Motor Thermal Management System (Part 2)

introduction

The National Renewable Energy Laboratory (NREL) is an important branch of the U.S. Department of Energy (DOE) Alliance. It mainly conducts research on new energy thermal analysis and materials through funding from the DOE. This article mainly introduces NREL's research on the new energy "motor thermal management system" and shares its results and technical routes.

With the continuous development of new energy vehicles, the demand for improved motor performance is increasing. There are two ways to improve motor performance under the condition of thermal management restrictions. One is to increase the size of the motor, and the other is to improve the high temperature performance of the material. These two ways are undoubtedly bumpy, so the importance of motor thermal management is highlighted. Through reasonable motor thermal management, the size of the motor can be reduced and the price of the motor can be lowered to achieve improved motor performance.

NREL's "Motor Thermal Management System" project was first officially launched by DOE in 2010, codenamed APE030 (2010-2013). Similar projects include EDT064 (2014-2017, ELT075 (2017-2019), and ELT214 (2019-2023).

"Motor Thermal Management System" Purpose: Optimize the selection and development of motor cooling technology to maximize motor indicators (weight, volume, cost, efficiency).

Motor thermal management consists of passive thermal management and active thermal management, including:

• Passive thermal management involves: motor design/material thermal properties/thermal boundaries

• Active thermal management involves: cooling medium/cooling location, etc.

Active Electric Motor Thermal Management

At present, there are two types of active thermal management for motors: air cooling and liquid cooling. Liquid cooling is further divided into water cooling and oil cooling. The current oil cooling system directly cools the motor through the transmission oil, which has a better cooling effect than traditional water cooling. At the same time, the design of oil cooling is also more divergent, and different manufacturers have different oil cooling solutions.

The active cooling research conducted by NREL is based on oil cooling. The test plan is as follows:

From the above test model, we can extract the parameters that affect the cooling performance: hole diameter d, injection distance S, injection area diameter D, oil flow rate, oil temperature, nozzle specifications, and sample state. For the above parameters, the research team performed parameter DOE optimization. Among them, 4 test samples with different states were selected, and the sample diameter, surface area, and surface roughness were different.

The test results show that: in the case of small flow rate injection, the state of the test sample (surface area, roughness, etc.) does not affect the heat dissipation, and can reach an approximate heat dissipation level; as the injection flow rate increases, the average heat transfer coefficient increases with the increase of the heat dissipation surface area of ​​the sample.

In addition, changing the oil temperature to a certain extent (50℃-70℃-90℃) has little effect on the heat transfer coefficient of the flat target surface.

Oil splashing occurred during the test. At an oil temperature of 70°C and a jet speed of 7.5m/s, the fluid hit the center of the sample surface and moved outward across the entire surface. In contrast, at 10m/s, some of the fluid deviated from the surface immediately after impact. In the 90°C data, fluid splashing was observed to occur at lower speeds. Since liquid splashing is more common at higher temperatures, this may be related to the lower ATF viscosity at high temperatures, which also explains the above "changing the oil temperature has little effect on the heat transfer coefficient of the flat target surface".

It is worth noting that when the oil temperature is 50°C, the heat transfer coefficient increases approximately linearly with the increase of the nozzle speed. At this temperature, the above-mentioned surface splashing does not occur (splashing increases the heat dissipation capacity to a certain extent). This means that there is an optimal temperature corresponding to different flow rates, and this corresponding relationship is related to oil splashing.

Considering the actual working process of oil-cooled motors, the oil jet does not spray vertically, so the research team designed a multi-parameter test. When the flow rate is constant, the heat transfer coefficient decreases as the orifice jet impact moves away from the center (S).

As the experiment progressed, the disadvantage of cylindrical injection gradually emerged, that is, uneven cooling (increasing the number of injection holes is a solution). Based on the existing model, the research team tried nozzle injection (the nozzle has a large injection area), and the cooling effect was very good.

For more details about the cooling scheme of the drive motor nozzle injection, please refer to patent number CN201721342611-Nozzle oil-cooled motor cooling system (SAIC passenger car). The advantages of the nozzle scheme are self-evident, and it can be implemented technically, but there is no cooling system using this scheme on the market. This involves a series of problems in product application (high working pressure of the oil pump, high price of the nozzle, large motor size, etc.). I believe that this scheme will appear in everyone's field of vision in the future.

Summarize

The work on the "motor thermal management system" has been going on for 10 years and is still ongoing. As a basic research, its results cannot be directly used in product applications, but it provides a solid theoretical basis and reference value for product applications. Sorting out GM's new energy motor cooling route, from the early VOLT to the mid-term SPARK, and then to BOLT, the motor cooling solution all uses oil cooling active cooling, which also shows the importance of oil cooling active cooling from the side.

Looking back at the purpose of the project at that time: to optimize the selection and development of motor cooling technology to maximize the motor indicators (weight, volume, cost, efficiency), it was basically achieved. According to incomplete statistics, the direct or indirect funds invested in the "motor thermal management system" reached 5 million US dollars, and it has been increasing year by year. It can be seen that DOE attaches great importance to basic research (this is only a small part of motor design)

So what is the next research direction? According to the latest news released by the research team, the "motor thermal management system" will focus on the following two aspects. Interested friends can continue to pay attention.

• Bonding interface thermal contact resistance (50℃–200℃)

- Typically, the insulating varnish penetrates into the slot lining, affecting the contact resistance between components

• System thermal validation and reliability

- Stator slot and stator interface, system verification