Title:

Analysis of Nphase induction machines using a generalised decoupled vector space theory

A generalised analysis method for Nphase induction machines is presented. The Nphase
system is analysed using 2N degrees of freedom. A new conceptual view of a
multiphase system using an Ndimensional vector space has been developed. It is
shown that such an Ndimensional vector space can be decoupled into 'h(N+PJ
independent two dimensional vector planes, where if N is odd, p=l, otherwise P=O.
Each vector plane is shown to be the combination oftwo positive and negative sequence
components ofthe same phase order. Each vector space is shown to be independent and
can be controlled separately by an Nphase inverter. Each inverter state is shown to
have a location in every vector space. A vector control strategy has been realised for
developing independent vectors in decoupled vector spaces, resulting in the desired
voltage waveforms at the inverter outputs. A fivephase induction motor is designed,
and constructed as an integral part of fivephase drive system. A fivephase decoupled
vector space controller is implemented in a fixedpoint digital signal processor and the
practical fivephase inverter based drive system validates the proposed decoupled vector
space theory.
The analysis of multiphase machines during imbalanced. electrical operation is
considered using decoupled vector space theory. It is shown that each independent
vector in the decoupled vector spaces produces unique flux distributions in an Nphase
AC machine, but only one vector space develops fundamental rotor linked flux and thus
fundamental torque. The interaction of decoupled vector spaces with winding
harmonics is developed in order to analyse the effects of unbalanced currents in a
i'
practical induction motor. It is shown that torque is produced only by the primary
vector space currents in a sinusoidally wound machine. The additional freedom
available from auxiliary vector spaces can be used to compensate for imbalance caused
by faulty machine phases. It is shown that there exists 2N3 degrees of freedom for an
Nphase system of fluxes of an Nphase machine with a floating neutral. These degrees
of freedom can be used to compensate for the phase imbalance created by opencircuited
inverter legs. The machine can still be fluxed to produce rotational power. At
the prefault to postfault transition, the current vectors of the operational phases are
reoriented to produce a rotating fundamental flux. A fivephase induction motor drive
system demonstrates the performance for the loss of one and two phases. Harmonic
winding function generated flux interaction is analysed and interpreted in terms of a
fivephase motor with single and two phase loss.
