| dc.description.abstract | Multi-phase machines (MPMs) have more than three windings in their stator, rotor, or both.
With the broader adoption of power-electronic converters for efficient driving of the machines,
MPMs are gaining attention in different applications due to their certain advantages over three-
phase machines. One such advantage is higher fault tolerance due to higher phase redundancy,
which makes it suitable for safety-critical applications like electric vehicles (EVs), ship propul-
sions, electric aircraft, etc. Another advantage is that MPMs allow power splitting across
multiple phases. Hence, the power rating per phase drive unit becomes low, making it suitable
for high-power applications like railway traction, pumps, compressors, etc. Recent literature
also proposes using the same multi-phase converter fed MPM, otherwise used for propulsion, as
an onboard battery charger; it substantially reduces space, weight, and cost. During charging
mode, the leakage inductance of the machine provides the required inductance for the grid
connection, and MPM’s higher degrees of freedom are used to lock the rotor electronically. An
asymmetrical six-phase machine (ASPM) or split-phase machine is one such MPMs and is very
common in EVs. This thesis aims to devise the pulse-width modulation (PWM) techniques of
a two-level six-phase inverter fed ASPM to improve the overall drive efficiency.
ASPM has two sets of balanced three-phase windings, which are spatially shifted by 30 de-
grees (electrical angle). In one of the popular configurations, the two three-phase winding sets
are connected in star fashion with two isolated neutral points. This machine is conventionally
analyzed in two two-dimensional (2D) orthogonal subspaces. One of these subspaces is associ-
ated with electromagnetic energy transfer and torque production. The other subspace doesn’t
transfer energy through the air gap and the equivalent circuit in this plane, consisting of wind-
ing resistance and leakage inductance, hence, provides a low impedance. Therefore, excitation
of this non-energy-transferring subspace causes a large current and associated copper loss. Any
PWM technique of ASPM aims to synthesize the desired voltages in the energy-transferring
plane and minimize the applied voltage in the non-energy-transferring subspace.
Linear modulation techniques (LMTs) of ASPM apply zero average voltage in the non-
energy-transferring subspace and synthesize the desired voltages in the energy-transferring
plane on an average over a switching cycle. It is expected that these LMTs should avoid
more than two switching transitions of an inverter leg within a carrier period to limit the
instantaneous switching loss. Through an innovative approach, our work finds a way to account
for all possible infinitely many LMTs that follow the rule of at most two transitions per leg.
But each of them results in a different current ripple performance. Ripple current is inevitable
in PWM converters and should be minimized through modulation to reduce the associated
copper loss. The total ripple current RMS of ASPM is contributed by both energy-transferring
and non-transferring planes. One machine parameter also impacts this performance: the ratio
of high-frequency inductances in these two subspaces. For all reference voltage vectors and the
whole feasible range of the machine parameter, our work finds the techniques with minimum
current ripple (RMS) among the above infinite possible LMTs through numerical optimization.
A hybrid PWM strategy is proposed with these optimal techniques, which outperforms all
existing techniques regarding current ripple performance.
Overmodulation (OVM) techniques of ASPM attain higher voltage gain in energy-transferring
subspace than LMTs by applying non-zero average voltage in the non-energy transferring sub-
space. This operation doesn’t cause any torque ripple, but the applied voltage in non-energy
transferring subspace should be minimised to reduce unwanted current and associated loss. The
existing OVM technique in the literature minimizes this average voltage from the space-vector
perspective with a pre-defined set of four active vectors. To find the best technique, one needs
to perform the above minimization problem with all possible sets of active vectors, which can
give higher voltage gain. So, this requires the evaluation of a large number of cases. In this
thesis, we have formulated the above minimization problem in terms of average voltage vectors
of two three-phase inverters, where active vectors need not be specified beforehand. Thus, the
analysis is more general. Following the above analysis, eight switching sequences in one part
and two in another part of the OVM region are derived, which attain the minimum average
voltage injection in the non-energy transferring subspace.
Although the above OVM sequences apply the same average voltages in the two subspaces,
they have different high-frequency ripple currents due to different switching strategies. The
current ripple study of the OVM techniques of ASPM is missing in the literature. Hence,
one of our works in the thesis studies the current ripple performances of the above optimal
PWM sequences in the OVM region, which apply minimum average voltage in the non-energy-
transferring subspace. We find the sequence with the best switching current ripple performance
for a given reference vector in the OVM region and the machine parameter. After that, a
PWM technique is proposed, which substantially improves the high-frequency current ripple
performance (RMS) compared to two existing OVM techniques for a given machine parameter
value.
Finally, simple carrier-comparison-based implementation methods of the proposed LMTs
and OVM sequences are found. The six-phase inverter is split into two three-phase inverters,
and the proposed strategy implements the PWM sequences per three-phase inverter basis. In
carrier-based implementations, the duty signal of the top switch of an inverter leg is compared
with a triangular carrier. The bottom switch’s gating pulse complements the top switch’s pulse
with a fixed dead time. The duty signal of the top switch of any leg has two components- a
modulation signal and a common-mode signal. Two 180-degree phase-shifted carrier signals
are required to implement the proposed sequences. The energy-transferring plane of ASPM is
divided into twenty-four equivalent sectors; the carrier signals and the expressions of modulation
and common-mode signals differ from one sector to another. Henceforth, a sector-independent
algorithm is proposed in this thesis to derive these duty signals that substantially reduce the
computational burden.
The proposed techniques are validated through simulation in MATLAB/Simulink and ex-
periments on a hardware prototype at a power level of 4 kW. | en_US |
