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Development of High Speed Inverter Rotary Compressor for the Air-conditioning System
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2015 IOP Conf. Ser.: Mater. Sci. Eng. 90 012038
(http://iopscience.iop.org/1757-899X/90/1/012038)
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Development of High Speed Inverter Rotary Compressor for
the Air-conditioning System
Seoung-Min Kang, Eun-soo Yang, Jin-Ung Shin, Joon-Hong Park, Se-Dong Lee,
Jong-Hun Ha, Young-Boo Son, Byeong-Chul Lee
*Energy Solution Research Lab., LG Electronics Inc., 327-23 Gasan-dong,
Geumchun-gu, Seoul, 153-802, Korea
Tel: +82-10-9930-3195 Fax: +82-2-867-3941 E-mail: seoungmin.kang@lge.com
(Seoung-Min Kang)
Abstract. In order to meet the various operating loads of an air-conditioning system, an
inverter compressor with a wide operational range is necessary. One of the ways to achieve a
wide operation range is to drive a small capacity compressor at high speed. Moreover, it is
possible to maximize the efficiency in part-load operation condition close to actual operating
conditions and to reduce the cost by compact design of a small capacity compressor. In
addition, the shortage of maximum capacity, due to the small rated capacity, is covered through
high speed operation. However, in general, if the compressor operates at high speed, problems
occurs such as reduced efficiency due to friction, increased noise, increased amount of oil
discharge and decreased durability of the main components. In order to solve these problems
the following have been investigated: optimized dimension parameters of the compression
chamber, enhanced shaft design and the structure for the reduction of oil discharge and noise at
high speed operation. Finally the high speed inverter rotary compressor with high efficiency
and more compact size has been developed as compared with the conventional rotary
compressor.
Keywords: Rotary compressor, High-speed, Oil discharge, High efficiency
1. Introduction
In recent years, energy saving in the residential and industrial area has become a big issue in the
world. In particular, the demand for energy-saving residential air conditioning systems, including
heating and cooling, is increasing. The inverter rotary compressor has a key role in energy savings and
reliability as the critical component accounts for 85% of the power consumption of air conditioning. In
response to the various operational loads in a residential air conditioning system, Inverter technology
to control the capacity by varying the rotation speed is important. Through inverter technology, we
have developed a high speed inverter rotary compressor capable of controlling up to 150rps (rotor
speed). In this development, we confirmed the problems of the rotary compressor at high speed
operating conditions and maximized the efficiency in the operating conditions of the air conditioner
through CAE analysis and experimental approaches. Also, it is possible to lower material costs
through reducing the shell diameter from Φ112mm to Φ101mm.
9th International Conference on Compressors and their Systems IOP Publishing
IOP Conf. Series: Materials Science and Engineering 90 (2015) 012038 doi:10.1088/1757-899X/90/1/012038
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Published under licence by IOP Publishing Ltd 1
In this paper, the way to choose component specifications are introduced and the CAE(Computer
Aided Engineering) / CFD(Computational Fluid Dynamics) analyses are performed for its reliability
of developing a high speed rotary compressor.[1-4]
2. Overview of high speed rotary compressor
We have developed a rotary compressor with a displacement volume of 13.0cm
3
, which is
employed in a residential air conditioner. R410A and PVE oil are used as a refrigerant and lubrication
oil. Figure 1 shows the structure of rotary compressor. The compressor consists of a cylinder, a
reciprocating vane, roller, crankshaft with eccentric parts, main and sub bearings and brushless DC
motor with rotor and stator. The lubrication oil in the sump is fed by the centrifugal force of the
propeller, which is installed in the hollow hole of crankshaft. The compressor has two compression
chambers which are operated alternately with a phase difference of 180 degree. The compression load
generated by the compression chambers is supported by main and sub bearings positioned in the upper
and lower parts of the compression chambers.
In a residential air conditioner, the high load occurs at the highest outdoor temperature in summer
and at the lowest outdoor temperature in winter. In that case, the compressor must provide large
capacity to fulfil the high load. In terms of a compact design compressor, it is necessary to achieve
high speed operation with the high load.
In this paper, the optimized compression part design, the brushless DC motor design, the shaft
reinforcement structure and oil discharge reduction structure to complete the high speed rotary
compressor are introduced.
Figure 1 Structure of rotary compressor
3. OVERVIEW OF HIGH SPEED ROTARY COMPRESSOR
3.1. Optimization of cylinder dimension
To enhance efficiency at high speed operating conditions, we focused on reducing the leakage loss
in the compression chambers and friction loss on the lubrication parts such as bearings. Figure 2
shows the design limitations of rotary compressor based on the cylinder inner diameter and height as
the main specifications. The area A is suggested the optimized design area limited by geometry of
compression chamber and reliability of main parts. These limitations are decided through
9th International Conference on Compressors and their Systems IOP Publishing
IOP Conf. Series: Materials Science and Engineering 90 (2015) 012038 doi:10.1088/1757-899X/90/1/012038
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implementation analyses and confirmation tests. As a result of adopting a lower cylinder height, the
volumetric efficiency of developed model is improved by reducing the leakage loss from the radial
clearance between the cylinder and roller.
Figure 2 Design limitations of rotary compressor specifications
3.2. Reduction of crankshaft deformation and sliding surface pressure
The load acting on the crankshaft is composed of the sum of gas forces acting on the roller, vane
force pushing the roller and centrifugal force of eccentric part(crank pin). The load deforms the parts
at high speed operation under overload condition and is the maximum in the vicinity of crank angle
when the discharge valve is opened. If a rotary compressor is operated with very high speed, the load
is distributed in all areas by increasing the centrifugal force of eccentric part of crankshaft and its
durability decreases by the high load acting on the sliding part.
Figure 3 shows the analytical result of crankshaft deformation between conventional and developed
models. ANSYS was used for CAE analysis on the crankshaft deformation. From the analytical results,
the shaft deformation was reduced by about 40% as compared to a conventional crankshaft at high
speed. We confirmed that the maximum stress is located at the upper part of the crank pin. In order to
verify the decrease of shaft deformation, we conducted an experiment on the basis of reliability
standard and the result met with the criteria. Figure 4 shows the specifications for reducing shaft
deformation. When we design the crankshaft considering the change to minimize crankshaft
deformation, we should also consider the insertion of roller and middle plate. The shorter crank pin
length is advantageous for shaft deformation and a certain length is required for a minimum of design
in the assembly state. The pump part can be assembled by this bending length when the middle plate
passes through the eccentric part. In other words, a middle plate can be assembled when this calculated
bending length is less than the period overlapped between the eccentric part and middle plate.
Cylinder inner diameter
Reliability Limit
(Bearing)
Reliability Limit
(vane)
Cylinder inner diameter [mm]
Cylinder height [mm]
Geometrical Limit
Reliability Limit
(Roller)
Conventional model
Developed model
″A″
9th International Conference on Compressors and their Systems IOP Publishing
IOP Conf. Series: Materials Science and Engineering 90 (2015) 012038 doi:10.1088/1757-899X/90/1/012038
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Figure 3 Comparison of crankshaft deformation at high speed
Figure 4 Specifications for reduction of shaft deformation
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3.3. Optimization of Discharge Valve
When the pressure in the compression chamber is higher than the pressure outside the cylinder, the
discharge valve is opened and then compressed gas is discharged through discharge port. The delayed
opening of the valve influences the increase of over-compression during discharge process. When the
rotary compressor operates in high speed, the over-compression becomes larger than normal speed. So
the valve reliability problems can occur due to larger impact stress and velocity of discharge valve.
For that reason, valve design is very important for reliability of compressor. The discharge valve
design was determined by CAE analysis and experiments for specifically configured conditions of
valve thickness and width, and L_mb to consider the operating and geometric conditions. L_mb is
seated area of discharge valve at main bearing and it is key factor of valve reliability. Figure 5 shows
the discharge valve structure and specifications. We performed the test with various valve
specifications and compared them with the simulation results of valve stiffness
Thickness
[mm]
Width
[mm]
L_mb
[mm]
Stiffness
[N/m]
Present
0.305
3.2
10
1413
Spec 1
0.254
3.2
10
816
Spec 2
0.254
5.0
7.6
850
Spec 3
0.254
5.0
6.5
682
Figure 5 Discharge valve structure and specifications
Figure 6 Simulation result of discharge valve
9th International Conference on Compressors and their Systems IOP Publishing
IOP Conf. Series: Materials Science and Engineering 90 (2015) 012038 doi:10.1088/1757-899X/90/1/012038
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Figure 6 shows the effective stress and impact velocity of discharge valve according to the
specifications. In the case of present and spec.1 with same valve width, if the discharge valve
thickness becomes thin, the noise of the compressor increases and the reliability of compressor
worsens due to increased stress and velocity. In spec.2 and spec.3, the effective stress of the discharge
valve is reduced by about 27%. The spec.2 was slightly smaller impact velocity than spec.3. We
selected spec.2 as the optimum design for reliability, noise and efficiency of the compressor.
3.4. Reduction of oil circulation ratio (OCR)
In order to keep a stable oil level in the compressor, it is necessary to optimize the circulation path
of discharged oil. Most of the oil discharged with the refrigerant gas from the discharge muffler goes
through the air gap between the motor stator and rotor and is separated by the centrifugal force caused
by the rotation of the rotor. The oil moving to the upper space of the compressor is separated and
dropped from the gas by the density difference. The separated oil is moved to the oil sump at the
bottom of the compressor through the stator outer peripheral space. If the compressor operates at high
speed, the oil circulation ratio (OCR : The amount of oil contained in the gas) increases due to lower
separation capacity by high flow velocity. If the OCR increases, the reliability of the compressor
worsens as the oil level decreases. Thus, even at the highest speed, OCR must be maintained at an
appropriate level. In addition, if the OCR increases, the amount of oil withdrawn together with the
refrigerant increases. For that reason, volumetric efficiency is decreased. In this study, we confirmed
the flow pattern inside the compressor through a CFD analysis to improve the oil separation structure
in the conventional compressor. Figure 7 shows the specifications for the reduction of OCR. The
distance between the end of the discharge pipe and crankshaft was investigated from 5 to 10mm. The
upper length and lower length of case were changed to decrease pressure difference between upper and
lower spaces based on the motor as well.
Figure 7 Design improvement of reduction of oil circulation ratio
Figure 8 shows the one example of CFD analysis result to understand the flow pattern inside the
compressor and the experimental result of OCR according to the operating condition before and after
changing the oil reduction specification. Finally, OCR could be reduced from 2.8 to 0.6wt% at150rps.
9th International Conference on Compressors and their Systems IOP Publishing
IOP Conf. Series: Materials Science and Engineering 90 (2015) 012038 doi:10.1088/1757-899X/90/1/012038
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Figure 8 Oil circulation ratio experimental result
3.5. High speed brushless DC motor
Generally, the bridge of the rotor shows the weakness at high speed rotation. Thus, we reinforced
the bridge to avoid rotor rib damage and prevent the escape of permanent magnet through CAE
analysis. Figure 9 shows the bridge stress of conventional and developed models. The bridge stress is
reduced from 314MPa to 79MPa with changing the rotor shape, and we acquired enough marginal
safety ratio(Yield stress/Max. stress) in spite of high speed operation.
Conventional model
Developed model
Stress
at 150rps
Shape
Max [MPa]
314
79
Figure 9 Maximum stress and safety ratio of bridge
Bridge
9th International Conference on Compressors and their Systems IOP Publishing
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3.6. Noise reduction at high speed
As the noise and vibration characteristics of the inverter rotary compressor are changed in
accordance with operating conditions, The countermeasures of those are important to develop an the
compressor. Especially, noise control at high speed operation is one of the most important factors
because it plays an important role in technology competitiveness.
The increasing noise level at high speed is not related to a resonance but an excitation force and
transfer path of the compressor. So, the study of noise characteristics and design optimization should
be carried out for the related component parts. First, in this study, noise level is measured by using
microphone. Then, frequency spectrum analysis is carried out to measure the signal to find the
countermeasures of the noise problem. Finally, motor design and welding method are modified to
improve the excitation force and the transfer path of the system, respectively.
The motor design which generates electromagnetic force is modified using computational simulation
to reduce the noise level. Table 1 shows the comparison of motor specifications between conventional
and developed motor. According to the simulation result, the excitation force of the developed motor
decreased while motor efficiency is equivalent to the conventional design. In order to verify the
computational analysis, the vibrations of conventional and developed models are measured.
Conventional model
Developed model
Motor Stack [mm]
55
45
Coil Diameter [mm]
Ф0.95
Ф0.85
Turn No.
51
65
Slot fill factor [%]
80.3/81
82.4/83
Resistance [Ω]
0.828
1.078
Table 1. Specifications for developed motor
(a) Vibration measurement position with accelometer (b) 1/3 Octave band at 130rps
Figure 10. Comparison of vibration level between conventional and developed motors
Frequency [Hz]
Vibration level [m/s
2
]
■Conventional
■Developed
9th International Conference on Compressors and their Systems IOP Publishing
IOP Conf. Series: Materials Science and Engineering 90 (2015) 012038 doi:10.1088/1757-899X/90/1/012038
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Figure 10 shows the vibration measurement position and the experiment result using accelerometer.
Radial vibration is measured at the half of motor stack height (hm) and 1/3 octave band frequency
spectra of conventional and developed models are compared. As a result, the vibration level of the
developed model is reduced by about 50%. Especially, 3~6 kHz frequency range is most effective
because high order harmonic components of electromagnetic force are reduced. Since vibration of
structure is the cause of noise radiation, reduction of vibration implies an improvement of noise level.
Additionally, welding position of case and pump part was investigated to improve the vibration
transfer path. In general, 3 points of main bearing are welded to the compressor case to support a
pump part. However, the thickness of main bearing was too thin and the combined structure was too
unstable to hold up the pump part. Thus, design modification was carried out by computational
simulation and 3 points of sub bearing are additionally welded.
(a) Noise measurement result
(b) Octave band at 130rps
Figure 11. Comparison of noise level between conventional and developed model
Speed [rps]
Frequency [Hz]
6
Frequency [Hz]
Sound Pressure Level [dBA]
Frequency [Hz]
■
Conventional
■Developed
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Figure 11 shows the noise reduction after design modification as compared with the conventional
design. A microphone to measure the noise is placed at a distance of 30cm from the half the
compressors body height and a noise signal is obtained after 15 seconds of stationary condition. Figure
11(a) is the color map of noise level with respect to operating speed. Both 1.25~1.6 kHz and 3~6 kHz
frequency ranges are reduced due to the improvement of motor design and welding method,
respectively. In order to check noise reduction at high speed, 1/3 octave band frequency spectra of
conventional and developed designs are compared at 130rps operating condition as shown in Figure
11(b) Noise level in the 1.25~1.6 kHz frequency range is reduced by about 10dB and that of the 3~6
kHz frequency range is reduced by about 3dBA. Total noise level is reduced by about 3dBA.
4. Conclusion
This paper described the technology to solve the problems in the high speed operation of the
compressor. For the better reliability and higher efficiency, the cylinder dimensions were compactly
optimized to reduce the leakage loss from the radial clearance between the cylinder and rolling piston.
The new crankshaft design concept of wide range application was confirmed and verified by CAE
analysis and the experiment. The design guideline was also determined to assure the sufficient rigidity
of the crankshaft. And blushless DC motor shape is modified for high speed operation and it is verified
through rigorous testing. As a results, the high speed(150rps) inverter rotary compressor was
developed with high efficiency and reliability for the air-conditioning system.
5. References
[1] Tsugio Itami, Masahiro Kubo, Makoto Sugiyama, Estimation of bearing load of Rolling piston
type Rotary compressors under high speed operation, Proc. Of Intl. Comp. Eng. Conf., Purdue,
Paper 547
[2] Mathias HARELAND, Anders HOEL, Stefan JONSSON, David LIANG, Guocai CHAI, 2014,
SELECTION OF FLAPPER VALVE STEEL FOR HIGH EFFICIENT COMPRESSOR, Proc. Of
Intl. Comp. Eng. Conf., Purdue, Paper 1408
[3] Jose Luiz GASCHE, Danilo Martins ARANTES, Thiago ANDREOTTI, 2014, Experimental
Analysis of the Fluid Structure Interaction in a Suction Valve Model, Proc. Of Intl. Comp. Eng.
Conf.,Purdue, Paper 1217
[4] Siva Rama Krishna Bolloju, Vamsi Tiruveedhula, Naveen Munnangi, Koteswara Rao Vaddadi,
Prathap Reddy M, 2014, Efficiency Improvement of Rotary Compressor by Improving the
Discharge Path through simulation, Proc. Of Intl. Comp. Eng. Conf.,Purdue, Paper 1543
[5] Jorg MAYER, Preben BJERRE, Fabian BRUNE, 2014, A Comparative Study of Different
Numerical Models For Flapper Valve Motion, Proc. Of Intl. Comp. Eng. Conf.,Purdue, Paper 1315
[6] H. C. Kim, M. G. Cho, J. Kim, J. H. Park and J. Shim, Coherence technique for noise reduction in
rotary compressor, 2012, Journal of Mechanical Science and Technology 26 (7) (2012) 2073~2076
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