Core Loss Analysis in PMSM and BLDC Motors

In permanent magnet synchronous motors (PMSM) and brushless DC motors (BLDC), core losses can no longer be ignored.

As motor systems move toward higher efficiency, higher speed, and higher power density, even small losses have a clear impact on thermal performance, reliability, and total system cost.

However, many technical references still give limited attention to this topic. Instead, they often suggest that engineers rely on alternative or simplified approaches. This gap creates risk during motor design, especially in electric vehicles, industrial automation, robotics, and aerospace applications.

To close this gap, engineers must understand where core losses occur, how to calculate them efficiently, and how to reduce them early in the design process.

What Are Core Losses in PMSM and BLDC Motors?

Core losses, also known as iron losses, occur when magnetic fields inside the motor core change over time.

In both PMSM and BLDC motors, alternating magnetic flux causes energy dissipation inside the laminated steel core. These losses convert electrical energy into heat and directly reduce motor efficiency.

Core losses mainly appear in the stator, where the magnetic field changes continuously during operation.

Two Main Approaches to Core Loss Analysis

Engineers generally analyze core losses using one of two methods:

1. Finite Element Analysis (FEA)

2. Analytical Calculation Methods

Each approach has its own advantages. However, their practical roles differ significantly during motor development.

Finite Element Analysis: Accurate but Slow

Finite element analysis uses commercial simulation software to calculate magnetic fields and losses.

This method works well when:

• Motor geometry is fully defined

• Material properties are known

• Final validation is required

With one simulation run, engineers can obtain detailed loss distributions across the motor.

However, FEA has a major drawback: long computation time.

In practice:

• Two-dimensional FEA often takes several minutes

• Three-dimensional FEA may require several hours

Because of this, FEA becomes inefficient during early-stage optimization, where many iterations are needed.

Why Analytical Methods Improve Motor Optimization Speed

Each calculation usually takes only a few seconds. As a result, engineers can quickly adjust:

• Geometric dimensions

• Material parameters

• Flux density targets

• Frequency assumptions

This speed makes analytical methods ideal for design exploration and optimization.

Instead of waiting for long simulations, designers can immediately see how changes affect core losses.

Why Core Loss Calculation Matters Early in Design

During motor design, engineers must decide:

• Where losses concentrate

• Which components need optimization

• Which materials offer the best trade-off

To answer these questions, core loss calculation requires accurate geometric and material data.

Analytical methods allow engineers to identify:

• Whether stator teeth or yoke dominate losses

• Whether lamination thickness needs adjustment

• Whether flux density exceeds material limits

This insight helps define where optimization should begin.

Focus on Stator Loss Mechanisms

To simplify the analysis and improve clarity, many motor design studies place greater emphasis on losses occurring in the stator.

In most practical designs, this focus is both reasonable and effective.

Under near-ideal operating conditions, the magnetic field inside the rotor yoke remains almost constant. As a result, energy dissipation within the rotor iron stays at a very low level. In contrast, most electromagnetic loss is generated inside the stator laminations, where the magnetic field varies continuously over time.

Therefore, during the early stages of motor design, engineers often exclude rotor-related loss terms from calculations. This simplification helps speed up analysis while maintaining acceptable accuracy for performance optimization.

When Rotor Core Losses Can Be Ignored — and When They Cannot

In low-speed and low-pole motors, rotor core losses are often negligible.

This assumption works well during:

• Initial motor sizing

• Concept design

• Preliminary efficiency estimation

However, this simplification does not always hold.

In high-speed motors, rotor core losses increase due to:

• Higher electrical frequency

• Stronger harmonic fields

• Non-ideal flux distributions

In these cases, rotor losses must be evaluated during later design stages.

Harmonic Effects and Permanent Magnet Losses

Another important factor is high-order harmonics generated by stator windings.

These harmonics create:

• Time-varying magnetic fields

• Eddy currents on permanent magnet surfaces

• Additional heat in the rotor

For low-speed motors, engineers often ignore these effects in early design.

However, in high-performance systems, magnet eddy current losses require careful evaluation, especially during final optimization.

Basic Mechanisms

Stator core losses consist of two main components:

1. Hysteresis loss

2. Eddy current loss

Both losses result from alternating magnetic flux inside the stator core.

Hysteresis Loss Explained

Hysteresis loss originates from the magnetic material itself.

Each time the magnetic field changes direction, energy is lost due to internal magnetic friction. This behavior depends on the material’s B–H curve.

Hysteresis loss is proportional to:

• Magnetic field frequency

• Magnetic flux density raised to a material-specific power

This relationship is often expressed using the Steinmetz equation, where the exponent reflects material properties.

Eddy Current Loss Explained

Eddy current loss occurs when changing magnetic flux induces voltage inside the core.

This induced electromotive force drives circulating currents within the laminations. These currents generate heat due to electrical resistance.

Eddy current loss increases rapidly with:

• Higher frequency

• Higher flux density

Because of this, thin laminations and proper insulation play a critical role in loss reduction.

Why Laminated Stator Cores Are Essential

Modern motors use laminated silicon steel cores instead of solid steel.

Laminations:

• Interrupt current paths

• Increase resistance to eddy currents

• Reduce heat generation

However, thinner laminations also increase manufacturing cost. Therefore, engineers must balance efficiency and production feasibility.

Flux Density Distribution in the Stator

Accurate loss calculation requires understanding flux density distribution across the stator.

In surface-mounted PMSM designs:

• Flux density changes smoothly

• No sharp discontinuities appear in the yoke

• Analytical approximation becomes easier

This smooth behavior explains why stator core losses dominate, while rotor losses remain limited under ideal conditions.

Summary: Why Analytical Core Loss Models Remain Essential

• Enables fast design iteration

• Supports early optimization

• Reduces development risk

• Complements FEA in later stages

For modern PMSM and BLDC motors, this approach remains a key engineering tool, especially in efficiency-driven applications.