Rakesh Dhawan, BTech, MSEE, MBA
Think stack motors are rarely encountered. However, they do exist. BionX, an erstwhile Canadian Company, launched one of the known thin-stack motors in the field of electric bicycles. The large diameter motor with a 12mm stack height had some unusual characteristics. I will discuss that design in a separate post.
Some of the concerns below are about thin stack motors.
- Increased Magnetizing Current
- A larger air gap increases the reluctance of the magnetic circuit.
- This requires a higher magnetizing current to maintain the same flux density, leading to higher copper losses and reduced efficiency.
- Reduced Flux Density & Torque Production
- The air gap is where the magnetic field transfers between the rotor and stator. A larger gap weakens this field, reducing torque production per ampere.
- This forces designers to compensate by increasing current, which raises I²R losses and decreases efficiency.
- Increased Leakage Flux
- More flux escapes into unintended paths, leading to poor power factor and additional losses.
- This can also result in weaker electromagnetic coupling between the stator and the rotor.
- Eddy Current Losses in Thin Laminations
- Thin stack designs rely on laminations to minimize eddy current losses, but a large air gap increases the fringing effect (flux spreading), which can induce additional losses in the edges of the laminations.
- This can cause localized heating and performance degradation over time.
- Structural & Mechanical Stability
- A large air gap can make the motor mechanically unstable, leading to vibrations and noise.
- It also demands tighter machining tolerances to maintain uniformity, increasing manufacturing complexity.
- Torque Harmonics
- Because of the non-linearities due to saturations in the laminations, the backEMF is no longer sinusoidal, introducing severe harmonics during torque production.
Balancing Stack Height vs. Air Gap
To optimize motor performance while maintaining thin stacks, designers must strike a balance between stack height and air gap:
- Increase Stack Height to Compensate for Air Gap Losses
- A taller stator stack provides more magnetic material, reducing the reluctance of the core and improving flux linkage.
- This helps recover lost torque due to an increased air gap.
- Optimize Air Gap Size
- A smaller air gap is preferable for efficiency, but practical manufacturing constraints (mechanical tolerances, rotor movement, cooling needs) must be considered.
- Using advanced high-permeability materials (e.g., silicon steel laminations) can help minimize the impact of air gap reluctance.
- Use High Energy Permanent Magnets (for PM Motors)
- In permanent magnet (PM) motors, stronger magnets (e.g., NdFeB) help counteract the effect of a larger air gap by maintaining sufficient flux levels.
- Improve Slot and Tooth Geometry
- Adjusting stator slot dimensions and tooth shape can help concentrate the flux density, making the motor more efficient even with a moderate air gap.
- Consider Air Gap Shape Optimization (or Flux Focus techniques)
- A graded or tapered air gap or flux focus technique (instead of uniform spacing) can help direct flux more efficiently, reducing unwanted leakage.
Conclusion
A large air gap in a motor with thin stacks leads to higher losses, lower torque, and efficiency issues. The optimal design requires balancing stack height and air gap size by:
- Increasing stack height to compensate for reluctance,
- Using high-energy magnetic materials,
- Optimizing slot, tooth, and air gap geometries,
- Keeping air gap tolerances tight while allowing for mechanical stability.
By carefully designing these factors, motor efficiency can be maximized without excessive material or energy loss.