Space Vector Modulated – Direct Torque Controlled (DTC – SVM ...

2. Voltage Source Inverter Fed Induction Motor Drive
2.4.4. Relation Between Carrier Based and Space Vector Modulation
All the carrier based methods have equivalent to the space vector modulation
methods. The type of carrier based method depends on the added ZSS, as shown in
section 2.4.2, and type of the space vector modulation depending on the time of zero
vectors t 0 and t 7 .
A comparison of carrier based method with SVM is shown in Fig 2.18. There is
shown a carrier based modulation with triangular shape of ZSS with 1/4 peak value.
This method corresponds to the space vector modulation (SVPWM) with symmetrical
placement of zero vectors in sampling period. In Fig. 2.18b is presented discontinuous
method DPWM1 for carrier based and for SVM techniques.
In the carrier based methods three reference signals U * Ac , U * Bc , U * Cc are compared
with triangular carrier signal U t , and in this way logical signals S A , S B , S C are generated.
In the space vector modulation duration time of active (t 1 , t 2 ) and zero (t 0 , t 7 ) vectors are
calculated, and from these times switching signals S A , S B , S C are obtained. The gate
pulses generated by both methods are identical.
The carrier based PWM methods are simple to implement in hardware. Through the
compare reference signals with triangular carrier signal it receives gate pulses.
However, a PWM inverter is generally controlled by a microprocessor/controller
nowadays. Thanks to the representation of command voltages as space vector, a
microprocessor using suitable equations can calculate duration time and realize
switching sequence easily.
It is possible to implement all carrier based modulation methods using the space
vector technique. The active vector times t 1 and t 2 equations are identically for all space
vector modulation methods. But every method demand suitable equation for the zero
vectors t 0 and t 7 .
The eight voltage vectors U 0 - U 7 correspond to the possible states of the inverter
(Fig. 2.13). Each of these states can be composed by a different equivalent electrical
circuit. In Fig 2.19 the circuit for the vector U 1 is presented.
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