It will be shown in section 4.3 that the rotor position can be detected
using additional sensors such as Hall-sensor and encoder, or form the signals
produced in the motor operation, such as back-EMF signals.
Similar to the DC motor, increasing the phase number of the BLDC wind-
ings can reduce the torque ripple of the motor. It will also be shown in section
4.4 that, the linear relationship of T (I) described by equation 4.47 can also be
used in the analysis of BLDC motor.
The current in armature windings of any electric machines also induces
magnetic fi eld, that is, both the fi elds produced by the magnets and armature
windings exist in the PM DC motors and PM BLDC motors. Therefore, the
Ampere's torque can also be considered as the result of the reaction between
the stator fi eld and rotor fi eld.
220.127.116.11 Reluctance Torque
Besides the Ampere's torque, another type of torque known as reluctance
torque may also be induced during the operation of an electric machine. The
mechanism behind the generation of the reluctance torque is quite complicated.
However, it can be explained using the concepts of the global electromagnetic
energy and virtual work. Let us use the EM system shown in Figure 4.6 to
explain this kind of torque. To simplify the description, it is assumed that
the magnet has an ideal demagnetization curve with in fi nite H c ,asshownin
Figure 4.28: The ideal demagnetization curve of the permanent magnet
The motor shown in Figure 4.6 is formed by a stator and a rotor. The
stator consists of permanent magnet and the stator core. The magnet is used
to produce the excitation fi eld. The rotor is simply a core capable of rotating