subject: Understanding Pm Motors In Detail [print this page] Motors are a part of our daily life, used in one form or another; from basic washing machines to high-profile industrial equipments. One such motor, which has found wide spread application in recent times, is the permanent magnet(PM) motors.
Based on DC motors working, these were introduced in the 19th century. However, they did not gain the required popularity or universal acceptance due to the poor quality of magnetic materials like steel and tungsten steel in those times. So, most designers and engineers continued using electromagnetic field excitation. Thankfully, advances in magnetic technology, like finding rare earth magnets, ensured that these motors demonstrated improvements. These aided in the motors steady state performance and power density. Lets learn how these motors work.
A PMDC motor does not have a field winding on the stator frame. Instead, it relies on permanent magnets to provide the magnetic field. The rotor field interacts with these magnets to produce torque. For large motors with high load, there may be compensating windings in series with the armature, which aids in improving commutation under load. Most permanent magnet stators are ideal for miniature motors to eliminate the power consumption of the field winding. This is because the field is fixed; it cannot be adjusted for speed control.
Often, for creating a PM motor high-energy magnets are used. Most of these are made with neodymium or others like neodymium-iron-boron alloys. This is to help minimize overall weight and size, for smaller applications. These magnets have higher flux density as compared to regular permanent magnets. Thus, electric machines with high-energy permanent magnets are easily compatible with all optimally designed singly fed synchronous and induction electric machines.
In cases of miniature motors, you may notice that they have at least three rotor poles along with the basic PM motors structure. These ensure starting of the motor, regardless of rotor position. Plus, their outer housing is a steel tube that magnetically links the exteriors of the curved field magnets.
Using permanent motors also offers quite a few benefits. First of all, they provide high efficiency for almost all applications, as little electrical energy is used. Plus, they offer high torque and power density along with better dynamic performance due to higher magnetic flux density in air gap, as compared to regular motors. Thus, it is of no surprise that these motors are amongst one of the most preferred for both industrial and domestic applications.
