IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO.

11, NOVEMBER 2015 8109804

Design and Analysis of a Novel Brushless

Wound Rotor Synchronous Machine
Qasim Ali1 , Thomas A. Lipo2 , Life Fellow, IEEE, and Byung-Il Kwon1
1 Department of Electronic Systems Engineering, Hanyang University, Ansan 426-791, Korea
2 Department of Electrical and Computer Engineering, Florida State University, Tallahassee, FL 32306 USA

This paper presents a new concept for brushless excitation of a wound rotor synchronous machine (WRSM) based on the
generation and utilization of a subharmonic component of the stator magnetomotive force (MMF). In this method, a dual inverter
topology for the stator winding is proposed. The idea is to generate and utilize an additional subharmonic stator MMF component
along with the fundamental component. The rotor carries two different windings: 1) excitation winding and 2) field winding. The
subharmonic component induces voltage in the excitation winding and feeds the field winding through a rotating rectifier on the rotor.
A 2-D finite-element analysis was performed to analyze and verify the proposed brushless WRSM.
Index Terms— Brushless excitation, finite-element analysis (FEA), synchronous machine (SM).

I. I NTRODUCTION force (MMF), some work was investigated in [7] and [8].
In [7], a fractional slot concentrated winding is used to
E LECTRIC machines play an important role in the modern
world of technology. Scientific research is being carried
out in this field for more than a century. Recently, the
develop 5th and 13th harmonic for an 18-teeth/10-pole
machine. The 5th harmonic is used as a main working
permanent magnet machines are under the spot light because harmonic, while the 13th harmonic is used to excite the rotor
of their high-accuracy fixed-speed drives. But the high price field winding, and hence achieving the brushless operation.
due to the permanent magnet material is a key consideration In [8], the brushless harmonic excitation principle is realized
while making these kind of machines. In contrast, wound rotor by injecting the third-harmonic current component, a high-
synchronous machines (WRSMs) are low price, permanent frequency single-phase current component, or a dc component
magnet-free machines, and they can be used for a wide range into the three-phase stator open windings, to generate a time
of speeds [1]. pulsating magnetic field which can induce back electromotive
In the WRSMs, field winding is used which is supplied with force (EMF) in the specially designed rotor harmonic coils.
a direct current (dc) that generates the rotor flux. A dc current The induced back EMF is rectified and supplies dc current to
can be applied to the rotor winding either using the brush and the rotor field winding.
slip-ring structure or brushless excitation method. The brushes This paper presents a new method for the brushless oper-
and slip rings have losses and maintenance issues, so they are ation of a WRSM based upon the generation of an addi-
used for small SMs. tional subharmonic stator MMF component, rather than a
To overcome this issue, different methods have been used. higher harmonic component. This subharmonic component
Hybrid excitation SMs were brought forward to combine the is utilized for the excitation of rotor windings without the
advantages of both permanent magnet synchronous machine need of brushes and slip rings. A 2-D finite-element analy-
(PMSM) and WRSM [2], [3]. The comparison study between sis (FEA) is carried out to analyze and verify the proposed
hybrid excitation topologies is discussed in [4]. However, a brushless WRSM.
rise in the price of rare-earth magnet caused by its scarcity is
a key consideration while developing such kind of machines. II. P ROPOSED M ACHINE T OPOLOGY
A brushless dc-excited flux-switching machine is studied A. Topology
in [5] with two set of windings in the stator. Torque producing
factors are analyzed to investigate about the true nature of The topology for the proposed brushless WRSM investi-
machine as compared with dc machines, SMs, and switched gated in this paper is shown in Fig. 1. Stator winding is
reluctance machines. divided in two portions inside the stator periphery. Inverter 1
For brushless excitation, conventionally, additional supplies the three-phase current to the winding in one portion
(auxiliary) stator windings were used to excite the rotor field of the machine, while the Inverter 2 supplies the three-phase
winding. Additional excitation winding on stator and a hybrid current to the winding in other portion of the machine. This
rotor to develop a brushless machine is studied in [6]. This dual inverter topology is implied to control currents in the
method faces issues, since both the stator windings are located two separate portions of the stator winding. Difference in
in the same stator core and require a large stator volume magnitude of currents in the two portions of stator winding
for making place for both the windings. Based upon the is responsible for the generation of additional subharmonic
utilization of higher harmonics of the stator magnetomotive component of stator MMF.
Two separate windings are placed in the rotor: 1) excita-
Manuscript received March 20, 2015; revised May 11, 2015, May 18, 2015, tion winding and 2) field winding. The excitation winding
and May 26, 2015; accepted May 26, 2015. Date of publication June 2, 2015; is responsible for the induction of voltage required for the
date of current version October 22, 2015. Corresponding author: B.-I. Kwon
(e-mail: bikwon@hanyang.ac.kr).
excitation of the rotor. The other winding is the field winding.
Color versions of one or more of the figures in this paper are available The induced voltage in the excitation winding is rectified and
online at http://ieeexplore.ieee.org. is supplied to the field winding. The diode bridge rectifier
Digital Object Identifier 10.1109/TMAG.2015.2440433 circuit is placed on the rotor between the two rotor windings.
0018-9464 © 2015 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.
8109804 IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015

Fig. 1. Proposed brushless WRSM with dual inverter topology.

Fig. 2. Machine layout with stator and rotor winding configuration.

B. Operation Principle The subharmonic component rotates with a different speed

Brushless operation for WRSM by the method of utilizing as compared with the synchronous speed. Therefore, to utilize
higher harmonics of the stator MMF is discussed in [7] and [8]. the additional subharmonic produced, an extra winding in
For the proposed machine, instead of using higher harmonics, the rotor is accommodated which has the same number of
a subharmonic is used to induce voltage in the excitation poles as that of poles generated by subharmonic component.
winding of rotor. The main component of the stator MMF is This winding is called the excitation winding, and has the
required for the torque production. responsibility to induce the voltage due to the subharmonic
Both the subharmonic component and the main component component. The induced voltage is rectified and fed to the
of the stator MMF are generated by the three-phase double rotor field winding by the rotating rectifier adjusted inside the
layer distributed windings without using any additional exci- rotor periphery. Field winding on the rotor is used to link
tation winding in the stator, which helps to reduce the machine with the main fundamental component of the stator MMF,
volume losses due to an additional winding and the overall cost so it has number of poles equal to the poles generated by the
of the machine. fundamental stator MMF component. In this way, the brushless
For the proposed topology, the generation of two substantial excitation of the proposed WRSM is achieved. The proposed
components of stator MMF from a single-stator winding lies topology is equally feasible for both motor and generator case.
in the fact that the stator winding is divided in two separate Due to the generation of subharmonics, there will be more
portions inside the stator periphery. Each portion connects to losses and higher torque ripple in the machine as compared
a separate current source inverter, as shown in Fig. 1. The with the conventional WRSM. But in the case of higher
current amplitude from the two inverters is different, while harmonics excitation, discussed in [6], there will be much
the phase and frequency of both the inverters are kept equal. more losses due to the higher frequency of harmonics used.
The currents in the two portions of the winding are given by
⎧ ⎫ III. D ESIGN AND A NALYSIS OF P ROPOSED M ACHINE
⎪ i a1 = I1 sinωe t ⎪
⎪
⎪ 2π ⎪ ⎪ A. Configuration
⎨ ⎬
i b1 = I1 sin ωe t − A 2-D model for the machine was developed based upon
 3  (1)
⎪
⎪ 2π ⎪ ⎪ the proposed WRSM, with 24 stator slots and basic 4-pole
⎪
⎩i c1 = I1 sin ωe t + ⎪
⎭ machine. The layout is shown in Fig. 2.
3
⎧ ⎫ Stator has double layer distributed winding. The left portion
⎪ i a2 = m ∗ I 1 sinωe t ⎪ has the currents described in (1) and the right portion has the
⎪
⎪ 2π ⎪⎪
⎨ ⎬ currents described in (2). The rotor has two separate windings:
i b2 = m ∗ I1 sin ωe t −
 3 ⎪ (2) 1) excitation winding and 2) field winding. The excitation
⎪
⎪ ⎪
⎪
⎩i c2 = m ∗ I 1 sin ωe t + 2π ⎪
⎭ winding is 2-pole winding, intended to induce the subharmonic
3 component and feed the rotor field winding. The field winding
is the 4-pole winding used to link the 4-pole stator field with
where I1 is the fundamental wave current amplitude, ωe is
rotor field.
the electrical angular frequency, t is time, and m is the ratio
between the magnitude of two inverter currents
B. Analysis
i a2
m= . (3) Winding distribution for the proposed machine is shown in
i a1 Fig. 3 along with the stator MMF. It is plotted for the case,
If the currents from the two inverters have the equal amplitude, where currents in A2, B2, and C2 are double in magnitude
i.e., m = 1, then only the fundamental component of the as compared with currents in A1, B1, and C1 and for the
stator MMF will be generated and no subharmonic component time moment when phase A is maximum. ( A1 = 1, A2 = 2,
will be observed. On the other hand, if the currents from the B1 = −0.5, B2 = −1, C1 = −0.5, and C2 = −1).
inverters have unequal amplitude, i.e., m = 1, the subharmonic The waveform has two different portions. For the first
component (with half the frequency of the fundamental com- 180 mechanical degrees, the MMF has a smaller maximum
ponent) and the fundamental component are generated. and minimum values due to the smaller current fed in the
ALI et al.: DESIGN AND ANALYSIS OF A NOVEL BRUSHLESS WRSM 8109804

TABLE I
D ESIGN PARAMETERS OF THE P ROPOSED B RUSHLESS WRSM

Fig. 3. MMF plot for 24-slot stator windings.

Fig. 4. Fundamental and subharmonic component of stator MMF.

Fig. 5. Stator flux density plot.
windings ( A1, B1, and C1). For 180 to 360 mechanical
degrees, the MMF waveform has larger maximum and
minimum peaks due to the higher current magnitude fed to
the windings ( A2, B2, and C2).
This waveform reflects that there are two dominant com-
ponents of the stator MMF (the main component of the
supplied frequency and the subharmonic of half the supplied
frequency), which are produced due to the proposed dual
inverter topology discussed above. These two components are Fig. 6. Induced voltage in rotor excitation winding for (a) transition period
and (b) steady-state period.
responsible for the generation of a 4-pole field and a 2-pole
field for the given configuration. some time (depending upon the temperature of the windings
In Fig. 4, the dominant components generated in the sta- in both portions), the Inverter 1 will increase its current and
tor MMF are shown. The fundamental component is 4-pole Inverter 2 will decrease its current.
component with frequency ω. The subharmonic component is To evaluate the induced voltage in the rotor excitation
2-pole component with frequency ω/2. The amplitude of the winding, stator winding was given currents for two different
subharmonic depends upon the required excitation of the rotor cases. For case 1, the currents in both the winding portions
winding. Both components rotate with different speeds and in were kept the same (m = 1), as given in
opposite directions.
i a1 = i a2 = 1 ∗ sin (120 ∗ πt). (4)
IV. 2-D FEA A NALYSIS In this case, no subharmonic is produced, so there in no
The 2-D FEA was performed for analyzing the performance voltage induced on the excitation winding.
of the proposed machine. The machine parameters are summa- For case 2, the current in one portion of winding was double
rized in Table I. Simulations are carried out for the motor case. as compared with the other portion of the winding (m = 2),
In Fig. 5, the flux density B is plotted for the stator core. as given in
The right side of the machine portion contains the winding
with double current magnitude, so it has higher flux density i a2 = 2 ∗ i a1 = 2 ∗ sin (120 ∗ πt). (5)
as compared with the left side. Although the use of different For this case, a subharmonic component is produced
levels of current in the two portions of the stator winding and voltage is induced in the rotor excitation winding. The
will obviously cause a heating unbalance, this problem can simulation results for both the cases are shown in Fig. 6.
be solved by exchanging the high-current and low-current In Fig. 6(a), voltages for transition period are shown. The
portions of the windings at fixed time intervals. If Inverter 1 voltages become uniform after 0.44 s. In Fig. 6(b), voltages
is initially supplying less current than Inverter 2, then after for steady state are shown.
8109804 IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015

Small deficiencies in average torque and efficiency of the

proposed machine compared with the conventional WRSM can
be justified by brushless and no slip-ring operation.

V. C ONCLUSION
This paper presents a new brushless WRSM based upon
the utilization of the subharmonic of the stator MMF. The
Fig. 7. Torque for m = 2 for the proposed brushless WRSM. brushless operation overcomes the problems associated with
the conventional brushes and slip rings, and also provides an
alternative for the highly expensive PMSM. The proposed
machine has a dual inverter topology feeding the two portions
of the machine stator winding. The difference in current
magnitude in the two portions of winding is responsible for the
production of additional subharmonic. The suitable applica-
tions for this machine are places where sparking from brushes
has to be avoided, i.e., oil and gas fields, vehicle fuel tanks,
and so on.
Fig. 8. Currents for the excitation (E) and field (F) windings. To verify the proposed machine, the 2-D FEA analysis was
carried out for the motor case. Simulations were performed
to validate and compare the presented topology with the
conventional one. The results validate the feasibility of the
proposed machine.

ACKNOWLEDGMENT
This work was jointly supported by the BK21PLUS pro-
gram through the National Research Foundation of Korea
funded by the Ministry of Education, and by the Human
Resources Program in Energy Technology of the Korea Insti-
Fig. 9. Field winding currents for different values of m. tute of Energy Technology Evaluation and Planning (KETEP),
granted financial resource from the Ministry of Trade, Industry
TABLE II
and Energy, Republic of Korea (20154030200730).
C OMPARISON OF THE P ROPOSED M ACHINE W ITH
THE C ONVENTIONAL WRSM
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