Fabrication, Parameter Evaluation and Testing of a surface mounted -Brush-less DC Motor

Transcription

1 Fabrication, Parameter Evaluation and Testing of a surface mounted -Brush-less DC Motor Pinaki Mukherjee 1, Mainak Sengupta 2 Dept. of Electrical Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah , W.B., India 1 pinaki.mukherjee.besus@gmail.com, 2 mainak.sengupta@gmail.com Abstract This paper presents the design data, fabrication, parameter evaluation and testing of a 0.75hp, 4-pole, 1500rpm surface mounted permanent magnet brush-less DC machine(spm- BLDC). The complete design of this machine has also done by the authors and has been published earlier. The fabricated machine was tested in the laboratory and its parameters were experimentally evaluated. There after it has been run at light load with open loop V/f control. The evaluated parameters are in excellent agreement with the analytically calculated values. Index Terms SPM-BLDC, BLDC Fabrication, Testing, Parameter evaluation. I. INTRODUCTION Brush-less DC Motor(BLDC)[1], [2] is emerging as an attractive alternative to induction motors(im) in different variable speed drive applications for reasons which are well known - the most important ones being high energy density and absence of brushed contacts. There are obvious challenges of rotor design using PM. The design of the BLDC should be optimised to have minimum volume of permanent magnets (for cost economy) together with reduced size and weight for the complete machine. The design should also ensure the protection of permanent magnet against demagnetising effect of armature current[2]. The demagnetising component of the armature current(daxis current) is normally considered to be zero, which points to the need for having closed loop current control operation. The machine has also been fabricated at the works of a small local machines manufacturer with imported magnets. The fabricated machine has coupled to a DC generator which will act as the load. The parameters have been evaluated [3], [4], [5] through direct tests and compared with predicted/ analytically calculated values. II. BLDC DESIGN DATA A 0.75hp (560 W), 3-phase, star connected, 400 V DC-link, 1500rpm, 4-pole surface mounted PM-BLDC(SPM-BLDC) was initially designed by the authors [1]. The BLDC (Fig.1&2) design started with an available stator lamination(of an Induction Machine of comparable power rating). This was done to reduce the tooling costs and time taken for fabrication. The additional tooling costs, which are exorbitantly high, therefore were required only for the rotor. The available lamination dimensions used for the calculation is given in TableII. This Fig. 1. BLDC motor power converter configuration. Fig. 2. Cross sectional view of the 0.75 hp, 4 pole surface mounted PM-BLDC motor(stator OD=103mm, ID=62.6mm, 24 slots). additionally may help in comparing the performance of the the designed and fabricated machine vis-a-vis an IM of similar rating since the stator laminations are of same dimensions. The details of the design procedure have not been included here since the same have been already reported elsewhere[1]. Table- I shows the design parameter of the 0.75 hp BLDC motor. The winding details of BLDC motor is given in tableiv. Full pitch double layer distributed type of winding has been

2 TABLE I SPICIFICATIONS AND DESIGN PARAMETERS OF THE 0.75HP BLDC DC-link voltage 400 V Rated power 0.75 hp. Rated speed (N) 1500 r.p.m. Phase 3 No. of pole (P) 4 Rated current(i ph ) 1.5 A Bore Diameter(D) 62.6 mm Stack length(l) 103mm B avg 0.6 T air gap length, l g 1 mm Magnet material Sm 2 CO 17 Magnet thickness, l m 3mm Fig. 3. Fabricated stator (OD=103mm, ID=62.5mm, Length= 103mm) of 0.75hp BLDC TABLE II STATOR LAMINATION DETAILS Item value Stator OD 103mm Stator ID(D) 62.6mm No. of slot(s) 24 Type of slot parallel Teeth hight 11mm Teeth width 4.8mm Slot area 55mm 2 Yoke depth 9.2 mm Lamination material M19 used here. TABLE III WINDING DETAILS Fig. 4. Dimensioned sketch of permanent magnet for rotor (side view) Item Value Coil pitch 6 slots Conductor per slot 70 Wire gauge SWG 24 No. of strands 1 Layer of winding 2 Rotor skew 0.5 slot pitch of stator slot III. BLDC FABRICATION A. Fabrication of stator-rotor assembly Fig.3 shows the fabricated stator of the 0.75 hp,4-pole BLDC motor. Length of the stator(l) is 103 mm and outer diameter of stator is 103 mm also. Fig.5 shows the fabricated 4-pole rotor with imported Sm-Co permanent magnet(pm). Length of the fabricated rotor is 115 mm to enable Hall position sensing and outer diameter(d) is 60.6 mm. Thus the (L/D) ratio for this machine comes as 1.7. The rotor is skewed at 0.5 stator slot pitch i.e. 2mm. Each pole of the rotor is axially made of 9 units of Sm-Co magnet(fig.4) of 0.5 inch length, 3-mm thickness. The PM magnets were fabricated on rotor with adhesive(araldite). Fig. 5. Rotor of BLDC motor(od=60.5mm, L=110mm, skew= 0.5 stator slot pitch) with Sm-Co arc magnet Fig.6 shows the fabricated PM-rotor sheathed by a protective. Finally, Fig.7 shows the complete fabricated 0.75hp BLDC motor. B. Position Hall sensor fabrication To run the BLDC motor, rotor position feedback[2] is essential. In this machine, rotor position feedback has been generated by using bipolar Hall-effect digital position sen-

3 Fig. 6. Rotor of 0.75 hp BLDC motor with sleeve Fig. 7. Completely fabricated 0.75hp, 1500 rpm, 4-pole BLDC motor sor(ss411p)which operates sensing the direction of magnetic flux. Three position hall sensors have been used for position sensing. It may be mentioned here that the placement of magnetic Hall sensor inside the rotor is a challenging issue. per 360o electrical rotation. 180o 120o elect= =60o. 3 The angle between adjacent Hall position sensor is 120o elect. For placing 3 Hall sensors, a total arc required is =2 60o m=120o. 60o So, gap between two position sensor IC= = 4 slot pitch 15o i.e. 4 slots between adjacent Hall position sensors. The inner diameter for position Hall sensor PCB is selected as 60.6mm. The PCB has a thickness of 15mm (less than stator width 20 mm.) with a arc of Fig. 8 shows layout of Hall sensor PCB. Fixing of Hall sensor PCB: The angle between Hall sensor pulses is 120o e. Placement of Hall sensor PCB should be such that positive going edge Hall sensor 1 pulse pulses (here the first one staring from the left end of the PCB) get synchronised with positive zero crossing of VRY voltage. The position sensor PCB board is mechanically fixed on the winding overhang of the BLDC motor at non-driving end side by means of adhesive with appropriate care such that position sensor IC has been placed directly above rotor magnet otherwise no output pulses will be generated from the sensor IC. IV. T ESTING OF BLDC MOTOR The BLDC machine thus fabricated and assembled was next tested through direct practical tests. The experiments conducted are of two categories viz. (i) tests for evaluation of parameters like Ld, Lq etc and elementary quantities like induced emf pattern/waveform etc. (for verification against designed or analytically predicted values/waveforms) and (ii) tests for verification of motor performance under different loading conditions. A. Experimental verification of emf waveform and Hall sensor pulses Fig. 8. Hall sensor PCB for 0.75 hp, 4 pole BLDC motor Position sensor placement calculations: A PCB has been made to accommodate position Hall sensor IC inside the air gap just above the extended portion of the rotor. PCB layout calculations are given below. In this case, no. of stator slot=24. No. of pole=4=2 pole pair. Rotor outer diameter(odr )=60.6mm. 1 full mechanical rotational cycle= 360o. Now, angle per stator slot=slot pitch angle=360o /24= 15o. Then, 360o / 2 pole pair=180o per electrical rotation. = 180o In generating mode, 0.75 hp BLDC has been driven at rated speed by means of a prime mover(coupled DC machine run as a motor). Fig.9 shows the FEM 2D waveform of per phase induce emf (peak 180 V, frequency 50Hz) at rated speed where as Fig.10 shows experimental waveform of the induced phase emf (RMS 160 V, peak 190V, frequency 50Hz ) in generating mode at 1500 rpm. Fig.11 shows the phase induced emf( VRN )(peak 100V) and line- to -line induced emf ( VRY )(peak 200V) at 750 rpm i.e. 25Hz frequency. Fig.12 shows position Hall sensor output at 750 rpm speed i.e. 25Hz frequency. From this figure, it is clear the sensor pulses are nearly 120o e apart. Fig. 13 shows the VRY waveform and position sensor 1 output pulse. The positive zero crossings are matched for VRY and Sensor 1 output explained earlier.

4 Fig. 9. Induced emf per phase of the designed BLDC at 1500 rpm in 2D FEM analysis Fig. 12. Oscilloscope trace of position hall sensor output pulses waveform for 0.75hp, 4-pole BLDC motor at 750 r.p.m. Fig. 10. Oscilloscope trace of phase induced emf waveform for 0.75hp, 4-pole BLDC motor at 1500r.p.m Fig. 11. Oscilloscope trace of V RY (yellow)(50v/div) and V RN (blue)(50v/div) for 0.75hp, 4-pole BLDC motor at 750 r.p.m. B. Parameter evaluation of the fabricated BLDC L d and L q determination : From generalised machine theory, total flux linkage of phase-a is given by [3], ψ a = (L l + L 1 + L 2 cos 2θ r )i a + ( L A comparison of calculated and experimentally determined L 2 cos 2(θ r 60 o ))i b value of machine parameters of fabricated BLDC is given +( L L 2 cos 2(θ r + 60 o ))i c + ψ 0 cos θ r Fig. 13. Oscilloscope trace of Line-to-line induced emf waveform and hall sensor 1 pulse output for 0.75hp, 4-pole BLDC motor at 750 r.p.m. 1) Case 1: A small DC voltage(2v) is applied to align phase-a and the d-axis (i.e., θ r = 0 o ). Now, at this rotor position a single phase variable AC(0-5V) is applied to phase-a and i a is measured. The condition gives,ω r = 0; i b = pi b = 0; i c = pi c = 0. v an = dψa dt = r a i a +(L l +L 1 +L 2 )pi a. From experiments, L l +L 1 +L 2 = mh 2) Case 2: AC supply is now applied to phase-b. Phase- B current and open circuit voltage at phase-a(v an ) is noted. Now, ω r = 0; θ r = 0 o ; i a = pi a = 0; i c = pi c = 0. v an = ( L1+L2 2 )pi b. From experiments, L 1 + L 2 = 16.82mH 3) Case 3: A small DC voltage is applied to phase-b, so that the B-phase is aligned with d-axis. Now, the variable AC voltage is applied to phase-c. Open circuit voltage of phase-a and phase-b current is noted. Under this condition, ω r = 0; θ r = 120 o ; i a = pi a = 0; i b = pi b = 0. v an = ( L1 2 +L 2)pi c. From test, 0.5L 1 L 2 =8.53mH. From above 3 cases, L l = 16.8mH; L l = mh; L 2 = 0.02mH. Hence L d = 43.75mH which is very close to estimated inductance (43.23 mh).

5 TABLE IV 0.75 HP BLDC: PARAMETER EVALUATION Parameter Estimated value Experimental (using design value calculation and FEM analysis) Ra(ohm)/phase Ld(mH) Lq(mH) EMF constant Ke (V L L /1000rpm) Torque constant K t (N-m/A) in Table-IV. Similarly, Table-V shows a comparison between the fabricated 0.75hp, 1500 rpm BLDC motor with a 3-ph Induction motor of similar rating. From this Table-V, it is clear that the power density and toque density are much higher for BLDC motor compared to 3-ph IM.Currently, the housing/shell of fabricated BLDC is made of MS material. If we use light(aluminium) housing/shell housing, then weight of the machine will reduce further(nearly by 40%) i.e. power & torque density will increase. Fig. 14. Phase voltage(v RN )(100V/div) and phase current(i R )(1A/div) at 300 rpm in V/f control sensor 1 output pulse is 10 Hz. Fig.17 shows motor phase voltage and hall sensor 1 pulse at 300 rpm speed. TABLE V COMPARISON BETWEEN 0.75HP, 1500 RPM BLDC MOTOR AND 3 PH- SQ INDUCTION MOTOR. Items Designed BLDC Available IM Effective weight(kg) 5 10 Total weight(kg) ( including housing) Effective power density(w/kg) Net power density(w/kg) Effective torque density(n-m/kg) Net torque density(n-m/kg) Fig. 15. Line voltage(v RY )(100V/div) and phase current(i R )(1A/div) at 10 Hz. i.e. 300 rpm speed in V/f control C. Experimental results of 0.75hp BLDC motor under open loop V/f control The BLDC motor has been tested in open-loop V/f control mode as a motor with low load. The DC-link voltage applied is nearly 200V at a switching frequency of 5kHz and at a motor speed is 300 rpm i.e. 10Hz electrical. Fig.14 shows motor phase voltage(v RN ) and phase current (I R ) at 10 Hz rotor frequency.the value of phase current is 0.6 A(rms). Fig.15 shows motor line to line terminal voltage (V RY ) and line current(i R ). Fig.16 shows motor line to line terminal voltage (V RY ) and position sensor 1 output pulse. From this figure, PM-rotor speed is confirmed. The frequency of position Fig. 16. Line voltage(v RY )(100V/div) and Hall sensor1 pulse(2v/div) at 10 Hz. i.e. 300 rpm speed in V/f control

6 [9] T. M. Hijazi and A. A. Arkadan,, Computation of winding inductances of permanent magnet brushless DC motors with damper windings by energy perturbation, IEEE Transactions on Energy Conversion, 3(3), , Fig. 17. Phase voltage(v RN )(100V/div) and Hall sensor1 pulse(2v/div) at 10 Hz. i.e. 300 rpm speed in V/f control V. CONCLUSIONS In this paper the design and fabrication of a 0.75hp, 1500 r.p.m, 400V (DC-link) surface mounted BLDC motor has been presented. For ease of fabrication and for cost reduction the design starts with the dimensions of an available IM stator stamping which has also been used finally for stator fabrication. The motor has been also completely fabricated at a local motor manufacturing company. After fabrication, the motor has been tested in generating mode(with coupled DCmachine) and motoring mode (with open loop V/f control). There after, parameter evaluation of the fabricated motor has been done experimentally. VI. ACKNOWLEDGEMENTS The authors wish to thank the staff of M/S GE motors Pvt. Ltd, Sheorapuli and Mr. Kausik Pyne, in particular, for the manufacturing support received in fabricating the BLDC motor. The authors also acknowledge the support received from the funding agency DeitY and the research colleagues particularly, Mr. Netai Dutta at the Advanced Power Electronics Laboratory, Dept. of EE, IIEST, Shibpur towards this work. REFERENCES [1] Mukherjee P., Sengupta M. Design, Analysis and Fabrication of a Brush-less DC Motor,IEEE conference, PEDES-2014, IIT Bombay. [2] K Venkata Ratnam, Special Electrical Machine,University Press, [3] D.O Kelly And S.Simmons, Generalized Electrical Machine Theory, McGRAW HILL, [4] S.K. Nanda, 1kW, 48V, 2000rpm, 4pole BLDC Motor for Electric Vehicle Application, M.E. Thesis, Department of EE, BESU, Shibpur, [5] Paitandi S., Sengupta M. Design, Fabrication and Parameter Evaluation of a Surface Mounted Permanent Magnet Synchronous Motor IEEE conference, PEDES-2014, IIT Bombay. [6] R.Krishnan, Permanent Magnet Synchronous And Dc Motor Drives,Crc Press,2005 [7] A.K. Sawhney, Electrical Machine Design, DhanpatRai and Co., Sixth edition, 2006 [8] N. A. Demerdash, R. H. Miller, T. W. Nehl et al,, Comparison between features and performance characteristics of fifteen HP samarium cobalt and ferrite based brushless DC motors operated by same power conditioner, IEEE Transactions on Power Apparatus and Systems, , 1983.