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ELECTRIC MACHINES
The speed and torque of electric machines can be regulated
very flexibly. Independent of the machine type, the speed and
torque of the machine can be adjusted to the requirements of
the drive using modern power electronics. Mechanical
transmissions or clutches are not essential. Depending on the
power requirement and the available space, brush DC motors or
induction motors can be used. The electrical drives for comfort
systems are mainly realised using brush DC drives, whereas
induction motors can also be used for electric auxiliary units,
depending on the power requirement. However, with regard to
electric auxiliary units, special emphasis on the development of
compact, highly integrated and low-cost components is needed.
The main advantages of electric drives for traction, compared to
conventional propulsion systems, are their higher efficiency,
robustness and better controllability. The efficiency of electrical
traction is about 90%, depending on the performance class.
Advantage
Drawback
AIM
+ Simple and robust assembly
+ Machine is passive, with no voltage or torque under
currentless conditions
+ Good short-term overload behaviour
+ No danger of demagnetisation during overload operation
+ Easy implementation of field weakening to expand the
speed range
+ Slightly lower costs
- Lower power density
- Higher weight
- Additional current required to magnetise the rotor
- lower power factor due to magnetization current
ELECTRIC DRIVES
The efficiency of propulsion systems using ICE is about 30%,
due to thermodynamic restrictions. A further advantage is that
electric drive systems require far less maintenance.
Due to their robust assembly, their power density and their high
dynamics, three-phase induction motors are preferable to direct
current machines. Permanent magnet synchronous motors
(PMSM) offer particular advantages due to their high power
density. However, the choice between an asynchronous
induction motor (AIM) or a PMSM depends largely on the
requirements and the design of the vehicle. The selection
process to identify the most appropriate machine type needs to
be based on a comprehensive vehicle simulation which takes
into account the given vehicle application (defined by the driving
cycle, range, acceleration, etc.), as well as the entire powertrain
concept and the interdependencies with other vehicle
components, such as the battery system or the combustion
engine.
PMSM
+ High power density
+ Lower weight
+ Short-term overload capacity
+ No power loss to excitation field production
+ better power factor below field-weakening
- Complex rotor assembly through use of various materials
such as metals, magnets, adhesives
- Temperature sensitivity of the rotor
- In case of rotating the machine mechanically the
permanent magnets cause induced voltages even if the
machine is not connected electrically
- Active field weakening due to additional current required
to expand the speed range
- Slightly higher costs
Table 1: Comparison between the main advantages and drawbacks of asynchronous induction machines (AIM) and permanent magnet synchronous machines (PMSM)
Cross section of an AIM Cross section of a PMSM
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