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

Voltage source PWM inverter for PMSM supply
3.3 Space vector based pulse width modulation (PWM) methods
In voltage source inverter the transistors are controlled in a on-off fashion. In order to
obtain a suitable duty cycle for each switches the technique pulse with modulation is
used. The modulation methods [18,21,22,23,24,26,28,29,30] have the influence on:
wide range of linear operation, low content of higher harmonics in voltage and current,
low frequency harmonics, minimal number of switching to decrease switching losses in
the power components.
The most important factor in PWM mode is modulation index defined as the ratio of the
reference voltage amplitude value to the maximum voltage amplitude value during sixstep
operation (see Fig 3.17) and is given by:
M
U
ref
= (3.8)
2
U
DC
π
where the
U
DC
is the DC link voltage (for three phase six diodes rectifier is 560 V ).
The modulation index varies between 0-1 and can be divided into two regions: the
linear ( 0< M ≤ 0.907) and the nonlinear ( 0.907 < M ≤ 1) as is shown in Fig. 3.17.
Im
U3(010)
U 2(110)
End of overmodulation
region (six-step mode)
2
π
Umax
= U DC
⇒ M = 1
sector = 2
sector = 3
2
U
3 dc
U ref
End of linear region
U
max
= U DC
⇒ M = 0.907
3
U 4
(011)
sector = 4
U 0
(000)
U 7
(111)
sector = 5
θ ref
T1
U
1
Ts
sector = 1
sector = 6
Re
U (100) 1
nonlinear
(overmodulation) region
linear region
U 5 (001)
U 6 (101)
Figure 3.17 Space vector diagram of the available switching vectors.
46