Revolutionizing E-Bikes with Advanced Motor Technologies

Introduction

Electric Bike (E-bikes) present an interesting potential market as being an alternative solution for the automotive field facing energetic and environmental problems. Therefore, industries invest in R&D projects to cope with the latest trends in mobility and meet the growing demand for e-bikes, as they are efficient, easy to master, and affordable. Battery-assisted bicycles are simply a viable alternative to private car ownership for medium distances and a preferred means of transportation especially for people suffering from health limitations.

The global Bicycle Market Size by Product 2015 – 2025 (USD Million) [1]
 

The above figure shows the global bicycle market size by product analyzed from 2015 until 2025. The statistic covers both conventional and electric bikes. E-bike is a fast-growing technology as being lightweight, flexible, and compact. Therefore, it is easy to fit into a car trunk or even an overhead compartment of an airplane. Considering its many merits, governments and climate advocacy organizations have been incentivizing consumers to adopt this technology as it holds the solution to traffic and climate problems.

E-Bikes Components

While conventional and electric bicycles have many components in common, several features set the latter distinct from the former:

• Motor: It generates torque and propels the rider forward when pedaling.

• Battery: It supplies the electrical power that the motor uses. The amount of battery power influences how far an electric bike can travel without pedaling.

• Sensors: These are the components of the bike that signal the motor you are pedaling and should begin assisting the rider.

• Controllers: It is used to manage the amount of power that the electric components of the bicycle supply.

• Display: It is an LCD screen that gives you the bike data. The LCD provides parameters such as speed, battery level, and other relevant information.

 

Hub Drive vs. Mid-Drive Motors

The main difference between mid-drive and hub motor e-bikes is the placement of the motor on the bike. In a hub drive motor, the motor is positioned in the front or rear wheel of the bicycle, placed in the wheel hub making the connection between the motor and the ground directly.

In the mid-drive motor, the motor is placed between the pedals at the bottom bracket of the bicycle. It uses the bicycle drivetrain to transfer the motor’s power to the rear wheel and move the bike forward.

 

 

Hub Motor in Front (a) and Rear Wheels (b) Mid Motor © [3]
 

Design Challenges

The enhancement of the efficiency and stability of an E-Bike motor raised many challenges including:

• Improvement of the cogging torque by increasing the magnet poles of the considered machine
• Enhancement of the back-EMF waveform by:
  • Changing the slot type from single layer to double layer
  • Changing the magnet shape from radial to rectangular form
• Presentation of the airgap flux density waveform before and after the optimization process
• Comparison of the electromagnetic performances at load conditions

Design Solutions for Electric Motors Design by EMWorks Inc.

EMWorks offers three electromagnetic products for electric motor design and simulation as shown in the following figure.

 

 

EMWorks Solutions for Electric Motors

 

• MotorWizard: is a template-based design software that allows users to generate different motor types and analyze their electromagnetic performances using the finite element and the semi-analytical methods based on different solver types such as the magnetostatic and the transient solvers.
• EMWorks2D: is an electromagnetic simulation tool that offers the capability of solving any 2D design based on electrostatic, magnetostatic, electric, and transient solvers. It enables challenging features such as parametrization and the coupling to motion and thermal studies leading to Multiphysics analysis.
• EMS: is a full 3D magnetic and electric field simulation software that provides the user with Multiphysics studies including thermal, structural, and motion analysis. It is accurate and efficient for low-frequency applications.

 

Design Specifications

An initial motor, used in an E-bike, is investigated in this study. This motor, with 24 slots and 16 poles, operates at a base speed equal to 1250 rpm. The aim is to enhance the efficiency and obtain more stable torque with fewer ripples. In fact, the main idea relies on improving the back-EMF waveform and decreasing the cogging torque amplitude. This is accomplished by increasing the pole pairs number, changing the slot type (windings type), and modifying the magnet shape  as illustrated in the table bellow.

 

 

Pole pair number variation

Many techniques were investigated to reduce the cogging torque magnitude such as skewing, shifting the angle, etc. In this study, we will focus on the pole number. As the number of slots per pole and per phase spp decreases, the magnitude of the cogging torque decreases too. In other words, fractional slot motors have a reduced cogging torque compared to integer slot topologies. Hence, for the sake of mechanical feasibility, we will consider 24 slots/20-poles motor design.

The following figures show the initial and the proposed topologies having respectively the slot per pole and per phase ratio spp equal to 0.5 and 0.4.

 

 

2D Design of the configuration with 24 Slots / 16 Poles having spp = 0.5

2D Design of the configuration with 24 Slots / 20 Poles having spp = 0.4

 

The cogging torque waveforms are presented for both rotor configurations at open circuit mode. Fig (a) shows the EMWorks2D results and Fig (b) exhibits the reference article results.

 

Cogging Torque Obtained: (a) EMWorkd2D (b) Reference Article [6]

 

As demonstrated, EMWorks2D simulations confirm the article's results. The peak-to-peak cogging torque shows an important reduction of 77 % compared to the first topology. 

  

Winding Arrangement Variation

To enhance the quality of the back-EMF waveforms, we switched to the double-layer winding configuration, as shown in the following figures, where the back EMF becomes more sinusoidal. Another advantage of the double-layer arrangement is  that there are fewer end turn windings, which reduces copper losses and thus improves efficiency.

 

 

Single Layer Slot                                                            Double Layer Slot

 

 

Three-Phase Back-EMF Waveforms of the 24 Slots /20 Poles Double Layer Winding Configuration: (a) EMWorkd2D (b) Reference Article

 

 

Single Layer vs. Double Layer Phase Back EMF Waveform

 

Magnet Shape Variation

The open circuit air gap flux density in surface radial and rectangular magnet shapes are evaluated and computed by finite element analysis tool as shown in the following figures. 

 

Radial Magnet                                      Rectangular Magnet

 

 

Airgap Flux Density for Radial Magnet Design Obtained with (a) EMWorkd2D (b) Reference Article

 

 

Airgap Flux Density for Rectangular Magnet Design Obtained with (a) EMWorkd2D (b) Reference Article

 

 

Electromagnetic Torque for (a) both Designs (b) Zoomed Torque Waveform

 

Based on a comparison between the electromagnetic torque produced by the studied machines, it is clear that the magnet's rectangular shape can reduce the torque ripples by 33 % compared to the radial magnet design.

 

Summary

The torque enhancement for no-load and on-load modes and the back-EMF waveform improvement are realized by manipulating the motor configuration, the winding arrangement, and the magnet shape. The table below summarizes results obtained by EMWorks2D software. As demonstrated, the following process leads to a significant decrease in the torque ripple and the peak-to-peak cogging torque that contributes to more stability, less noise, and better efficiency.