ENB205 Lecture Notes - Lecture 3: Brushless Dc Electric Motor, Ferromagnetism, Arc Length
Document Summary
Can only generate small amount of torque because the field relies upon residual" magnetic flux in the permanent magnets. Current in the machine tends to cause a reaction in the field, demagnetising the field poles. Current in the rotor works to increase or decrease flux density at trailing/leading edges of the magnet. This reaction works to weaken the magnets in a pmdc motor. By increasing the air-gap between rotor and poles at the edges of the pole faces, the magnetic field lines are less inclined to bunch up" reducing the armature reaction. Ferromagnetic b-h curves are not the same going up, as down. Pm material needs to have a high residual flux density and a high magnetic intensity to demagnetise. Neodymium-iron-boron (rare earth magnets) have both a very high h, and high maximum residual flux density. Armature reaction works to weaken flux, lower flux means lower induced emf and hence the motor can spin faster for the same terminal voltage.
