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

Sensorless Speed Direct Torque Control with Space Vector Modulation (DTC-SVM)
Therefore, for commercially manufactured drives are impractical and further a simple
open loop based techniques will be considered.
6.5.2 Back electromotive force (BEMF) technique
This technique uses the back electromotive force to estimate the rotor speed [70]. The
velocity signal could be integrated to generate a position estimate. However, this signal
is sensitive to parameter variations and tends to drift and have offset problem. Another
problem with using BEMF to estimate position is that at zero speed the BEMF goes to
zero and at low speed the signal to noise ratio can not be ignored.
6.5.3 Stator flux based technique
Generally, the calculation of rotor speed is based on the simple relationship:
θ = θ − δ , (6.20)
r Ψs Ψs
where θ
r
is electrical position, θ Ψ s
is stator flux position and δ Ψ s
is torque angle.
After differentiation equation (6.20) and taking into account that θ r
= p b
γ m
the
mechanical speed of PMSM rotor can be expressed as:
⎛dθ
⎜
⎝ dt
Ψs
Ω
m
= −
dδΨ
⎞
⎟/
pb
, (6.21)
dt ⎠
dθ s
where Ω Ψ
Ψs
= is angular speed of stator flux vector and δ Ψ
is torque angle.
dt
As we can observe form equation (6.21) in order to calculate the mechanical rotor speed
it is necessary to calculate separately two components. One of them is angular speed of
stator flux vector Ω
Ψs
and the second one is change of the load angle d δ Ψ (see Fig.
dt
6.12).
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