Introduction - Permanent Magnet Synchronous Motor Design

Introduction

Brushless DC motors are typically classified by having trapezoidal back emf and are typically driven by rectangular pulse current

PM synchronous motor differed from BLDC motor by having sinusoidal back emf and driven by sinusoidal current

Motors are usually designed for inside rotor and outside rotor. In this magnetic field travels in radial direction which are usually called as radial flux motor

If magnetic field travels between stator and rotor in axial direction, called axial motor

All electrical motor is constructed with winding on stator and permanent magnet on rotor

Magnet poles and Motor phases

It is possible to build PMSM with any even number of magnet pole and any number of phases greater than or equal to one

Usually most of the PMSM motor are designed for 3 phases

The choice of magnet depends on application and space available for magnet

For high speed motor it’s better to choose low number of pole count and for high torque motor its better to choose high pole number.

For high speed motor D:L ration will be 1:1 or less and for high torque motor forms pan cake like structure since it can accombadate more magnets

Stator

Stator usually has teeth that protrude towards magnet on the rotor from outer ring of steel called stator back iron. In between teeth are called slots where electrical winding is placed

Electrical and Mechanical measures

Mechanical speed – rotor shaft makes one complete revolution it travels 360 deg mechanical

Electrical speed – movement of rotor which puts back the rotor in same identical magnetic orientation

Fe is the fundamental electrical frequency, which determines the speed at which the commutation must occur to run at given speed. Inverse of the frequency is commutation time period which determines the time over which to energize the phase completely. 

Fe determines the design of power electronics to keep the motor running

It is Common to use fewer magnet poles for high speed motor. Higher the magnet poles increase, torque production efficiency decreases