Permanent-Magnet Rotor Gap Structure for Eddy Current Mitigation

Solution Overview

Problem

Permanent-magnet-embedded electric motors experience losses due to eddy currents, which generate Joule heat and can lead to irreversible demagnetization, especially when using rare earth magnets, as the magnetic flux variations cause significant eddy currents on the surface of the magnets.

Innovation Solution

The motor design includes a rotor core with magnet insertion holes where the gaps between adjacent permanent magnets are larger on the radially outer side than on the radially inner side, reducing electrical resistance and mitigating eddy currents by ensuring that rotating-direction side surfaces are apart on the inner side, thus minimizing heat generation and demagnetization risks.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If permanent magnets are divided into multiple pieces and inserted into a single magnet insertion hole, then eddy currents are reduced, but electrical resistance between adjacent magnets decreases when they are in close contact

Engineering Contradiction:

Improveeddy current lossVSAvoidelectrical resistance between magnets

Core Design Contradiction:

Loss of energyVSReliability

Solution Approach 1:

An insulating layer is introduced between adjacent permanent magnets to prevent direct electrical contact. This intermediary layer maintains electrical isolation, preserving high electrical resistance between magnets while allowing the magnets to remain in close contact for mechanical stability and compact design.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The insulating layer is applied locally only at the contact surfaces between adjacent permanent magnets, rather than coating the entire magnet surface. This localized application provides the necessary electrical isolation precisely where needed (at the interfaces between magnets) while minimizing additional complexity and material usage.

Inventive Principle:

Principle #3Local quality

2Device complexity

If rotating-direction side surfaces of permanent magnets are in close contact, then structural compactness is improved, but eddy currents increase due to reduced electrical resistance

Engineering Contradiction:

Improvestructural compactnessVSAvoideddy current loss

Core Design Contradiction:

Device complexityVSLoss of energy

Solution Approach 1:

An insulating layer is introduced between adjacent permanent magnets to prevent direct electrical contact. This intermediary layer maintains electrical isolation, preserving high electrical resistance between magnets while allowing the magnets to remain in close contact for mechanical stability and compact design.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The insulating layer is applied locally only at the contact surfaces between adjacent permanent magnets, rather than coating the entire magnet surface. This localized application provides the necessary electrical isolation precisely where needed (at the interfaces between magnets) while minimizing additional complexity and material usage.

Inventive Principle:

Principle #3Local quality

3Temperature

If Joule heat is generated in permanent magnets, then temperature rises, but coercivity drops leading to irreversible demagnetization

Engineering Contradiction:

Improvetemperature of permanent magnetVSAvoidcoercivity and magnetic stability

Core Design Contradiction:

TemperatureVSReliability

Solution Approach 1:

The insulating layer, while primarily serving to reduce eddy currents, also indirectly benefits magnetic stability by preventing the chain reaction that would otherwise occur: reduced eddy currents mean reduced Joule heating, which in turn prevents temperature rise that would otherwise cause coercivity drop and demagnetization.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This design effectively reduces heat generation in the permanent magnets, minimizes losses, and enhances the motor's resistance to demagnetization, allowing for improved efficiency and motor downsizing while maintaining high magnetic force.

Implementation Method 1

variations in the magnetic flux inside the rotor result in eddy currents flowing on the surface of the permanent magnet

Methodology Applied

Scientific EffectEddy currents: Eddy Currents

Implementation Method 2

The eddy currents result in Joule heat in the permanent magnet and the Joule heat causes a loss in the electric motor

Methodology Applied

Scientific EffectJoule heating: Joule Heating

Implementation Method 3

a rotor core disposed on an inner diameter side of the stator core and including a plurality of magnet insertion holes corresponding to the number of magnetic poles

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Data Source

PatentUS10110077B2Permanent-magnet-embedded electric motor and compressor

Publication Date: 2018.10.23 MITSUBISHI ELECTRIC CORP

AI summary

A permanent-magnet-embedded electric motor includes a stator core; a rotor core disposed on an inner diameter side of the stator core and including a plurality of magnet insertion holes corresponding to the number of magnetic poles; and a plurality of permanent magnets arrayed in a rotating direction in the magnet insertion holes. Gaps between two permanent magnets adjacent to each other among the permanent magnets disposed in the magnet insertion holes are formed such that the gaps on the radially outer side are larger than the gaps on the radially inner side.