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

Pinaki Mukherjee1, Mainak Sengupta2Dept. of Electrical Engineering,

Indian Institute of Engineering Science and Technology, Shibpur,Howrah - 711103, W.B., India

E-mail: 1pinaki.mukherjee.besus@gmail.com, 2mainak.sengupta@gmail.com

Abstract—This paper presents the design data, fabrication, pa-rameter evaluation and testing of a 0.75hp, 4-pole, 1500rpm sur-face mounted permanent magnet brush-less DC machine(SPM-BLDC). The complete design of this machine has also doneby the authors and has been published earlier. The fabricatedmachine was tested in the laboratory and its parameters wereexperimentally evaluated. There after it has been run at lightload with open loop V/f control. The evaluated parameters arein excellent agreement with the analytically calculated values.

Index Terms—SPM-BLDC, BLDC Fabrication, Testing, Pa-rameter evaluation.

I. INTRODUCTION

Brush-less DC Motor(BLDC)[1], [2] is emerging as anattractive alternative to induction motors(IM) in different vari-able speed drive applications for reasons which are well known- the most important ones being high energy density andabsence of brushed contacts. There are obvious challenges ofrotor design using PM. The design of the BLDC should beoptimised to have minimum volume of permanent magnets(for cost economy) together with reduced size and weightfor the complete machine. The design should also ensure theprotection of permanent magnet against demagnetising effectof armature current[2].

The demagnetising component of the armature current(d-axis current) is normally considered to be zero, which points tothe need for having closed loop current control operation. Themachine has also been fabricated at the works of a small localmachines manufacturer with imported magnets. The fabricatedmachine has coupled to a DC generator which will act asthe load. The parameters have been evaluated [3], [4], [5]through direct tests and compared with predicted/ analyticallycalculated 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 Induc-tion Machine of comparable power rating). This was done toreduce the tooling costs and time taken for fabrication. Theadditional tooling costs, which are exorbitantly high, thereforewere required only for the rotor. The available laminationdimensions 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 mountedPM-BLDC motor(stator OD=103mm, ID=62.6mm, 24 slots).

additionally may help in comparing the performance of thethe designed and fabricated machine vis-a-vis an IM of similarrating since the stator laminations are of same dimensions. Thedetails of the design procedure have not been included heresince 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

TABLE ISPICIFICATIONS AND DESIGN PARAMETERS OF THE 0.75HP BLDC

DC-link voltage 400 VRated power 0.75 hp.

Rated speed (N) 1500 r.p.m.Phase 3

No. of pole (P) 4Rated current(Iph) 1.5 ABore Diameter(D) 62.6 mm

Stack length(L) 103mmBavg 0.6 T

air gap length, lg 1 mmMagnet material Sm2CO17

Magnet thickness, lm 3mm

TABLE IISTATOR LAMINATION DETAILS

Item valueStator OD 103mm

Stator ID(D) 62.6mmNo. of slot(S) 24Type of slot parallelTeeth hight 11mmTeeth width 4.8mm

Slot area 55mm2

Yoke depth 9.2 mmLamination material M19

used here.

TABLE IIIWINDING DETAILS

Item ValueCoil pitch 6 slots

Conductor per slot 70Wire gauge SWG 24

No. of strands 1Layer 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-poleBLDC motor. Length of the stator(L) is 103 mm and outerdiameter of stator is 103 mm also. Fig.5 shows the fabricated4-pole rotor with imported Sm-Co permanent magnet(PM).Length of the fabricated rotor is 115 mm to enable Hallposition sensing and outer diameter(D) is 60.6 mm. Thus the(L/D) ratio for this machine comes as 1.7 . The rotor is skewedat 0.5 stator slot pitch i.e. 2mm. Each pole of the rotor isaxially made of 9 units of Sm-Co magnet(Fig.4) of 0.5 inchlength, 3-mm thickness. The PM magnets were fabricated onrotor with adhesive(Araldite).

Fig. 3. Fabricated stator (OD=103mm, ID=62.5mm, Length= 103mm) of0.75hp BLDC

Fig. 4. Dimensioned sketch of permanent magnet for rotor (side view)

Fig. 5. Rotor of BLDC motor(OD=60.5mm, L=110mm, skew= 0.5 stator slotpitch) with Sm-Co arc magnet

Fig.6 shows the fabricated PM-rotor sheathed by a pro-tective. Finally, Fig.7 shows the complete fabricated 0.75hpBLDC motor.

B. Position Hall sensor fabrication

To run the BLDC motor, rotor position feedback[2] isessential. In this machine, rotor position feedback has beengenerated by using bipolar Hall-effect digital position sen-

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 magneticflux. Three position hall sensors have been used for positionsensing. It may be mentioned here that the placement ofmagnetic Hall sensor inside the rotor is a challenging issue.

Fig. 8. Hall sensor PCB for 0.75 hp, 4 pole BLDC motor

Position sensor placement calculations: A PCB hasbeen made to accommodate position Hall sensor IC insidethe 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 outerdiameter(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

per 360o electrical rotation.

⇒120oelect=180o

3=60o.

The angle between adjacent Hall position sensor is 120oelect.For placing 3 Hall sensors, a total arc required is=2×60om=120o.So, gap between two position sensor IC=

60o

15o= 4 slot pitch

i.e. 4 slots between adjacent Hall position sensors.The inner diameter for position Hall sensor PCB is selected as60.6mm . The PCB has a thickness of 15mm (less than statorwidth 20 mm.) with a arc of 2100. Fig. 8 shows layout of Hallsensor PCB.

Fixing of Hall sensor PCB: The angle between Hall sensorpulses is 120oe. Placement of Hall sensor PCB should besuch 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 thewinding overhang of the BLDC motor at non-driving endside by means of adhesive with appropriate care such thatposition sensor IC has been placed directly above rotor magnetotherwise no output pulses will be generated from the sensorIC.

IV. TESTING OF BLDC MOTOR

The BLDC machine thus fabricated and assembled wasnext tested through direct practical tests. The experimentsconducted are of two categories viz. (i) tests for evaluationof parameters like Ld, Lq etc and elementary quantities likeinduced emf pattern/waveform etc. (for verification againstdesigned or analytically predicted values/waveforms) and (ii)tests for verification of motor performance under differentloading conditions.

A. Experimental verification of emf waveform and Hall sensorpulses

In generating mode, 0.75 hp BLDC has been driven at ratedspeed by means of a prime mover(coupled DC machine runas 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.10shows experimental waveform of the induced phase emf (RMS160 V, peak 190V, frequency 50Hz ) in generating mode at1500 rpm.

Fig.11 shows the phase induced emf( VRN )(peak 100V) andline- to -line induced emf ( VRY )(peak 200V) at 750 rpm i.e.25Hz frequency.

Fig.12 shows position Hall sensor output at 750 rpm speedi.e. 25Hz frequency. From this figure , it is clear the sensorpulses are nearly 120oe apart.

Fig. 13 shows the VRY waveform and position sensor 1output pulse. The positive zero crossings are matched for VRYand Sensor 1 output explained earlier.

Fig. 9. Induced emf per phase of the designed BLDC at 1500 rpm in 2DFEM analysis

Fig. 10. Oscilloscope trace of phase induced emf waveform for 0.75hp, 4-poleBLDC motor at 1500r.p.m

Fig. 11. Oscilloscope trace of VRY (yellow)(50V/div) andVRN (blue)(50V/div) for 0.75hp, 4-pole BLDC motor at 750 r.p.m.

B. Parameter evaluation of the fabricated BLDCLd and Lq determination : From generalised machine

theory, total flux linkage of phase-A is given by [3],

ψa =

(Ll + L1 + L2 cos 2θr)ia + (−L1

2+ L2 cos 2(θr − 60o))ib

+(−L1

2+ L2 cos 2(θr + 60o))ic + ψ0 cos θr

Fig. 12. Oscilloscope trace of position hall sensor output pulses waveformfor 0.75hp, 4-pole BLDC motor at 750 r.p.m.

Fig. 13. Oscilloscope trace of Line-to-line induced emf waveform and hallsensor 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 alignphase-A and the d-axis (i.e., θr = 0o). Now, at this rotorposition a single phase variable AC(0-5V) is applied tophase-A and ia is measured. The condition gives,ωr= 0; ib = pib = 0; ic = pic = 0. van = dψa

dt =raia+(Ll+L1+L2)pia. From experiments, Ll+L1+L2

= 35.82 mH2) Case 2: AC supply is now applied to phase-B. Phase-

B current and open circuit voltage at phase-A(van) isnoted. Now, ωr = 0; θr = 0o; ia = pia = 0; ic = pic =0. van = (−L1+L2

2 )pib . From experiments, L1 + L2 =16.82mH

3) Case 3: A small DC voltage is applied to phase-B, sothat the B-phase is aligned with d-axis. Now, the variableAC voltage is applied to phase-C. Open circuit voltageof phase-A and phase-B current is noted. Under thiscondition, ωr = 0; θr = 120o; ia = pia = 0; ib = pib = 0.van = (−L1

2 +L2)pic . From test, 0.5L1−L2 =8.53mH.From above 3 cases, Ll = 16.8mH; Ll = 20.426 mH; L2

= 0.02mH. Hence Ld = 43.75mH which is very close toestimated inductance (43.23 mH).

A comparison of calculated and experimentally determinedvalue of machine parameters of fabricated BLDC is given

TABLE IV0.75 HP BLDC: PARAMETER EVALUATION

Parameter Estimated value Experimental(using design value

calculation and FEManalysis)

Ra(ohm)/phase 7.2 8.1Ld(mH) 43.23 43.75Lq(mH) 43.23 43.75

EMF constant Ke 178.5 180(VL−L/1000rpm)Torque constant 2.38 2.38

Kt (N-m/A)

in Table-IV. Similarly, Table-V shows a comparison betweenthe fabricated 0.75hp, 1500 rpm BLDC motor with a 3-phInduction motor of similar rating. From this Table-V, it isclear that the power density and toque density are muchhigher for BLDC motor compared to 3-ph IM .Currently, thehousing/shell of fabricated BLDC is made of MS material. Ifwe use light(aluminium) housing/shell housing, then weightof the machine will reduce further(nearly by 40%) i.e. power& torque density will increase.

TABLE VCOMPARISON BETWEEN 0.75HP, 1500 RPM BLDC MOTOR AND 3 PH- SQ

INDUCTION MOTOR.

Items Designed BLDC Available IMEffective weight(kg) 5 10

Total weight(kg) 10 16.4( including housing)

Effective power 112 54.94density(W/kg)

Net power 55 33.5density(W/kg)

Effective torque 1.226 0.8856density(N-m/kg)

Net torque 0.613 0.54density(N-m/kg)

C. Experimental results of 0.75hp BLDC motor under openloop V/f control

The BLDC motor has been tested in open-loop V/f controlmode as a motor with low load. The DC-link voltage appliedis nearly 200V at a switching frequency of 5kHz and at amotor speed is 300 rpm i.e. 10Hz electrical. Fig.14 showsmotor phase voltage(VRN ) and phase current (IR) at 10 Hzrotor frequency.The value of phase current is 0.6 A(rms).

Fig.15 shows motor line to line terminal voltage (VRY )and line current(IR). Fig.16 shows motor line to line terminalvoltage (VRY ) and position sensor 1 output pulse. From thisfigure, PM-rotor speed is confirmed. The frequency of position

Fig. 14. Phase voltage(VRN )(100V/div) and phase current(IR)(1A/div) at300 rpm in V/f control

sensor 1 output pulse is 10 Hz. Fig.17 shows motor phasevoltage and hall sensor 1 pulse at 300 rpm speed.

Fig. 15. Line voltage(VRY )(100V/div) and phase current(IR)(1A/div) at 10Hz. i.e. 300 rpm speed in V/f control

Fig. 16. Line voltage(VRY )(100V/div) and Hall sensor1 pulse(2V/div) at 10Hz. i.e. 300 rpm speed in V/f control

Fig. 17. Phase voltage(VRN )(100V/div) and Hall sensor1 pulse(2V/div) at10 Hz. i.e. 300 rpm speed in V/f control

V. CONCLUSIONS

In this paper the design and fabrication of a 0.75hp, 1500r.p.m, 400V (DC-link) surface mounted BLDC motor hasbeen presented. For ease of fabrication and for cost reductionthe design starts with the dimensions of an available IMstator stamping which has also been used finally for statorfabrication. The motor has been also completely fabricated ata local motor manufacturing company. After fabrication, themotor has been tested in generating mode(with coupled DC-machine) and motoring mode (with open loop V/f control).There after, parameter evaluation of the fabricated motor hasbeen done experimentally.

VI. ACKNOWLEDGEMENTS

The authors wish to thank the staff of M/S GE motorsPvt. Ltd, Sheorapuli and Mr. Kausik Pyne, in particular, forthe manufacturing support received in fabricating the BLDCmotor. The authors also acknowledge the support receivedfrom the funding agency DeitY and the research colleaguesparticularly, Mr. Netai Dutta at the Advanced Power Electron-ics Laboratory, Dept. of EE, IIEST, Shibpur towards this work.

REFERENCES

[1] Mukherjee P., Sengupta M. “Design, Analysis and Fabrication of aBrush-less DC Motor”,IEEE conference , PEDES-2014, IIT Bombay.

[2] K Venkata Ratnam, Special Electrical Machine,University Press, 2008.[3] D.O’Kelly And S.Simmons, Generalized Electrical Machine Theory,

McGRAW HILL, 1968.[4] S.K. Nanda, 1kW, 48V, 2000rpm, 4pole BLDC Motor for Electric Vehicle

Application, M.E. Thesis, Department of EE, BESU, Shibpur, 2006.[5] Paitandi S., Sengupta M. Design, Fabrication and Parameter Evaluation

of a Surface Mounted Permanent Magnet Synchronous Motor IEEEconference, PEDES-2014, IIT Bombay.

[6] R.Krishnan, Permanent Magnet Synchronous And Dc Motor Drives,CrcPress,2005

[7] A.K. Sawhney, Electrical Machine Design, DhanpatRai and Co., Sixthedition, 2006

[8] N. A. Demerdash, R. H. Miller, T. W. Nehl et al, , Comparisonbetween features and performance characteristics of fifteen HP samariumcobalt and ferrite based brushless DC motors operated by same powerconditioner, IEEE Transactions on Power Apparatus and Systems, 104-112, 1983.

[9] T. M. Hijazi and A. A. Arkadan, , Computation of winding inductancesof permanent magnet brushless DC motors with damper windings byenergy perturbation, IEEE Transactions on Energy Conversion, 3(3),705-713, 1988.