In comparison to the 24 phase design, the 15 phase design was found to have the following disadvantages:
3.7 Even and Odd Number Topology Comparisons
Fig. 3.55 Figure of Merit Summary: Comparison of 24 Phase and 15 Phase Designs
1. Commutating Inductance: Higher commutating inductance due to the high number of turns per phase winding, bearing in mind that inductance is directly proportional to the square of the number of turns.
2. Auxiliary Clamp Capacitance: Higher clamp capacitance required to achieve same clamp voltage due to the increased commutating inductance requiring more commutation energy absorption, so the clamp capacitors are bigger. 3. Power Dissipation: Owing to higher commutating inductance, more power
(LcI2
2 ) dissipated into the clamp discharge circuit. However, the clamp discharge
resistor power dissipation represents only 10% of the total electronic commutator losses in the case of the 24-phase machine. This drawback can be mitigated by use of energy recovery circuits.
4. Fault Detection: Clamp Voltage Imbalance protection scheme proposed to detect electronic commutator failure or malfunction has reduced response bandwidth due to the asymmetric nature of the commutator operation with an odd phase numbered machine.
Figure 3.55 gives a figure of merit summary of the comparisons between even and odd number of stator phases for a 24 and a 15 phase machine. The comparison
3.8 Summary
is normalised on the 24 phase design parameters. Although the analysis and com- parison was done on two specific design, the parameters used in the figure of merit assessment follow a similar trend for generic even and odd number topologies of similar performance. In general, the benefits of odd-phase number topology in terms of cost, reliability and air gap torque ripple outweigh its drawbacks and is an attractive alternative proposition to the even numbered topology.
3.8
Summary
This chapter has presented the operating principles and analysis of the electronic com- mutation process of the multiphase electronically commutated dc machine topology with an even number of stator phases. The simulation and analysis was extended to a topology odd number of stator phases. The current commutation processes for both topologies were analysed and key factors affecting electronic current commutation presented. It was shown that symmetric current commutation control proposed for the even stator phase number electronic commutator topology can not be used for the odd phase number topology. Instead, the study has shown that an interleaved current commutation control strategy proposed aided suppression of net circulating current in the stator polygonal winding for the odd number topology. Interestingly, the fast Fourier transform of the phase winding current of a 15 phase machine revealed
the absence of the 15th harmonic current component, which would otherwise have
resulted in a net circulating current in the stator polygon. Fast Fourier transforms of the simulated dc link voltage and machine torque has confirmed that despite the use of squarewave phase current excitation, low order harmonics had no detrimental effect on the machine torque ripple signature. In fact, the lowest torque ripple harmonic order
components where shown to beN fs and 2N fsfor even and odd number respectively,
whereNis the stator phase number and fsis the machine fundamental frequency.
A comparative analysis based on two similarly rated even and odd number topolo- gies was given and figure of merit summary highlighted that similar or even better performance to the even stator phase number topology can be obtained using odd stator phase number topology with significantly less number of stator phases.
3.8 Summary
Its envisaged that the two level topologies introduced in this chapter will be applicable to low and medium voltage applications where commercially off the shelf power electronic switching devices can be directly applied without the need for series connection of switching devices. However, a limitation of these two level topologies is that in applications that require high dc link voltages, series connected power electric devices will be required, as such dynamic and steady state voltage sharing of seriesed devices will be required. The next chapter will analyse a variant of the multiphase electronically commutated dc machine topology that is better suited to high voltage applications and circumvents the need for direct series connection of electronic commutator devices.
Chapter 4
Multi-Level Multiphase Electronically
Commutated DC Machine Topology
4.1
Introduction
The two level multiphase electronically commutated dc machine presented in the preceding chapters is well suited for low voltage applications. For high voltage ap- plications, the two level topologies require series connection of commutator devices which brings some challenges in ensuring transient, dynamic and steady state voltage sharing across the string of series connected devices. An alternative topology suit- able for high voltage dc application which benefits from most of the advantages of multiphase electronic commutated dc machines alluded to earlier is the multilevel mul- tiphase electronically commutated dc machine topology. This topology was invented by Allan Crane as reported in [92]. The work presented in this chapter is a scientific analysis of the operational attributes of this topology. This work seeks to analyse and evaluate suitable electronic commutator configurations and their associated control schemes for this multilevel topology. In this analysis, suitable control schemes and simulation models are developed and energy recovery circuit topologies are proposed for recovering commutation energy back into the drive circuit. Control strategies aimed at enhancing the fault tolerant operation of this topology are proposed and simulation results presented.