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CEDRAT News - N° 69 - March 2016
Comparative Study of Concentrated and Distributed
Winding Using Flux®
Ghania Bara - CEDRAT.
T
he paper presents a comparative study of 3-phase
permanent-magnet (PM) synchronous machines (PMSM) with
concentrated and distributed windings. The purpose of this
study is to identify the machine that gives the better electromagnetic
performance (torque, efficiency, back electromotive force…). Two
PMSM with concentrated and distributed windings having identical
output power, stator and rotor outer diameter, airgap, axial length,
are designed. Machine performance of the two machines is
compared using finite element analysis (Flux 2D).
Comparative study
a) Concentrated winding
The advent of new materials such as permanent magnets,
developments in power electronics (great powers, frequencies, new
topologies ...) and progress in the manufacture of semiconductor
materials are the source of a new generation of electric actuators
and improve the performance of electric drive systems and various
applications.
Among AC motors used in variable speed we find permanent
magnet synchronous machines, adopted in many applications
such as robotics, production of electrical energy (wind turbines),
electric vehicles, railway traction, etc. Synchronous machines with
permanent magnets tend to replace induction machines thanks
to their characteristics, especially with the development of rare
earth magnets (Neodymium Iron Boron and Samarium-Cobalt).
Some advantages:
• Less rotor losses compared with synchronous machines with
wound rotor
• Lower temperature
• Higher efficiency
• Less cooling required compared to induction motors
b) Distributed winding
Fig.Ϯ: tiŶĚiŶg iŶ boƚŚ ŵacŚiŶes
» A - Cogging torque
Cogging torque results from the interaction of the rotor permanent
magnets with the stator teeth [see Fig.3].
This torque produces vibration and noise which is considered
undesirable in most permanent magnets machines. The cogging
torque period can be calculated as follows:
Where LCM is the least common multiplier, Ns the number of slots
and NP the number of poles.
Three types of permanent magnets synchronous machines exist:
• Mounted surface permanent magnets
• Inset permanent magnets
• Buried permanent magnets
The study compares two permanent magnets synchronous
machines with distributed (DW) and concentrated winding (CW)
[see Fig.1].
A+
C-
C-
B+
B+
A+
A+
A+
C-
C-
B+
B+
Fig.ϯ: oggiŶg ƚorƋƵe.
A: Distributed winding
A+
A-
A-
A+
C-
C+
C+
C-
B+
B-
B-
B+
B: Concentrated winding
Fig.1: tiŶĚiŶg iŶ boƚŚ ŵacŚiŶes
All the characteristics are the same for both machines (geometrical
and physical). The differences are in the number of slots in the stator
(12 for C and 36 for DW) and the number of poles (10 poles in CW
and 6 poles in DW).
The study involves doing two simulations (no-load and load) and
for each simulation, computation of the performances of both
machines.
Fig.ϰ: oggiŶg ƚorƋƵe coŵparisoŶ beƚǁeeŶ ƚŚe boƚŚ ŵacŚiŶes.
The cogging torque for the two machines is shown in Fig.4.
It should be noted that the machine with distributed winding
presents a higher cogging torque value than the one with
concentrated winding. This difference can be interpreted as being
due to the higher number of slots (teeth) in the machine with
distributed winding.
» B - Back EMF and open-circuit magnetic field
dŚe cŚoice of ƚŚe ƚLJpe of ǁiŶĚiŶg ĚepeŶĚs oŶ ƚŚe ƚLJpe
of applicaƟoŶ. For edžaŵple iŶ ƚŚe applicaƟoŶ ƚŚaƚ ŶeeĚs
loǁ Ŷoise aŶĚ ǀibraƟoŶ ǁe Ƶse coŶceŶƚraƚeĚ ǁiŶĚiŶg͕
aŶĚ iŶ applicaƟoŶs ƚŚaƚ ŶeeĚ ŚigŚ eĸcieŶcLJ ǀalƵe ǁe
prefer ƚo Ƶse ĚisƚribƵƚeĚ ǁiŶĚiŶg.
The three-phases Back EMF harmonics content for the both
machines with concentrated and distributed winding, is shown
in Fig.5. The value of the no-load voltage depends on the flux
produced by the magnets in the air gap, the speed of the rotor
and the number of stator coil turns. In our test, we used the same
(see continued on page 19)
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CEDRAT News - N° 69 - March 2016
speed (1000 rpm) and adjusted the number of turns in the case of
the machine with concentrated winding to obtain the same Back
EMF as the machine with distributed winding.
» E - The inductance
The self-inductance phase can be calculated as follow:
Fig.5 shows that both machines have nearly the same Back EMF
amplitude.
Where E is the back EMF, Icc short circuit current and f frequency.
Machine A (CW)
Fig.ϱ: acŬ DF FFd coŵparisoŶ beƚǁeeŶ ƚŚe boƚŚ ŵacŚiŶes.
» C - Flux density
The Fig.6 compares the flux density harmonic content of the tow
machines. To compute the normal component of flux density we
created a path in the middle of the air gap.
We see in the case of the machine with concentrated winding
that the 5th harmonic (synchronous harmonic) is very significant
compared to other harmonics.
Machine B (DW)
Short circuit current (A)
440
595
Back EMF (V)
145
146
Frequency (Hz)
83.33
50
Inductance (mH)
0.63
0.78
dable /: calcƵlaƟoŶ of iŶĚƵcƟoŶ.
The table I show the comparison between the inductance for
concentrated and distributed winding, we see little difference.
This difference is due to the value of the short circuit current and
the back EMF.
» F - Iron losses in the stator
Iron losses are calculated by finite-element analysis. The induced
losses in the stator provoke temperature increase that must be
limited.
For iron losses in the stator we find:
• Eddy current
• Hysteresis
• Excess
All these losses are due to flux density harmonics.
It can be seen that iron losses increase as the speed increases. The
losses in the machine with concentrated winding are higher than
in the machine with distributed winding, because the first is rich
in harmonics.
Fig.ϲ: FlƵdž ĚeŶsiƚLJ coŵparisoŶ beƚǁeeŶ ƚŚe boƚŚ ŵacŚiŶes.
» D - Back MMF
Once the winding layout is defined, it is possible to calculate the
magneto-motive force (MMF) created by the stator currents.
The magneto-motive force (MMF) is given by Ampere circulation
law:
Fig.ϴ: /roŶ losses coŵparisoŶ beƚǁeeŶ t aŶĚ t.
Where Ns is the number of turns.
Conclusion
The aim of this study was to compare two machines
(concentrated and distributed winding). The choice of
the type of winding depends on the type of application.
For example in the application that needs low noise and vibration we
use concentrated winding (low cogging torque), and in applications
that need high efficiency value we prefer to use distributed winding.
In the case of concentrated winding the synchronous harmonic is
the number rank per pole.
Fig.ϳ: DDF coŵparisoŶ beƚǁeeŶ ƚŚe boƚŚ ŵacŚiŶes.
Fig.7 shows an MMF comparison between the two machines. As
we can see in the case of the distributed winding the synchronous
harmonic (same frequency as the rotor) is the first harmonic. But in
the case of the concentrated winding, the synchronous harmonic
is the fifth harmonic.
Further options:
• Do the same study load (with and without current harmonics)
• Change the position of the magnet (inset and buried magnets)
• Change the number of the layers in the case of concentrated
winding (single layer)
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