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

Voltage source PWM inverter for PMSM supply
As a consequence when the phase current I sA
is positive, the output voltage is reduced,
and when the current I
sA
is negative the output voltage is increased (see Fig. 3.11).
U0
I
sA
> 0
ideal voltage
real voltage
U I < 0
0
sA
U DC
2
U DC
2
−U DC
2
deacresing
−U DC
2
increasing
Figure 3.11 Dead time effect on the inverter output voltage: (fat line real voltage, doted line
ideal voltage).
Voltage drop across power devices
In real voltage source inverter power switches do not conduct ideally. When they are
conducting the voltage across them is not zero and equal the voltage drop on the
conducted transistor V T
. Also in blocking mode the power switches have voltage drop
on the conducted diode V D
. More details about real IGBT module you can find in
Appendices.
The voltage drop across the power devices is dependent on the direction of the phase
current. It has influence on the output voltage, especially at low speed operation of
motor and high load current [17,20,27,30]. Fig. 3.12 shows the voltage drop influence
on the output voltage. Also shows that the output voltage is asymmetric (with offset)
and the voltage drop decreases the output voltage when the phase current is positive and
increases the output voltage when the phase current is negative.
U 0
I > 0
sA
U 0
I < 0
sA
U DC
2
V T
U DC
2
V D
V D
V T
−U DC
2
−U DC
2
Figure 3.12 Output voltage in voltage source inverter due to voltage drop across the power
devices a) for I
sA
> 0 , b) I
sA
< 0 (fat line real voltage, doted line ideal voltage).
The influence of dead time effect and voltage drop across power devices on the output
voltage from inverter is illustrated in block diagram (Fig. 3.13). The ideal reference
*
voltage components in stationary reference frame ( U α
,
_ ideal
*
U β _ ideal
) are equal real
40