The thesis deals with construction, control design and operating characteristics of a 9-phase electric drive, with special emphasis on field-weakening operation. The central element of the drive is a 9-phase synchronous machine with surface mounted permanent magnets, whose phase windings are divided into three 3-phase winding sets, that are spatially (magnetically) aligned. Two variants of the machine were used, differing in the degree of magnetic coupling between the winding sets: a machine with strongly magnetically coupled sets (9SSstrong) and a machine with weakly magnetically coupled sets (9SSweak).
In the introductory section, the development of a 9-phase machine from a conventional 3-phase machine is explained. Winding schemes for a conventional 3-phase machine and for 9SSstrong and 9SSweak are provided. The dynamic model of a 3-phase machine is given and then extended to a dynamic model of a 9-phase machine using an inductance matrix, which takes into account the magnetic coupling of winding sets.
The central part of the thesis explains torque control principles of a 3-phase machine in rotor reference frame, on which also control principles of a 9-phase machine are based. Next, control scheme of a 9-phase machine is explained with a special emphasis on a part necessary for operation in instances where one or two winding sets must be turned off. Special attention is given to the field weakening control, which undergoes some crucial changes when applied to a 9-phase machine, especially in the event of turning off of one of the winding sets. The control algorithm was implemented to the laboratory model of a 9-phase drive, which turned out to be a challenge by itself due to the complexity of the drive system and numerous cross influences and connections between the building blocks of the algorithm.
Then measurements were performed on the laboratory model of a 9-phase drive (both variants of the machine were used, i.e. 9SSstrong and 9SSweak) in different operating states. The emphasis was given to the operation in field-weakening region, where we switched off one or two winding sets. We recorded current conditions before and after one winding set was switched off. Because the control system is still able to follow speed reference, a voltage is induced in the inactive winding set. We found out that regardless of the machine design (9SSstrong or 9SSweak), diode current appears in the inactive winding set, terminating via power converter diodes. Its amplitude is increasing with the increasing of the rotor speed and is highly dependent on parasitic effects of the whole circuit. In case of 9SSweak the diode current has quite typical waveform, similar to that of a 6-pulse diode rectifier, whereas in the case of 9SSstrong the current has an irregular, pulsating waveform. Measured phase currents were then transformed into rotor reference frame, which enabled us to better estimate the influence of the diode current on the operation of the drive. We discovered that the diode current in the inactive winding set in 9SSweak generates a negative torque-producing component of the current and therefore has a braking effect. Diode current in 9SSstrong lacks that characteristic.
The same measurement was repeated for a case where two winding sets of the machine were turned off. Results were expectedly very similar to the previous measurement; the only difference were in the amplitudes of the currents. The currents in the active winding sets had a slightly bigger amplitude while the diode currents in inactive winding sets had smaller amplitude, which is true for both the 9SSstrong and 9SSweak.
We have also performed a measurement in which we studied an influence of decreasing the flux-producing current on the diode current in the inactive winding set. Having drive operating at constant speed we decreased (i.e. in negative direction) the flux-producing component of the stator current. Surprisingly, in 9SSstrong as well as in 9SSweak we detected an increase in the amplitude of diode currents in inactive winding sets. These results were contrary to what was expected as one would expect that the increase in flux-producing component of current would dampen the diode current in inactive winding sets. Therefore, the described phenomenon would be an interesting subject of further research.
Finally, a derating measurement was performed on 9SSstrong. We measured the machine speed torque characteristic when one or two winding sets were switched off. In this way we showed the ability of the machine to continue to operate even in a case of a fault, that would require turning off one or two winding sets, which is one of the main advantages of multi-phase drives in comparison to conventional 3-phase drives.
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