Direct Torque Control with Space Vector Modulation (DTC-SVM) of ...

Control methods of PM Synchronous motor
4.2 Field oriented control (FOC)
During many years a DC motor has been mostly used. Because of simple control
method, which based on fact that flux and torque can be controlled separately using
current control loop with PI controllers. However the weak point of this drive was DC
motor, which could not worked in aggressive or volatile environment and required
cyclical maintenance. This disadvantages has been eliminated, when instead of DC
machine a three phase PMSM motor were used.
In searching new control method for induction machine in 1971 was developed vector
control method known as field oriented control (FOC) [31,38,49]. This method allows
control the flux and the torque in the AC machine in similar way as for DC motor. It
was achieved by transform current vector in stationary reference frame ( α,
β ) into new
coordinate system ( dq) , with respect to rotor (magnet) flux vector. So the flux
produced by permanent magnet is frozen to the direct axis of the rotor (see Fig. 4.4).
q−axis
β −axis
I s
I sq
δ I
γ I
I sd
γ m
Ω s
Ψ PM
d−axis
α −axis
Figure 4.4 Vector diagram illustrated the principle of FOC.
Further, stator current vector can be split into two current components: flux current I sd
and torque producing current I sq
. In analogy to separate commutator motor, the flux
current components corresponds to excitation current and torque-producing current
corresponds to the armature current. Therefore, the goal of the control system is to
reference the
I
sd_
ref
,
sq_
ref
I stator current components in respect to requirement of
references torque and flux. The flux and torque producing stator current references are
obtained on the output of the reference current generation block (see Fig. 4.5).
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