DESIGN AND CONSTRUCTION OF LINEAR MOTOR FOR LINEAR COMPRESSOR USED IN HOUSEHOLD REFRIGERATOR Dr. Priya Nitin Gokhale, IEEE Member Professor, Dept of Electrical Engineering Jayawantrao Sawant College of Engineering Pune, India Lalit Bacchav Dept of Electrical Engineering Jayawantrao Sawant College of Engineering Pune, India Abstract- In contrast to the conventional rotational compressors, the linear compressor is driven by a linear motor directly coupled with a piston. So the performance of the linear compressor is strongly dependent on the characteristics of the linear motor. In addition to high efficiency, the parameters of a linear motor should be nearly constant regardless of the amount of the current flow and position of the piston in order to control the position of piston without an additional sensor. High reliability of the linear motors is required for use in compressor operated under severe conditions for long period. Several types of the linear motors are investigated to meet those requirements. A simple structure of moving coil linear motor is designed for its high power density and efficiency. Keywords linear motor,moving coil type, linear compresor I. INTRODUCTION A Linear motor is a special type of motor which gives linear motion instead of rotational motion, as in the case of conventional induction motor. The application of this linear motor is in linear compressor used in household refrigerator.it operates on the principle of which a conventional induction motor operates. This project describes the design and construction of linear motor. General induction motor is having radial force but in this linear motor the force is axial, so the shaft moves axially. The principle of the linear motor is a current carrying copper coil is placed inside a magnetic field produces force. The Lorentz force thus developed is used to drive the connecting rod of the piston of compressor. The direction of the Lorentz force is given by the Flemings left hand rule. There are two flexure springs attached at two ends of the connecting rod. There are two types of linear motor 1. Moving coil linear motor 2. Moving magnet linear motor Because of low weight, high efficiency the moving coil type is often used for linear compressor. II. MOVING COIL LINEAR MOTOR A. Schematic of Linear Motor Figure 1 shows the schematic of a moving coil linear motor. It consists of an electric coil (fixed to the piston drive 1
DESIGN AND CONSTRUCTION OF LINEAR MOTOR FOR LINEAR COMPRESSOR USED IN HOUSEHOLD REFRIGERATOR shaft) free to reciprocate in annular working gaps formed by a stationary assembly of an axially magnetized permanent magnet and appropriately shaped pole pieces. Fig. 1. Schematic diagram of linear motor A4 paper size. If you are using US letter-sized paper, please close this file and download the file MSW_USltr_format. B. Assumptions for Design of linear Motor 1. Magnetic field in the air gap is radial, constant and uniform, with no leakage flux outside the gap. 2. The impressed voltage has only the first harmonic given as 3. The motion of the piston is simple harmonic in nature and is given as 4. The self-inductance and the resistance of the coil are constant and independent of coil position 5. Eddy current and hysteresis losses are negligible. Operating voltage is 20 Volts (AC) linear displacement required is. Considering this required mechanical output following design procedure is followed. The design of moving coil linear motor consists mainly of two parts. Figure 2. Design Layout of Linear Motor C. Desgin of Magnetic Circiut The magnet and pole piece sizing is to be done so as to yield one or two working gaps of appropriate height and width, while minimizing the quantum flux leakage through paths other than the working gaps and thus minimizing the flux loss. In the present work, single magnetic gap motor is considered. Inner diameter of the magnetic gap is same as the outer diameter of the magnet. Gap inner diameter, Dgi= 75.0 Length of the magnetic gap, Lg = 6.0 (in the direction of gap flux) Gap outer diameter, Dgo=Inner diameter of the magnet +2* Length of magnetic gap Gap height, hg= 15 (in direction normal to gap flux) Coil height, hs = 20 (this value is greater than the fringing flux of 1.2 times the gap height to utilize maximum gap flux) Fringing flux is the flux bulging out of the magnetic gap and is useful flux, which can be considering by assuming the coil height just greater than 1.2 times the gap height. PermeanceP1 and P2 give idea about the fringing flux. Amplitude of piston displacement, Xp= 5 Flux density is considered as 0.8285 Tesla [1]. D. Coil Design Generation of necessary power output at the required efficiency level forms the major goal in the design of motor. The parameters, which require careful consideration, are 1. Generation of required flux density in the magnetic gap volume. 2. Coil parameters inductance, resistance, height of coil, active length of wire. 3. Voltages, current, supply frequency, and the phase difference between supply voltage and piston movement. 4. Amplitude of piston movement, ratio of coil height and magnetic gap height. Wire diameter (27 BSW gauge, copper wire) = 0.45 Specific wire resistance, = 0.1326 Ω/m at Temperature coefficient of resistance of copper (TCRC) = 0.00395 /K Frequency, F = 50 Hz Angular velocity = 2* π*f= 314 rad/sec Yoke intermediate diameter, = 102.0 Yoke outer diameter, = 120.0 Useful gap in the coil former for the winding = length of the gap - 2*clearance with pole piece - wall thickness of coil former = 6- (2*0.5) -0.5 = 4.5 Number of layers = useful gap / diameter of the wire = 4.5/0.45 = 10 layers Number of turns= height of the coil / diameter of the wire = 20 / 0.45 = 44 Total number of turns = Number of layers * Number of turns = 440 Wire length, = 110.93 m (calculated from number of turns and number of layers possible on the coil former) Total resistance of the coil = Total length of the coil ( ) * specific resistance 2
The resistance of coil in operation at a temperature of will be total resistance of wire, = 0.1326 * 110.93 * (1+0.00395 * 30.0) =16.45 Ω III. EQUIVALENT CIRCUIT OF LINEAR MOTOR For the electrical circuit of the linear motor, shown in Figure3,equating the voltage drop with the source emf gives Sr. No. Parameter Value 1 Operating frequency FHz 50 Hz 2 Power output of motor, 1.4 watt 3 Magnet material Nd-Fe-B 4 Pole piece material Soft iron 5 saturation flux density,t 2.1 Figure 3. Equivalent circuit of Linear Motor From the cyclic analysis the power input to the opposed piston linear compressor is around 2W.This is the total power input the compressor with a linear motor efficiency of 70%. Thus the power output of the opposed piston compressor would be, Total power output of the compressor= 2*0.70 = 1.4 W 6 Inner diameter of magnet,, 7 Outer diameter of magnet, 8 Length of the magnet, 9 MMF loss factor, 10 Magnet flux density, T 11 Gap flux density, T 12 Coil height, 13 Diameter of copper wire,, 14 Total length of the coil 8 75 15 1.45 0.8285 0.545 20 0.45 110.93 winding,, m 15 Total resistance of coil, 16.45 ohm 16 Power input required,, w 40.79 17 Efficiency of the motor, η, % 70.7% IV. FABRICATION OF LINEAR MOTOR Figure 4 shows magnetic pole piece assembly. Figure 5 shows coil former with winding. Figure 6 shows the assembled linear motor. 3
DESIGN AND CONSTRUCTION OF LINEAR MOTOR FOR LINEAR COMPRESSOR USED IN HOUSEHOLD REFRIGERATOR Input electrical power = 22 20 = 2 Watt Efficiency of Motor = (output mechanical power / Input electrical power) * 100 = (1.414/2) * 100 =70.7 % Figure 4. Magnetic pole piece Assembly le Figure 5. Coil Former with Winding Output mechanical power Output mechanical power = Force * Maximum velocity Force = Mass * Gravity = 0.1Kg * 9.81 1 N Maximum Velocity = 2 *Average Velocity Average velocity = (Average distance travelled in one cycle) * (Number of cycles per sec) = 20 * 50 cycle per sec =1000 meter/sec Table 1 Parameters of Linear Motor Figure 6, Assembly of Linear Motor Maximum velocity = 1.414 * 1 = 1.414 meter/sec Output mechanical power =1 N * 1.414 = 1.414 Watt 5.3.2 Input electrical power Input electrical power on 230 V side = Measured input - Measured input power with load power without load Measured power with mass = 22 Watt Measured power without mass = 20 Watts Figures 6 Experimental Setup for efficiency of linear Motor 4
Conclusion 1. The moving coil type linear motor is selected for the linear compressor of household refrigerator as this type of construction gives low weight, moderate efficiency and zero radial forces in the linear motor. 2. Linear motor specifications are 20 Volt AC, 1.06 Amp, 50 Hz, 2 Watt which are appropriate to drive the compressor of household refrigerator. 3. The full load efficiency of the linear motor is determined experimentally which is closer to the designed efficiency 4. The permanent magnet material used is Nd-Fe-B of high quality to get better magnetization characteristic. 5. The increase in operating voltage gives increase in displacement and frequency. However total 20 displacement in simple harmonic motion is required for the linear compressor hence the operating voltage is limited to 20 Volt.. References [1] Report of IIT Bombay, Advanced technology Development Of Two Stage Cryocooler For IRS Prograe. [2] Horn, S.B and Lumpkin, M.E, and Walter B.T. pneumatically driven split cycle cryogenetic refrigeration, Advances in cryogenic Engineering.19,pp.216-230(1974). [3] Lalit Bachhav etal.,ug Project report,ay 2014-15, " Design And Construction Of Linear Motor For Linear Compressor Used In Household Refrigerator ",in Department of Electrical Engg, JSCOE, Pune under the guidance of Dr. P.N. Gokhale. 5