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In general, 3-phase current machines can be categorized as syn-
chronous or asynchronous motors based on the operating beha-
vior of the stator and rotor. For the latter, since there is a time lag
between generation of the magnetic field in the rotor and in the
stator, the rotor turns with a time delay, and thus runs asynchro-
nously. However, the motor scores with good power density, a sim-
ple structure and simple regulation, and the fact that it does not
require magnets. On the other hand, the synchronous motor has
even higher efficiency and higher power density. The speed of the
rotor synchronously follows the specified frequency and the rota-
ting magnetic field in the stator. However it requires more complex
regulation. The rotor can be designed with expensive permanent
magnets (buried or surface magnets) or with external excitation in
coils (building a magnetic field using current-carrying coils) at the
expense of efficiency. There are specific rotor and stator const-
ructions, depending on motor polarity; these are presented in Fig.
11 [Franke (2011a)].
The various design forms are shown below [for example see Frei-
aldenhoven (2009); Mathoy (2010); Hofmann (2010)].
Asynchronous machine (ASM):
One characteristic of the asyn-
chronous machine, also referred to as an induction machine, is
the cage winding in the rotor (inductor). The rotor, structured as
12 The Authors‘ own illustration based on Franke (2011a)
Fig. 11: Motor topologies
Chapter 2