New technologies Development of Motor Fan Noise Prediction Method in Consideration of Operating Temperature during Engine Idling Yasuhito Suzuki* Masahiro Shimizu* Abstract In these years there is an increasing demand for improvement in vehicle fuel efficiency. In light of this trend, more and more motor fans are designed to adopt multi stage control including operation at low rotation speed for power saving. Furthermore HEV models with idling stop function are increasing, which requires motor fans to be quieter. Since these motors have to operate at lower temperature with low background noise under the engine room, the motor fan design needs to cover the wider operating temperature range. To meet these requirements, we have developed a fan noise prediction method at all operating ranges in terms of both temperature and speed. Key Words : Heat exchanger, Electric motor, Temperature/Fan noise 1. Introduction 1.1. Outline of Motor Fan The function of the motor fan is to feed cooling air to heat exchangers with rotation speed control in response to various vehicle conditions (vehicle speed, water temperature in the radiator, refrigerant pressure in the condenser, etc.). It typically consists of motors, plastic fans, and a plastic shroud. (Fig. 1) 1.2. Motor Fan Operation Environment Recently certain changes in motor fan operation environment, such as rotation speed range, operating temperature range and air flow rate required for given fan diameter, have been observed. This trend is mainly due to power saving for improving vehicle fuel efficiency and layout requirements related to vehicle styling, which is described below in detail. (1) Power saving for improvement in vehicle fuel efficiency Multi-stage control is employed for the fan rotation speed to contribute to power saving and noise reduction by lower rotation speed. (Fig. 2) Fig. 1 Structure of motor fan system Blowing capacity and noise reduction performance are mainly required for a motor fan. In the blowing capacity, sufficient air flow needs to be provided in accordance with required engine cooling performances of the radiator as well as air conditioning performances of the condenser. On the other hand, a high degree of quietness during vehicle idling is an important demand on the noise reduction performance. In general motor fan product development has been aimed at meeting those requirements. (2) Changes in engine room temperature Motor fans tend to be used in wider ambient temperature range than ever. For instance engine room temperature decreases in such cases that HEV is driven in EV mode, and more frequent and longer engine stops by idling stop function. (Fig. 2) * Global Technology Division 52
Development of Motor Fan Noise Prediction Method in Consideration of Operating Temperature during Engine Idling Fig. 2 Example of motor fan operating Range (3) Adaption of dual fan system for vehicle layout requirements Engine hoods are lowered in recent vehicle design trends. Accordingly, the height of the heat exchangers is restricted while higher air flow performances are required. To meet these requirements, adoption of dual fan systems with advantage in better velocity distribution for the same heat area is increasing in many vehicle model cases. (Fig. 3) noise by modifying the control system (avoiding the fan rotating speed from the resonance frequency ranges at all ambient temperature conditions) than modifying motor fan designs for each vehicle model because motor fan is one of widely standardized components. However, evaluating the fans at all the possible temperature ranges and operating rotation speeds for all the vehicle models and market regions may not be a practical approach because of excessive complexity and scope. Instead, the fan rotation speed range in which noticeable beat noise occurs can be accurately predicted and reflected into the motor fan control system. To take this approach, the following points are identified that need to be clarified: mechanism of the beat noise, and relation between the ambient temperature and the operating rotation speed. Fig. 4 Generation of beat sound Fig. 3 Comparison of single and dual fan system The motor fan noise needs to be evaluated in consideration of the above (1) to (3). 1.3. Influence on Motor Fan Noise Reduction A motor fan generally has a resonance frequency in the operating range due to its shape and material. When the fan rotates at the resonance frequency, resonance occurs, resulting in high amplitude peak noise. In the dual fan system especially, the right and left fans rotate at the nearly same speeds for air velocity distribution in the heat exchangers. Hence, if the right and left fans simultaneously generate resonances, the noise peaks within the narrow frequency band result in noticeable beat noise. (Fig. 4) It is more rational and effective to prevent the beat 2. Verification Result 2.1. Influence of operating temperature range To understand the above points, noise measurements were conducted with subjective evaluation on a dual fan system under a range of ambient temperature levels. In the subjective evaluation, beat noise was not identified within the motor fan operating range for most of the temperature conditions except at a high temperature condition, which represents idling state after high load driving. From the measured results in Fig. 5, the followings are identified in the motor fan noise characteristics while ambient temperature increased. (1) The rotation speed corresponding to the noise peak decreased. (Shaded in red in Fig. 5) (2) The operating rotation speed increased for the same voltage condition. (Shaded in gray in Fig. 5) Implication of the mechanisms of those phenomena are discussed in the following sections. 53
CALSONIC KANSEI TECHNICAL REVIEW vol.11 2014 modulus and subsequently resonance frequency (f), resulting in reduction of the rotation speed at which the beat noise occurs. (2) Verification of the hypothesis The fan resonance frequency was measured under different ambient temperature. (Fig. 6) Fig. 6 Frequency response function of motor fan Calculation Young's modulus Measurement 92% 90% Fig. 5 Characteristic of motor fan noise (Bench test) 500MPa 20 Temperature 54 2.2. Change in Noise Peak Rotation Speed (1) Hypothesis on noise peak shift The noise peak occurs due to the resonance of a fan s certain vibration mode, which is excited by the motor rotation. The resonance frequency is given by the equation (1) below. = 1 2 f : Resonance frequency [Hz],k : Spring constant [N/m],m : Mass (1) The spring constant (k) depends on the fan structure and Young s modulus of the fan material. Hence, increase of the temperature may reduce the Young s Fig. 7 Changing rate of Youngʼs modulus Then, the measurement result shown in Fig. 6 was compared with the calculation result of equation (1). As a result, the noise-peak occurrence point due to the thermal influences decreased to 92% in the actual experiment, which is almost equal to the calculated 90%. This result leads to the assumption that the noisepeak rotation speed was changed by the decreased Young s modules. (Fig. 7) 2.3. Change in Motor Fan Operation Range The motor fan rotation speed is determined by the combination of the fan rotation torque (resistance) and
Development of Motor Fan Noise Prediction Method in Consideration of Operating Temperature during Engine Idling the motor rotation torque (output). Thus, hypotheses are formulated and verified for changes in the fan rotation torque characteristics of each part due to thermal 109% influences. (1) Hypothesis on motor fan torque shift The fan rotation torque (resistance) depends on the blade surface area and its angle. The fan rotation 90% 100% torque at the blade edge in relation to the material properties is expressed by the equations (2) and (3). 500rpm High temperature Room temperature = ( ) (2) 0.1 N m (Constant static pressure) ω : Specific angle of torsion [rad/m], T : Rotation torque at the blade edge [N m] G : Modulus of transverse elasticity [Pa], Ip : Second moment of area [m 4 ] = (3) 2 (1+ ) Fig. 8 Characteristic of fan torque (Bench test) Calculation Measurement E : Young s modules, ν: Poisson s ratio As described in the section 2.2, when the Young s modules of the plastic material decreases, the material is softened due to increased temperature. Then the air flow rate decreases because the rotating blade angle becomes shallower. As a result, the fan rotation torque could decrease while the fan rotation speed increases. 0.1 N m 90% 85% 20 Temperature Fig. 9 Changing rate of fan torque (2) Verification of the hypothesis To verify the hypothesis, the fan rotation torque was measured under the conditions where the fan rotates at constant speed under different ambient temperature conditions. (Fig. 8) In this experiment, raising temperature from low to high condition increases the rotation speed by 9% at the same rotation torque, and decreases the rotation torque by 10% at the same rotation speed. Then, the measurement result shown in Fig. 8 was compared with the calculation result of the equations (2) and (3). As a result, the fan rotation torque decreased to 90% due to the thermal influences in the measurement result, and to 85% in the calculation result. Both of the results indicate that the fan rotation torque decreased with increased temperature. (Fig. 9) (3) Hypothesis on motor rotation torque shift The motor rotation torque is generated by electromagnetic forces. Factors of thermal influences are magnetic forces from magnet components in the motor and electric current flowing through the coils. The following equations show the governing relations. = 2 (4) 60 P : Motor output [W],N : [rpm], T : Rotation torque [N m] = 2 (5) F : Electromagnetic force [N],R : Distance between rotation center and coil [m] 55
CALSONIC KANSEI TECHNICAL REVIEW vol.11 2014 2.4. Predictability of Rotation Speed = (6) B : Magnetic flux density [T],I : Electric current [A] The equation (4), (5) and (6) can be transformed into the following equation. = 30 (2 2 ) (7) To judge whether the motor fan rotation speed can be predicted under the influence of ambient temperature based on the test results of each component, the calculation result was compared with the actual test result of the motor fan assembly. The operation points are intersections of the rotation and torque lines of the components. (Fig. 11) This equation indicates that the magnetic force and electric current are inversely proportional to the fan rotation speed. Since magnetic force generally decreases with electric currents as temperature rises, Motor line (Constant load voltage) Fan line (Constant static pressure) Operating point the denominator of the equation (7) is considered to decrease as well. Consequently temperature rise leads to decrease of the fan rotation torque and increase of the fan rotation speed. 200rpm 0.1N m Operating point (4) Verification of the hypothesis To verify the hypothesis, the fan rotation torque was measured under different ambient temperature conditions. (Fig. 10) 500rpm 0.05 N m Fan torque range High temperature Room temperature Low temperature (Constant load voltage) Fig. 10 Characteristic of motor torque (Bench test) The measurement result in Fig.10 indicates that the fan rotation speed increases at high temperature within the fan operation range. Fig. 11 Operating point analysis As a result of comparison, the resultant operation point appeared to shift to 104% in the measurement and to 106% in the calculation due to the thermal influences, both of which provided almost equivalent results. (Fig. 12) Measurement 100rpm 20 Temperature Calculation Changing rate:6% 106% 104% Changing rate:4% Fig. 12 Comparison of operating point 2.5. Estimation of Beat Noise Generating Range The above study result proves the two hypotheses described in section 2.1, and demonstrates predictability on the shifts of the noise-peak rotation speed range and 56
Development of Motor Fan Noise Prediction Method in Consideration of Operating Temperature during Engine Idling the fan rotation speed. Noticeable beat noise generating zones were estimated in relation to the rotation speed and temperature ranges by mapping these data. Fig. 13 shows the map of these zones. The range (A) indicates the shift of the fan resonance frequency due to the thermal influences. The range (B) indicates the shift of the fan rotation speed. Then the overlapped areas (C) of the ranges (A) and (B) are the zones in which noticeable beat noise is generated. Fig. 13 Peak noise map of fan To prevent the beat noise from falling into noticeable zones, the motor fan operating speed range (B) must be controlled in certain spaces where the noise-occurrence zone (C) does not overlap with the operating temperature range (D) in the engine room. Fig. 14 shows the result of actual test on a dual fan system operated in the overlapped range (colored in yellow). Beat noise was clearly audible as predicted with the mapping estimation. (Fig. 14) This study result indicates that the beat noise generating range can be estimated based on rotation-torque characteristics of the fan and motor depending on temperature conditions, along with noise characteristics and material properties in particular. 3. Conclusion To prevent an increase of vehicle noise due to the beat noise during engine idling, a motor fan noise prediction method for all operation conditions has been developed. (1) Noticeable beat noise generating range and the rotation speed of the motor fan are shifted with respect to ambient temperature. (2) The shift is caused by the following factors. Shift of the fan resonance frequency and rotation torque along with changes in fan stiffness Shift of the fan rotation torque along with changes in magnetic force and electric current of the motor (3) The noticeable beat noise generating range can be estimated by mapping the noise characteristics at normal temperature, material properties, and rotation-torque data. (4) This prediction method allows us to design a fan system that avoids noticeable beat noise generation in the operation range. 4. Acknowledgments Sound pressure The authors would like to thank all the contributing parties both inside and outside Calsonic Kansei Corp. for their support in this study. Time 0.2sec (Constant load voltage, static pressure and temperature) Yasuhito Suzuki Masahiro Shimizu Fig. 14 Beat noise data of dual fan 57