SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

with misconceptions. The primary issues are machine construction, current control, and “torque
ripple,” with secondary issues being efficiency, “harmonics,” and audible noise. While some
references are weakly worded (as if the authors are uncertain of their claims), it is not difficult to
find sources with clear wording from both extremes of each issue. For example, “there is
absolutely no difference between sine and trap motors,” and “sine and trap motors are entirely
different machines.” Or, “sine and trap machines require different types of current commutation,”
versus “either motor may be driven by similar means.” Or, “motor and drive types may be
mixed,” versus “mixing motor and control types may cause serious issues.” Without field
experience it is difficult to determine the truth regarding each issue. The largest difficulty seems
to be that the amount of popular literature exceeds the amount of academic literature (concerning
this issue). The author has found that although there are excellent resources to help a newcomer
distinguish the difference, it takes a steady research effort to locate these resources, it takes time
to work through and comprehend these resources, and it is very difficult to “unlearn” solid
misconceptions. Some references include [68], [69], [63], [65], [76].
The author’s understanding is summarized here. Sine and trap motors may be constructed a
number of different ways but as far as torque production and bEMF generation is concerned, the
motors differ only in the shape of the rotor-stator flux linkage. This difference can be created by
changing the way the winding is distributed in the stator slots, by adjusting the magnetic design
(airgap and tooth/slot dimensions), by adjusting the construction of the rotor and physical shape
of the rotor magnets, and by adjusting the magnetization profile of the magnets [68], [69]. As
shown earlier, the bEMF/torque function is the derivative of the rotor-stator flux linkage with
respect to rotor position and in the general case torque is produced as shown in Figure 2.28 and
Equation (2.54). It is clear that the fundamental mechanism by which torque is produced is
identical regardless of the type of motor. If a motor can be categorized as trap or sine—and ideal
drive currents are forced into it—the motor will produce “ideal” torque. There is much
misinformation about the possibility or impossibility of driving a motor with non ideal current
waveforms. Quite simply, either type of motor (or any arbitrary motor not fitting either
classification) can be driven by any arbitrary current waveform. The torque produced in each case
will be different, the efficiency will be affected, and the performance of the current controller will
be affected; these issues will be discussed later.
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