Chuck Yung
Senior Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "Air Gap: What Is It, What Does It Do, and Why Is It Important?" presented by Chuck Yung at the EASA Convention 2018 explores the critical role of the air gap in AC and DC machines. The air gap is the physical space between the rotor and stator in AC machines, or between the armature and fields/interpoles in DC machines. This gap is crucial for the machine's magnetic performance and efficiency. Yung explains that the magnetic force decreases with the square of the distance, meaning that as the air gap increases, the magnetic force diminishes significantly. This principle underscores the importance of maintaining a small and uniform air gap to minimize magnetizing current, which is the current required to drive the magnetic flux across the air gap.
In AC machines, the air gap must be within a 10% tolerance of the average gap to avoid issues such as electrical noise, unbalanced currents, and heating due to circulating currents. A smaller air gap is desirable as it reduces magnetizing current, but practical limits are imposed by manufacturing tolerances. For example, a 4-pole motor requires a smaller air gap than a 2-pole motor. An uneven air gap can lead to significant problems, including increased electrical noise and the risk of the rotor striking the stator bore during high current conditions.
DC machines have similar requirements for air gap uniformity, but the roles of the field and interpole air gaps are distinct. The field air gap affects the machine's speed and torque, while the interpole air gap is adjusted to match the interpole flux to the armature flux, minimizing field flux distortion. Adjusting the interpole air gap is more effective by interrupting the flux path with non-ferrous shims rather than changing the physical gap.
Yung emphasizes that rotor runout, which is the deviation of the rotor from its ideal circular path, should not exceed 5% of the average air gap to maintain machine performance. Any repair or modification that increases the air gap beyond the manufacturer's specifications can lead to increased magnetizing current and reduced efficiency. The paper concludes that maintaining a uniform air gap is essential for preventing a range of issues, including circulating currents, reduced efficiency, increased electrical noise, and the risk of rotor-stator contact.
Key Points Covered:
- Definition and importance of the air gap in AC and DC machines
- Relationship between air gap size and magnetic force
- Impact of air gap on magnetizing current and machine efficiency
- Tolerance limits for air gap variation in AC and DC machines
- Consequences of an uneven air gap, including electrical noise and unbalanced currents
- Specific considerations for field and interpole air gaps in DC machines
- Importance of maintaining rotor runout within acceptable limits
Key Takeaways:
- A small and uniform air gap is crucial for efficient machine operation.
- Air gap size directly affects magnetizing current and machine performance.
- Uneven air gaps can lead to significant operational issues.
- Proper maintenance and repair practices are essential to preserve air gap integrity.
- Understanding the distinct roles of field and interpole air gaps in DC machines is important for optimal performance.
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