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Introduction

Eddy current brakes are based on the braking torques (forces) generated by induced Eddy Currents inside an electrical conductor by electromagnetic induction. Stationary electric conductor immerged inside a time-varying magnetic flux and/ or a moving electrical conductor inside a static magnetic flux will lead to the creation of eddy currents in this conductor. The magnetic flux source in an eddy current brake can be produced by permanent magnets and/or electromagnets. According to the differences of the flux sources, eddy current brakes could be divided into three categories: 

• Electrical Excitation ECB: In this type, the magnetic flux is generated by exciting one or several electromagnets. The braking force in this case can be controlled by adjusting the current of the electromagnets. To further increase the braking torque while keep limiting the heat generated and power consumption, permanent magnets can be added as well.

• Permanent Magnet ECB: This type eliminates the need of the external electrical supply, but on the other hand, braking force/torque control is not allowed, and magnet corrosion and relatively low temperature tolerance are potential risks.  
• Hybrid Excitation ECB: It is a combination of the PM ECB and the EE ECB and exhibit the advantages of both. Hence, the amplitude of the braking force/torque density is high and adjustable, and the excitation loss is reduced. 

Problem Description 

In this analysis, a Radial Flux Hybrid Excitation ECB system is presented. The brake design is shown by the Fig. 1. The Stationary part is made of 2 radially polarized PMs pairs and copper windings inside inner Back iron. The moving part consists of a conductive layer attached to the outer cylindrical back iron. An air gap of 1mm is used between the stationary and rotary parts. 

Motion analysis coupling of EMWorks2D inside SOLIDWORKS is used to explore the studied ECB System using a Transient Magnetic study type. The assigned materials for each BS component are defined by Table 1. 

Electrical Excitation ECB Mode 

For this analysis part, Electrical excitation mode is activated by exciting all windings with 14 A/mm² current density and replacing PMs by Iron back. A first run with a speed of 3000 rpm allows to plot the next Magnetic flux mapping shown by Fig 2. As noticed, the maximum flux is concentrated across the primary core to achieve 1.63T at 30 ms time step.

This second figure is showing the maximum attained braking torque versus different speeds after 30 ms. As clearly noticed from the plot, maximum braking torque is achieved for a speed of around 1500 rpm. 

Permanent Magnet ECB Mode  

Same work done for  this second analysis part, PM-ECB mode is activated by excluding all windings and keep PMs. A first run with a speed of 3000 rpm allows to plot the next Magnetic flux mapping shown by Fig. 3. Maximum magnetic flux is concentrated across the PMs to achieve 1.09T at 30 ms time step. 

The next 2D plot is showing the maximum attained braking torque versus different speeds after 30 ms. Maximum braking torque of 1.27 N.m is achieved for a speed of 2000 rpm. 

Hybrid Excitation ECB Mode  

For this third part we are considering the Hybrid excitation mode of the studied ECB system design. Both windings and PMs are kept. By rotating the outer Core with the attached conductive layer at 3000 rpm speed, the simulation revealed the next Flux mapping at 30ms. 

For the same speed, the Eddy Current density distribution across the conductor layer is plotted and shown below. High induced current achieving 2.25 E+7 A/mm². Eddy currents are associated with Induced solid losses which are represented by the second figure showing its variation versus time to achieve maximum value around 1150W. 

The next 2D plotting of the braking torque versus speed of the Hybrid Excitation mode is compared to the other modes. As seen, the maximum attained braking torque values are obtained for this combined ECB system design. The second 2D plot is showing the braking torque progress versus time steps for all studied speeds. The torque achieved its permanent status after almost 10 ms of rotating for all speed cases. 

After checking the effect of excitation mode and speed on the attained braking torque, the final analysis is dedicated to vary the air gap between the stationary and rotary parts using parametric analysis of EMWorks2D. For a fixed speed of 3000 rpm, the maximum achieved braking torque versus air gap is represented by the following figure: as clearly noted, reducing air gap automatically increases the braking torque.  

References 

[1]. Yazdanpanah, Reza. "Design and Analysis of Radial-Flux Hybrid Excitation Eddy Current Brake." 2019 10th International Power Electronics, Drive Systems and Technologies Conference (PEDSTC). IEEE, 2019.