Session: 17-13-01: Plant Performance, Fluid Dynamics and Thermal Hydraulics in Energy and Power Applications

Paper Number: 172500

A Study on Mechanical Loss Reduction to Improve the Efficiency of Synchronous Motors 

1. Objectives  

Large synchronous motors are key power sources in the oil and gas industry. Improving their efficiency during long-term operation is critical for reducing energy consumption and operating costs. For a 27.5 MW, industrial synchronous motor, a 0.1 percentage point improvement in efficiency can reduce annual electricity consumption by approximately 230 MWh and CO₂ emissions by 100 tons.  

This study proposes a mechanical loss reduction approach to improve the efficiency of the motor. Thermal-fluid analysis and simplified dummy model testing were conducted to quantify mechanical losses, and a correlation analysis between simulation and experiment was performed to derive a loss reduction design, which was then validated for its effectiveness. 

2. Contributions 

 

1) Structure and Mechanical Loss Characterization 

The subject of this study is a 27.5 MW, 4-pole, 60 Hz synchronous motor, which consists of a stator, rotor, and exciter. The cooling system is classified as IC81W (Totally Enclosed Water-to-Air Cooled), where internal air is forcibly circulated by axial fans mounted on both ends of the rotor.  The axial fans of the rotor continuously drive the internal airflow along the cooling paths, and this airflow is closely related to one of the key mechanical losses during motor operation, known as windage loss. 

The total losses in the motor are categorized into electrical losses (e.g., copper loss and core loss) and mechanical losses. Mechanical losses occur due to friction between the rotating components and surrounding air, and mainly include windage loss and bearing friction loss. Based on the loss measurements test, approximately 40% of the total losses were mechanical, and the windage loss—excluding bearing friction—was measured to be approximately 81% (137.1 kW) of the mechanical losses. 

 

2) CFD Analysis and Validation of Windage Loss  

Windage loss arises from the energy dissipation caused by pressure differences between rotating surfaces and the surrounding air. It can be decomposed into friction loss due to viscous resistance and pressure loss resulting from vortex generation. 

In this study, windage loss was estimated through CFD analysis of the synchronous motor, yielding a predicted value of approximately 139 kW, with less than 2% deviation from the experimentally measured value. Excluding the 6.8 kW loss generated in the exciter, the remaining windage loss—approximately 132.2 kW—originated from the rotor parts. This includes 10.2 kW of friction loss and 122 kW of pressure loss, accounting for approximately 95% of the total windage loss. 

 

3) Rotor Component Contributions to Windage Loss 

Among the rotor windage losses, the rotor coil accounted for approximately 41%, representing the largest contribution. The pole caps and axial fans on both ends contributed around 25–28%, respectively. These results indicate that the internal flow characteristics of the motor are highly influenced by the rotor geometry, particularly the fan and coil structures. Accordingly, effective windage loss reduction requires structural optimization primarily focused on the rotor components. 

3. Methodology and Results 

 

1) Simplified Dummy Model Design 

To experimentally evaluate windage loss, a geometrically simplified dummy model with the same dimensions as the actual synchronous motor was designed to replicate internal flow characteristics while minimizing structural complexity.  

The rotor geometry and internal cooling path were preserved, while the stator was replaced with perforated plates and bracing bars for structural reinforcement. The number and total area of the perforations were designed to match those of the duct slots in the actual stator. Additionally, an exit duct was installed on the top of the frame to enable airflow measurement through the motor. 

 

2) Experimental Evaluation of Rotor Geometry Effects 

To measure the mechanical losses of the dummy model, a dynamometer equipped with a torque meter was used. The windage loss, excluding bearing friction, was approximately 141 kW, with a deviation of less than 3% compared to the actual motor.  

Based on the dummy model, experiments were conducted to evaluate changes in windage loss and airflow resulting from modifications to the rotor geometry. Reducing the number of rotor fan blades led to approximately a 15% decrease in airflow, which significantly reduced fan windage and resulted in a 5.2% reduction in total windage loss. In contrast, reducing the number of rotor support blocks caused little change in airflow but led to a 28% increase in total windage. These results indicate that optimizing flow resistance around the rotor is more critical than simple geometric modifications. 

 

3) CFD-Based Rotor Design Modifications to Reduce Windage Loss 

To reduce windage loss generated in the rotor, a CFD-based geometry optimization was conducted. First, the edge shape of the rotor pole cap was modified from an angular to a curved form to reduce vortex formation in the surrounding region. Second, a chamber-type structure was applied to the multiple rotor support blocks. This design minimizes the effective flow area facing the airflow at the front of the rotor, thereby improving unstable flow and reducing pressure loss. 

As a result of applying the improved geometry, windage losses in each rotor component were reduced to the range of 2–10 kW, with the most notable reductions observed in the internal fan and rotor coil regions. In total, windage loss was reduced by approximately 25 kW, resulting in a 19% reduction from the pre-improvement model. 

4. Conclusion  

1) Windage Loss Characterization in Large Synchronous Motors 
Loss measurement tests showed that approximately 40% of the total losses were mechanical, with windage loss accounting for about 81% (137.1 kW). CFD analysis showed that nearly 95% of the windage loss originated from the rotor parts, primarily from the rotor coil and pole cap. These findings suggest that rotor-focused structural optimization is essential for improving motor efficiency. 

2) Experimental Validation Using a Dummy Model 
A simplified dummy model replicated internal flow and assessed rotor geometry effects. Measured windage was ~141 kW with <3% deviation. Tests with varied fan blades and supports confirmed rotor geometry strongly affects airflow and windage loss. 

3) CFD-Based Rotor Geometry Optimization 
CFD-driven optimization was conducted to reduce windage. Curved pole caps and chambered supports minimized vortex and pressure loss. As a result, rotor windage dropped to 2–10 kW, with total windage reduced by 25 kW (19%). 

Presenting Author: Eunjin Bak HD Hyundai Electric Co., Ltd.

Presenting Author Biography: I am a Senior Researcher in the Rotating Machinery R&D Department at HD Hyundai Electric. My main responsibilities include thermal-fluid analysis and cooling performance testing of high-voltage induction motors. I have been involved in several projects related to the development of high-power and high-efficiency electric motors:

1. Design improvement of cooling fans to enhance cooling performance in high-speed induction motors (2021–2023)
2. Optimization of internal cooling flow paths to improve output performance in high-speed induction motors (2023–2024)
3. Reduction of mechanical losses to improve efficiency in synchronous motors (2024–2025)

Through these projects, I have gained practical experience in cooling fan design, internal cooling flow path optimization, and windage loss analysis. Currently, I have been conducting research on reducing aeroacoustic noise in induction motors by improving the geometry of sound absorbers and fan covers.

Authors:

Eunjin Bak HD Hyundai Electric Co., Ltd.
Jupil Kim HD Hyundai Electric Co., Ltd.