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

Sensorless Speed Direct Torque Control with Space Vector Modulation (DTC-SVM)
Ψ PM
I sA
ABC
Is
α
αβ
Isd
L d
Ψ sd
dq
Ψ sα
Ψ s
I sB
αβ
Is
β
dq
I sq
L q
Ψ sq
αβ
Ψ sβ
θ Ψs
γ m
Figure 6.7. Current model based stator flux estimator.
6.3.3 Voltage model based flux estimator with ideal integrator
The stator flux linkage can be obtained by using terminal voltages and currents. It is the
integral of terminal voltages minus the resistance voltage drop:
dΨ sα
= ( Us α − RsIs
α
)
(6.7)
dt
dΨ sβ
= ( Usβ
− RsIsβ
)
(6.8)
dt
However, at low speed (frequencies) some problems arise, when this technique is
applied, since the stator voltage becomes very small and the resistive voltage drops
become dominant, requiring very accurate knowledge of the stator resistance R s
and
very accurate integration. The stator resistance can vary due to temperature changes.
This effect can also be taken into consideration by using the thermal model of the
machine. Drifts and offsets can greatly influence the precision of integration. The
overall accuracy of the estimated flux linkage vector will also depend on the accuracy
of the monitored voltages and currents.
The most know classical voltage model obtains the flux components in stator
coordinates ( α,
β ) by integrating the motor back electromotive force E , E
sα
sβ (see Fig.
6.8). The method is sensitive for only one motor parameter, stator resistance R s
.
However, the application of pure integrator is difficult because of dc drift and initial
value problems.
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