2000-01-0132 Integrated Motor Drive Unit A Mechatronics ... - Delphi

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Figure 2. Early Integrated Motor The goal of this design study is to integrate an electric motor and its associated electronics to the maximum extent possible. DESIGN DESCRIPTION This design approach, illustrated in figures 3 and 4, mounts semiconductor switches, normally MOSFET's, on a power distribution bus that is mounted between the motor and the motor drive unit. Also, motor position sensors rest directly on the system control board. 1 2 3 4 5 6 Figure 3. Cutaway View 7 8 9 14 13 12 11 10 2 5 11 8 10 Figure 4. Exploded View 3 The semiconductor switches (2) are attached directly to a power distribution bus (11). This power distribution bus is made of a highly thermally and electrically conductive material such as copper. This power distribution bus is attached to the bottom of the case (5) using an electrically isolating, but thermally conductive, double sided tape. A second electrical isolation pad is placed between the distribution bus (11) and the motor endcap(8). This pad must be highly thermally conductive. Heat dissipated in the semiconductor switches (2) is conducted through the power distribution bus (11) to both the motor end cap (8) and the case (5). From these two parts the heat conducts through various parts of the integrated motor structure until it is dissipted into the surrounding air. Inside the case (5) an interconnect board (4) is used to connect various electrical parts (6) and the power distribution bus (11) together to form electrical circuits. The semiconductor switches (2) are connected to the interconnect board (4) with wire bonds through windows in the case (5). Above the interconnect board (4) a system control board (13) is mounted. This control board (13) contains various electronic devices that together control the motor system. Also on the control board are sensors that together with the encoder wheel (14) monitor the angular position of the motor. 6 1 14 13 4
The motor winding terminations (3) pass through the motor end cap (8), the case (5), and the system control board (4). The power distribution bus (11) has tabs that are bent upward and connect to the motor winding terminations (3). This connects the motor to the drive unit. Parts that dissipate a large amount of heat (12) may be mounted to the inside of the case. Some of the parts of the motor include the outer sleeve (10), end cap (8), bearing (9), and motor core (7). The drive unit is enclosed with a cover (1). FINITE ELEMENT ANALYSIS In this design both the semiconductor switches and the motor dissipate heat into the same structure. An obvious concern is that the two parts will heat each other. To address this question a Finite Element Analysis has been performed. Dielectric FET Frame Solder Axis of Symmetry FET Figure 5. Schematic of EPS System Level Model (2D) Table 1. Factors Considered Motor End Cap Motor Case Heat Sink Housing Winding/Core Load on End Cap Motor Case Convection He at Sink Motor/FET Thicknes Material Coefficient (Block) (%) (mm) (W/m2-C) 100 6 Aluminum 0 With 60 12 Steel 7 Without 20 20 A 2D cross-section model is schematically presented in figure 5. The maximum temperature reached by the FET is the primary response variable. Because this 2D-model will overestimate the actual die temperatures, the model is primarily useful for assessing the relative effects of various design options. A full factorial experiment with 60 distinct cases was completed using the factors listed in table 1. The object labeled "heat sink" in figure 5 represents a gearbox. Because the thermal sinking abilities of the gearbox were not known it was included as a constant temperature sink in some models. For the rest of the models it was assumed be an isothermal boundary. 3 The motors power consumption and duty cycle are specified according to table 2. Table 2. Power Input Case 1 Case 2 Case 3 % Max Power 100 60 20 On Time (s) 7 90 Infinity Off Time (s) 180 1000 N/A Power in FET (W) 38.1 13.7 1.5 Power in Motor (W) 210 75.6 8.4 N o r m a l i z e d T e m p e r a t u r 1.200 1.000 0.800 0.600 0.400 With Sink 0 With Sink 7 Temperature vs LC With Sink 20 Heat Sink Condition Convection Coefficient Figure 6. 2D Analyses Results No Sink 7 No Sink 20 The results of all 60 cases were analyzed using graphs such as shown in figure 6. From these graphs it is evident that the percent of power applied is the most important variable affecting the FET temperature. The 100% load case always produces the highest temperatures. Furthermore, at 100% load all the other variables have little effect on the temperature. This occurs because during the short high-power pulse the energy dissipated in the FET does not have time to dissipate into the motor. This burst of energy is then falowed by a long cool down time that allows the energy to be dissipated even through a less than optimal structure. The variables in the model also have little effect on the 20% load case because the energy applied is small enough to easily be dissipated. The presence of the heat sink has a large effect in the 60% power load case. This occurs because the power off portion of the cycle is not long enough to dissipate the energy through convection alone. It is also evident that for the 100% load only a small part of the structure, immediately around the FET, warms up during the power on cycle. With this in mind, a 3D model of the area immediately around the FET was constructed. The geometry for this model is pictured in figure 7. This is a 1/8’Th symmetry model of a square cutout around the FET. Two versions of the model were created. One model has a one mm thick copper layer to which the FET is mounted. In the other model the copper layer is two mm thick. Heat is applied to the top surface of the silicon chip. Because of the short duration power pulse, it is not necessary to consider any heat output from the model. The heat rise at the model extremities is small and consistent Load: 20 % 60 % 100 %
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2000-01-0132 Integrated Motor Drive Unit A Mechatronics ... - Delphi

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