Proceeding of International Conference on Electrical Machines and Systems 2007, Oct. 8~11, Seoul, Korea
A Study on the Design and the Characteristics
in Single-phase Line-start Permanent Magnet Motor
1
Dae-Sung Jung 1, Seung-Bin Lim1, Jin-Hun Lee1, Sang-Hoon Lee1, Hyung-Bin Lim1,
Youn-Hyun Kim2, Ju Lee1
1
Department of Electrical Engineering, Hanyang University, Korea Department of Electrical Engineering,
Hanyang University, Seoul, 133-791, Korea
2
Department of Hanbat National University San16-1, Duckmyoung-dong, Yuseong-Gu Daejeon 305-719,
Korea
E-mail: jungds61@hanyang.ac.kr
Abstract — The purpose of this paper is the optimal design of
a single-phase LSPM (Line-Start Permanent magnet Motor). A
single-phase LSPM has a permanent magnet in the rotor that is
same as induction motor. For that reason, the transient
characteristics are very different from an induction motor.
Therefore, we need the PM design to consider transient and
steady-state characteristics. In this paper, we designed a singlephase LSPM that has a high power and a good starting
characteristic, and analyzed the performance of designed LSPM
by varying each parameter. Finally, we examined and compared
the simulation results and the prototype motor`s characteristics.
I.
INTRODUCTION
In recent years, high-efficiency motors have been needed
in a large variety of industrial products in order to save
electrical energy. For many applications, a permanent-magnet
synchronous motor can be designed which is smaller in size
but more efficient as compared to the asynchronous machines
such as induction motors [1]. Also the decreasing price of
permanent magnets (PMs) and their improved performances
make PM motors even more interesting in an industrial point
of view as there is an increasing demand for high efficiency
motors, such as compressors for house appliances. Single
phase line-start synchronous motor with interior permanent
magnet and flux barrier inside the rotor has high efficiency
and almost unity power factor in the steady state, and robust
starting performance caused by induction cage [2].
In this paper, for precise analysis and design by coupled
Finite-Element Analysis (FEA), Design of Experiments and
electric circuit analysis in steady state, the variation of d-q
axes inductances by load condition are calculated by FEM
then applied to performance analysis. Moreover, the results
are verified as to compare with the experimental results.
II.
transient state. Therefore, a careful study on the parameters
should be done to make a structure that has good starting
characteristics. Additionally, during start-up, the machine
should show the characteristic of the induction motor,
however, because of the permanent magnet generated
breaking torque and cogging torque, there are cases for
starting motor to have difficulties with starting. In short, a
trade off exists between the permanent magnet size and power
generated and the starting characteristics. Therefore, a design
considering both starting and power should be done [3], [4].
ANALYSIS MODEL
The LSPM motor, which is used in this paper, has a rotor
structure of interior type PM with a higher efficiency, as
shown in Fig. 1. Single phase LSPM has a permanent magnet
inserted rotor which generates reluctance torque caused by the
magnetic resistance difference. The reluctance torque also
varies due to the inductance variance caused during the
Fig. 1. Structure of the single-phase LSPM motor
A. Magnetic circuit analysis of LSPM
The equivalent magnetic circuit is used to design the
permanent magnet of LSPM, and we assumed the flux path as
Fig 2(a). In Fig 2(a), five flux paths are split by cage bars, and
some flux paths may be saturation. So, we assumed the
reluctance of a path 1 and a path 5 is saturation, and the
reluctance of others in rotor are not a saturation. A Fig 2(b) is
a complete equivalent magnetic circuit by the Fig 2(a).
We simulated many models by varying the geometrical
component such as an angle or a radius of magnetic pole. And
we can get some information from simulations that some
models do not make desired torque though same size of
magnetic pole and others have problems to self start.
So, we used the D.O.E to make self starting and desired
torque.
The main specifications of the single phase LSPM is
shown in table I
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TABLE I
The Specification of the Single LSPM
Stator
Rated torque[Nm]
0.34
Rated speed [rpm]
1800
Frequency[Hz]
60
Rotor
Number of pole
4
Air gap
Length[mm]
PM
Residual flux density[T]
0.15
1.12@20 C
o
Fig.3 Tendency analysis of the design factors
(a) Flux pattern of LSPM
C. Flux barrier design of LSPM
Fig 4 shows non barrier, Barrier I and Barrier II.
(b) Magnetic circuit of LSPM
Fig. 2. Simple magnetic circuit analysis of LSPM
B. Optimum rotor design by DOE
The permanent magnet to be inserted in the rotor has
impact not only on the output power but also on the starting
time. For the design, thickness, insert angle, insertion position
and the rotor bar resistance are selected as the main design
parameters. Based on this assumption, the analysis models are
minimized by DOE and a FEA was done for the selected
models. The design factors and their level are shown in table 2.
Fig 3 shows the influence and tendency of the starting time
and torque depending on the selected four parameters.
TABLE II
Design Level and Design Factor of Each Factor
Min
Max
Design
level
Magnet arc[deg]
30
35
2
Magnet thickness[mm]
1
2
2
Magnet deep[mm]
14
15
2
Resistant
1
2
2
Design factors
Fig. 4. Design of barrier
The flux distribution of the non barrier model with a
magnet installed in a arc shape is shown in Fig 5(a).
Permanent magnet design is an important design factor for
efficiency and synchronization of the single phase LSPM.
With a magnet showing high density, the efficiency may
increases but can how loss of synchronization. With a magnet
with low density, the motor can show better synchronization,
however the efficiency will be degraded. Therefore, optimal
magnet design is necessary. Even with optimal designs,
inserting a permanent magnet without a flux barrier, as in Fig
5, the flux does not link with the windings. This increases flux
leakages at the end of the permanent magnet which degrades
the back-emf characteristics. The back-emf waveform is as in
Fig 5(b) and the maximum value is 194[V].
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preventing heat demagnetization caused by high temperature
and reverse magnetic field. Additionally, reviewing heat
demagnetization is essential for a reliable motor design. The
demagnetization characteristic of the NdFeB with different
temperatures is shown in Fig 8. The knee-point exists in the
second quadrant at normal temperature. However, with higher
temperature, even though the knee-point is in the second
quadrant, the residual flux density and coercive force
decreases. A finite element analysis supported by the B-H data
showed that a B lower than the knee-point does not exist. The
results show that non-irreversible demagnetization does not
happen until 140 C.
(a) Vector of magnetic flux density
(b)Result of EMF
Fig. 5. Magnetic flux density and EMF of Non barrier model
To prevent the flux leakage at the ends of the permanent
magnet, a flux barrier can be installed besides the permanent
magnet as shown in model a of Fig 6(a). As it is shown, the
flux leakage at the end of the permanent magnet are decreased,
however, the flux shows a leak through the other poles. The
maximum value of the back-emf is 195[V].
Fig. 8. Influence of temperature of the demagnetization curve
(a) Vector of magnetic flux density
(b)Result of EMF
Fig. 6. Magnetic flux density and EMF of barrier model I
III.
Model II of Fig 7(a) extended the flux barrier to the point
where the squirrel cage were located, with the bars excluded
from the model. This is to prevent the flux leakage to the
adjacent poles. Compared to the non barrier model and the
barrier model 1, the flux leakage has decreased and the flux
path has been made for the flux to link through the armature
windings. This shows that considering flux barriers is
important for designs with optimal permanent magnet designs.
Additionally, this design can reduce the magnet size which
reduces the production cost of the motor. The maximum value
of the back-emf is 263[v] which is higher than the previous
two models also showing a more sinusoidal waveform.
COMPARISON BETWEEN THE ANALYSIS RESULTS AND THE
EXPERIMENTAL RESULT
A. EMF
The back-EMF of simulation and experiment are shown in
Fig 9. The result of main winding is 157 and 168. As it is
shown in Fig 9, the results of simulation and experiment
match well. The motor was connected to the dynamometer to
rotate at the rated speed, 1800rpm
(a) Vector of magnetic flux density
(b)Result of EMF
Fig. 7. Magnetic flux density and EMF of barrier model I
D. Analysis of LSPM considering demagnetization
NdFeB is known to have outstanding magnetic
characteristics. However, carefulmagnet design is required for
Fig. 9. EMF
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B. Speed-Time response
The starting characteristics at no-load is shown in Fig 10.
It can be seen that simulation and experimental results match.
A large overshoot is observed when the motor enters steady
state from transient state. This is because of the initial position
of the rotor is different. A breaking torque is made when the
motor enters synchronization. This breaking torque can be a
positive value or a negative value depending on the initial
position of the rotor. If the breaking torque has a value larger
than 0, overshoot is observed. If the breaking torque has a
value lower than 0, a synchronization without overshoot will
be observed.
IV.
CONCLUSION
In this paper, a single phase LSPM was designed for
energy saving and high efficiency. The stator, bearing,
housing and shaft of the single phase induction motor was
adopted and only the rotor was optimally redesigned. The
permanent magnet inserted to the rotor was selected by DOE
which optimizes the starting time, steady state power output.
The insertion position and permanent magnet size was
selected. For an efficient use of the permanent magnet flux, a
flux barrier was designed and the optimal position of the
squirrelcage was selected. Additionally, the possibility of
thermal demagnetization was also reviewed. To verify the
design results, an experiment was done with a prototype.
Through this paper, the size and the inserted position of
magnet which makes synchronization possible were predicted.
V.
[1]
[2]
[3]
[4]
REFERENCES
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pp.1555-1562, Novemver/December 2000.
T.J.E Miller, "Single-phase permanent-magnet motor analysis.", IEEE
TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. IA21, NO.4, pp.651-658, May 1985.
K.J.Minns and M.A.Jabbar, "High-filed self-start permanent magnet
synchronous motor," Proc. IEE, vol. 128, no.3, pp.157-160, 1981.
Byung-Taek Kim, Young-Kwan Kim and Duk-Jin Kim, "Analysis of
Squirrel Cage Effect in Single Phase LSPM", KIEE International
Transactions on EMECS, Vol. 4-B No.4, pp. 190~195, 2004
Fig. 10. Simulation and measured speed-time responses
C. Result of efficiency and power factor
Fig 11 shows the power factor and efficiency
characteristics of the designed single phase LSPM. PF above
0.95 was achieved for the entire drive range and the efficiency
was 72.3% maximum at load angle 72.
Fig. 11. Measured results of load performance characteristics
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