Mathematical Problems in Engineering

Volume 2016 (2016), Article ID 8783615, 8 pages

http://dx.doi.org/10.1155/2016/8783615

## Loss Prediction and Thermal Analysis of Surface-Mounted Brushless AC PM Machines for Electric Vehicle Application Considering Driving Duty Cycle

^{1}Jinhua Polytechnic, Zhejiang 321007, China^{2}Beijing Institute of Technology, Beijing 100081, China

Received 11 May 2015; Revised 21 June 2015; Accepted 5 July 2015

Academic Editor: Xiaosong Hu

Copyright © 2016 Tianxun Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper presents a computationally efficient loss prediction procedure and thermal analysis of surface-mounted brushless AC permanent magnet (PM) machine considering the UDDS driving duty cycle by using a lumped parameters’ thermal model. The accurate prediction of loss and its variation with load are essential for thermal analysis. Employing finite element analysis (FEA) to determine loss at every load point would be computationally intensive. Here, the finite element analysis and/or experiment based computationally efficient winding copper and iron loss and permanent magnet (PM) power loss models are employed to calculate the electromagnetic loss at every operation point, respectively. Then, the lumped parameter thermal method is used to analyse the thermal behaviour of the driving PM machine. Experiments have been carried out to measure the temperature distribution in a motor prototype. The calculation and experiment results are compared and discussed.

#### 1. Introduction

The massive application of electric vehicle (EV) is a significant way to reduce the emission and to settle the energy crisis [1]. In general, the types of EVs can be categorized as pure electric vehicle (PEV), hybrid electric vehicle (HEV), and fuel cell electric vehicle (FCEV) [1–3]. To all these subcategories, electric machine is always the key traction component, which needs to be well designed and manufactured.

Among all the types of electric machine, brushless AC PM machine is a promising candidate for EV traction motor due to its high power density, high efficiency, fast dynamics, and compactness [4, 5]. However, this kind of machine could easily suffer insulation failure of coils and irreversible demagnetization due to the poor cooling condition on the rotor side and excessive heat generation on the stator side, especially for variable-speed application [6]. Therefore, the accurate temperature prediction for traction brushless AC PM machine is of great importance at the design stage.

In fact, one of the key elements of accurate thermal analysis is the accurate losses calculation. Generally, there are two main sources of loss within an electric machine: mechanical and electromagnetic. Mechanical loss is attributed to the frictional effects within the bearing assembly (bearing loss) and fluid dynamics or aerodynamics effects within the motor body (windage or drag loss) [7]; it can be easily obtained according to the manufacturer’s manual. Electromagnetic losses, which effect thermal analysis heavily, are usually associated with active parts of the motor assembly and include the iron, winding, and permanent magnet (PM) loss components [8–10].

For electromagnetic losses prediction, two approaches are widely used: analytical and numerical [11–19]. The analytical approach for iron loss calculation was first developed by Steinmetz [11] and then further modified by Bertotti [12]. The Bertotti formulation which divides the iron loss into three individual parts, that is, eddy current loss, hysteresis loss, and supplementary loss, is commonly used at current electric machine design stage. The analytical winding loss approaches have been well developed to account for the AC effect, for example, the skin effect and the proximity effect [13–15]. The AC equivalent resistance is the commonly used element to account for the AC effect. However, it is frequency dependent and would change with the change of operation point. Therefore, it would be difficult to calculate the winding losses of all operation points accurately by a single value of AC equivalent resistance. For PM power loss, a variety of analytical techniques have been developed. These are based on simplified assumptions of the field distribution and their use is limited to the selected machine topologies for which the assumptions hold [16, 17].

The numerical approach including time-stepping or frequency domain FEA is a more accurate way to calculate the electromagnetic losses [4, 18, 19]. However, it is time consuming and computationally intensive. Therefore, the numerical approach would not be suitable in the case that a loss map or an efficiency map is required.

Besides the losses prediction approaches, the thermal model is another important element for thermal analysis. Two main models could be found from literatures: FE based thermal model and lumped parameter thermal model [20, 21]. The FE based thermal model is accurate and capable of predicting the hottest pot within a motor, while it is time consuming. The lumped parameter thermal model is quick, while it could be only capable of calculating the mean temperature of each motor component. Thus, the choice of thermal model strongly depends on the design goals.

In addition, there has been increased interest in predicting temperature distribution under the driving duty cycle [4, 22]. A lumped parameter thermal model [20] has been used to calculate the temperature distribution under the Chinese city driving duty cycle in [4]. Good agreement is visible between the analytical and experimental results. However, the procedure presented is time consuming, since the FEA is employed to calculate the loss at each operating point. Some accurate and computationally efficient loss scaling techniques [23–25] have been presented, and some of them have been introduced into the analysis procedure and the equivalent-circuit lumped parameter thermal model has been adopted in the literature [22], while it does not take the PM loss into account, which may be small but can directly heat up magnets.

This paper proposes a computationally efficient loss prediction procedure and a lumped parameter thermal analysis of surface-mounted brushless AC PM machine considering the UDDS driving duty cycle. The machine is applied on a 10-meter motor-direct-driving large coach bus. The FEA based iron loss [23], copper loss [24], and PM loss [25] scaling techniques are utilized to obtain the loss distribution under the driving duty cycle in a timely manner. The equivalent thermal parameter of winding [26] is calculated and introduced into the thermal model. Finally, the temperature of winding, stator, and PM is predicted and is compared with the results from experiment.

The remainder of the paper is organised in the following manner: Section 2 outlines the machine design exemplar and the selected coach bus configuration; Section 3 describes the analysis procedure and the electromagnetic loss scaling technique; Section 4 details thermal modelling; Section 5 describes the experimental setup and results; Section 6 summarizes the research findings.

#### 2. Study Machine and Bus Model

The analysed motor is a radial-flux, integer-slot, distributed-wound internal-rotor PM machine with water cooling jacket, as shown in Figure 1. Selected details of the driving motor are given in Table 1. And basic traction parameters of selected coach bus are given in Table 2. Please note that the aim of this paper is to accelerate the analysis speed by using computationally efficient loss mapping techniques. Therefore, some of the coach bus traction parameters have been modified to ensure that torque-speed characteristic of analyzed machine can cover the torque-speed requirement.