Synchronous Motor with Solid Rotor Salient Poles for Direct-On-Line Starting

Vili Matošević

Synchronous Generator

February 27, 2025

Synchronous Motor with Solid Rotor Salient Poles for Direct-On-Line Starting

Introduction

Synchronous motors are widely used in industrial applications, particularly for driving compressors, pumps, and other heavy-load equipment. For this project, a  16-pole synchronous motor with a DOL starting method was developed to power a compressor with a specific torque-speed characteristic. The key design requirement was to ensure a starting torque while maintaining reliable performance during steady-state operation.

Traditionally, similar motors have been designed with damper windings to facilitate asynchronous starting. However, for this motor, we opted for a solid rotor pole design. This choice aligns with solutions used by many leading manufacturers but represents a novel development for our team, as it expands our product range and expertise in synchronous machine design. Solid rotor poles acting as damper windings during asynchronous starting.

This paper outlines the process of analyzing and designing the motor, including the computational methodologies employed to predict the torque-speed curve accurately. Numerical simulations were validated using experimental data from a similar motor design, providing confidence in the proposed approach and paving the way for further innovations in synchronous motor design.

Solid Rotor Pole Design

The decision to use solid rotor poles instead of traditional damper windings was driven by the need to simplify the motor structure while ensuring robust performance. Solid rotor poles effectively replace damper windings by inducing eddy currents during the asynchronous start, generating the required starting torque. This approach has been successfully implemented by leading manufacturers and offers several advantages:

  1. Mechanical Robustness: Solid poles eliminate the need for complex damper windings, reducing mechanical complexity and potential failure points.
  2. Simplified Manufacturing: The solid rotor design simplifies the rotor assembly, reducing manufacturing time and costs.
  3. Enhanced Reliability: The absence of damper windings minimizes maintenance requirements and improves long-term reliability.

Computational Analysis

To design and evaluate the motor, we used our software for transient simulations of electrical machines. This software enables the calculation of eddy currents, electromagnetic torque, and other critical parameters during the startup process. The model includes:

  1. External Electrical Circuit: The stator winding supply and external grid impedance are incorporated into the simulation.
  2. Rotor and Load Dynamics: The rotor’s inertia and the compressor’s torque-speed characteristic are modeled to capture the transient response accurately.
  3. Transient Electromagnetic Simulation: The software computes the electromagnetic torque at each time step, considering the eddy current distribution in the solid rotor poles.

Iterative Design Optimization

Using the simulation results, multiple designs were evaluated to identify the optimal dimensions and parameters for the motor. Key factors in the design process included: rotor geometry, including pole height, poles shoes shape and air-gap length:

The final design was selected based on the best compromise between starting torque, efficiency, and manufacturability.

Validation

To verify the numerical approach, the transient simulation results were compared with experimental data from an existing synchronous motor with similar characteristics. The comparison confirmed the accuracy of the simulation, validating the use of the developed methodology for future designs.

Results

Torque-Speed Characteristic

The simulated torque-time curve for the final motor design is shown in Fig. 1. The motor provides sufficient torque throughout the asynchronous start. The simulated speed-time curve for the final motor design is shown in Fig. 2.

Eddy Current Distribution

The distribution of eddy currents in the poles of the solid rotor during the starting process is also analyzed. The currents are concentrated at the pole surface, providing the necessary damping and torque production.

Discussion

This project represents a significant step forward in our synchronous motor design capabilities. The use of solid rotor poles eliminates the need for traditional damper windings, offering a mechanically robust and efficient solution for DOL starting.

The validated computational methodology provides a reliable tool for future motor designs, enabling accurate prediction of torque-speed curves and other critical parameters.

 

Future Work

The success of this project opens the door to new challenges, including:

  1. Design of Brushless Exciters: Developing brushless exciters suitable for asynchronous starting.
  2. High-Power Applications: Scaling the solid rotor pole design to motors with higher power ratings.
  3. Thermal Analysis: Investigating the thermal performance of solid rotor poles during startup and steady-state operation.
  4. Resistance in rotor circuit: The added external resistance in the rotor circuit affects the torque characteristic and the current-voltage conditions in the rotor winding. It is necessary to find the optimal resistance value.

 

Conclusion

A synchronous motor with solid rotor salient poles was successfully designed and analyzed for DOL starting. The use of solid rotor poles provides the required starting torque while maintaining mechanical robustness and simplifying the motor structure. The validated design methodology ensures reliable performance and sets the stage for further advancements in synchronous motor technology.

Recent articles

Data to be checked when replacing the AVR