ANALYSIS OF SIX-PHASE SYNCHRONOUS MOTOR

Abstract

The area of multiphase (more than three phase) drive is well explored and remained in research focus during last two decades. This is because of the presence of several potential advantages when compared to its three phase counterpart; making it suitable for many applications like pumps, induced draft fans, ship propulsion, rolling mills, cement mills, paper and textile mills etc. Applications power range may vary from lower to higher where, both synchronous and induction motor are used in voltage source inverter fed drive. Such drives are still limited to the lower end of higher power application range. But, for the applications where power may vary in the range of multi MW, synchronous motor is employed, fed from current source inverter or cycloconverter. This is because of the limitation offered by lower power rating of power electronics switches and therefore, motivated the researchers towards multiphase system. In the new system of multiphase, power sharing by power electronics switches is substantially reduced. Hence, making it suitable for higher power applications. Usually, the winding end terminals of ac motor are provided at terminal box, which can be connected either in star or delta. These existing motor windings can be reconfigured to a six phase winding structure, embedded in machine stator. The most usual winding configuration is the two sets of three phase winding which are physically displaced by 300 (asymmetrical six phase winding) such that each winding set can be fed from independent three phase source. The major advantage of asymmetrical winding configuration has been noted in the reduction in torque pulsation, but with increased stator current. This is due to the substantial reduction of lower order time harmonics (such as 5th, 7th, 17th, 19th…) as well as space harmonics (no harmonics less than 11th order). Therefore, a smooth motor operation is obtained with less mechanical vibration and noise in rotor assembly. However, heating due to increased stator current can be conveniently handled with a suitable cooling arrangement in stationary part of machine. Motor having the above six phase winding configuration offers yet more attractive features. Such drive system is very suitable for the applications where reliability is of prime importance. This is because it will start and continue to operate even during the supply outage to some phases without significant performance degradation. Infact, effect of supply outage to few input phases will not be noticeable for motor having larger number of phases. Furthermore, a substantial reduction in phase current without increase in its voltage level was also noted at a particular output power. In this respect, power handling capability of six phase motor becomes ideally double, when compared to its three phase counterpart. But due to losses associated with actual machine operation it increases to 1.73 times, approximately. Thus increasing its power to weight ration. Hence, not only making it suitable of higher power applications but also in some other applications where space availability is extremely important (like aircraft). The well developed general theory for the conventional three phase machine may be effectively extended for the modeling and analysis of six phase motor under balanced operating condition. A limited literature can be found in this regard, dealing with six phase synchronous motor. But for the motor operation under unbalanced condition (created due to fault at input terminals), the developed method of symmetrical component is usually adopted, during steadystate with sinusoidal excitation. This method is not suitable for evaluation of dynamic response, because of the non-existence of interaction between lost phase (due to fault condition) and remainder of machine winding, altering its dynamic behavior drastically. Another alternative approach is to carry out the analysis for unbalanced condition using Park's dq0 variables, but has not been reported for six phase synchronous motor. Few authors have also developed the concept of space vector decomposition, applicable for unbalanced condition and has been reported for six phase induction motor only. From a comprehensive literature survey, it was revealed that a very limited research activity has been carried out for six phase synchronous motor, as this field is still in its primitive stage. Therefore, the present work is dedicated for its extensive analysis of six phase synchronous motor in many aspects. Motivation for the selection of synchronous motor in this work is due to the existence of some important advantages when compared with other ac motors, particularly induction motor. Perhaps, the major advantage of a wound rotor (field excited) synchronous motor is its ease to control the operating power factor by just controlling its field excitation. Motor can be operated easily not only at lagging power factor, but also at unity as well as leading power factor, if required. This feature is not available with induction motor, until some complex scheme is employed. Secondly, speed of the synchronous motor is remain constant (synchronous speed) irrespective to the change in operating conditions. This is not the case with induction motor operation and requires some control scheme to keep the speed constant (slightly less than synchronous speed). Furthermore, owing to the higher efficiency, use of synchronous motor is more economical. These factors together with its possibility to operate in leading power factor make it suitable to be used in load commutated inverter fed drive without employing the complex and expensive commutation circuitry. Hence, suited in higher power applications like electric ship propulsion, cement mills, rolling mills etc. In the present work, an extensive exploration and analysis of six-phase synchronous motor has been reported in following aspects: 1: Initially, mathematical modeling together with its development of equivalent circuit has been carried out, wherein the effect of mutual coupling between two three-phase winding sets abc and xyz are considered, employing dq0 approach. A detailed dynamic analysis of motor has been then carried out under sudden change in load torque, change in input voltage magnitude and field excitation. Motor operation was found to be stable with less swing in stator current and rotor speed. 2: Usually, any ac motor operates at steady-state at a particular output power. Therefore, steady-state analysis is extensively important in order to understand its operational behavior. It has been carried out in details for six-phase synchronous motor during its normal operation (input supply to both winding sets) as well as the operation with input supply to only one winding set. Analysis includes the development of mathematical model as well as phasor diagrams from which performance may be directly analyzed. 3: Fault analysis of motor drive system is an important step to develop a suitable protective scheme. The most frequent faults which are encountered at input terminals of machine are open-circuit and short-circuit. An exclusive analytical treatment together with its experimental validation has been carried out for open-circuit fault to explore the motor’s redundancy characteristic followed by highlighting the machine operation both as motor as well as generator (motor-generator). It has been noted that open-circuit fault at few input terminals can be sustained without much performance degradation. Whereas, in the case of short-circuit fault, an abrupt increase in current was noted, irrespective to its nature (asymmetrical or symmetrical). This requires it to be cleared/isolated within prescribed time limit. The dynamic behavior of motor under such condition has been carried out by developing a suitable computer program in the work. 4: Stability of a machine is an important factor which is dependent directly or indirectly on many design factors and its operating conditions during steady-state. The present work reports a detailed small-signal stability analysis of a six-phase synchronous motor by developing a linearized mathematical model wherein the mutual leakage between both the stator winding sets abc and xyz is considered, using dqo approach. The developed linearized model was used to evaluate the system eigenvalues for its stability analysis under small disturbance/excursion during steady-state. An association between eigenvalue to the machine parameters has been established. It has been carried out by calculating eigenvalues by varying motor parameters within a particular range. Also, based on the variation of dominant eigenvalue, few stabilization technique is suggested, which can be considered during design stage. Further, a simple but effective stabilization technique (based on state feedback technique) has also been developed to ensure a stable operation at all operating region (both stable and unstable region). Finally, formulation of transfer function between input and output variables has also been carried out, where different stability plots (root locus, nyquist plot, bode plot) can be easily drawn. 5: Use of synchronous motor is suited in load commutated inverter (LCI) fed drive, used in higher power applications. The LCI drive has been investigated together with the development of a closed loop scheme for motor operation in self-controlled mode. The developed scheme ensures the motor operation at leading power factor at all load conditions. System dynamic simulation has been carried out for a step change in load torque. 6: Key analytical results have been experimentally investigated, as per the available facility in machine laboratory. Research papers published/accepted in refereed journals 1: A. Iqbal, G.K. Singh and V. Pant, “Steady-state modeling and analysis of six-phase synchronous motor”, System Science & Control Engineering, 2 (1), 2014. 2: A. Iqbal, G.K. Singh and V. Pant, “Stability analysis of asymmetrical six-phase synchronous motor”, Turkish Journal of Electrical Engineering & Computer Sciences, (accepted, SCI journal). 3: A. Iqbal, G.K. Singh and V. Pant, “Some observation on no-load losses of asymmetrical six-phase synchronous machine”, Turkish Journal of Electrical Engineering & Computer Sciences, (accepted, SCI journal). In addition to the above, following two more papers are submitted to journals which are under review process: 4: Analysis of asymmetrical six-phase synchronous motor under fault condition. 5: Linearized modeling of asymmetrical six-phase synchronous motor for stable operation.