Design and Testing of a Universal Motor Using a Soft Magnetic

Composite Stator Alan Jack, Barrie Mecrow and Phil Dickinson

The University of Newcastle upon Tyne Newcastle upon Tyne, NE 17RU

UK

Abstract - Design and test results are reported for a vacuum cleaner universal motor with the stator core manufactured from compacted iron powder. The isotropic magnetic properties allow freedom to create better-shaped windings and savings in copper. The core is separated into poles and half-yokes, split on the axial centreline, allowing bobbin winding of the field and easy assembly. The motor uses the existing production rotor, brushes and bearings. Tests show the new motor to be near identical electrically, but slightly better than the existing machine when placed in the vacuum cleaner.

I. INTRODUCTION

Series connected ac commutator motors (so called universal motors) continue to be built in many millions every year and still have a dominant market position in power tools and domestic appliance applications. The machines have reached a very high level of sophistication, both in the power density and in their manner of production. The result is a very cheap drive, which continues to constitute a very competitive benchmark against which low cost, electronically commutated, brushless drives must be compared. Despite their continued market dominance, surprisingly little research is reported on universal motor development.

This paper reports on the design and measured performance of a new universal motor in which compacted iron powder (soft magnetic composite or SMC) has been used to form the magnetic core of the stator. The motor has been built to be directly comparable with an existing commercial motor for a domestic vacuum cleaner. It uses the same armature, brushes, bearings and fan as the original laminated motor. The aim was to produce a motor of comparable performance but lower cost in volume manufacture.

Soft magnetic composites have been produced since the earliest days of electrical machines [ 11, but recent research and development has produced a sharp improvement in the magnetic properties of the material [2-41. Even with these advances, soft magnetic composites remain inferior to typical lamination steels, with a rather low maximum permeability, reduced saturation flux density and higher iron loss. Nevertheless, in this application the isotropic properties, close tolerance and the possibility of smooth rounded three- dimensional components allows significant improvements to be made that more than compensate for the deficiencies. The results described in this paper show that at least comparable performance has been achieved.

Patricia Jansson, and Lars Hultman Hoganas AB

Hoganas S-263 83 Sweden

In essence the new motor allows much more efficient usage of the field winding copper (around 60% less than the established laminated machine). The cross section of the core can also be increased in critical areas by employing the three- dimensional flux carrying capability of powdered iron, without increasing the overall envelope of the machine.

11. CONCEPT

This work had the benefit of an earlier universal motor prototyping exercise [ 5 ] , which showed that axially recessing the pole necks of the stator (i.e. field), whilst also making them circumferentially wider, produced substantial savings in the required field copper. This was because the pole neck cross section was made more nearly circular, reducing the peripheral coil length for the same core cross section. The saving was magnified because the reduction in coil length required thinner wire to be used to match the original field winding resistance. In that earlier work little attention was paid to the practicalities of manufacture of the machine. This study considered volume production from the outset, and it is reflected in the assembly method.

Fig. 1 Exploded view of stator core

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The concept for the powder core is shown in Fig. 1. The core is spit into four sections - two identical core backs and two identical poles pieces. These parts have a low aspect ratio with no small features, both of which are important features for a die compacted powder core which makes them both practical and cheap to compact. The design also allows easy winding of the field coil (either separate from the core or using the pole section as a former) and, via axial clamping, mechanical integrity of the core without recourse to glue. Fig. 1 also shows that the core back is substantially axially longer than the poles. This allows the core to be thinner, freeing up space for the field winding and hence realising further reductions in the average length of a turn.

The general concept for the core can be seen as a development of earlier work on powder iron cores for servo motors [6]. The core back of those motors was slotted to allow insertion of the teeth after assembly of the core, relying on a glue joint to retain the tooth. In this case the core back is split axially to allow a clamping action. The axial split does not lie across a magnetic field path and hence the small clearance, required to force interference between the poles and the core back, does not affect the electromagnetic performance. The clearance between poles and the core is very small as a natural consequence of the accuracy inherent in the die compaction process.

The material chosen for the stator core, Somaloy 550 TM [2], has been especially formulated for low frequency applications. The powder grain size is relatively large to give higher permeability at the expense of increased iron loss at higher frequencies. In this application maximising the saturation flux density is by far the most important property. Whilst this material is better than previous soft magnetic composites, it still has much lower permeability and saturation flux density than the low carbon, silicon free, steels typically used for universal motors.

2.5

2

1.5 h

t m

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0 4 I I I I I 1 0 10 20 30 40 50 60

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Fig. 2 Somaloy 550 and low carbon lamination steel B-H curves.

111. ELECTROMAGNETIC DESIGN

The aim of the design was to match or improve upon the performance of the commercial machine. The original rotor was used, so it was necessary to match the flux in the machine in order to give the same terminal voltage at a given speed. A significant increase in flux was not worthwhile, since it would cause excessive saturation in the armature. It was decided to use the same number of the field winding turns as in the laminated machine; hence the design devolved into choosing the stator magnetic circuit to minimise material usage without infringing production requirements.

The finite element method was used for the design of the machine. Three-dimensional models are strictly necessary, but the field is planar for the majority of its path and hence the simple artifice of an effective “space factor” of 1.8 was used for the core back of the powdered iron motor, commensurate with the ratio in the axial length of the core back and poles. The field distribution at maximum current is compared in Fig 2 for the laminated and SMC machines. For an rms current of 6.3 amps the peak core back flux density in the laminated machine was calculated to be 1.74 Tesla, and the powdered iron stator was designed for the lower value of 1.33 Tesla.

The most critical area in the stator of the machine is the pole shoe on the trailing side. Armature reaction reduces the flux on the leading side of the pole and increases it on the trailing side (as is clearly apparent in Fig 2). Any increase in pole shoe depth takes away from winding area and hence a balance needs to be struck. In the SMC machine the core back is shallower (because it is axially longer) which allows the pole shoe to be substantially increased in depth.

Fig. 2a Field at load for the powdered iron machine

Fig. 2b Field at load for the original laminated machine

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IV. MACHINE COMPARISON

The difference in stator arrangements is shown in Fig. 3 and the completed machine is shown in Fig. 4. The two stator cores have the same width and therefore the two pictures may be compared. It may be noted that the stator core back of the SMC machine (Fig. 3a) is split in quarters rather than the halves shown in Fig. 1. This was purely to facilitate manufacture of the prototype and is not regarded as a good choice for production. The very large reduction in field winding copper is clear. The field winding was produced via simple bobbin winding methods using the pole as a former. It will be apparent from Fig 3a that the room necessary for the field winding has been rather overestimated and that a second generation machine can be made substantially smaller.

Before dynamometer testing and final testing with the vacuum cleaner fan installed, the two motors had their standstill characteristics measured. The results for terminal voltage against current are shown in Fig. 5, along with the predicted results from the finite element study. These results are dominated by the field reactance. The resistance of the field and armature and the reactance of the armature are all relatively small. Hence to a large extent Fig. 5 is showing the size of the flux produced by the field. The agreement between finite element and measured results is generally good. The largest area of difference is in the laminated motor at mid-voltages, where the saturation characteristics of the actual motor are a little sharper than the predictions. This is probably an inaccuracy in the BH curve used in the finite element calculations. The curves show that at low currents the laminated motor has significantly more flux but at working values, i.e. at 4 amps and above, the two are rather similar. This is a function of the far lower maximum

permeability of the SMC at low currents followed by near equality at high currents as the balance of a slightly reduced saturation flux density combined with a larger iron cross section in the SMC machine becomes dominant.

Because the SMC machine produces similar field flux at working currents then similar torque can be expected from the two machines, thereby obtaining the desired design goal. The laminated machine, with its very large end winding, creates a significant leakage reactance. This has been estimated at 2mH using a simple end region model, which constitutes about 5% of the field flux linkage at 5A.

Fig 3b Original laminated stator

Fig 4 Completed motor

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70

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a 40

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- L

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0 1 2 3 4 5 6 7

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Current (A) Fig. 5 Terminal voltage against current (at standstill, 50 Hz)

The torque was measured as a function of rms current at standstill and compared with finite element predictions, as shown in Fig. 6. The agreement between finite element and

Fig. 6 Torque at standstill, 50Hz

SMC measurement once more is good. The measured results are a l_l__l Laminated ~ " _ "

29600 29100 little lower than the predictions, possibly due to the bearing %::::$ 775 762 friction or simply error in the finite element results. The current(^) 4.7 4.6 measured results are not smooth, which probably points to Field resistance @2OC 2.4 2.46

580 620 The basic torquelcurrent characteristics have the classical ~~~~~~ f:;;:::$)(kg) 240 98

Efficiency (") 73 71 random error in the measurements.

shape of a series commutator motor, showing torque variatior with current squared at low current followed-by a linear variation with current once saturation sets in. The measurements show that there is very little to choose between the two motors in standstill tests.

The machines were tested on the dynamometer without their fans fitted and the results are shown in Fig. 7. Unfortunately the laminated machine was tested at 230V whilst the SMC machine was tested at 240V (both 50Hz). This makes true comparison rather difficult. At reasonable current levels (i.e. above about 3A) the machines torques are more or less linear with volts and taking the ratio 2401230 into account it is clear that the two machines are very similar in electromagnetic performance across most of the load range. It is interesting to note that the increased iron loss that might be expected in the SMC stator does not show up in the overall loss results. This only demonstrates that stator iron loss is not a dominant source of loss in these machines.

The major parameters of the two motors are compared in Table I, with the running electrical figures taken from dynamometer tests (without the fan) at a torque of 25Ncm, typical of loaded operation and scaling the laminated motor speed, power and current by the ratio 2401230 to approximately account for the differences in test voltage. The electrical performance is very similar. The reduction in

field winding copper (by 60%) is the most obvious feature. The laminated core is 7% lighter, yet the overall envelope of the SMC machine is the same as the laminated machine because of the three dimensional nature of the SMC magnetic circuit.

The final comparison of the two motors is in the product. The motors were installed in a vacuum cleaner and run with varying throttle restriction in the inlet. Measurements were taken of air flow, vacuum and input power at a fixed terminal voltage of 240 volts, 50Hz. The results are shown in Fig 8. As may be seen, the new motor slightly outperforms the laminated motor across the full load range, yet has more or less identical input power. The two motors are obviously very close to each other, although arguably the SMC machine is marginally better. Detailed examination of the results show that the increase in aerodynamic performance is slightly in excess of the increase in electromagnetic performance. In this style of vacuum cleaner all of the air passes directly through the motor. The reduction in copper and the thinner and longer core back probably form a better duct and account for this extra performance.

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I-II 1

-X- Power (laminated) - - -& - -Power (SMC)

& durreni (I&.) . . . -x- . . - current (sMC) . . . -0.. . - Efficiency (SMC) # Efficency (lam.)

,

1 I

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0 5 I O 15 20 25 30 35 40 45 50 Torque (Ncm)

Fig. 7 Dynamometer tests

350000

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V. CONCLUSIONS

The possibilities for improvements in universal motors using SMC for the stator have been demonstrated by the prototype described in the paper. Copper savings can be made without loss of performance. The proposed motor is simple to construct and none of the components are particularly challenging to manufacture. The isotropic nature of SMC coupled with the ability to manufacture fully three dimensional shapes with good tolerance and surface finish allow design freedoms which can more than offset the intrinsically poorer magnetic performance.

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0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

Flow Rate (m3/s)

Fig. 8 Flow and power test with the motors in the vacuum cleaner

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REFERENCES

Fritts, US Patent 874908, 1886 W. Rutkowski, and B. Weglinski, “PM composites for magnetic cores of electrical converters”, Powder Metallurgy, 1979, No. 2 P. Jansson, “Advances in soft magnetic composites based on iron powder”, Soft Magnetic Materials 98, Barcelona, Spain, April 1998, Paper 7. R. Krause, “Development of a composite material for high-density, three dimensional, soft magnetic components’: Sofi Magnetic Materials 98, Barcelona, Spain, April 1998, Paper 17. A.G. Jack, “Experience with the use of soft magnetic composites in electrical machines”,fCEM ConJ 1998, Istanbul, Sept 1998, pp1441- 1448. Jack, A.G., Mecrow, B.C., Dickinson, P.G., Stephenson, D., Burdess, J.S., Fawcett, J.N., Evans, T., “Permanent magnet Machines with Powder Iron Cores and Pre-Pressed Windings”, 1999 fEEE Industry Applications Conference, 34Ih Annual Meeting, Phoenix Arizona, 3-7 October 1999, pp97-103.

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