Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...
Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...
Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...
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Application and Verification 155<br />
7.2.2 Motor drives<br />
There are at least two fundam<strong>en</strong>tally differing motor control approaches:<br />
1. Direct control of motor quantities by direct application of suitable states. No<br />
classical pulse width modulation as such is used, but of course the resulting pulse<br />
pattern ess<strong>en</strong>tially also is PWM signal. DTC is ABB’s preferred approach to motor<br />
control. None of the pres<strong>en</strong>ted NP control schemes can be applied directly to<br />
DTC. However, DTC does automatically make use of the full physically<br />
controllable range based on the real time NP curr<strong>en</strong>t function if a proper state<br />
selection for NP control is done. This is not true for the modified and virtual<br />
vectors. An application of such a combination of states would be pure coincid<strong>en</strong>ce.<br />
New NP control schemes for DTC can be <strong>de</strong>fined by simply including modified<br />
and virtual vectors in the set of possible output vectors. The timing of the partner<br />
states of a virtual vector would be tricky as the overall application time of any<br />
vector is not known beforehand. DTC can also be tuned to have constant<br />
switching frequ<strong>en</strong>cy (e.g. [108]). Yet other approaches would be required to make<br />
use of virtual vector in this case. There is also research work on the<br />
implem<strong>en</strong>tation of DTC based on MPC done (e.g. [52]). MPC will automatically<br />
make use of state combinations like the virtual vectors if the prediction horizon is<br />
long <strong>en</strong>ough. However, this is computationally very <strong>de</strong>manding and has not be<strong>en</strong><br />
pres<strong>en</strong>ted so far.<br />
2. Vector control schemes or U/f control schemes make use of an approximated<br />
output voltage of the converter. Classical modulation schemes can be used for that<br />
purpose (SVM, carrier based). Consequ<strong>en</strong>tly, all of the proposed modulation and<br />
NP control schemes are applicable to this kind of motor control.<br />
Motor drives g<strong>en</strong>erally have less string<strong>en</strong>t requirem<strong>en</strong>ts regarding output waveform harmonics.<br />
But in any case, a low distortion is b<strong>en</strong>eficial to keep losses in the machine low, reduce torque<br />
ripple and avoid mechanical oscillations in the system.<br />
Bearing <strong>de</strong>gradation due to bearing curr<strong>en</strong>ts is an important topic in drive systems. A lot can<br />
be done by proper bearing <strong>de</strong>sign. On the other hand it is clear that low average CM voltage and<br />
low CM transi<strong>en</strong>ts are always b<strong>en</strong>eficial for the drive system. Some of the proposed schemes (e.g.<br />
6 th harmonic injection) perform better than others (e.g. virtual vector scheme) in that respects. A<br />
hysteresis based control approach can provi<strong>de</strong> both a lot of NP control capacity and low CM<br />
voltage operation in steady state.<br />
7.2.3 Back to back configurations<br />
Back to back refers to the connection of two or more converters of the same type to the same<br />
DC link. This may be a drive system with active front <strong>en</strong>d, a power g<strong>en</strong>eration system with line<br />
inverter, a power quality <strong>de</strong>vice with multiple connections (e.g. UPFC), a multi-drive system with<br />
several motor inverters on the same DC link, etc. NP control in all these systems can either be<br />
<strong>de</strong>dicated to one inverter or shared by several. In the case of sharing the control task, differ<strong>en</strong>t<br />
concepts are possible: Master slave concepts can be use, where one controller is taking care of the<br />
task using multiple inverters. Multiple controllers can also work in<strong>de</strong>p<strong>en</strong>d<strong>en</strong>tly, but in this case care<br />
has to be tak<strong>en</strong> with stability of the control loops, as multiple controllers may counteract.<br />
Regardless of the differ<strong>en</strong>t possibilities of approaching the control problem, physical limitations do