM is 0.61 Wb, while the flux linkage in stator
winding of existing BL-WRSM and regular PMa-BL-WRSM
is 0.57 and 0.571 Wb, respectively. Moreover, the leakage
flux in the proposed machine is small as compared to the
existing BL-WRSM and regular PMa-BL-WRSM. Therefore,
the proposed machine exhibits a higher back-electromotive Fig. 7. Output torque. (a) Existing BL-WRSM. (b) PMa-BL-WRSM.
(c) Proposed CPH-BL-WRSM. (d) Torque comparison at steady state.
force (EMF), and higher torque density as compared to the
BL-WRSM and regular PMa-BL-WRSM. In order to further
compare the validity of the CPH-BL-WRSM, its performance, a stator current of 3.4 Arms at 60 Hz. Both the PM-assisted
including the back EMF, starting torque, torque density, machines required 24.4% less stator current compared with
PM usage, and efficiency, is compared with the existing the BL-WRSM because of the PMs used to provide the rotor
BL-WRSM and the regular PMa-BL-WRSM. flux in assistance with the induced field current.
The sub-harmonic component of the stator MMF induces
III. P ERFORMANCE A NALYSIS BY FEA an alternating voltage in the harmonic winding. This induced
A. No-Load Analysis voltage is rectified to provide a dc current to the field winding
The back EMF was simulated for all three machines at the of the machine. The rotor currents in the harmonic and the
rated speed of 900 rpm by supplying the same dc current of field windings are shown in Fig. 6(a) and (b), respectively.
9 A to the rotor field winding. The back EMF comparison The existing BL-WRSM achieves steady state after 150 ms,
is given in Fig. 5. It can be seen that the back EMF of the while the regular PMa-BL-WRSM and the proposed CPH-
proposed CPH-BL-WRSM is 34.5% higher than that of the BL-WRSM achieves the steady state after 300 ms. This is
existing BL-WRSM because the proposed machine is assisted due to the small magnitude of the sub-harmonic compo-
by PMs. When comparing the back EMF of the proposed nent in both PM-assisted machines. The output torque of
CPH-BL-WRSM with the back EMF of the regular PMa-BL- the existing BL-WRSM, PMa-BL-WRSM, and the proposed
WRSM, the proposed machine 9.5% higher back EMF due to CPH-BL-WRSM is shown in Fig. 7(a)–(c), respectively.
the small flux linkage in the regular PMa-BL-WRSM. It can be observed that the starting torque of the existing
BL-WRSM is zero due to the BL operation. However, the start-
B. Load Analysis and Performance Comparison ing torque of the PMa-BL-WRSM and the proposed CPH-
The three machines studied in this paper are designed for BL-WRSM is 4.16 and 5.69 Nm, respectively. The proposed
a 1-hp rating with a rated speed of 900 rpm. A current machine exhibits a 26.9% larger starting torque than the
of 4.5 Arms at a frequency of 60 Hz was applied to the stator PMa-BL-WRSM because of the high flux linkage. The
winding of the existing BL-WRSM. Part of this stator current overall torque performance comparison at steady state is
is used to induce the voltage in the harmonic winding, and shown in Fig. 7(d). It can be observed that the proposed
the rest is used for torque production. However, PMa-BL- CPH-BL-WRSM exhibits higher torque ripple compared to the
WRSM and the proposed CPH-BL-WRSM are supplied by existing BL-WRSM because the air-gap reluctance variation
 This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

4 IEEE TRANSACTIONS ON MAGNETICS

TABLE II
P ERFORMANCE C OMPARISON BY 2-D FEA

Fig. 8. Proposed CPH-BL-WRSM prototype. (a) Stator. (b) Rotor.

Fig. 9. Experimental setup.

in the proposed machine is greater compared to the existing

BL-WRSM. However, it can be seen that the magnet depth
in the regular PMa-BL-WRSM is greater compared to the
proposed CPH-BL-WRSM, which results in greater air gap
reluctance variation compared to both the existing BL-WRSM
and the proposed CPH-BL-WRSM. Therefore, the torque
ripple of the regular PMa-BL-WRSM is greater as compared
to both the proposed machine and existing BL-WRSM.
The effectiveness of the proposed CPH-BL-WRSM is Fig. 10. Torque at the synchronous speed.
examined using a performance comparison with the existing
BL-WRSM and regular PMa-BL-WRSM. All the design para-
meters for the three machines are the same, including the outer IV. P ROTOTYPE AND E XPERIMENTAL V ERIFICATION
dimensions, air-gap length, field winding turns, stator and rotor In order to validate the analysis of the proposed CPH-BL-
coil diameters, and the number of turns per phase of stator WRSM, a prototype machine has been built as shown in Fig. 8.
winding. However, the rotor types are different in the three Fig. 8(a) shows the eight-pole, 48-slot stator with double-
machine topologies, and in the case of PMa-BL-WRSM and layer distributed winding. The stator winding is composed
CPH-BL-WRSM, the number of harmonic winding turns is 32 of series-connected windings: winding ABC and XYZ. The
rather than the 48 turns in the existing BL-WRSM. Because end connections of winding XYZ are joined to make the
of this, the resistance of the harmonic winding is greater in neutral of the machine, whereas the stator currents are supplied
the existing BL-WRSM. Table II summarizes the performance through winding ABC. Fig. 8(b) shows the rotor of the CPH-
comparison in terms of torque performance including the BL-WRSM prototype with the PM inserted on the rotor pole
starting torque, torque density, torque per magnet usage, and face. The rotor field and harmonic windings are connected in
efficiency. The total torque of the proposed CPH-BL-WRSM is parallel through the bridge rectifier.
10.9% higher than that of existing BL-WRSM and 9.5% higher Fig. 9 shows the complete experimental setup with the
than the torque of the regular PMa-BL-WRSM. The torque prototype machine connected to the load machine, dc power
per magnet usage in the proposed machine is 0.73 Nm/cm3 supply, inverter, oscilloscopes, and torque sensor used for the
as compared to the 0.3 Nm/cm3 in PMa-BL-WRSM, which is torque measurement.
mainly due to the high flux linkage in the proposed machine. To achieve the desired speed, a closed-loop speed and
Moreover, the efficiency of the proposed machine is 1.91% current control are implemented for the motor drive in the
and 4.22% greater than the efficiency of the PMa-BL-WRSM synchronous frame of [13]. The measured torque for the
and the existing BL-WRSM, respectively. The torque ripple is prototype at the rated load conditions is shown in Fig. 10.
significantly lower than that of the regular PMa-BL-WRSM, The prototype machine 7.8 Nm of torque, compared with the
but it is still a bit higher in the proposed machine as compared 8.18 Nm obtained by FEA. A torque ripple of 35.5% was
to the existing BL-WRSM. However, the proposed machine observed in the prototype. Moreover, the efficiency of the pro-
exhibits higher torque density which makes it more suitable totype is 82.56%, compared to 83.24% analyzed by 2-DFEA.
as compared to the existing BL-WRSM and the regular The performance of the prototype machine is summarized
PMa-BL-WRSM. in Table III.
 This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

HUSSAIN et al.: CONSEQUENT-POLE HYBRID BRUSHLESS WOUND-ROTOR SYNCHRONOUS MACHINE 5

TABLE III R EFERENCES

C OMPARISON OF S IMULATION AND M EASURED R ESULTS
[1] F. Yao, Q. An, X. Gao, L. Sun, and T. A. Lipo, “Principle of
operation and performance of a synchronous machine employing a new
harmonic excitation scheme,” IEEE Trans. Ind. Appl., vol. 51, no. 5,
pp. 3890–3898, Sep./Oct. 2015.
[2] F. Yao, Q. An, L. Sun, and T. A. Lipo, “Performance investigation
of a brushless synchronous machine with additional harmonic field
windings,” IEEE Trans. Ind. Electron., vol. 63, no. 11, pp. 6756–6766,
Nov. 2016.
[3] G. Jawad, Q. Ali, T. A. Lipo, and B. I. Kwon, “Novel brushless
wound rotor synchronous machine with zero-sequence third-harmonic
V. C ONCLUSION field excitation,” IEEE Trans. Magn., vol. 52, no. 7, pp. 1–4, Jul. 2016.
A CPH-BL-WRSM was presented and developed to [4] Q. Ali, T. A. Lipo, and B. I. Kwon, “Design and analysis of a novel
brushless wound rotor synchronous machine,” IEEE Trans. Magn.,
improve the starting torque, torque density, and efficiency of vol. 51, no. 11, pp. 1–4, Nov. 2015.
the machine. The proposed machine has the hybrid excited [5] A. Hussain and B.-I. Kwon, “A new brushless wound rotor synchronous
rotor with the winding and PMs. The sub-harmonic component machine using a special stator winding arrangement,” Elect. Eng.,
pp. 1–8, Nov. 2017, doi: 10.1007/s00202-017-0662-8.
of the stator MMF was used to excite the rotor field winding [6] T. A. Lipo and Z. S. Du, “Synchronous motor drives-a forgotten option,”
along with the PMs, which acted as a constant source of in Proc. Int. Aegean Conf. Elect. Mach. Power Electron. (ACEMP),
the rotor flux. The advantages of the CPH-BL-WRSM were Int. Conf. Optim. Elect. Electron. Equip. (OPTIM), Int. Symp. Adv.
Electromech. Motion Syst. (ELECTROMOTION), Sep. 2015, pp. 1–5.
determined by comparing the FEA results of the proposed [7] C. H. T. Lee, J. L. Kirtley, and M. Angle, “A partitioned-stator flux-
CPH-BL-WRSM with the recently developed BL-WRSM and switching permanent-magnet machine with mechanical flux adjusters for
the regular PMa-BL-WRSM. The proposed machine exhibits hybrid electric vehicles,” IEEE Trans. Magn., vol. 53, no. 11, pp. 1–7,
Nov. 2017.
higher back EMF, starting torque than that of the BL-WRSM [8] Q. Ali, S. Atiq, T. A. Lipo, and B. I. Kwon, “PM assisted, brush-
due to the PM-assisted rotor in the proposed machine. The less wound rotor synchronous machine,” J. Magn., vol. 21, no. 3,
torque per magnet usage in the proposed machine is 58.9% pp. 399–404, Nov. 2016.
[9] D. Li, R. Qu, J. Li, and W. Xu, “Design of consequent pole, toroidal
greater than that of the regular PMa-BL-WRSM due to the winding, outer rotor Vernier permanent magnet machines,” in Proc.
high flux linkage in the proposed CPH-BL-WRSM. The IEEE Energy Convers. Congr. Expo., Pittsburgh, PA, USA, Sep. 2014,
experimental test of the proposed machine revealed the output pp. 2342–2349.
[10] H. Wang et al., “A novel consequent-pole hybrid excited Vernier
torque within 4.65% error and torque ripple within 7.9% error machine,” IEEE Trans. Magn., vol. 53, no. 11, pp. 1–4, Nov. 2017.
at the synchronous speed. [11] S.-U. Chung, J.-M. Kim, D.-H. Koo, B.-C. Woo, D.-K. Hong, and
J.-Y. Lee, “Fractional slot concentrated winding permanent mag-
ACKNOWLEDGMENT net synchronous machine with consequent pole rotor for low speed
direct drive,” IEEE Trans. Magn., vol. 48, no. 11, pp. 2965–2968,
This work was supported in part by the Human Resources Nov. 2012.
Program in Energy Technology of the Korea Institute of [12] S.-U. Chung, J.-W. Kim, Y.-D. Chun, B.-C. Woo, and D.-K. Hong,
“Fractional slot concentrated winding PMSM with consequent pole
Energy Technology Evaluation and Planning, granted financial rotor for a low-speed direct drive: Reduction of rare earth permanent
resource from the Ministry of Trade, Industry and Energy, magnet,” IEEE Trans. Energy Convers., vol. 30, no. 1, pp. 103–109,
South Korea under Grant 20154030200730, and in part by the Mar. 2015.
[13] S. Atiq, T. A. Lipo, and B. I. Kwon, “Wide speed range operation of
BK21PLUS Program through the National Research Founda- non-salient PM machines,” IEEE Trans. Energy Convers., vol. 31, no. 3,
tion of Korea within the Ministry of Education. pp. 1179–1191, Sep. 2016.