Transient Analysis and Modelling of Sixphase Asynchronous Machine

Multiphase Induction machine offers numerous advantages compared to the conventional three phase induction machine. Among the different types, the six-phase induction machine is most common, due to the flexibility in converting a three phase to six phase. By splitting the phase belt of a three phase induction machine, a six-phase induction machine is realized. The areas of application of multiphase (>3) machines are enormous, mostly where reliability is paramount. This paper presents the analysis, modeling and simulation of a 5Hp, 50HZ, 4 pole, 24 slots asymmetrical six-phase induction machine for submarine application. With the help of MATLAB software, the models developed are simulated and results in the form of computer traces of the dynamic performance of the machine at start-up are presented and discussed. This investigation shows that the replacement of three phase with a six phase machine is both technically and economically advantageous.


Introduction
Three phase induction machines are asynchronous speed machines. They are operated as motors and generators, comparatively less expensive to equivalent synchronous or dc machines. It is considered as the workhorse of the industry. Induction machines has almost replaced the DC machine in the industry due to the simplicity of design, ruggedness, low-cost, low maintenance cost and direct connection to AC power source compared to the DC motors [1,2,3]. In fact 85% of the total power consumption in the industrial sector is from induction motors. Due to the above, new strategies, new methods of analysis and design are being sought that will improve the performance of this machines and increase efficiency. Investigation has shown that multiphase machines are possible and advantageous [4]. Among the groups of multiphase machines, the six-phase has received more attention due the simplicity in converting a three phase machine to six-phase machine [5]. This is achieved by splitting the phase belt of three phase machine, that is, two sets of three phase stator winding of the original three phase, with set I spanning 30 0 electrical from set II, having a common magnetic structure [6][7].
The use of a common magnetic structure shared by two sets of stator winding dated since 1930 [4,8], where in an attempts to increase the power capability of a large synchronous generator, the stator winding has to be doubled. From that time, research activities of the dual stator induction machine (DSIM) increased. These type of machines are normally constructed by "splitting" the stator winding into two identical windings [9][10][11][12]. Phase orders of multiples of 3, i.e. 6,9, etc, are very possible. Presently, there is evidence of current research on higher phase AC machines of phase order 5, 6, 9 even 15 [13][14][15]. Multiphase induction machine finds its applicability in the area of high degree of reliability demand as in more electric aircrafts, electric ship propulsion, electric vehicles, (EH) and hybrid electric vehicles (HEV).
Particularly for ship propulsion, there is an ongoing research in the application of six phase induction motor by this author, where a six phase motor is to replace two three phase motors.
Analysis of multiphase induction machine is carried out in [16,17]. Lipo in [8] presented a six-phase induction machine model in d-q transformation, the analysis here uses a split-phase configuration where the 60 0 phase belt was split into two portions each spanning 30 0 . The simulated results for three phase and six-phase were compared. B. Kundrotas et al [12] modelled a six phase induction motor in the dynamic mode. In [18], a six-phase induction motor was used to reduce the noise level in an electric traction system. Improved reliability is guaranteed in submarine with the application of two sets of star connected stator winding spatially shifted by 30 0 electrical with isolated neutral point as reported in [19]. A transition design based on the Markov chain was used to analyse the availability index of multiphase induction machine, [20,21]. The submarine is rapidly emerging in the last couple of years as the main potential application area for multi-phase motor drives. The idea of multiphase suggests a replacement of two electric machines with a single machine of DSIM. A lot of research is ongoing in multiphase induction machines. The concept of multiphase is still in the infant stage, though much research is ongoing. It is the aim of this paper to investigate the transient performance of a six-phase split wound induction machine for submarines.

Modelling of Six-Phase Induction Machine
Multiphase electric machine (MPEM) which is more advantageous and has improved performance compared to the three phase counterpart come as both synchronous and asynchronous machines. Figure 1, shows the V abcxyz voltages of a six-phase induction machine with their phase displacement. In order to model the six-phase induction machine, the dq transformation is applied, and the rotor reference frame is adopted [16].

Experiment
In order to get data for simulation, a 5.5 Hp 3Ø, 24 slot, 4 pole induction motor was reconfigured into a split phase (6-phase) motor with two set of 3-phase displaced 30 0 elect from each other. The new motor maintains all specifications of the old, except that, the motor is now a split winding motor (6-phase). The construction motor tested on No-load and On-load. Retardation test was also carried out. The experimental results and the computed results is shown in table 1.

Simulation Results
The simulation results in the form of computer traces are presented of the performance of a six-phase induction machine is presented in figs 5-8. The simulation for phase currents and qd currents, mechanical rotor speed and Electromagnetic Torque are simulated and presented using the data in table1.

Discusion of Results
Electromagnetic Torque in fig. 8, stabilizes at 0.7s. The mechanical rotor speed reaches synchronous speed at 0.6s, this agrees favorably with the theoretical concept. The phase currents also stabilize at 0.6s. Looking at the simulations, the redesigned motor can work as a three phase machine or/and as six phase. In submarines, the idea here is to use a six phase motor to replace two three phase motors, and money is saved redesigning the machine with higher reliability. The loss of a phase does not stop the motor from running. In submarines mostly Naval War ships, reliability is a criterion in designing a submarines.

Conclusion
The simulation result shows that, the split six phase is a dual stator induction machine with sets of three phase currents; I abc and I xyz . This is expected because DSIM is like paralleling two three phase induction motors. So, instead of using two three phase induction motors for propulsion (submarines), a single DSIM can replace the two and the cost is reduced and reliability increased which is the actual aim of this work.