⚡ electrical engineering

Influence of Annealing Treatment on Electrical Steel Characteristics and Electric Machine Performance

This study demonstrates that applying multi-stage controlled or high-temperature rapid annealing treatments to electrical steel significantly enhances the magnetic properties and reduces core losses of a BLDC motor compared to the untreated as-cut condition, thereby improving overall machine performance.

Original authors: Vikas P. Patil, K. K. Gupta, Atul Jain

Published 2026-10-22

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CC BY 4.0

Original authors: Vikas P. Patil, K. K. Gupta, Atul Jain

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Electricity powers our modern world, but it does not travel without cost. Inside the motors that drive industrial machines and the transformers that deliver power to our homes, a silent struggle occurs every time the magnetic field flips direction. The material at the heart of these devices, a special kind of steel, resists this change. As the magnetic field cycles back and forth, the steel's internal structure fights the motion, generating heat and wasting energy. This wasted energy, known as core loss, accumulates into massive amounts of power lost globally every year. To make these devices more efficient, engineers have long sought ways to make the steel more willing to change its magnetic state. The key lies in how the steel is treated after it is cut and shaped, a process called annealing, which involves heating and cooling the metal to relieve internal stresses and improve its magnetic flow.

Researchers at Varroc Tech. Center and BITS Pilani recently set out to test two very different approaches to this heat treatment. They wanted to see if a slow, careful method could outperform a faster, more aggressive one, and whether the choice of method truly mattered for the final electric motor. They began with standard sheets of non-oriented electrical steel, the kind used in countless motors and transformers. To ensure a fair test, they prepared three groups of samples. One group was left exactly as it was after being cut, serving as a baseline. A second group underwent a multi-stage, controlled heating process that took nearly ten hours, allowing the metal's internal structure to settle gradually. The third group was subjected to a rapid, high-temperature process that took about six and a half hours, prioritizing speed and efficiency.

The team then built small electric motors using these treated steel cores to see how the different heat treatments affected real-world performance. They measured how easily the steel could be magnetized, how much energy it wasted as heat, and how the motor behaved when electricity was applied. The results showed that both heat treatments improved the steel significantly compared to the untreated version. The steel that received the slow, multi-stage treatment showed the most dramatic gains. Its ability to conduct magnetic fields increased by nearly fifty percent, and the energy wasted as heat dropped from an average of 5.90 watts per kilogram to 4.90 watts per kilogram. This means the motor core lost less energy and ran cooler. The steel treated with the faster, high-temperature method also improved, though slightly less. Its magnetic ability rose by a similar margin, and its energy loss fell to 5.10 watts per kilogram.

Beyond the material itself, the changes rippled through the entire motor's electrical behavior. The motors built with the treated steel showed higher inductance, a measure of how well the coil stores magnetic energy, while their electrical resistance remained virtually unchanged. This shift altered how the motor handled sudden surges of electricity. The untreated motor allowed a short-circuit current of 52.36 amperes to flow, but the motor with the slow-treated steel limited this surge to 48.15 amperes. The motor with the fast-treated steel sat in the middle at 49.85 amperes. This reduction in current surge is a sign of a more stable and efficient magnetic core.

The study also looked closely at consistency, a critical factor for mass production. While the slow, multi-stage process produced the best average results, it showed slightly more variation from one sample to another. The faster process, while yielding slightly lower peak performance, produced results that were remarkably consistent across all samples. This suggests that the choice between the two methods depends on the specific needs of the manufacturer. For applications where maximum efficiency is the priority, such as electric vehicles or premium renewable energy systems, the slower, more thorough treatment offers the best performance. For applications where cost and production speed are paramount, the faster method provides a reliable and consistent improvement over untreated steel. The research confirms that the way steel is heated and cooled is not just a manufacturing step, but a decisive factor in how much energy a motor saves over its lifetime.

Technical Summary: Influence of Annealing Treatment on Electrical Steel Characteristics and Electric Machine Performance

Problem Statement
Electrical steel is a critical component in modern electromagnetic devices, including power transformers and electric motors. While the material possesses inherent soft magnetic properties, its performance is often degraded during the manufacturing process. Mechanical stresses introduced during stamping and shrink-fitting alter the material's magnetic characteristics, leading to increased iron losses and reduced efficiency. Although heat treatment (annealing) is known to relieve these stresses and restore magnetic properties, existing research often focuses on optimizing single parameters (such as temperature or time) rather than comparing fundamentally different annealing strategies. Furthermore, a gap exists in the literature regarding the direct correlation between complete annealing cycles and the final performance of the electromechanical product (the motor), as most studies remain limited to material-level characterization.

Methodology
This study experimentally investigated the effects of two distinct annealing routes on non-oriented electrical steel used in a Brushless DC (BLDC) motor. Three sample conditions were evaluated:

Sample C (Reference): Retained in the "as-cut" condition without heat treatment.
Sample A (Process A): Subjected to a multi-stage, controlled annealing process (Process A). This "patient" approach involved five stages: surface cleaning (430°C), pre-heating (450°C, 120 min), primary treatment (770°C, 120 min), controlled cooling, and natural cooling. The total cycle time was approximately 9.5 hours.
Sample B (Process B): Subjected to a high-temperature, rapid annealing process (Process B). This "fast-track" approach involved rapid pre-heating (450°C), high-temperature soaking (770°C), and controlled cooling. The total cycle time was approximately 6.5 hours.

To isolate the effects of annealing, all samples were prepared using a low-stress stamping tool to minimize edge damage. Before winding, samples underwent independent magnetic characterization (B-H curves and core loss) at 50 Hz and 1.5 T. Subsequently, the materials were wound into stator assemblies under identical conditions and coupled with permanent-magnet rotors to form complete machines. Key performance metrics measured included line inductance, winding resistance, Back Electromotive Force (BEMF), short-circuit current, and the rate of current change (di/dt). Statistical analysis, including standard deviation and coefficient of variation, was employed to assess process consistency and repeatability.

Key Contributions

Comparative Analysis of Annealing Philosophies: The study provides a direct, head-to-head comparison between a multi-stage controlled annealing route and a rapid, high-temperature route, evaluating not just peak performance but also process consistency.
System-Level Validation: Unlike studies limited to material properties, this research links the annealing cycle directly to machine-level electrical characteristics, demonstrating how material processing influences the final motor performance.
Trade-off Evaluation: The work quantifies the trade-off between magnetic performance enhancement and manufacturing robustness, highlighting that while one process may yield superior average properties, another may offer better batch-to-batch consistency.

Results
The experimental data revealed significant improvements in both annealed samples compared to the untreated reference (Sample C):

Magnetic Properties:
- Sample A: Showed a 2.44% increase in RMS magnetic flux density ( $B_{rms}$ ), a 35.83% reduction in RMS magnetic field strength ( $H_{rms}$ ), and a 48.25% increase in permeability.
- Sample B: Produced comparable results with a 2.20% increase in $B_{rms}$ , a 34.73% reduction in $H_{rms}$ , and a 46.54% increase in permeability.
Core Loss:
- Average core loss decreased from 5.90 W/kg (Sample C) to 4.90 W/kg for Sample A (a 16.95% reduction) and 5.10 W/kg for Sample B (a 13.56% reduction).
- Consistency: Process B demonstrated higher consistency with a coefficient of variation (CV) of 1.27%, matching the untreated Sample C. Process A showed slightly higher variability with a CV of 2.08%, suggesting greater sensitivity to process control parameters.
Electrical Characteristics:
- Inductance: Line inductance increased from 104.90 $\mu$ H (Sample C) to 109.38 $\mu$ H (Sample A) and 107.15 $\mu$ H (Sample B). Winding resistance remained nearly unchanged across all groups.
- Short-Circuit Current: The short-circuit current decreased from 52.36 A (Sample C) to 48.15 A (Sample A, an 8.04% reduction) and 49.85 A (Sample B, a 4.79% reduction).
- BEMF: Back EMF increased from 4.73 V (Sample C) to 4.99 V (Sample A) and 4.82 V (Sample B).

Significance and Claims
The paper claims that annealing treatment significantly influences the magnetic and electrical behavior of BLDC motor cores. The results establish a clear link between the annealing process, improved magnetic behavior, increased machine inductance, reduced core loss, and reduced short-circuit current.

The study concludes that the choice of annealing strategy depends on the specific application requirements:

Process A is identified as better suited for high-efficiency applications (e.g., electric vehicle motors, renewable energy systems) where superior magnetic properties and maximum energy savings justify higher processing costs and longer cycle times.
Process B is presented as a balanced solution offering greater manufacturing consistency, lower production costs, and reliable performance, making it suitable for conventional industrial motors and cost-sensitive products.

The authors emphasize that while Process A yielded the largest improvements in core loss and permeability, Process B achieved magnetic properties close to those of Process A with superior repeatability. This provides a practical experimental basis for optimizing annealing conditions based on the desired trade-off between efficiency, cost, and manufacturing robustness.

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