Bringing Down the Volume

By Brandon Grainger, Student; Oreste Scioscia, Student; and Gregory Reed, Director, Electric Power Initiative, University of Pittsburgh, U.S.A.

The University of Pittsburgh and Carnegie Mellon University have teamed up with the Advanced Research Project Agency - Energy and others to develop novel high-frequency, magnetic nanocomposite materials for power applications with material performance proven by ANSYS simulation.

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Diagram of a transformer

With any modern technology, the key to success is to make the product smaller, faster, simpler and more cost-efficient.

With any modern technology, the key to success is to make the product smaller, faster, simpler and more cost efficient. Engineers need to find a way to integrate renewable energy (wind and solar primarily) into the existing grid while considering these design goals. Engineers at the University of Pittsburgh (Pitt) and Carnegie Mellon University (CMU) are working with patented nanocomposites (HTX-012B) that are designed to compete with ferrites used in modern power transformers.

Areas and priorities for power electronics improvements determined by ARPA-E in 2010
Areas and priorities for power electronics improvements determined by ARPA-E in 2010

The Advanced Research Projects Agency -Energy (ARPA-E) — part of the U.S. Department of Energy (DOE) — advances high-potential, high-impact energy technologies that are too early for private sector investment. The agency exists to help fund energy research and technology developments that break through technological and scientific barriers and accelerate technology to market. Awardees of ARPA-E funds are developing entirely new approaches and methods for solving existing energy challenges. ARPA-E focuses on transformational energy projects by providing funding, technical assistance and aid in preparing products for the market.

In conjunction with ARPA-E, the Magnetics Division of Spang & Company and Los Alamos National Laboratory (LANL) are helping to transform the power electronics industry. Spang specializes in research, design and production of a broad range of powder, ferrite and strip-wound magnetic cores for such applications as transformers, inductors and power supply components. Spang is developing novel manufacturing techniques for the HTX-012B core materials. LANL is a federally funded research and development center, which aligns its strategic plan with priorities set by the DOE and other energy administrations. LANL uses the HTX-012Bbased transformer core to develop next-generation power electronics systems that convert DC solar power into readily available power for distribution to end users.

Diagram of a transformer
Diagram of a transformer

Power electronics system design is a multiphysics application, and ANSYS software plays a critical role. Engineers at Pitt are using ANSYS PExprt and ANSYS RMxprt to compare the performance of the materials developed by CMU in the ARPA-E program to ferrites in various applications, including electric machines and transformers. 

PExprt is a tool for design, modeling, and analysis of transformers and inductors. Using a combination of classical and finite element analysis techniques, PExprt determines the core size and shape, air gaps and winding strategy for a given power converter topology. RMxprt is a template- based design tool used to calculate machine performance, to make initial sizing decisions and to perform hundreds of “what if” analyses. RMxprt can then automatically set up the ANSYS Maxwell project (2-D/3-D) including geometry, materials, boundary conditions (plus appropriate symmetries) and excitations with coupling circuit topology for rigorous electromagnetic transient analysis.

Carnegie Mellon University Explores Other Materials

In another study, CMU colleagues explored a new class of Fe-Co-based materials in permanent magnet machine applications. Cobalt-rich compositions offer an alternative to iron-rich alloys for high-frequency operation when material strength is critical. With ANSYS simulation, 10 kW-rated machines showed a 70 percent size reduction when using HTX- 005C (Fe-Co nanocomposite material) in place of non-oriented silicon steel.

Illustration of the framework used to provide analytical results for optimizing the transformer volume
Illustration of the framework used to provide analytical results for optimizing the transformer volume
Optimal manufacturer core volume solutions of the top ten ferrite core solutions shown below.
Optimal manufacturer core volume solutions of the top ten ferrite core solutions shown below.
ANSYS PExprt solutions for a ferrite-based core that can be improved by adopting the HTX-012B core
Top ten ferrite core solutions rated for 1 kW
ANSYS PExprt solutions for a ferrite-based core that can be improved by adopting the HTX-012B core.

Reducing materials

Using advanced magnetic materials that operate at a high switching frequency will result in significant reduction of materials for components. HTX-012B is a novel nanocomposite magnetic material with chemistry optimized to operate at high temperatures (above 200 C), high switching frequency, tunable permeability that depends on the desired application, and a design applicable to high-power applications (kW to MW scale magnetic core designs). If the material volume is minimized, the result is a weight reduction in magnetic-based components with observed power density improvements. In addition, space restrictions may no longer apply when installing equipment — all benefits for electric ship design, as one example.

 

TAKING TURNS

The bidirectional DC/DC power converter interests engineers because it can transform DC voltages in shipboard power systems, accommodate electric vehicle design and be used for other power engineering applications. The converters use high-frequency transformers in their design.

When designing a 300 V, 20 kHz, 1 kW-rated transformer for a bidirectional DC/DC converter, PExprt optimizes the magnetics by minimizing the total system loss associated with the core and windings. The software also provides the top ten off-the-shelf ferrite-core solutions that ANSYS users can purchase to use in their designs and prototypes.

Comparison of optimized core results
Comparison of optimized core results

Engineers compared optimized solutions for a transformer core designed with traditional ferrites and the designed nanocomposite magnetic material. By operating at higher switching frequency, component volume can be reduced, resulting in improved power density. This is one of the key milestones of the ARPA-E program. Hence, the core volume, total loss and number of wire turns on the core are parameters of interest in this analysis, with core volume as the highest priority.

Before the nanocomposite material could be evaluated in a charger application, the team needed to experimentally obtain loss characteristics of various cores as a function of frequency and induction level. Once engineers obtained the ferrite core parameters, they evaluated Steinmetz coefficients and used them as inputs into PExprt for HTX- 012B. Six circular cores of HTX-012B were manufactured, annealed, impregnated and tested by technicians at Spang to determine loss characteristics. The material properties from the tested HTX-012B cores were obtained, provided to PExprt, and a similar analysis was conducted as previously described.

Engineers compared an estimated analytical solution, the simulated ferrite core and the simulated HTX-012B core. The analytical result is an approximation, whereas PExprt provides a more finite solution from a larger family of core shapes and sizes that are widely available. By using the HTX-012B material, the losses are more evenly distributed between the core and windings. The number of turns required to achieve the converter rating is reduced in comparison to the ferrite design, as is the core’s magnetizing inductance. The ferrite material does not exhibit any noticeable advantage in comparison to HTX-012B from an engineering design perspective. 

With the power of PExprt and its selection algorithms, the team found that the HTX-012B was a more suitable core structure than the ferrite core. ACADEMIC The design of power electronics systems is a multiphysics application and ANSYS software plays a critical role in the project.

The design of power electronics systems is a multiphysics application and ANSYS software plays a critical role in the project.

New materials for induction motors

Although HTX-012B is an iron-rich alloy, the intent was to show power density improvements for induction motor applications. Using the ANSYS RMxpt motor library, the researchers selected an example motor model to observe possible power density improvements for induction motors. By increasing the driving frequency of the motor, the motor’s size will be shown to decrease.

When increasing the driving frequency without changing any of the motor’s other parameters, the mechanical speed increases proportionally. The team considered two motor models — one motor with two magnetic poles and another with eight poles, both operating at the same mechanical speed. The two-pole and eight-pole machines operated with a driving frequency of 50 Hz and 200 Hz, respectively. HTX-012B material replaces the stator material but not the rotor material to maintain mechanical integrity. M600-50A, a high frequency (2 kHz or less) electric steel, is used in the rotors of both machines.

Hardware of the ETD-49 core which was considered the best option
Hardware of the ETD-49 core which was considered the best option. ANSYS Maxwell simulation shows a cross section of 72 windings configured in two parallel turns around the ferrite core.

RMxprt is able to quickly optimize a motor's geometric parameters including stator and rotor diameters, and slot dimensions for given system constraints. These factors can be varied so that a decrease in the overall size (volume) of the motor becomes apparent. The team wanted the motor to operate at rated output power with efficiency greater than 80 percent and motor power factor above 0.90 while improving its overall power density. For the eight-pole motor, the magnetic flux has less distance to travel to get from one end of a pole to the other. The width of the return path in the stator core must be wide enough so the motor’s ability to produce torque is not diminished and the magnetizing current increased. If the magnetizing current increases, overall system losses will increase, the power factor will decrease and the output power will not meet rated conditions.

ANSYS RMxprt results
ANSYS RMxprt results show that a 56 percent size reduction and 57 percent weight reduction can be obtained when operating an eight- pole compared to a two-pole machine if HTX-012B is incorporated into the machine stator.

Winding Down

Using ANSYS simulation products, the engineering teams provided numerical proof that smaller may, in fact, be bigger. Material innovations for power magnetics allow those units to operate at higher switching frequency, resulting in drastic volume reductions and improved power density performance. With the help of ANSYS simulation solutions, specifically PExprt and RMxprt, engineers can evaluate many designs rapidly. These design improvements in bidirectional converters and electric machines could not have been economically determined any other way.

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