Revolutionizing E-Bikes with Advanced Motor Technologies

Motors and Generators
By Hajer Jmal | 23/01/2023

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.


Components of an E-Bike [2]

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]
The tables bellow show respectively the advantages and limitations of Hub Drive and Mid-Drive motors [4,5].

Pros Cons
  • Affordable
  • Requires less maintenance than a mid-drive motor
  • Throttle-assist is widely available: Suitable for beginner cyclists as there's the throttle option
  • Heavy
  • Consumes more power than mid-drives.
  • Need for a visible controller and a pedal-assist sensor
  • Wheel spokes can work themselves loose over time as the power transition goes straight to the rear wheel
Hub Drive Motor Pros and Cons


Pros Cons
  • Quiet
  • Lightweight
  • Long-lasting
  • Smooth power transition
  • Better torque at slower speeds
  • Easier maintenance on moving parts
  • Improved weight distribution as the weight is at the center of the bike
  • Less powerful
  • Expensive since the whole bike frame is built around the mid-drive motor
  • In case of the chain snaps, there is no throttle-assist rendering the drivetrain useless
  • Suitable for experienced cyclists as you’ll have to pedal more to get efficient speed for optimized pedal-assisted range
Mid-Drive Motor Pros and Cons

As has been stated above, determining the most appropriate e-bike motor depends on the range you need, the type of terrain you ride, as well as your budget.  Hub motor is suitable for riders who just need a reliable and affordable bike to get around town. Whereas, mid-drive can be the best option for more range, better performance,  and a high budget.

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.

Initial Topology Final Topology
  Configuration 24 slots /16 poles 24 slots /20 poles
  Slot type Single Double
  Base speed 1250 rpm 1000 rpm
  Winding configuration Concentrated
  Rotor position Outer
  Stator outer diameter 66.3 mm
  Stack length 20 mm
  Core material M-36
  Permanent magnet Ferrite (Grade 8)
  Conductor

Copper


The no-load and on-load torque waveforms of the initial machine are compared using EMWorks2D and a reference article [6]. The results of the two studies present a good agreement. The electromagnetic torque, presented in blue, is almost equal to 2.35 Nm at 1250 rpm. It shows ripples equal to 28.7 % which is an undesirable index that causes vibration and acoustic noise in on-load operating mode. The cogging torque, shown in red, is considered one of the important contributors to the output torque ripples. It is evaluated at open circuit operating mode, and it results from the interaction between the stator tooth and the rotor PMs. Thus, it should be reduced to minimize the torque ripples. 

No-Load and On-Load Torque of the Initial Machine: (a) EMWorks2D (b) Reference Article [6]

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.


Initial topology Optimized topology
  Configuration 24 slot/ 16 pole 24 slot/ 20 pole
  Winding distribution Single Double
  Magnet shape Radial Rectangular
  Average torque 2.21 Nm 2.38 Nm
  Torque ripples 28.7 % 0.6 %
  Peak-to-peak cogging torque 0.55 Nm 0.001 Nm
Performance Comparison of the Initial and the Optimized Topology

Conclusion

A permanent magnet motor used for electric bike applications is simulated using  EMWorks2D software. Three parameters have been investigated to enhance the machine's electromagnetic performances including the number of poles, the slot type, and the magnet shape. It has been demonstrated that minimizing the slot per pole and per phase ratio by increasing the rotor pole pairs leads to a decrease in the cogging torque magnitude by 77 %.  Furthermore, a comparison based on different winding configurations has proven that the double-layer winding arrangement can improve the back EMF waveform quality leading to a more sinusoidal back EMF shape. Indeed, a study carrying out the influence of varying the magnet shape has shown that the magnet rectangular shape has the advantage of reducing the torque ripples by 33 %. 

References
[6]: Bhat, S. B., S. P. Nikam, and B. G. Fernandes. "Design and analysis of ferrite based permanent magnet motor for electric assist bicycle." In 2014 International Conference on Electrical Machines (ICEM), pp. 106-111. IEEE, 2014.
[7]: Krishnan, Ramu. Permanent magnet synchronous and brushless DC motor drives. CRC press, 2017.
[8]: T. J. E. Miller and J. R. Hendershot, Design of brushless permanent magnet motors, USA: Oxford University Press, 1995.