Thermal Management of High Voltage E-Drives

Quoting the European Automobile Manufacturers Association (ACEA), EETimes Europe reported in November 2016 that 137,423 vehicles with alternative powertrains were newly registered in the third quarter of 2016, representing a growth of 7.0 % over the same period last year.

About 50 % of these vehicles were hybrid electric cars, representing an increase of 29 % year over year. The ACEA divides electric vehicles into two groups: Electrically Chargeable Vehicles (ECVs) and Hybrid Electric Vehicles (HEVs). Thus, battery-electric vehicles and plug-in hybrids are combined in one group. In this combined group plug-in hybrids accounted for 26.4 % of the growth.

Because these EVs and HEVs require higher powered electric motors, the market for high voltage systems is also showing significant growth. HV drive systems of this type are categorized as voltage class B in the PAS, where the maximum voltage is defined as: 60 < U ≤ 1500 V DC, and 30 < U ≤ 1000 V AC rms.

Under these conditions, thermal management of the motor and the associated power electronics is not only critical for the reliability of the motor, the temperatures of the components also affect material properties that have a direct bearing on the torque production, control, and efficiency of the motor.

For this reason, designers need accurate thermal models of the electric motor during the design and control development of the motor.

THE HOTTER THE MOTOR THE COOLER THE MANAGEMENT

Thermal management of electric motors is a complicated process because of the multiple heat transfer paths within the motor and the different materials and thermal interfaces through which the heat must pass to be removed.

The key to accurately controlling the thermal behavior of the motor is access to data describing the critical thermal characteristics of the machine. Such data includes direction-dependent thermal conductivity measurements of non-uniform motor components such as lamination stacks and windings. It also includes data to quantify thermal contact resistances between components in the construction. Finally, it also includes data to support the modeling and design of active cooling of the motor and the convective heat transfer coefficients available through alternative cooling approaches.

In a report on “Electric Motor Thermal Management R&D” funded by the National Renewable Energy Laboratory (NREL) and issued in April 2016, Kevin Bennion investigated the challenges associated with thermal management of power electronics and electric motors.

He concluded that the thermal performance of an electric motor largely depends on the passive stack thermal resistance within the motor as well as the convective cooling performance of the selected cooling technology. 

Before deciding to address the convective cooling, it’s necessary to optimize the passive thermal stack elements through developing new materials, fabrication methods, and materials processing. The areas of particular interest include:

The stator-to-case thermal contact resistance Lamination through-stack and in-plane thermal conductivity Winding cross-slot thermal properties Thermal interface resistance between ground insulation materials and the respective motor elements that come into contact with the slot liner or ground insulation.

Thereafter, to improve convective cooling, it’s common to introduce an external cooling medium. The most common of these are:

• Directly cooling the windings with oil or ATF thereby directly cooling the motor windings or rotor. However, heat from the stator must still pass through several interfaces. The resulting changes in the temperature distribution lead to hot spots within the motor that may be difficult to identify.
• Cooling the motor with a cooling jacket surrounding the stator. A motor cooled with a stator cooling jacket will require heat generated within the slot windings to pass through multiple material layers and material interfaces before the heat is extracted through the cooling jacket. The thermal properties of the materials and the thermal contact resistances due to the material interfaces impact the temperature distribution inside the motor as heat flows into the cooling jacket.

The effectiveness of either cooling approach depends on the application's coolant availability, the motor geometry, and the motor loss distribution. 

Directly addressing the difficulties surrounding thermal management, American startup DHX Electric Machines has developed an advanced direct-winding heat exchange cooling technology that it claims removes ten times the heat of a standard coolant channel. 

DIRECT-WINDING HEAT EXCHANGER COOLS THINGS DOWN TO THE CORE

The DHX traction motor technology is based on proprietary direct-winding heat exchange cooling technology that is able to remove motor heat at the source—the stator windings. The technology relies on the advanced micro-feature heat transfer research and development efforts of Dr. J. Rhett Mayor (DHX CEO) and Dr. S. Andrew Semidey (DHX VP of Engineering) at the George W. Woodruff School of Mechanical Engineering at Georgia Tech.

“Our DHX electric motor features standard materials, not exotic steels and magnets,” said DHX president and co-founder Rhett Mayor. “It achieves power densities of 25 kilowatts per liter and extraordinary torque of 70 Nm/l.”

The direct-winding heat exchange system uses micro-feature technology to increase the area of the cooling surface by up to four times of that of a standard cooling channel. The micro-feature technology also helps to increase the relative flow velocity of the coolant, by a process of localized turbulence. 

As a result, DHX claims that the cooling technology removes more than ten times the heat of a standard coolant channel. More heat removal means more current (about four times more) leading to four times the torque. Conversely, the DHX motor is four times smaller than a standard motor of the same power, the company says.

However, in an HV electric drive system, it’s not only the motor that’s subject to unwanted temperature build up, the controllers, which handle massive electrical loads, are also vulnerable.

According to electrification and intelligent controller specialists AVID Technology, the heat flux from an insulated-gate bipolar transistor (IGBT) semiconductor device can reach around 400 W/sqcm, like the sun’s surface!

Under these conditions, cooling technologies using standard heat sinks and fans are rapidly approaching their cooling capacity limit, and thermal management is becoming a critical step in enabling enhanced product functionality.

COOLING THE POWER ELECTRONICS IS JUST AS IMPORTANT

Over the past few years, significant advances have been made in cooling technology for power electronics. 

Thermal management has developed from air cooled heatsinks with copper bases and heat pipes embedded in the bases to the array of cooling techniques now available to the industry, including:

• Single and double sided cooling with liquid cold plates 
• Micro-channel liquid coolers built into power module base plates or integrated with the DBC substrate
• Jet impingement and direct contact liquid cooling of module base plates or DBC substrates
• Two-phase liquid cold plates with boiling of dielectric refrigerant coolant

In high power electronic controllers, liquid cooled systems are finding favor, including liquid cold plates that provide localized cooling of power electronics by transferring heat to liquid before being conveyed to a remote heat exchanger which dissipates the heat to air or to another fluid in a secondary cooling system. 

Liquid cold plates provide more efficient cooling and enable greater levels of integration, and are therefore able to achieve major reductions in the volume and weight of power electronics systems. 

Existing and emerging liquid cold plate solutions include:

• Tube type and fin type liquid cold plates to cool packages with DBC substrates with and without copper base plates.
• Liquid flow through fins formed directly on copper and AlSiC base plates.
• Single and double sided cooling using liquid jet impingement.
• Direct cooling of the base plate or DBC substrate using concepts such as the Danfoss “shower power” design using jet impingement or liquid flow through meandering channels. 
• Direct double sided cooling of back to back modules with and without fins integrated with DBC substrates.
• Micro-channel coolers built into base plates or integrated with DBC substrate or into specially customized package designs.
• Stacked power modules and liquid cold plates using extruded channel type, folded fin type and micro-channel type cold plates.

In many applications where liquid filtration is not desirable, flow channels are required to be of a sufficiently large size (~2-3 mm) to avoid clogging by particles and precipitates in the cooling liquid. High fluid velocities and “turbulator” inserts are commonly used to enhance the heat transfer coefficient in such large size channels including in tube type liquid cold plates. 

However, this kind of approach provides limited opportunity for heat transfer area enhancement. An alternative approach uses helicoidal flow paths to create strong secondary flow with high vorticity to achieve high heat transfer coefficients at the flow channel wall and parallel flow paths to reduce liquid velocities and the corresponding pressure gradient. Area enhancement is achieved by orienting the helical path so that its axis is normal to the base. This design concept, named the “vortex liquid cold plate” or VLCP, is ideal for cooling a stack of press pack IGBT’s, thyristors or diodes in series. 

For liquid cooling systems where liquid filtration is possible, high heat transfer coefficients may be obtained using high shear flow in narrow channels. The liquid is distributed through a parallel manifold system and forced to flow at a low velocity through narrow flow channels in direct contact with the surface to be cooled. 

Furthermore, the HSDC cooling plate may be designed to provide double sided cooling so that power electronics devices interface with the cooling plate on both the upper and lower surfaces. In addition, depending on the requirements of the application, the cooling plate may be made from non-conducting plastic or electrically conducting metal. This double-sided cooling is an attractive solution for lower cost applications.

In a significant development, at the 9th International Conference on Integrated Power Electronics Systems (CIPS 2016) held in March 2016, Hannes Stahr / AT&S Austria, presented an investigation into a power module with double sided cooling using a new concept for chip embedding.

In order to reduce thermal resistance and inductance, the technique relies on a full contact area of the MOSFETs, IGBTs and power diodes with copper termination in a power core using an isolated metal substrate with silver sinter paste.

When compared to current Surface Mount Technology the new embedded technology promises improved reliability, reduced space requirements and advances in electrical and thermal performance: This makes it very attractive for power modules in high power EV applications. 

With the reduction in packaging volumes and the significant increase in power demands placed on High Voltage EV drives we can expect to see exciting new technologies being applied to thermal management over the next few years. This is critical for the advancement and acceptance of EVs and HEVs into the broader marketplace.

Sources

• IChristoph Hammerschmidt; EETimes Europe, Automotive; HEVs, BEVs gain popularity – at the expense of other alternative drives; November 2016; http://www.automotive-eetimes.com/news/hevs-bevs-gain-popularity-expense-other-alternative-drives
• Kevin Bennion; National Renewable Energy Laboratory; Electric Motor Thermal Management R&D; April 2016; http://www.nrel.gov/docs/fy16osti/64944.pdf
• AVID Technology; Electric and Hybrid Vehicle Thermal Management; http://avidtp.com/electric-and-hybrid-vehicle-thermal-management/
• S. Kang; Aavid Thermalloy LLC; Advanced Cooling for Power Electronics; Paper delivered at the International Conference on Integrated Power Electronics Systems
• Hannes Stahr; AT&S Austria; Conference on Integrated Power Electronics Systems (CIPS 2016); March 2016; https://conference.vde.com/cips/2016/Pages/Report.aspx