Magnet Flux Leakage and How to Reduce It in Electric Motors

If you've ever designed or worked with electric motors, you’ve likely encountered something called flux leakage. But what exactly is flux leakage, and how can we minimize it to improve motor performance? Let me break it down for you.

What is Magnet Flux Leakage?

In simple terms, magnet flux leakage happens when some of the magnetic field generated by the motor's magnets doesn’t follow its intended path.

Ideally, the magnetic flux should flow through the rotor and stator, generating torque and power. But in reality, some of that magnetic field “leaks” out, bypassing the motor’s active components and not contributing to its performance. This leakage can lead to lower efficiency and reduced power output.

Several factors cause flux leakage, including:

Material saturation: When the magnetic materials (like the iron core) can’t handle any more flux, excess flux leaks out.

Air gaps: The space between the rotor and stator allows some flux to escape into the surrounding air.

Magnet positioning: Misaligned magnets create uneven flux distribution, which can also lead to leakage.

Tips for Reducing Magnet Flux Leakage

Here are some straightforward tips to help reduce flux leakage in your motor designs.

Tip 1. Keep the Air Gap Small, But Not Too Small

The air gap between the rotor and stator is one of the main places where flux can leak.

Keeping this gap as small as possible ensures more magnetic flux goes through the intended path.

But here’s the catch—you don’t want the gap to be too small, as it could lead to mechanical wear, increased noise, or even potential damage. It’s about finding that sweet spot where the gap is minimal but still safe for the motor's operation.

Tip 2. Use High-Quality Magnetic Materials

Magnetic permeability refers to how well a material conducts magnetic flux. The better the material, the less leakage you’ll have. So, choosing high-permeability materials like silicon steel can help. These materials guide more magnetic flux through the motor’s core, minimizing the amount that leaks out.

Make sure to choose materials with a high saturation point too, so they don’t get overwhelmed by the flux and allow leakage.

Tip 3. Magnetic Shields Can Help

If you're working with a design where flux leakage is unavoidable, magnetic shielding might be the solution.

Using materials like mu-metal for shielding can help keep the magnetic field contained within the motor. Think of it as creating a barrier that directs the stray flux back into the motor’s useful areas.

This is especially helpful in high-performance motors where every bit of flux counts.

Tip 4. Choose the Right Magnet Configuration

The way magnets are arranged inside the motor can also affect how much flux leaks out. For example, using a Halbach array—a specific way to arrange magnets—can concentrate the magnetic field on one side and cancel it out on the other. This reduces leakage significantly. Alternatively, in interior permanent magnet (IPM) motors, embedding the magnets inside the rotor helps keep the flux contained and reduces stray fields.

Tip 5. Slot-Pole Combinations Matter

Another design factor that influences flux leakage is the combination of stator slots and rotor poles. Certain combinations result in more uniform flux distribution, which can reduce leakage.

For instance, motors with fractional-slot concentrated windings are often better at minimizing flux leakage compared to those with full-pitch windings.

Wrapping Up

Magnet flux leakage might seem like a small detail, but addressing it can have a big impact on motor performance. Whether you’re optimizing the air gap, selecting high-permeability materials, using magnetic shields, or tweaking the magnet geometry, there are plenty of ways to cut down on flux leakage.

Next time you’re designing a motor, keep these tips in mind. Small design changes can make a big difference in performance, efficiency, and reliability. And, after all, who doesn’t want to get the most out of their motor designs?

Sumeet Singh, PhD - Product Manager for eMotor Electromagnetic Simulation at EMWorks Inc., Montreal