E-mobility Technology Winter 2020

e-mobility Technology International | Vol 7 | Winter 2020
Influence of the
resolution on
performance
Evaluations of the torque curve show, as expected, a
low-resolution leads to higher torque fluctuations in
the starting phase.
Especially when a vehicle has to accelerate
under a high-torque condition avoiding
sliding wheels and vibration and thus
material-friendly, the resolution of the
speed-sensor is of decisive importance.
The graph on he left clearly shows that, for
example, a sensor resolution of 8 bit and a
typical inverter switching frequency of 10kHz
leads to a resolution of the speed of 2344
RPM. During run-up at very low speed the
speed controller does not measure the real
gradient of speed and therefore the torque
controller overdrives. This inevitably leads
to vibrations in the drive and ultimately
to noises when starting up. That’s what we
know from trams or other railways where
this does not play a major role due to the
few starting phases and the very stable
mechanical design of the engines. In electric
vehicles with many stop and go phases
vibrations and noises are undesirable.
A typical situation in a passenger vehicle
is, for example, driving over a curb or a
fully occupied bus, which has to start on an
ascending road.
Blue line: 8-bit resolution
high torque ripple causes
high vibrations and loud
noises.
Red line: 10-bit resolution
lower torque ripple causes
lower vibrations and less
noises
Comparison of the power consumption shows
in the start-up area that a higher resolution
(red line) contributes to a gain in efficiency.
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