(DaeSung Jung)
†iD
Copyright © The Korean Institute of Illuminating and Electrical Engineers(KIIEE)
Key words
DOE(Design of Experiments), FEM(Finite Elements Method), High efficiency, LSPM(Line-Start Permanent Magnet Motor), Trade-off
1. Introduction
Recently the volatility of oil prices and limitation of energy resources made society
focus on energy saving. Additionally, the growing needs for well-being has made discussions
on a cleaner environment more active.
As the demand for environmentally-friendly products has increased, the importance
of motor efficiency has emerged and is currently a very important factor for home
appliances. For this, industrial research proposed reducing the core lamination thickness
or adopting materials with less iron loss. Currently in the home appliance market,
the single phase induction machines are the dominant machines in use. The ease of
manufacturing and low cost are the main reasons for this. However, the oscillation
made by the asymmetric magnetic field of the main windings and the auxiliary windings
and the efficiency reduction caused by the core loss of the rotor have been always
noted as the limiting factors for efficiency. Therefore, single phase LSPMs are being
highlighted to overcome this inefficiency problem. A single phase LSPM does not need
a separate power supply device, as it can be line-started, and shows steady-state
characteristics, similar to a permanent magnet motor, which has improved efficiency
as there is no loss caused due to slip induction[1-3].
Single phase LSPMs have a permanent magnet inserted rotor, which generates reluctance
torque caused by the magnetic resistance difference. Reluctance torque varies due
to the inductance variance caused during the transient state. Therefore, a careful
study on the parameters should be conducted to devise a structure that has good starting
characteristics. Additionally, during start-up, the machine should maintain the characteristics
of the induction motor, however, because of the permanent magnet generating breaking
torque and cogging torque, the starting motor can have difficulties. In short, a trade
off exists between the permanent magnet size and power generated, and thereby the
starting characteristics. Therefore, a design considering both, starting and power
should be done[4, 5].
In this paper, analysis using DOE and FEM was conducted to obtain an optimum design
of a motor showing good performance, power, and starting characteristics. Also, a
flux barrier was considered to set the flux path of the permanent magnet; a prototype
was manufactured and verified the simulation results.
2. Model and analysis method
The characteristic equation of the single phase LSPM can be known from the d-q axis
voltage equation, flux linkage of the d-q axis, voltage equation of the rotor, and
the equivalent circuit of the d-q axis. The torque can be expressed as follow:
In the equation, Ti is the induction torque generated by the squirrel bars (2), Tr is the reluctance torque generated by the inductance difference between the d-axis
and q-axis (3). Tm is the torque made by the permanent magnets as shown in (4).
2.1 Optimum rotor design of single phase LSPM by DOE
The single phase LSPM studied in this paper reuses components (stator, shaft, bearing
etc.) of a conventional single phase induction machine and adopts a magnet inserted
rotor for optimal design. This can make the LSPM more cost effective and easier to
mass produce. The magnet used in this study requires a linear demagnetization characteristic
to avoid demagnetization caused by high stator current during starting. NdFeB is considered
as the appropriate material as it has a high coercive force and residual flux density.
The main specifications of the single phase LSPM are shown in Table 1.
For designing the permanent magnet, common design methods of a permanent magnet motors
using the magnetic equivalent circuit are not adaptable, as they do not consider the
transient state and the steady state simultaneously. Thus, the finite element method
(FEM) can be more effective, as it can recongize the two states simultaneously. The
initial permanent magnet size was designed using the FEM considering the starting
torque and efficiency.
The permanent magnet to be inserted in the rotor has an impact not only on the output
power but also on the starting time. For the design, thickness, insertion angle, insertion
position, and rotor bar resistance are selected as the main design parameters. Based
on this, the analysis models are minimized by DOE and FEA for the selected models.
Fig. 2 shows the influence and tendency of the starting time and torque depending on the
selected parameters.
Table 1. Specifications of the single LSPM
|
Section
|
Specification
|
|
Stator
|
Input voltage
|
220V
|
|
Rated torque
|
0.34Nm
|
|
Rated speed
|
1800rpm
|
|
Stack length
|
45mm
|
|
Frequency
Main winding R
Aux winding R
|
60-Hz
47.5Ω
60.2Ω
|
|
Rotor
|
Number of poles
|
4
|
|
PM
|
Residual flux density
|
1.12T
|
|
Air gap
|
Length
|
0.15mm
|
Fig. 1. Structure of the single-phase LSPM motor
Fig. 2. Tendency analysis of the design factors
2.2 Flux barrier design of single phase LSPM
The flux distribution of the non-barrier model I with a magnet installed in a arc
shape is shown in Fig. 3(a).
The permanent magnet design is an important factor for efficiency and synchronization
of the single phase LSPM. With a high density magnet, efficiency may increases, but
it can show a loss of synchronization. With a magnet with a low density magnet, the
motor can show better synchronization, however the efficiency will be degraded. Therefore,
an optimal magnet design is necessary. Even with an optimal design, inserting a permanent
magnet without a flux barrier, as in Fig. 3, the flux does not link with the windings.
This flux leakage at the end of the permanent magnet degrades the back-emf characteristics.
The back-emf waveform is as in Fig. 3(b) and the RMS voltages of main windings and auxiliary windings are 2.03V and 2.19V,
respectively.
To prevent flux leakage at the ends of the permanent magnet, a flux barrier must be
installed besides the permanent magnet, as shown in model II a of Fig. 4(a). As shown, the flux leakage at the end of the permanent magnet is decreased, however,
there is flux leakage through other poles. The RMS voltages of main windings and auxiliary
windings are 3.58V and 3.88V, respectively.
Model III of Fig. 5(a) extends the flux barrier to the point where the squirrel cage is located, with the
bars excluded from the model. This is to prevent flux leakage to the adjacent poles.
Compared to the non-barrier model I and the barrier model II, the flux leakage decreased
and a flux path has been made for the flux to link through the armature windings.
This shows that considering flux barriers are important for optimal permanent magnet
designs. Additionally, this design can reduce the magnet size, which reduces the production
cost of the motor. RMS voltages of main windings and auxiliary windings are 94.1V
and 101.93V, respectively.
Fig. 3. Magnetic flux density and EMF of non-barrier model I
Fig. 4. Magnetic flux density and EMF of barrier model II
Fig. 5. Magnetic flux density and EMF of barrier model III
2.3 Analysis of LSPM considering demagnetization
NdFeB is known to have outstanding magnetic characteristics. However, careful magnet
design is required for preventing demagnetization caused by high temperatures and
reverse magnetic fields. Reviewing heat demagnetization is essential for reliable
motor design. The demagnetization characteristics of NdFeB at different temperatures
are shown in Fig. 6. The knee-point exists in the second quadrant at normal temperature. However, with
higher temperatures, even though the knee-point remains in the second quadrant, the
residual flux density and coercive force decrease. A finite element analysis supported
by B-H data showed that a B lower than the knee-point does not exist. The result
shows that non-irreversible demagnetization does not happen until 140oC.
Fig. 6. Influence of temperature of the demagnetization curve
3. Results and discussions
Fig. 1 shows the design model of this paper, a single phase LSPM. The squirrel cage is identical
to conventional single phase induction motors and 16 bars were selected considering
both starting characteristics and output power.
For mechanical reasons, the single phase LSPM needs a rib, which acts as a magnetic
flux path for the axial direction flux. This causes the saliency ratio to decrease
and degrades the torque performance. To prevent leakage, the rib should be designed
as small as possible, however, to fulfill mechanical stability, the rib size can not
be minimized. In this design, the squirrel bars are designed to be close to the flux
barriers to reduce flux leakage through the rib. This causes the magnetic flux to
be saturated. Also, the magnetic flux of the air gap has been made to be sinusoidal
by making the squirrel cages close to the flux barrier.
Fig. 7 shows the rotor of the prototype of the motor designed. An experiment was conducted
to verify the motor design.
Fig. 7. Prototype of single-phase LSPM
3.1 The variation of magnetizing pattern
The back-EMF of the simulation and experiment are shown in Fig. 8. As shown, the results of simulation and experiment match well. The motor was connected
to the dynamometer to rotate at the rated speed, 1800rpm.
Fig. 8. EMF generated by permanent magnets
3.2 Results of current versus time
The current of the main windings and the auxiliary windings under no load starting
are shown in Figs. 9 and 10, respectively. A 220V, 60Hz power was supplied to both windings. The difference between
the simulation and experiments are due to the current probe offset, error in measurements,
and the decrease of back-EMF.
Fig. 9. Simulation and measured results of main winding current versus time
Fig. 10. Simulation and measured results of aux winding current versus time
3.3 Speed-time response
The starting characteristics under a no-load condition is shown in Fig. 11. It can be seen that the simulation and experimental results match. A large overshoot
is observed when the motor enters the steady state from the transient state. This
is because the initial position of the rotor is different. A breaking torque is created
when the motor enters synchronization. This breaking torque can have a positive or
negative value depending on the initial position of the rotor. If the breaking torque
has a value larger than 0, an overshoot is observed. If the breaking torque has a
value lower than 0, synchronization without an overshoot will be observed.
In Fig. 12, the starting features of the LSPM and induction motor are shown. When the rotor
turns, a and c are positive torques, and b and d are negative torques. The LSPM is
better than the induction motor in terms of starting characteristic at interval a,
however, it is not at interval d. In the between e and f, because the LSPM has positive
torque and higher inertia than the induction motor, there is an overshooted phenomenon
at the rated speed. However, the speed can be stabilized and damping is caused by
the induction torque. These features can be changed by the initial position of the
permanent magnets.
Fig. 11. Simulation and measured speed-time responses
Fig. 12. LSPM and IM speed-time responses
3.4 Results of efficiency and power
Fig. 13 shows the power and efficiency of the designed single phase LSPM. The efficiency
of the single phase induction motor is 62% at the rated load torque 0.35Nm. The efficiency
of the single phase LSPM is 70.8% under the same load. The efficiency is improved
about 9% compared with induction motor. The maximum efficiency of this motor is 72.5%
at 0.4Nm load torque.
Fig. 13. Simulation and measured results of load performance characteristics
4. Conclusions
In this paper, a single phase LSPM was designed for energy savings and high efficiency.
The stator, bearing, housing, and shaft of the single phase induction motor were adopted
and only the rotor was optimally redesigned. The permanent magnet inserted to the
rotor was selected by DOE, and optimizes the starting time and steady-state power
output. The insertion position and permanent magnet size were selected. For an efficient
use of the permanent magnet flux, a flux barrier was designed and the optimal position
of the squirrel cage was selected. Additionally, the possibility of thermal demagnetization
was also reviewed. To verify the design results, an experiment was conducted using
a prototype. Through this paper, the size and the insertion position of the magnet
which makes synchronization possible was predicted.
References
Knight Andrew M., McClay Catherine I., 2000, The Design of High-Efficiency Line-Start
Motors, IEEE Trans. Ind. Applicant, Vol. 36, No. 6, pp. 1555-1562
E Miller T. J., et al. , 2003, Line-Start Permanent-Magnet: Single-phase Starting
Performance Analysis, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, Vol. 39, No. 4,
pp. 1021-1030
Kim Hyunwoo, Park Yeji, Liu Huai-Cong, Han Pil-Wan, Lee Ju, 2020., Study on Line-Start
Permanent Magnet Assistance Synchronous Reluctance Motor for Improving Efficiency
and Power Factor, Energies, Vol. 13, No. 2, pp. 384
Kurihara Kazumi, Rahman M. Azizur , 2004, High Efficiency Line-Start Interior Permanent
Magnet Synchronous Motors, IEEE Trans. Ind. Applicant, Vol. 40, No. 3, pp. 789-796
Kim Byung-Taek, Kim Young-Kwan, Kim Duk-Jin, 2004, Analysis of Squirrel Cage Effect
in Single Phase LSPM, KIEE International Transactions on EMECS, Vol. 4, No. 4, pp.
190-195
Biography
Dae-Sung Jung received a Ph.D. degree from the Department of Electrical Engineering,
Hanyang University, Seoul, Korea, in 2009. From 2009 to 2014 he worked as an traction
motor design engineer for HYUNDAI MOBIS. Since 2014, he has served on the faculty
in the Department of IT Engineering, Yonam Institute of Technology.