352
value. Fig. 19 depicts the results of a speed step from standstill to rated speed. These results
353
Figure 3.17: Experimental setup of the proposed motor emulator.
3.4. INVERTER CUMULATIONIMPLEMENTATION ANDVIRTUAL MACHINEEXPERIMENTAL
VERIFICATION 31
Figure 3.18 shows the evolution of stator currents, back-EMF, rotor flux linkage and angular velocity during a slow acceleration process from standstill to nominal speed under no load, when the IUT is operated in speed control mode. The field oriented controller of the IUT enforces speed and current, while keeping the amplitude of the rotor flux constant. As the actual speed increases up to nominal speed, the back-EMF also rises and reaches its nominal value. Fig.
3.19 depicts the results of a speed step from standstill to rated speed. These results are entirely consistent with the behaviour of a real asynchronous machine. As expected from acceleration tests, the IUT delivers its nominal current only during the short acceleration phase and hence, cannot be tested properly without load.
0 2 5 5 0
Fig. 18. Stator currents, back-EMF, rotor flux linkage and angular velocity when accelerating from standstill to nominal speed
t[s]
n[min−1]
Fig. 19. Reference and actual values of speed and q current component resulting from a speed step from standstill to rated speed at no load
Fig. 22 displays the evolution of the stator currents of the VM during a speed reversal
368
from -120% to 120% of the nominal speed. Again, the results are absolutly consistent with
369
the behaviour of a real motor.
370
VI. CONCLUSION
371
The investigations carried out with the motor emulator presented in this paper attest to the
372
possibility of switching magnetically coupled standard VSIs sequentially. This method does
373
Figure 3.18: Stator currents, back-EMF, rotor flux linkage and angular velocity when accelera-ting from standstill to nominal speed.
0 2 5 5 0
Fig. 18. Stator currents, back-EMF, rotor flux linkage and angular velocity when accelerating from standstill to nominal speed
t[s]
n[min−1]
Fig. 19. Reference and actual values of speed and q current component resulting from a speed step from standstill to rated speed at no load
Fig. 22 displays the evolution of the stator currents of the VM during a speed reversal
368
from -120% to 120% of the nominal speed. Again, the results are absolutly consistent with
369
the behaviour of a real motor.
370
VI. CONCLUSION
371
The investigations carried out with the motor emulator presented in this paper attest to the
372
possibility of switching magnetically coupled standard VSIs sequentially. This method does
373
Figure 3.19: Reference and actual values of speed and q current component resulting from a speed step from standstill to rated speed at no load.
32 CHAPTER 3. FIRST ATTEMPT OFINVERTER CUMULATIONANDVIRTUAL MACHINE
The diagram in Figure 3.20 displays the evolution of angular velocity and q current while applying a load step with an amplitude equal to 75% of the rated load torque at nominal speed.
At timet = 0.08s, the load torque step is emulated by the VM. The speed controller of the IUT reacts with a currentiqto counteract the resistive torque and keep the speed constant. Therefore, the possibility of emulating mechanical energy transfers with the VM enables to test the IUT under realistic operating conditions.
Fig. 20. Load step of 75% rated torque at nominal speed
0 1 2 3 4 5
Fig. 21. Evolution of d current and angular speed at constant flux and in the flux weakening region
not only allow increasing the switching frequency but also leads to an appreciable reduction in
374
overall losses since power electronics devices exhibit a soft switching behaviour under these
375
circumstances. Experimental results show that the performance of five inverters connected in
376
parallel in terms of power and dynamics is sufficient to realize an efficient motor emulator.
377
A crucial feature of the Virtual Machine is its ability to be operated with an IUT working
378
with a field oriented controller. This has been made possible by using a model computing the
379
back-EMF as a result of the currents enforced by the IUT. Doing so, the electrical behaviour
380
of the VM is in perfect agreement with the one of a real machine. Furthermore, mechanical
381
torque steps can be reproduced, allowing the IUT to be tested under almost every possible
382
operating conditions.
383
Figure 3.20: Load step of 75% rated torque at nominal speed.
Figure 3.21 shows the ability of the VM to operate not only at rated flux amplitude but also in the flux weakening region. As expected, the current controller of the IUT keeps the d current constant at its nominal values as long as the speed reference does not exceed its rated value.
However, when the speed reference is increased beyond this threshold, the rotor flux has to be reduced to limit the back-EMF.
0 0 2 0 4
Fig. 20. Load step of 75% rated torque at nominal speed
0 1 2 3 4 5
Fig. 21. Evolution of d current and angular speed at constant flux and in the flux weakening region
not only allow increasing the switching frequency but also leads to an appreciable reduction in
374
overall losses since power electronics devices exhibit a soft switching behaviour under these
375
circumstances. Experimental results show that the performance of five inverters connected in
376
parallel in terms of power and dynamics is sufficient to realize an efficient motor emulator.
377
A crucial feature of the Virtual Machine is its ability to be operated with an IUT working
378
with a field oriented controller. This has been made possible by using a model computing the
379
back-EMF as a result of the currents enforced by the IUT. Doing so, the electrical behaviour
380
of the VM is in perfect agreement with the one of a real machine. Furthermore, mechanical
381
torque steps can be reproduced, allowing the IUT to be tested under almost every possible
382
operating conditions.
383
Figure 3.21: Evolution of d current and angular speed at constant flux and in the flux weakening region.
3.5. SUMMARY 33
Fig. 22. Evolution of the stator current and the angular velocity during a speed reversal from -120% to 120% of the nominal speed