POLITECHNIKA WARSZAWSKA

4. ANN based Current Controllers (CC)
switching frequency, well-defined harmonic spectrum, optimum switch pattern and
DC link utilisation. Also, full independent design of the overall control structure as
well as open loop testing of the inverter and load can be easily performed. In the
linear group, the following controllers are described: PI stationary and synchronous,
state feedback, predictive with constant switching frequency.
A. Stationary controller PI
The Stationary Controller, also called the Ramp Comparison Current Controller,
uses three PI error compensators to produce the voltage commands u Ac ,u Bc , u Cc for a
three-phase sinusoidal PWM (Fig.4.7) [5].
i Ac +
i Bc + -
i Cc + -
-
Carrier
u Ac
u Bc
u Cc
S A
S B
S C
U DC
i A
i B
i C
Three-phase
Load
Fig. 4.7. Linear current controller: stationary PI
In keeping with the principle of sinusoidal PWM, comparison with the triangular
carrier signal generates control signals S A
,S B
, S C
for the inverter switches. Although
this controller is directly derived from the original triangular suboscillation PWM
[19], the behaviour is quite different, because the output current ripple is fed back and
influences the switching times. The integral part of the PI compensator minimizes
errors at low frequency, while proportional gain and zero placement are related to the
amount of ripple. The maximum slope of the command voltage u Ac (u Bc , u Cc ) should
never exceed the triangle slope. Additional problems may arise from multiple crossing
of triangular boundaries. As a consequence, the controller performance is satisfactory
only if the significant harmonics of current commands and the load EMF are limited
at a frequency well below the carrier (less than 1/9 [4]). The main disadvantage of this
technique is an inherent tracking (amplitude and phase) error. To achieve
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