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sheet metal elements usually has a bar winding, in which the bars
are connected by two short-circuiting rings. In some cases this
results in extremely high currents in the rotor. The torque achieved
is proportional to the difference (slip) of the circumferential speed
to the grid frequency. The machine has a high rpm range, constant
power output at uniform inverter load and high overall efficiency.
In this way the ASM dispenses with sliding contacts to the rotor
(as opposed to the direct current machine or the externally ex-
cited synchronous machine). The asynchronous machine has an
extremely simple and cost-effective structure with low volume
and weight. Further, only a relatively cost-effective controller and
pulse generator are required. The ASM is a so-called high-speed
machine, as opposed to a high-power machine) and thus requi-
res a gearbox. However, at higher rpm the asynchronous machine
exhibits a quadratically decreasing torque curve. Moreover, multi-
pole motors require extremely precise and small air gaps for a
good current phase angle. Also, rotor losses at low rpm and high
torques cause high rotor temperatures. However, the efficiency is
below the level for a synchronous machine. It is considered safe,
economical and is suitable for urban applications. It has already
been used in the Tesla Roadster, for example.
Permanently excited synchronous machine (PSM):
The stator
structure of a permanent-magnet excited synchronous machine is
similar in structure to an asynchronous machine. In addition to dis-
tributed windings, concentrated windings can also be used here.
Moreover, the rotor (also referred to as the inductor) is equipped
with permanent magnets. These can be fixed in place on the sur-
face (surface magnets) or they can be located in pockets within
the rotor (buried magnets). It has a simple mechanical and electri-
cal structure (no brushes, no sliding contacts, and no complicated
windings). Multi-pole motor types can also be easily manufactu-
red. With short-circuited windings, the machine develops a high
braking torque (safety function, however this can also entail risks)
and has an excellent efficiency level in the lower rpm range and
partial load range. Thus it is considered to be one of the high-pow-
er machines. The PSM achieves an extremely high torque density
and power density. Particularly with use of rare earths, (Nd-Fe-B
magnets) an extremely compact design is possible - even more
compact than it is the case for externally excited synchronous ma-
chines. The necessary use of rare earth metals however results
in high material costs and dependence on imports. Likewise a
cost-intensive rotor position encoder is also necessary. However,
every permanent-magnet excited motor also has a limit torque that
is defined by its magnet content. Moreover, at high rpm there are si-
gnificant losses and only moderate power. The high rpm range (field
weakening range) requires extremely high reactive current compo-
nents for the inverter. Likewise extremely high short-circuit currents
and idle voltages are possible if inverter defects arise. Further, a
complex manufacturing process must be assumed for mass pro-
duction. The permanent magnet excited synchronous machine is
currently the most widely distributed machine for hybrids. However,
it is being increasingly used as a traction motor, as well.
Current excited synchronous machine:
Externally excited syn-
chronous machines are employed to provide larger synchronous
motors, in particular. Although the stator has the same structure
as a PSM, the rotor is magnetized by current supplied from the
outside (instead of magnets, for the PSM). To do this the rotor
has salient poles that have windings or non-salient poles with
retracted windings. The rotor (also referred to as an inductor) is
magnetized via the current-excited poles. The exciter windings
are supplied with current via slip rings (in some cases brushes).
For larger machines (diameters greater than 400 mm) contactless
transmission is also possible. However, as opposed to direct cur-
rent motors, only a little current must be transmitted into the rotor.
Current excited synchronous motors achieve 2.5x the nominal va-
lue for approximately 30 seconds, and up to 4x the nominal value
for 5 seconds. A major advantage for them is the fact that magnets
do not need to be used for this. Likewise, the machine does not
have a power drop at higher rpm. It also has an extremely high ef-
ficiency level; however, it has lower power density than the PMS.
In addition, there is a minor worsening of the efficiency level and
a need for additional power electronics for provision of the exciter
current in the rotor. A special design of the synchronous machine
is the hybrid synchronous machine: Through the additional use of
permanent magnets, an increase of the reluctance torque can be
achieved. This is advantageous in enabling adequate torque in
emergency operation even without current excitation.
Synchronous reluctance machine (SyR):
Reluctance motors are
based on the principle that a body that can bemagnetized (but which
itself isnotmagnetic) aligns itself in thedirectionof theexternal field.
For the synchronous reluctancemachine excitation is provided by a