Modular Brushless Machine

1.3.1 Brushless Machine

Brushes and slip rings showed in Figure 1.8 which are usually used to excite the field current of WRSM is a demanding problem. Its disadvantages are losses, maintenance and debris issues.

Figure 1. 8: Brush excitation [35]

In this study, a brushless system with a rotary transformer which have successfully demonstrated system efficiency > 90% with a lower lifetime cost is used. It causes lower frictional losses, and an easier maintenance compared to a brush system. A rotary transformer shown in Figure 1.9 is a transformer with an axial symmetry and an air-gap between the primary side and the secondary side. The air-gap allows the rotation of one half of the core, without influencing the flux lines and the inductive power transfer between the primary and secondary side [34].

1.3. Modular Brushless Machine 13

1.3.2 Modular Machine

Fault tolerance of a system is a key issue in embedded systems. Safety must be considered in the event of a failure into the system. As consequence of the failure, some damages of the equipment and danger for the system operators can occur [23]. In WRSM drives, several faults, both in the machine and in the power converter must be considered. [24], [25] show many potential faults that can occur in electrical machines such as:

- Winding open circuit,

- Winding short circuit: phase to phase, phase to ground, turn to turn, - Short circuit at the machine terminals.

Phase to phase short circuit faults are among the most common faults in AC machine. The combination of the above faults can occur because they tend to be cumulative and therefore, one fault induces other faults. Here, a modular system is proposed in order to prevent this type of short circuit.

The use of modular machine drive increases the redundancy and improves reliability of the system [29]- [31]. The integration of the drive electronics into the housing of the machine shown in Figure 1.10 has been widely investigated in a wide range of applications [26].

Figure 1. 10: Single phase modular configuration

Integration of the machine and drive has several advantages in [27], [28] as: - Reduction of the total drive volume and weight,

- Elimination of interconnects and reduce the risks of generating high voltage transients on the winding.

However, the use of integrated motor drive can cause an increase of losses and therefore, of the temperature.

In this study, each phase of the machine is connected to a separate single-phase converter (see Figure 1.11). Each phase drive module is considered as a single module and every phase can operate independently of the others. These structures are highly redundant and the effects on the other phases are minimized in case of having fault at one phase. It means that the system is well able to continue the operation when a fault occurs.

Figure 1. 11: Integrated machine and power supply concept

1.3.2.2 Multiphase machine

The multiphase machine has recently become a competitive candidate for safety critical applications due to its enhanced fault tolerant capacity compared to a conventional three-phase machine. The higher number of phases increases the number of freedom degrees for the machine supply which is used to achieve a better performance and a higher fault tolerance. There are several considerable benefits from the use of multiphase machines [23], including:

- Improved fault tolerant behavior

Multiphase machine has an improved fault tolerant capability because more phases can provide partial retention of functionality even in the eventuality of a power component die.

- Reduced phase current

In a multiphase AC drive, the phase current can be reduced without increasing the voltage per phase since the power is evenly divided among a larger number of phases. This reduces the size and the power rating of the individual power semiconductor switches.

- Improved armature magneto-motive force waveform

Multiphase machines can improve Magneto-Motive Force (MMF) waveforms. The reduction in the armature air-gap MMF harmonic components has several advantages such as the torque ripple reduced, acoustic noise reduced and lower rotor losses.

1.3. Modular Brushless Machine 15

Nevertheless, the multiphase machine has several drawbacks such as:

- Higher number of power semiconductor switches are needed. It can increase the volume, the weight and the cost of the power converter, leading to a higher complexity and possibly to a lower reliability of the overall system,

- Non fundamental harmonics of the current must be considered by the control.

Finally, the increase in the number of phases could finally make the whole system more complex and expensive. However, it increases the fault tolerant capability as well as better reliability.

1.3.2.3 Stator segmentation

An interesting concept for achieving higher modularity in the machine is the segmentation in the stator core. For this structure, high fault tolerant capability can be achieved due to physical separations. These advantages are presented in [26]. Indeed, the faulty components can be replaced quickly and cheaper. Moreover, the copper slot fill factor is higher and the machine can be wound automatically. One of different types of the segmented stator is a ‘T-core’ segment as shown in Figures 1.12 and 1.13.

Figure 1. 12: Stator segmentation Figure 1. 13: Modular motor in LG compressor

Mitsubishi Electric Corporation developed this type of machine called POKI-POKITM _{shown in Figure}

1.14. The stator slots are fully open (e.g. the teeth have no tip) and therefore, it is easier to pre-wind the coil for each segment of the stator core with non-overlapping concentrated winding. After the coils are wound, the stator is formed into a cylindrical shape and is press fitted into a frame. Its advantages are the modularity, the fast automatic winding manufacturing, the easiness to assemble and to remove, and therefore, it is easier to replace faulted components and this simplifies the connections in the machine. Moreover, the slot fill factor can be increased, leading to a decrease of copper losses [32]. It should be noticed that the POKI-POKITM _{machine is only suitable for short active length due to the mechanical}

Figure 1. 14: Modular structure of POKI-POKITM _{machine [32]}