Direct Power and Torque Control of AC/DC/AC Converter-Fed ...

7. Summary and Conclusion
should be stressed that the system has to be equipped with fast PF loop and/or DClink
chopper (see Fig. 6. 18).
The detailed investigation has shown that PF loop with power estimation based on
motor voltage and current sensing gives better results as in the case of estimation
based on torque and speed sensing (see Chapter 4.2.2).
For proper operation of the closed control loops in DPTC-SVM scheme, the
digital PI controllers design is very important. Therefore, in Chapter 3 synthesis of
active and reactive power as well as DC-link voltage controller has been given. This
design is based on continuous transfer function approach (symmetric optimum – SO),
because fast sampling used in the system (dominated time constant – line inductor
T
RL
- is much higher as sampling time T S
), and has been confirmed by both
simulated and experimental results. The flux and torque control design has been
implemented similarly (for more details see [152]).
The main features and advantages of developed DPTC-SVM scheme (Fig. 3. 10)
for AC/DC/AC converter-fed IM can be summarized as follows:
• four-quadrant operation (bi-directional power flow) (Fig. 6. 4),
• fast flux and torque response (Fig. 6. 17),
• available maximum output torque in wide range of speed operation region,
• constant switching frequency and uni-polar voltage pulses (thanks to use
SVM),
• low flux and torque ripple (Fig. 6. 12b),
• robustness to rotor parameter variations,
• motion sensorless operation,
• nearly sinusoidal line current (low THD typically below 5%) (Fig. 6. 3),
• direct adjustable reactive power (including unity power factor – UPF),
• controllable DC-link voltage (well stabilized at desired level),
• AC side voltage sensors are eliminated (only DC-link voltage sensor is
required),
• wide speed range operation of the IM connected to the output of the VSI,
• reduction of DC-link capacitor, thanks to active power feedforward – PF
(gives possibility to use a foil capacitor),
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