Hidden Noise Surfacing

1. The curtain opens

For most people, “transportation electrification” refers to the different aspects of traction.  This typically includes the drive system as a whole as well as its elements, such as the electric machine(s), the inverter(s), and the energy storage.  Cost, packaging, system layout the energy flow, the electric sizing, but also mechanical stability of the overall chassis and driving behavior are common aspects of interest.  The noise developed by such electric traction drives has also gained significant attention.  For example, the absence of the noise combustion engine’s noise may allow driving during early morning hours in noise-sensitive neighborhoods, but also affects the pedestrians’ awareness of approaching cars.  While theoretically less noisy than a combustion engine, inverter-based drive systems introduce a different frequency spectrum of force excitations which may trigger resonances within the mechanical structure.  Such reduction has found wide attention, notably by modifying the control strategy, such as reducing the supply’s harmonic spectrum. 

While most of such discussions have focused on the traction drives themselves, the road noise and especially the noise of the combustion engines have nicely hidden much of the noise developed by the auxiliary drives in a car in the past.  As a matter of fact, as the general noise level in a car has been constantly decreasing (Fig. 1), the noise of auxiliary drives may surface.  In addition, the number of auxiliary drives has been increasing continuously for reasons of performance, safety, and comfort enhancements.  Even though the total number of small electric drives in a car strongly depends on the vehicle’s equipment (and hence price range), it amounts to more than 100 for a mid-size executive car (1,2) with a rising tendency.

1

Fig. 1: Development of in-car noise over time, as per (1).

2. Fractional horsepower, auxiliary drives by themselves

Three types of such auxiliary drives are distinguished between:  

(1) Performance-related drives which are directly linked to the engine or power train, such as the oil pump or the engine cooling pump.  
(2) Safety-related drives which are directly or indirectly connected to the car’s drivability and handling, such as the windshield wipers or the headlamp beam height control; and
(3) Comfort-related drives which are not necessary for the functioning of the car, but which enhance the driving experience, such as ventilation and air conditioning or seat actuators.

As the expectations on the comfort of driving have been continuously increasing over time, the comfort-related drives are the largest share of auxiliary drives.  As per (2, citing 3), they contribute to more than ¾ of all the auxiliary drives of a mid-size executive car.  In 2013, Mercedes Benz published the number of comfort-related electric drives in their S-class, see Table I.
 

TABLE I: NUMBER OF AUXILIARY DRIVES TO INCREASE THE COMFORT
OF THE MERCEDES S-CLASS IN 2013, AS PER (4).

 

Function

Quantity

Ventilation, air conditioning system

21

Seat actuator

54

Steering wheel adjustment

2

Magnetic valves for massage seat (two seats)

28

Window lifter

4

Exterior mirror

5

Door and tailgate closer

5

Sound system (tweeter adjustment)

2

Steering wheel vibrator

1

Fan, main display

1

Sunroof and window shade

5

Seat belt bringer and stretcher

4

DVD-deck

2

∑ 132

 

In contrast to “larger,” integral horsepower drives, fractional horsepower drives are not subject to standardization.  As a matter of fact, customized solutions of such small drives are developed for each case of utilization, which, in turn, often is a mass-application.  As such applications are very much cost-driven, suboptimal motor behavior is often accepted, as long as the functionality of the product is not compromised.  This may result in, e.g., single-phase instead of three-phase motors, fluctuating power and torque, or high cogging torque (which is a possible noise source), as higher performance designs, or mitigation techniques, such as cogging torque reduction, are expensive.

3. New design challenges

The increase in the number of auxiliary drives has also resulted in renewed interest not only in the performance parameters of these small drives, such as energy conversion efficiency, size, and cost, but also in electromagnetic interference (EMI), noise, and fault tolerance.  As part of this process, electronic-based brushless direct current (BLDC) drives have recently been replacing their brushed counterparts, notably for long-term applications, such as ventilation or pumping systems.  Table II shows the motor types and their applications, as identified by different sources, illustrating the trend towards the increased usage of BLDC motors; see, for example, the use as engine cooling fan and fuel pump.  These recent developments have opened up a large field of research that is yet to be addressed, as illustrated in Fig. 2.

TABLE II: TYPES OF MOTOR USED AS AUXILIARY DRIVE-IN AUTOMOTIVE APPLICATIONS
AND CORRESPONDING APPLICATION. (6, 7, 8, 9, 10, 11, 12) 

Application

BLDC/

BLAC

Brushed DC

Stepper

ESP/ABS motor

X

EVP (electric vacuum pump)

X

EMB (electric-mechanical breaking)

X

Sunroof motor

X

Closure motor

X

Window lift motor

X

Seat adjustment motor

X

Steering column adjuster

X

Engine cooling fan

X

X

Seat ventilation

X

Sensor ventilation

X

Main heat- & AC fan

X

Thermo-management fan

X

Sensor blower

X

Active steering support

X

Wash water pump

X

Cooling water pump

X

Fuel pump

X

X

Oil pump

X

Air throttle valve

X

Windscreen wiper motor

X

Expansion valves

X

Mirror adjustment

X

X

Headlight range adjustment

X

Fuel and speed gage

X

 

 

2

Fig. 2: Research problems as the number of auxiliary drives in automotive applications as well as
the performance requirements have been constantly increasing. 

The reduction of the noise that has been hidden for long and that is now surfacing is tightly related to the other research questions identified.  As the different sources of noise are part of the design of the electric machine, its drive, and the overall driven system, trade-offs have to be made, and performance spaces for different design spaces identified.  

Noise generation in electric drives is indeed very complex, including three main sources: electromagnetic, mechanical, and aerodynamic.  While electromagnetic and mechanical sources primarily cause structure-borne noise, which may indirectly lead to air-borne noise, aerodynamic sources directly emit air-borne noise.  

4. Example case and conclusions 

To illustrate the sensitivity of the structure-born noise caused by a rotor imbalance, different small masses were applied to a small commercially available fan (12 V, 0.2 A, 2600 rpm, seven blades, 32 mm inner and 75 mm outer radii, 26 g), see Fig. 3: 

• a mass of 0.7 g at position 1 (inner radius)
• a mass of 0.7 g at position 2 (outer radius)
• a mass of 1.05 g at position 2 (outer radius)

3

Fig. 3: Example case fan and positioning of the added masses.

The increase of the fundamental component of the acceleration is determined by an accelerometer.  For the studied imbalance, this fundamental corresponds to the drive’s rotational speed.  The measured increase of the fundamental is remarkable, amounting to more than 300%, 1600%, and 2350% respectively.

4

Fig. 4: Measured increase of fundamental component of the acceleration due to imbalance of example case fan.

Such illustrations demonstrate well the challenge in meeting the demands of increased performance on the rising number of auxiliary drives used with automotive applications, and the concerted effort of different disciplines that is required to address these.

 

References:
[1] P. Zeller, Handbuch Fahrzeugakustik–Grundlagen, Auslegung, Berechnung, Versuch [Manual of vehicle acoustics – design, computation, experiments; in German] Springer, 2009; p. 394.
[2] H. Hembach, “Systematischer Vergleich von BLDC-Motorkonzepten mit Anwendung auf nass laufende Wasserpumpen kleiner Leistung,” [Sytematic comparison of BLDC motor concepts for low power wet running water pump applications; in German] Ph.D. Dissertation, Bundeswehr Universität München, Germany, 2007.
[3] R. Kennel: Tagungsunterlagen: „Elektrische Kleinantriebe im Kfz“, HdT Kleinantriebe, Essen, Januar 2004
[4] P. Upadhayay, “Mit über 100 kleinen Stellmotoren – Die neue S-Klasse,” [With more than 100 actuator motors – The new S-class; in German] July 2013, [Accessed: February 08, 2019]. [Online]. Available: https://blog.mercedes-benz-passion.com/2013/07/ mit-uber-100-kleinen-stellmotoren-die-neue-s-klasse/
[5] R. Krishnan, Permanent Magnet Synchronous and Brushless DC Motor Drives. Florida: CRC Press, Taylor and Francis Group, LLC, 2010.
[6] Nidec Corporation, “Nidec motors and actuators,” [Accessed: February 12, 2019]. [Online]. Available: http://nidec-ma.de/en/productlist.html
[7] ebm papst, [Accessed: February 08, 2019]. [Online]. Available: https://www.ebmpapst.com/de/overview-industries/automotive/ applications/applications.html
[8] Hella Tech World, [Accessed: February 12, 2019]. [Online]. Available: https://www.hella.com/techworld/de/Technik/ Beleuchtung/Leuchtweitenregulierung-838/
[9] Pierburg pump technology, [Accessed: February 14, 2019]. [Online]. Available: https://cdn.rheinmetall-automotive.com/fileadmin/media/kspg/Broschueren/ Poduktbroschueren/Pierburg_Pump_Technology/Oelpumpen/ppt_oelpump_ verbrennung_d.pdf
[10] Pierburg, [Accessed: February 14, 2019]. [Online]. Available: http://www. pierburg-service.de/ximages/pg_pi_0034a_de_web.pdf
[11] Bosch, “bosch-mobility-solutions,” [Accessed: February 14, 2019]. [Online]. Available: https://www.bosch-mobility-solutions.de/de/produkte-und-services/ pkw-und-leichte-nutzfahrzeuge/innenraum-und-karosseriesysteme/ komfortantriebe/
[12] Johnson Electric, [Accessed: February 14, 2019]. [Online]. Available: https://www. johnsonelectric.com/en/product-technology/motion/actuators-automotive

 

Authors:

Muetze photo Annette Muetze (S'03-M'04-SM'09-F'16) received the Dipl.-Ing. degree in electrical engineering from Darmstadt University of Technology, Darmstadt, Germany, in 1999, the Diploma degree in general engineering from the Ècole Centrale de Lyon, Ècully, France, in 1999, and the Dr.-Ing. degree in electrical engineering from Darmstadt University of Technology in 2004.  She is currently a Full Professor with Graz University of Technology, Graz, Austria, where she heads the Electric Drives and Machines Institute and the Christian Doppler Laboratory for Brushless Drives for Pump and Fan Applications.  Prior to joining Graz University of Technology, she was an Assistant Professor with the Department of Electrical and Computer Engineering, University of Wisconsin, Madison, WI, USA, and an Associate Professor with the School of Engineering, University of Warwick, Coventry, U.K.  Dr. Muetze was the recipient of the NSF CAREER award in 2004.
Hofmann photo Stefan Hofmann received the B.Sc. degree in electrical engineering from Graz University of Technology, Graz, Austria, in 2017.  He is currently working on his Master’s Thesis within the framework of the Christian Doppler Laboratory for Brushless Drives for Pump and Fan Applications, at the Electric Drives and Machines Institute at Graz University of Technology, Graz, Austria.  His research interests include the noise, vibration, and harshness (NVH) behavior of fractional hp electric drives. 
Leitner Stefan Leitner (S'16-GS'17) received the B.Sc. degree and the Dipl.-Ing. degree in electrical engineering from Graz University of Technology, Graz, Austria, in 2014 and 2016, respectively.  He is currently pursuing his Ph.D. degree in electrical engineering at the Electric Drives and Machines Institute at Graz University of Technology, Graz, Austria.  He spent an academic year at the University of Tennessee, Knoxville, USA, in 2014-15, via the International Student Exchange Program (ISEP).  He was a visiting scholar at Washington State University, Pullman, WA, USA, and a recipient of the Marshall Plan Scholarship in 2016.  He was awarded Oesterreichs Energie-Preis 2017 for his Master's Thesis.  His research interests include the design of fractional hp drives, finite element analysis, and microgrids.  Mr. Leitner is a member of the Christian Doppler Laboratory for Brushless Drives for Pump and Fan Applications.

 


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