Chuck Yung
Senior Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "DC Theory and Design 101" by Chuck Yung, presented at the EASA Convention 2016, provides a comprehensive overview of the fundamental principles and design considerations of DC motors. The paper begins by explaining the basic laws of magnetism, which are crucial for understanding the operation of DC machines. These laws include the attraction of opposite poles and the repulsion of like poles, as well as the inverse square relationship between magnetic force and distance.
Yung describes how DC motors utilize these magnetic principles by strategically placing magnets (field poles) around an electromagnet (armature) that pivots around a shaft. The continuous rotational motion is achieved by reversing the polarity of the armature coils, a process known as commutation. The commutator, a collection of copper bars connected to the armature leads, facilitates this process by allowing DC power to be supplied through carbon brushes.
The paper delves into the properties of magnetic fields, emphasizing the importance of flux density and the role of ferrous materials in enhancing magnetic flux. It explains how the strength of an electromagnet can be adjusted by varying the current or the number of turns in the coil. The concept of ampere-turns is introduced, highlighting its significance in determining the strength of the magnetic field.
Yung also discusses the construction of DC machines, noting that they must have an even number of poles and that symmetry is critical for optimal performance. The paper explains the importance of maintaining a consistent air gap between the armature and the field poles to ensure steady torque production.
The concept of compound fields is introduced, combining the advantages of shunt fields (good speed control) and series fields (high torque). The paper explains how the relative strengths of the shunt and series fields determine the overall field strength and the motor's performance characteristics.
The role of interpoles in DC machines is thoroughly examined. Interpoles are additional electromagnets placed between the main field poles to counteract the distortion of the field flux caused by the armature flux. The correct polarity and strength of interpoles are crucial for preventing arcing and ensuring stable motor operation. The paper provides methods for adjusting interpole strength, including adding or removing shims and adjusting the air gap.
Yung emphasizes the importance of verifying the relative polarity of the interpoles and the armature after assembling a DC machine. He describes a simple test using AC voltage to ensure the correct polarity relationship. The paper also discusses the significance of interpole shims and their impact on interpole strength.
The design of the armature is another key topic covered in the paper. Yung explains how to determine the coil pitch and the difference between lap and wave windings. He highlights the importance of equalizers in lap wound armatures to balance the current and prevent circulating currents that cause heating and arcing.
The paper concludes with a discussion on improving armature design by recognizing clues that indicate potential issues. Yung suggests using split-pitch coils to address less-than-ideal slot-pole combinations and improve motor performance.
Key Points Covered:
- Basic laws of magnetism and their application in DC motors
- Properties of magnetic fields and flux density
- Construction and design principles of DC machines
- Importance of maintaining a consistent air gap
- Advantages of compound fields
- Role and adjustment of interpoles
- Verifying interpole and armature polarity
- Significance of interpole shims
- Armature design, including coil pitch and winding types
- Use of equalizers in lap wound armatures
- Improving armature design with split-pitch coils
Key Takeaways:
- Understanding magnetism is crucial for DC motor operation.
- Magnetic field strength can be adjusted by varying current or coil turns.
- Symmetry and consistent air gaps are essential for optimal motor performance.
- Compound fields combine speed control and high torque.
- Correct interpole polarity and strength are vital for preventing arcing.
- Verifying polarity and using interpole shims ensure stable operation.
- Proper armature design and the use of equalizers prevent circulating currents.
- Recognizing design issues and using split-pitch coils can improve motor performance.
Print
