Low Cost Solutions to Reduce Cogging Torque and

Acoustic Noise of Small Brushed DC Motors for

Automotive Radiator Cooling Fan Modules Aldo Boglietti, Fellow IEEE, Andrea Cavagnino, Senior Member IEEE, Alberto Tenconi, Senior Member IEEE

Politecnico di Torino, Dipartimento Energia, Corso Duca degli Abruzzi 24, 10129 Torino, ITALY aldo.boglietti@polito.it, andrea.cavagnino@polito.it, alberto.tenconi@polito.it,

Abstract - The paper focus is on the reduction of the acoustic noise emitted by radiator Cooling Fan Modules (CFMs) equipped with small brushed DC electrical motors. Initial

measurements performed on standard production CFMs proved that the main part of the emitted noise is excited by specific cogging torque harmonics of the motor. Then, modifications in

the motor production have been analyzed by means of electromagnetic Finite Element Method (FEM) simulations. The solutions held compatible with the cost-sensitive automotive component market and the actual automatic production lines have been implemented in special prototypes. The reported noise measurements results of CFMs equipped with these prototypes show the effectiveness of the proposed low-cost solutions for the cogging torque and emitted noise reduction.

Index terms: Electric motors, automotive applications, brushed DC motors, finite element method, acoustic and aerolic noise, cogging torque, vibrations, cooling fan modules.

1. INTRODUCTION

Tn automotive applications, including the electrical vehicle ones, severe noise emission limits are imposed by standards and costumer acceptation tests, with the obvious consequence that 'noiseless' components have to be mandatory used. With respect to electrical motors to be employed as automotive component, the Permanent Magnet (PM) brushed DC motors still play a key role in the market, even if the AC machines are used more and more [1], [2]. As it is well-known, one of the important drawbacks of the PM machines is the cogging torque [3] which involves, if it is not adequately filtered by the mechanical system, shaft vibrations and acoustic noise. The cogging torque has a periodic shape versus the mechanical position [4] and its negative effects especially appear at low speeds and light loads [5].

Focusing now the attention on small brushed DC motors to be used in radiator Cooling Fan Modules (CFMs), any system modifications (devoted to the reduction of the cogging torque or to the mitigation of its effects) could represent a binding challenge for the manufactures. Tn particular, the feasibility of these modifications requires the compatibility with the actual automatic production lines and the pressing cost constraints typical of mass production.

To the best of the authors' knowledge, very few technical references concerning the vibro-acoustic analyses of small PM brushed DC motors can be found ([6] is an example for what concern the vibrations), while for AC motor typologies several studies and researches are documented [7]-[11].

Nowadays, the available analytical/numerical simulation tools allow to compute in an accurate way the mechanical forces on the different electric motor parts and the consequent vibration modes of the structure. Anyway, the computation of the emitted acoustic noise by the whole CFM it is a very difficult task. For this reason, the authors decided to approach the problem by means of a full experimental approach. In fact, a close correlation between vibrations and acoustic emission can be easily observed [12]-[15].

The paper is structured in the following way. In section 11 the results of an initial test campaign performed on CFM of normal production are reported in outline in order to address the problem of the emitted noise in the considered application. Section III deals with low-cost solutions for the cogging torque reduction (in particular, its 36th harmonic). Finally, the solutions held compatible with the cost-sensitive automotive component market and the actual automatic production lines have been implemented in special prototypes and the related results are included in detail in Section IV. These measurements prove that, for the considered CFM application, the noise emissions can be reduced to acceptable levels.

(a)

(b)

ui� ------- --------�-------- -------

Fig. 1: (a) brushed DC motor under test, (b) patented rotor lamination [16].

978-1-4673-2421-2/12/$31.00 ©2012 IEEE 1870

(a) (b)

Fig. 2: Two of the considered test layouts used for the acoustic noise measurements in the semi-anechoic chamber.

IT. NOISE SOURCE IDENTIFICATION IN CFMs

A. Radiator coolingfan module

The company involved in this research selected the brushed DC motor type and the specific CFM under analysis. Fig. I.a shows the 300 W, 13 V, 4 poles electric motor, while Fig. l.b shows the detail of the patented 18 slots, asymmetric rotor lamination [16], [17]. The complex rotor slot shapes allow to considerably reduce the length of the annature endwinding with respect to more conventional laminations. The CFM unit is constituted by the electrical motor, an eight symmetrical blades fan, the shroud and the radiator, as shown in Fig.2.b.

The acoustic noise measurements have been performed in the semi-anechoic chamber available in the factory' s laboratories, changing the rotational speed of the motor from o up to 3500 rpm. The noise spectra have been acquired using a linear FFT, with a resolution of 800 lines, in the frequency range 0-5 kHz. The noise levels are measured in dB in the sonograms and in dB(A) in the overall/hannonic noise curves.

B. Noise measurements on standard production CFMs Objectives of this initial test campaign perfonned on three

standard production motors were:

• the identification of the different noise sources (e.g. noise due to brushes, aerolic noise, cogging torque hannonics, blade resonance effects, etc.);

• to investigate the possibility to have reasonable infonnation about the acoustic noise emissions without assemble and test the whole system, as done during the normal acceptation tests.

For these reasons different microphone positions around to the motor/CFM, several test layouts and different load conditions were considered. The complete results of these measurements are presented in detail in [18]. Anyway, for the sake of completeness, the noise measurements for the layouts shown in Fig.2 are reported here following for one of the tested motors. In particular, the two considered layout are:

RPM

Fig. 3: Sonogram for the motor fixed on a rigid support (Fig.2.a).

Fig. 4: Sonogram for the CFM in elastic suspension (Fig.2.b).

• motor in no-load condition (without fan), fixed on a rigid mechanical support (Fig.2.a);

• motor with fan and radiator (complete CFM system), in elastic suspension (Fig.2.b).

For both of these layouts, the microphone is positioned at a distance equal to 30 cm on the shaft axis direction in the front of the motor/system (fan side).

Fig. 3 shows the sonogram for the motor fixed on the rigid support in the whole speed and frequency ranges. This figure highlights a huge number of harmonics (oblique lines) plus a mechanical resonance of the rigid support. This • confusing' situation has been considered useless to predict the real behavior of the machine when it is mounted in the CFM. For example, as shown by the cursors, a 90th harmonic is active but it has not practically effects on the whole CFM system.

The emitted acoustic noise measurements for the complete CFM system (motor with fan and radiator, in elastic suspension) are reported in FigA and Fig.5. In particular, the overall noise curve is shown in Fig.5 together its main harmonics. For the application in hand, it is evident that the three peaks in the overall noise curve are due to the 36th harmonic mode. Since a 18 slots, 4poles PM DC motor is used, this mode can be only excited by the 36th harmonic of the cogging torque [3], [19].

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1.2

1.0

5

Fig. 6. Standard production motor (a), and motor with PMs having chamfered edges (b).

Cogging torqueharmonics Ip.u.1 - Standard production - Chamfered PM edges

9 �8

Fig. 5. Emitted acoustic noise of the CFM in elastic suspension: overall 0.6

characteristic (red), 18th harmonic (blue), and 36th harmonics (green).

In order to reduce the noise emission, some modifications in the fan (e.g. number, profile and symmetrical! asymmetrical pitches of blades) or in the shroud (e.g. shape, stiffness) could be used. Anyway, the target of the study is to directly reduce the noise sources, not to filter them through the mechanical system.

TIT. FEM ANALYSES RESULTS

To reduce the vibrations/noise in the considered CFM application the motor cogging torque harmonics should be reduced properly, by means of suitable modifications of its electromechanical structure. Tn particular, including specific 'asymmetries' and 'part shift' in the machine, the cogging torque negative effects can be mitigated [4]. As aforesaid reported, only low-cost geometrical and/or technological modifications (with respect to the motor of normal production) can be considered in the ambit of the automotive component market. From this point of view, it seems reasonable to mainly act on the PM stator structure instead of the rotor armature one, even if a special armature lamination will be considered in the following, because it is compatible with the actual automatic lines adopted for the armature winding production.

It is evident that the impact on the cogging torque of the proposed modifications can be analyzed by means of Finite Element Method (FEM) numerical simulations only. Tn particular, bi-dimensional transient FEM analyses have been used (Flux 2D, by Cedrat Group). Before going into the details, it is important to remark that in the actual study the focus is on the reduction of the cogging torque harmonics (in particular the 36th one) and, for the moment, any possible machine performance degradation is not investigated. Hereafter, the proposed motor modifications are reported and discussed.

A. Baselines for the FEM result evaluations The cogging torque spectra of standard production PM

brushed DC motors (Fig.6.a) and the same motors having PMs with chamfered edges (Fig.6.b) are assumed as baselines to judge the FEM results of the proposed solutions.

0.4

0.2

0.0 A /\. /\.

I" o 5 10 15 20 25 30 35 40 45 50 55 60

harmonic order

Fig.7. Cogging torque harmonics for the baseline machines.

Fig.8. Symmetrical (left) and asymmetrical (right) position shift of the PMs.

The use of PMs with chamfered edges is the typical solution that the factory adopts when noiseless motors are requested by the costumers, but it involves a non negligible production cost increase. In Fig.7 the cogging torque harmonics computed for the two baseline machines are reported. In this figure, such as in the figures reported in the following, the base value is the 36th harmonic magnitude of the standard production motors. In addition, in the following spectra, only the main harmonics are shown.

B. Circumferential shift of the P Ms In this solution, the relative position of the PMs is changed

along the stator circumference. As shown in Fig.8, both the 'symmetrical' and 'asymmetrical' shift of two magnets (with

respect to the other two ones) have been considered. In the FEM simulations, different values of the shift angle have been imposed. The cogging torque harmonics spectra are reported in Fig.9 and Fig. l0 for the symmetrical and asymmetrical shift respectively. It is possible to observe that for these solutions a large 18th harmonic appears. Without doubts, the 18th harmonic would introduce noise problems at lower rotational speed of the fan of the CFM.

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1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0 o

Cogging torqueharm nics Ip.u.1 - Symmetrical shift 7.5°

5

- Symmetrical shift 10°

- Symmetrical shift 15°

A 10 15 20 25 30 35 40 45 50 55 60

harmonic order Fig.9. Cogging torque harmonics for the symmetrical shift of PMs and for

three different values of the shift angle. 1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0 o

Cogging torque harmonics [p.u.] -Asymmetrical shift 100

5

-Asymmetrical shift 15°

A 10 15 20 25 30 35 40 45 50 55 60

harmonic order Fig.IO. Cogging torque harmonics for the asymmetrical shift of PMs and for

two different values of the shift angle.

Tn addition, in order to obtain an effective reduction of the cogging torque harmonics, very large values of the shift angle are requested. As a consequence, since the magnets are very close one to the other, non-negligible detriments of the machine performances are expected. For the asymmetrical shift solution, it should be noted that an unbalanced force on the rotor is active, introducing serious problems in the motor bearing-brass system.

C. Stator yoke with double thickness The stator yoke of the standard production motor is heavy

saturated by the PM magnetic flux. Starting from this observation, a FEM simulation adopting a double thickness for the stator yoke have been done. The results reported in Fig.11 show a worsening in the 36th cogging torque harmonic.

It should be noted that, due to mechanical constraints, it is not possible to reduce the yoke thickness with respect to the original machine.

D. Circumferential gap or dummy slots inside the PMs Tn the solution shown in Fig.12.a the magnets are radially

cut in their center and the two obtained parts are shifted on the two sides realizing the wanted gap width.

The other three drawings in Fig.12 sketch some example of studied dummy slots. Due to cost reason, no more than two slots can be cut in the magnets. Since at prototyping stage the dummy slots have to be produced by grinding, the shape of the dummy slot shape is imposed by the used tool and it is shown in Fig.12.d.

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Cogging torqueharmonics [p.u.] - Stator yoke with douhle thickness

- -

r7 '1 t= I

l'.. .LJ �

-

A AA o 5 10 15 20 25 30 35 40 45 50 55 60

harmonic order Fig.II. Cogging torque harmonics for a motor having a double yoke

thickness with respect to the standard production machine.

(a) (b)

(e) (d)

Fig.12. (a) gap between the magnet parts, (b) single rectangular dummy slot, ( c) two dummy slots, (d) single dummy slot (used in the prototype).

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0 o

Cogging torqueharmonics Ip.u.1 -Gap width 0.75 mm

5

-Gap width 1 mm -Gap width 2 mm

Gap width 4 mm

1\ " 10 15 20 25 30 35 40 45 50 55 60

harmonic order Fig.13. Cogging torque harmonics for solution using a circumferential gap

between the magnet parts.

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0 o

Cogging torqueharmonics [p.u.] Single dummy s lot

-Two dummy s lots

- Single dummy slot, prototype

1\ A A 5 10 15 20 25 30 35 40 45 50 55

harmonic order Fig.14. Cogging torque harmonics for the dummy slot solutions

60

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1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Fig.IS. Optimal arc span of the PMs having radial edges.

Fig.16. Relative shift between two groups of nine slot openings.

Cogging torque harmonics [p.u.]

-Opening shift

-Optimal PM arc s pan

A /\ o 5 10 15 20 25 30 35 40 45 50 55 60

harmonic order Fig.17. Cogging torque harmonics for the opening shift and

optimal PM arc span solutions.

The related results are shown in Fig.13 and Fig.14. It has been found that the minimum value of the 36th harmonic is for a circumferential gap width equal to 1 mm. Unfortunately, to mechanical feasibility reasons, in the dummy slot solution a little be larger slot opening width have to be used.

£. Optimal PM arc span This solution has been already presented in literature

together guidelines to select the optimum value of the arc span [19]. The optimal PM arc spam technique requires to use PMs with radial edges, as shown in Fig. 15. As a consequence, the mechanical system (springs) to retain in position the magnets cannot be used and they have to be glued on the stator yoke. So, even if this solution leads to very good results (Fig. 17), the change in the production process has to be carefully evaluated from the involved cost point of view.

F. Relative shift between a group of nine slot openings with respect to the other nine ones

Also this method of cogging torque reduction has been already presented in literature and it is known as Slot­Opening Shift [3].

Fig.IS. Prototype with shifted slot openings (Prototype I, on the left), prototype with a single dummy slot in the PMs (Prototype 2 , on the right).

Tn the application in hand, nine slot openings, shown in red in Fig.16, have been rotated in counterclockwise direction, while the nine blue ones have been rotated in clockwise direction of the same angle. As a consequence, the relative shift angle between the red and blue slot opening groups is twice as the adopted angle. Tn theory, each slot opening can be shifted in independent way from the other ones inside the slot pitch angle [3]. Anyway, in order to use the available automatic tool machine for the annature winding production, this shift techniques can be just implemented gathering the 18 slot openings in two groups.

For this solution, the obtained results are very good (see Fig.17) and they are comparable with the optimal PM arc span method.

Finally, for the sake of completeness, other methods to reduce the cogging torque could be adopted [19]. For example, single or multiple dummy slots in the annature lamination tooth tips or the skewing of the PM lateral edges along the axial direction can be cited. Looking at the complex tooth tip geometry (Fig.l.b), the use of dummy slots in the rotor teeth is not possible due to mechanical constraints. Tn addition, this method moves the cogging torque hannonic toward higher orders, involving, for the considered CFM application, noise problems at lower rotational speed of the fan. With respect to the PM skewing, a prototype characterized by a discrete 'two-steps' axial skewing of the PMs have been realized and tested, as reported in [18]. Anyway, since the related production cost increases have been judged unacceptable, this solution is not longer presented.

IV. PROTOTYPE PRODUCTION AND RELATED ACOUSTIC NOISE MEASUREMENTS

On the basis of the previous FEM analysis results and in accordance with the factory involved in this research, the more interesting solutions for the prototyping activities are:

• the relative shift between a group of nine slot openings with respect to the other nine ones (Prototype 1, on the left side of Fig. 18);

• single dummy slots inside the PMs (Prototype 2, Fig.12.d and on the right side of Fig.18);

• optimal PM arc span.

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58 5S

54

RPM

Fig. 19. Sonogram for the CFM equipped with a standard production motor.

Fig. 20. Sonogram for the CFM equipped with the Prototype 1.

Fig. 21. Sonogram for the CFM equipped with the Prototype 2.

Unfortunately, at the time of the writing the prototype with the optimal PM arc span is still under production and the related results cannot be included in the actual report.

Case-by-case, the two prototypes have been mounted in the same CFM used in the initial test campaign (Section ll) together a further motor of normal production. The elastic suspension layout for the CFM in the semi-anechoic chamber are used during the fmal tests too. For the prototypes, no specific optimization of the brushes positioning angle has been done with respect to the standard production machine ( equal to 5 degrees).

100 dB

90

80

70

60

50

40

30

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36th '" -72th

20

10 Speed [rpm] 0 o

500 1000 1500 2000 2500 3000 Fig. 22. Emitted acoustic noise of the CFM equipped with a standard

production motor: overall characteristic and main harmonics. 100 � �---,-----,------,------,------,---�

dB 90 t------t------�----4------+------�-----80 t------t------�----4---������-----70 t-----�-----+���------��--���-

20 t-�--_t----�------+------+------�-----10 t-----�-----+-----4------�----+-----� o +-____ -+ ______ �----�------�--�S�pe= e� d�lr �pm� I�--

o 500 1000 1500 2000 2500 3000

Fig. 23. Emitted acoustic noise of the CFM equipped with the Prototype 1: overall characteristic and main harmonics.

100 � �---,-----,------,------,------,-----­dB

90 t-----i-----_+-----4------�----+-----� 80 t-----�-----t----�--��}-���-----70 t-----_t------����-----+--�--����

30 t--y��----�----�----�-=��-------

20 t-�--�-----+-----4------�----+------

l0 t------r----�------+------+------�--� o +-____ -+ ______ �----�------�--�S�pe= e� d�[r�pm= ]�--

o 500 1000 1500 2000 2500 3000

Fig. 24. Emitted acoustic noise of the CFM equipped with the Prototype 2: overall characteristic and main harmonics.

The measured sonograms for the standard production motor, the Prototype 1 and the Prototype 2 are reported in Fig.19, Fig. 20 and 21 respectively, while the overall noise characteristics together the related main harmonics that g?vem the phenomenon are reported from Fig.22 up to Flg.23. Tn order to avoid the attenuation in the overall and harmonics characteristics due to the A-weighting (in the low speed range), the authors decided to show the noise levels directly in dB, instead of dB(A) as done in Fig. 5.

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1.2 Coggingtorque harmonics [p.u.] 1.0

-Normal Production -Prototype 1

0.8 -Prototype 2

0.6

0.4

AAA M! A s.::.. 1> .A � A 0.2

0.0 o 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

harmonic order Fig.25. Measured cogging torque harmonics for a machine of normal

production. the Prototype I and the Prototype 2.

Both the sonograms and the noise curves put in evidence that the proposed prototypes are able to reduce the effects of the cogging torque 36th harmonics on the overall acoustic noise. In fact, the three peaks active in Fig.22 are reduced to acceptable levels in Fig.23 and Fig.24. From the emitted acoustic noise point of view, the best solution seems to be the Prototype 1, because for the Prototype 2 a non negligible 72th harmonic plays a role in the overall noise characteristic at lower speed. The 72th harmonic appearance is reasonable because, from the cogging torque theory [19], the dummy slots in PMs leads to 8 'elementary' pieces of magnet (Least common multiple( elementary PMs, slots) = LCM(8, 18)=72).

Measurements of the cogging torque spectra have been performed in the factory' s laboratory and they are shown in Fig.25. These measurements further confirm the possibility to reduce the 36th harmonic by means of the proposed low-cost solutions implemented in the two prototypes. Comparing Fig.25 with Fig. 7, Fig.14, and Fig.17 it is possible to observe smaller reductions in the measured cogging torque harmonic components than expected by the 2D-FEM simulations. This fact could be reasonable, taking into account that the three prototypes have been produced 'by hand' as unique pieces, and spreads in the production processes and material characteristics are always possible. In addition, it is an authors opinion that 2D-FEM analyses are an useful tool to understand general tendency trends, but the actual benefits of the proposed modifications can be rigorously quantified through measurements only.

V. CONCLUSIONS

Initial measurements performed on standard production PM brushed DC motors showed that for the considered CFM application the acoustic noise problems were directly correlated to the 36th harmonic of the cogging torque.

Several low-cost modification solutions suitable to mitigate the problem have been proposed and analyzed by numerical simulations. The two most promising solutions have been implemented in special prototypes. The reported experimental results prove that the noise emissions can be reduced to acceptable levels. As general conclusion, the designers of small PM brushed DC motors for noiseless CFM applications should take into account the necessity to reduce the machine cogging torque until during the electromagnetic design stage.

ACKNOWLEDGMENT

The authors gratefully thanks Mr. Renato Poncini (GATE S.r.l. company, Asti, Italy) for the support during protos production and Mr. Ermanno Procopio (GATE S.r.l. company, Asti, Italy) for the noise emission measurements.

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