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Brushless Motors: Deconstructing the Secrets of a Versatile Machine




Brushless motors usually go by several names like BLDC, PMSM, Axial flux, Out-runner, Slotless etc. They have several layers of complexity depending upon the construction, geometry and control strategies but the two key common factors across all the different configurations are in the use of permanent magnets on the rotor and an electronic controller for commutation.


Brushless permanent magnet motors are increasingly becoming popular in several applications like electric vehicles, drones, HVAC systems, robotics, industrial automation etc. They enjoy unique advantages like high efficiency, high power density, high torque-to-inertia ratio, high-speed operation capability, precise speed control, high reliability, noiseless operation and a long lifespan. They are thus highly adaptable and versatile.


In this article we will explore the different construction options of brushless motors and briefly look at how they operate. Finally we will discuss the key differences between Brushless Direct Current (BLDC) motors and Permanent Magnet Synchronous Motors (PMSM) and hopefully clear some common misconceptions.


Construction

Brushless Permanent magnet motors consists of three main components

  1. Rotor – The rotating part of the motor and it contains permanent magnets

  2. Stator – The stationary part of the motor containing electromagnets and thus holds the windings

  3. Electronic controller – The controller is an electronic circuit that provides power to the stator and electronically commutates the motor thus removing the need for any mechanical brushes.

Brushless permanent magnet motors come in many shapes, forms and configurations. Depending upon the end application they can be constructed using a combination of different parameters. A machine designer thus has several degrees of freedom to create the most optimum machine depending upon the performance characteristics demanded.



Fig: Different construction parameters of a brushless motor


One can design a radial flux motor with concentrated windings in a slotted stator with interior permanent magnets on the rotor in an in-runner configuration with a hall sensor FOC based control strategy.


Brushless motors can further be constructed according to the number of phase windings. From single phase to multi-phase windings. Three phase brushless motors are the most common.


Further several configurations are derived by choosing one of the above parameters. For example in a yokeless axial flux motor there are double rotors and a single stator while for a yoked axial flux motor there is a single rotor with double stators.


Once the key design parameters are chosen as per the form factor, torque, speed and power requirements, the machine designer come up with even further variations with the use of different types of materials used.

  • Type of Conductors: Copper Vs Aluminium windings

  • Type of Iron Core in Stator: Steel Laminations Vs Soft Magnetic Cores

  • Type of Permanent Magnet in Rotor: Ferrites Vs AlNiCo Vs SmCo Vs NdFeB

The possibilities are thus immense and endless enabling great versatility.


Operating Principle

A brushless motor produces torque due to the interaction between the magnetic fields generated by the rotor and the stator. When current is applied to the stator windings it generates a magnetic field which interacts with the rotor’s permanent magnet causing the rotor to rotate. The speed and direction of the rotor’s rotation is controlled by the current flow in the stator windings.


Ideally, maximum torque occurs when these two fields (stator magnetic field & rotor magnetic field) are 90 degrees to each other.


A microcontroller-based electronic circuit detects the rotor position (either through back-emf in a sensorless control or through use of sensors/encoders) and controls the current in the windings through electronic switching thus determining the speed and the direction.


BLDC V/s PMSM

BLDC motors are often confused with PMSMs. Interestingly BLDC motors have a construction similar to that of PMSMs (an AC motor) while its electrical characteristics are similar to those of a DC motor (refer our blog article here for brushed DC motor characteristics). This makes BLDC motors singularly unique and extremely adaptable in different applications.


The three key differences between these two motors are:

  • Back-emf: BLDC motors have a trapezoidal back-emf waveform while PMSMs have a sinusoidal back-emf waveform.

  • Stator Current: The stator current in a BLDC is in a quasi-square waveform (making it neither a DC nor an AC motor) while a PMSM motor has a pure sinusoidal waveform stator current.

  • Control method: BLDC motors have a 120 degrees conduction interval implying that the phase currents flow only in two of the three-phase winding at a time while in a PMSM the phase currents flow in all three phase windings at a time thus the switching happens at a 180 degrees conduction interval.

The other difference could be in the winding structure, where BLDC motors have concentrated windings while PMSMs can have both concentrated as well as distributed windings.


The above differences between these two motors lead to performance variations (torque ripples, noise etc), control complexity and system cost.


To conclude we can say that Brushless motors can be designed in several topologies and driven by an intelligent controller to achieve diverse requirements. This is what makes them a preferred choice.


Alphasine Technologies is a technology start-up designing and manufacturing energy-efficient permanent magnet brushless motors and their controllers in Lucknow, India. It is our vision to be a global leader in the space of ultra-high efficiency motors, drives & controllers providing cutting-edge technology to numerous industries like electric vehicles, aerospace and defence, industrial machinery & consumer appliances.


In our next article we will talk more about the operating principles of brushless motors and examine different control strategies and how they are being adopted for brushless motors’ optimum performance.



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