Gradual Electronic Pole Changing Technique to Minimize the Circulating Currents During Pole/Mode Transition in Induction Motor Drive

This paper proposes a control strategy for the electronic pole changing (EPC) induction motor drive (IMD) for the transition between 3-ϕ,12-pole mode to 9-ϕ,4-pole mode. The proposed control strategy employs a gradual electronic pole changing (GEPC) technique which linearly demagnetizes the IMD winding from 3-ϕ,12-pole and magnetizes progressively to 9-ϕ,4-pole mode. The proposed GEPC technique is also compared with the instantaneous EPC technique for its performance. The proposed GEPC solution eliminates the issue of the circulating current, which exists in the instantaneous EPC technique and is responsible for negative torque generation during pole transition from 3-ϕ mode to 9-ϕ mode. Apart from eliminating the circulating current, the proposed GEPC strategy also sorts out various typical issues which exist in instantaneous EPC technique during pole/mode transition. The various issues in the instantaneous mode transition are (i) absence of interlocking between stator and rotor fields, (ii) oscillations in the flux between the 12-pole (3-ϕ) and 4-pole (9-ϕ), (iii) strengthening and weakening 4-pole formations in stator and rotor, (iv) distorted flux and pole formations, (v) dip in speed, and (vi) high oscillations/ripple content in torque and speed. Ansys simulation is done using Maxwell 2-D integrated with Twin-builder for EPC IMD for instantaneous and proposed GEPC techniques. Further, the paper presents the experimental results to validate the simulation studies.

Kelly [5] and Osama et al. [7], [8] have proposed an effective solution for EPC-IMD by injecting two different voltages (3-ϕ and 9-ϕ) simultaneously to the stator of IMD. The two operating voltages are applied in such a-way that the magnitude of upcoming and existing mode voltage is instantaneously increased and decreased, respectively with the step-change in the same proportion. However, the above said method still suffers from the drawbacks like high in-rush currents, flux distortion, loss of interlocking between stator-rotor fluxes, and negative torque disturbances. Kelly et al. [11] have given another interesting solution by simultaneous control of torque and flux during mode transition. In order to achieve this, the magnitude of torque in the 12-pole mode is decreased and simultaneously the magnitude of torque for next 4-pole mode is increased. While maintaining the fluxes of 3-ϕ and 9-ϕ kept at peak value during mode transition. Nevertheless, still the given solution suffers with same issues as discuss above. Above all, since both 3-ϕ and 9-ϕ rotational fluxes exist with peak value, so there is a high probability for saturation of the IM core. Umesh et al. [6] has given another elegant solution by segregating the IMD winding as three, 3-ϕ balanced equipotential groups and isolating them during pole changeover [6], [12]. In particular, the said method breaks the path of the circulating currents instantly during pole changeover with the help of a bi-directional back-to-back switch. By this solution, some of the adverse problems with respect to circulating current during pole changeover are minimized. But even this method doesn't give a complete solution. This can be attributed to instantaneous step change from one mode to another. Furthermore, the given solution doesn't take into account the smooth change in flux with respect to new pole formation. Thus, the given method is a bit rigid in its operation, as it isolates the equipotential windings in 9-ϕ operation during machine running conditions which may not be desirable. Also, the given method doesn't concentrate on the magnetizing and demagnetizing effects on stator and rotor when moving from one mode to another.
Hence, there is a requirement of a solution for the above discussed issues. This paper presents a solution for the same. The proposed solution presents a required control strategy for gradual pole changing during a mode transition from 12-pole to 4-pole mode in EPC-IMD. The gradual electronic pole change (GEPC) is achieved by progressively changing the fluxes from 3-ϕ to 9-ϕ mode. The progressive change in fluxes during mode transition is carried out by gradually phase shifting the voltage fed to IMD. The other benefits of the proposed GEPC technique during mode transition are: 1) It helps to have a progressive change in fluxes which minimizes distortion in the stator and rotor peripheries. 2) It avoids the oscillations of flux between the 12-pole and 4-pole; apart from that, it also eliminates the strengthening and weakening nature of 4-pole formations in the stator and rotor. 3) It maintains synchronism/interlocking between the poles of the stator and rotor; by this, the problem of pole distortions in IMD can be eliminated. 4) It also avoids abrupt raises in the current that is drawn by the motor during mode transition. Subsequently, this reduces the overall losses in IMD. 5) It avoids the zero or negative torque state, dip in speed, and also high ripple content in torque and speed. The paper is sectionized as follows; Section II briefs inverter schematic for EPC of IM. Section III presents the working mechanism of the inverter for the proposed GEPC technique of IMD. Section IV gives the details of the results obtained using the Ansys simulation. The results part also gives the experimental results and gives a detailed discussion of the experimental results validating simulation results. Finally, the last section gives the conclusions for the manuscript, followed by references.

II. WORKING OF INVERTER SCHEMATIC FOR ELECTRONIC
POLE CHANGING TECHNIQUE OF IMD The given system for the electronic pole changing of IMD (EPC-IMD) consists of an input dc source, 9-ϕ, 2-level inverter, and 9-ϕ, 4-pole induction motor (IM). The input terminals of the inverter are connected to the DC source 'V dc ' via electrolytic buffer capacitors 'C1' & 'C2' and reconfigurable IMD, as shown in Fig. 1. The reconfigurable IMD has 36 stator slots with windings segregated as 9-ϕ. The 9-ϕ of the IM are connected to nine output terminals of 9-legs/phases of the 2-level inverter, as shown in Fig. 1. Each leg of the inverter independently controls the pole voltages applied at the input terminals of the induction motor. The applied pole voltage can be for 3-ϕ or 9-ϕ operation of the EPC-IMD. Thus, the machine is operated in two modes; (i) 3-ϕ, 12-pole mode and (ii) 9-ϕ, 4-pole mode, using the same set of arrangements without any mechanical reconfiguration as shown in Fig. 1. The phase angle difference between the adjacent leg pole voltages of the inverter defines the operating mode. This can also be noted from Fig. 1 that the phase angle difference between the adjacent legs of inverter corresponds to 120°and 40°for 3-ϕ and 9-ϕ operations, respectively. In addition to the phase angle difference, the pole formation in stator periphery is also essential. As this IMD operates in two modes (3-ϕ and  [5], [6] 9-ϕ mode) the 36 slots in the stator segregated as 3 sets in which each set forms 4-poles resulting in 12-pole. Similarly for 9-ϕ operation, the windings are segregated as one set to form 4-pole as shown in Table I. Along with the details of the pole formation, the current directions in the stator winding for half stator periphery of IMD in both modes i.e., 3-ϕ, 12-pole mode and 9-ϕ, 4-pole mode is tabulated in Table I [5], [6].
The Table I also gives the required connections for the 18-slots with nine stator windings of IMD having two terminals (one is entering while the other is leaving). The entering terminals are connected to the inverter output, and the leaving terminal is shorted with the neutral point 'n', as shown in Fig. 1. Thus, the stator currents in the winding have two polarities. The positive polarity indicates current entering and negative polarity indicates current leaving, as shown in Fig. 1 and Table I. Pole formation using the inverter terminal voltage for the two modes of operation is given in Table I. It can be observed that stator slots with odd number 'S 1 , S 3 , S 5 …. S 17 ' are positive polarity with entering the current direction. Besides, the slots with even number 'S 2 , S 4 , S 6 …. S 18 ' are negative polarity with leaving current direction as shown in Fig. 1. As discussed, in 3-ϕ operation, the windings are segregated as 3-sets.
The winding 'a, b, c' (0°, 120°, 240°) is grouped as Set-1, 'a 1 , b 1 , c 1 ' (360°, 480°, 600°) is grouped as Set-2, and 'a 2 , b 2 , c 2 ' (720°, 840°, 960°) is grouped as Set-3. These sets form 12-poles in the stator periphery, as shown in Table I. Similarly, in 9-ϕ mode the windings are grouped as one set and are denoted as 'a, b, c, d, e, f, g, h, i' (0°, 40°, 80°, 120°, 160°, 200°, 240°, 280°, 320°) forms 4-poles in stator periphery as shown in Table I. In the given inverter schematic for EPC-IMD in Fig. 1, the IGBT/MOSFET switches are denoted as 'S pq ', where p ∈ {a, b, c} and q ∈ {12, 34, 56} six switches of 3-ϕ inverter. Here, 'a' denotes for Set-1, 'b' denotes for Set-2 and 'c' denotes for Set-3 as shown in Fig. 1. Thus, this inverter is able to operate in both 3-ϕ and 9-ϕ mode by exciting the windings with 120°and 40°phase sifted voltages respectively. Hence, the given EPC-IMD can independently operate in 3-ϕ mode and 9-ϕ mode without having any mechanical reconfiguration. However, requirements with respect to change of operating mode may also arise during the running condition. In other words, it may be required that IMD running in 3-ϕ, 12-pole mode at steady state need to be operated in 9-ϕ, 4-pole mode. Therefore, a solution is required for switching the operating mode from one mode to another. In other words, a switching strategy is required to switch the modes between 3-ϕ mode and 9-ϕ mode during the motor operation. Therefore, for pole change in EPC-IMD the inverter switching technique is briefed in next section.

III. PROPOSED METHODOLOGY FOR EPC-IMD
As discussed in the above section, the given EPC-IMD system can operate independently in two modes. A Kim-Sul-based SVPWM [20] is employed for the generation of inverter pulses from the reference waveforms to operate the IMD in 3-ϕ and 9-ϕ mode. However, a solution is also needed with respect to switching from one mode to another during running condition. This can be done by employing instantaneous switching from one mode to another. Thus, the inverter will be instantaneously switched from 3-ϕ to 9-ϕ operating mode instantaneously. However, the given method is quite harsh and doesn't take dynamics of the IM into account while changing from one mode to other. To analyze the adverse effects for mode change from 3-ϕ, 12-pole to 9-ϕ, 4-pole in EPC-IMD an instantaneous changing considered. This is carried by instantaneous change in the applied phase voltage from 3-ϕ (120°phase shift) to 9-ϕ (40°phase shift) mode using the inverter PWM control. It is realized by the instantaneous change in reference waveform from existing 3-ϕ to 9-ϕ of next mode using Kim-Sul based SVPWM [20] as shown in Fig. 2(a). The motor running in 3-ϕ, 12-pole mode in steady state is fed instantly with phase voltage of 9-ϕ at time t = 750 ms using the inverter PWM control as shown in Fig. 2 ). This sudden change in applied pole/phase voltage stabs the sudden change in the stator magnetizing current (flux distribution) of the motor. However, the sudden change in flux distribution 'doesn't happen, as it requires high neutralizing flux to neutralize existing mode flux with the simultaneous addition of next mode flux distribution. Since, the simultaneous demagnetization and magnetizing of flux in the stator and rotor is not happening, it results in various adverse effects in the EPC IMD as listed below.
1) Distortion of flux in the stator and rotor periphery.
2) Loss of interlocking between stator flux and rotor flux.
3) Distorted pole formation in stator and rotor peripheries. 4) Abrupt raise in the current drawn by the motor. 5) Enormous raise in the overall losses of the machine. 6) Subsequently, leading to zero or negative torque 7) Following this, the motor loses its control leading to speed drop till it gains stator-rotor flux interlocking. As discussed, in the methods given in the literature one or more above-mentioned adverse effects still exist. Based on the ongoing discussion, proper care to be taken in control strategy, which alleviates the above issues during mode change in EPC-IMD. Therefore, there is a requirement of smooth transition during the new pole formation. The gradual pole/mode transfer is carried out by linearly decreasing the voltage of existing mode and simultaneously injecting the increasing voltage of next mode during transition. This is done using the PWM control of the inverter which defines the switching strategy to control the required phase voltage at the output of the inverter. Hence, the applied switching strategy linearly decreases the phase voltage of the existing mode while simultaneously injecting the increasing phase voltage corresponding to the next mode during transition. Precisely, it is executed by adding linearly decreasing reference waveform of existing mode with subsequently increasing reference waveform of next mode using Kim-Sul based SVPWM [20] as shown in Fig. 3(a). In brief, the next mode voltage content is gradually injected in increasing nature to linearly decreasing existing mode reference waveform. During the mode transition, the balanced 3-ϕ and 9-ϕ voltage waveforms are injected into the stator simultaneously with one (3-ϕ) decreasing and while the other (9-ϕ) increasing in nature. Hence, the overall voltage waveforms may appear to be distorted. However, they are balanced voltage waveforms, since the voltage magnitude and phase difference between the different phases of 3-ϕ and 9-ϕ are maintained to required value using Kim-Sul based SVPWM. Consequently, this PWM technique helps in injection of next mode voltage content in increasing nature and simultaneously decreasing the present mode voltage content of inverter that feed IMD during the transition. This results in progressive shift in output voltage phase angle of inverter to next mode. Precisely, for 3-ϕ to 9-ϕ transition the '120°' phase shifted reference wave gradually changes to '40°' phase shift and can that be seen in phases 'a' and 'b' voltage waveforms given in Fig. 3(b) and (c). This progressive change helps in reduction of high circulating currents during pole change. However, the decrease rate of existing mode reference waveform and simultaneous increase of next mode reference waveform needs to be considered by taking into account the system dynamics. The gradual change time can be defined as the time period that is taken for linear decrease of existing mode reference waveform and simultaneous increase of the upcoming mode reference waveform as shown in Fig. 3(a). The optimum value of time for gradual mode/pole change is maximum electrical time constant of IMD. The electrical time constant 'τ c ' of the IMD is given by the sum of stator and rotor electrical time constants [2]. Therefore, the ramp-up and ramp-down time for GEPC is given in (1).
where, 'τ s ' and 'τ r ' are the time constants of stator and rotor, respectively. The time constants 'τ s ' and 'τ r ' are given in (2) where, L s is the stator inductance given by L s = L m + L ls , and L r is the rotor inductance given by L r = L m + L lr . Here, L m is the mutual inductance between stator and rotor. L ls , L lr are the winding self-inductances of stator and rotor, respectively. R s , R r are the winding resistances of the stator and rotor, respectively. The gradual pole changing time for the given IMD can be calculated from (1). The gradual time for the designed IMD is 120 ms (approx..,) and the corresponding reference waveform can be observed in Fig. 3(a) which shows the reference waveforms employed to generate PWM pulses of inverter for gradual EPC control strategy. Waveforms of the applied phase voltage during transition for gradual pole transferring to 9-ϕ, 4-pole mode between the time t = 0.75 s to t = 0.87 s as in Fig. 3(b) and (c) given with reference to phase 'a' and 'b', respectively. Therefore, this simultaneous linear decrease and increase of phase voltage at both modes indirectly supports smooth flux distribution for new pole formation. Specifically, the simultaneous linear decrease and increase of phase voltage controls the motor current which further controls the flux as shown in Fig. 4. Thus, simultaneous injection of voltages of two modes leads to the linear decrease of currents of one mode and increase the currents of next mode in the stator and rotor periphery of IM. From Fig. 4 [4], the 3-ϕ stator current 'i 3s ' followed by flux 'λ 3s ' linearly decreases, simultaneously the 9-ϕ stator current 'i 9s ' followed by flux 'λ 9s ' linearly increases. Therefore, this gradual decrease and progressive increase of flux helps the stator to magnetize and demagnetize simultaneously. Hence, by this simultaneous magnetization and demagnetization of fluxes in stator helps in smooth pole transition in IMD. This simultaneous change in currents progressively gives the smooth pole formation both in the stator and rotor periphery as described in the next simulation and experimental result section.

IV. SIMULATION AND EXPERIMENTAL RESULTS
In this section, a comparative and detailed analysis by using simulation and experimental results is given for both the instantaneous EPC and proposed GEPC technique. To demonstrate this, a 7.5 HP 3-ϕ, 12-pole IMD with stator/rotor slot ratio 36/46, is designed in Ansys RM Xprt [9], [18], [21], [22], [23], [24]. The design parameters of the inverter, IMD, including the materials considered for its design, in order to carry out simulation analysis of EPC technique is given in Table II. The designed machine is further analyzed for stator current, losses, flux distribution, pole distribution, torque and speed characteristics for both instantaneous EPC and GEPC in Maxwell 2D  From the waveforms given in Fig. 5(a) and 6(a), it can be seen that there is a quick rise in peak value of stator current to 64 A (approximately) for the instantaneous EPC. However, there is no such quick rise in current for the proposed GEPC technique, as can be seen from Fig. 5(b) and 6(b). From Fig. 5(b) the stator current is progressively increased to 44 A (approx.). Further, the lower order harmonic content and also DC offset component in  the current waveform is improved for the proposed gradual EPC technique. The DC offset content in proposed GEPC technique of IMD is reduced nearly by 7-10 time (approx.,) in comparison with instantaneous pole changing technique which can be evidenced from Fig. 6(a) and (b). Therefore, in the proposed GPEC technique the DC offset of current is reduced and harmonic spectrum during the pole changing is improved. In addition, the current raises gradually and its peak value is reduced by 33% in comparison with instantaneous EPC this can be supported by Fig. 6(a) and (b).
Further, the rapid rise in the peak value of stator current in case of instantaneous EPC also results in quick rise in the flux from 0.78 Wb to 1.0 Wb (approx.) during pole transition as can be seen in Fig. 5(c) and 7(a). Also, the flux distortions in the form of non-sinusoidal nature, oscillating peak and DC offset in the flux waveform for instantaneous EPC can be seen in Fig. 7(a). However, in case of proposed GEPC, there is no abrupt rise in flux and its peaks value gradually decreases without any DC offset during pole transition as can be seen in Fig. 7(b). Thus, there is a minimal distortion of the flux in the proposed GEPC. This is further supported by Figs 8 to 10. Fig. 8 shows the d-q plot of the flux 'λ d vs λ q ' obtained from simulation during the pole transition period recorded from time t = 0.72s to t = 1.0s.
From the d-q plot given Fig 8(a), it is clear that the flux in the periphery of stator is highly distorted, passes through origin and also have high peak value which may results in IM saturation (near to 1.21 Wb). However, in the case of proposed GEPC, there is progressive or gradual flux change which never crosses origin and peak value (0.78 Wb) as can be seen from Fig. 8(b).
Figs. 9 and 10 show the flux distribution lines using finite element analysis (FEA) [23], [24], [25], [26], [27], [28] in Maxwell 2D environment for instantaneous EPC and proposed GEPC. The results given in Figs. 9 and 10 are divide into two parts: (I) for instantaneous EPC, and (II) for proposed GEPC. In the case of instantaneous EPC and proposed GEPC, the stator is required to change its pole formation from 12-pole to 4-pole. This pole change for instantaneous EPC and proposed GEPC is done by using the inverter which changes its output from 3-ϕ to 9-ϕ instantly and gradually respectively. The flux distribution lines obtained from FEA in Figs 9 and 10 give details of the pole formation in rotor and stator. It can be easily verified that for instantaneous EPC, the stator forms the 4-pole which is typically controlled by the inverter output. However, same is not true with respect to rotor pole formation. The formation of the poles in the rotor oscillates between 12 and 4 as can be seen in subplots (b), (c) of Fig. 9(I) and subplots (a), (b) of Fig. 10(I). This oscillation between the pole ends after time t = 870 ms, as after this time the rotor has only 4-pole flux lines. Apart from this, an intermediate state also exists where the rotor has both 12-pole and 4-pole formation as can be seen from subplots (b), (d), (e) of Fig. 9(I) and also subplot (a) of Fig. 10(I). Further, from the subplots (d), (e) in Fig. 9(I) and (c), (d) in Fig. 10(I), the dense and sparse 4-pole flux lines can be observed, which actually represent the strengthening and weakening of the 4-pole formation in the rotor. Thus, the oscillations between 12-pole and 4-pole apart from the strengthening and weakening of 4-pole flux lines, further results in the distortions and fluctuations in peak value of the flux. The same can depicted in the results given in Fig. 7(a). However, in case of proposed GEPC technique, the stator is fed with the 9-ϕ and 3-ϕ voltages via inverter which are progressively increased and decreased simultaneously. Thus, the stator is fed via inverter with both 9-ϕ and 3-ϕ voltages simultaneously with increasing and decreasing amplitude respectively between 750 ms and 870 ms. Also, at time t = 810 ms the stator is fed with 50% of both 9-ϕ and 3-ϕ voltage simultaneously via inverter. It can be seen from flux lines given in subplots (a) to (e) in Figs. 9(II) and 10(II), that the flux lines in the rotor slowly adjust towards the 4-pole formation. The rotor has both 12-pole and 4-pole flux lines after t = 850 ms and till t = 920 ms as can be seen in subplots (a) to (d) of Fig. 10(II). Also, it can be easily verified from Figs. 9(II) and 10(II) (between time t = 750 ms till t = 920 ms), the flux lines alignment for the 4-pole formation is progressively strengthening, while the 12-pole formation is simultaneously weakening in the same proportion. Thus, in the proposed GEPC the oscillations between 12-pole and 4-pole, strengthening and weakening of 4-pole flux lines further never happened. Thus, in the proposed GEPC, the 4-pole are formed in the rotor gradually. Now to further analyze the interlocking of the poles between stator and rotor, same FEA results are employed to represent   of interlocking may result in oscillations and distortion in motor torque as observed in Fig. 13(a). The oscillations and distortions in torque can be attributed to different pole combinations rotating with different speeds in the stator and rotor during period of mode transition. Further, due to sudden transition from 3-ϕ to 9-ϕ, a negative torque can be observed in Fig. 13(a) which is mainly due to the flow of circulating currents flowing within the stator windings [8]. Also, a dip in the rotor speed can be observed in Fig. 13(a), which can be attributed to negative torque and its distortions. Whereas in the case of proposed GEPC, due to progressive development of 4-poles both in stator and rotor, there is no distortion in both stator and rotor poles. Also, there is a strong interlocking between stator and rotor poles, till the motor speed ramps (accelerating region) up by moving into the field weakening region. In the field weakening/ accelerating region, behavior of the motor in instantaneous EPC, and proposed GEPC is nearly same. The same can be observed from the subplots shown in Fig. 5(c) and (d) after time t = 920 ms. Further, there is no negative torque and its distortion. The motor torque is maintained as can be seen in Fig. 13(b). Thus, there is no dip in speed as can be observed in Fig. 13(b). Also, from the subplots (d)-(f) of Fig. 12, it can be observed that both the stator and rotor tend towards the 4-pole formation. Another important thing to be noted that the steady state 4-pole formation in the stator and rotor is achieved earlier for the proposed GEPC compared to instantaneous EPC. The same can be observed from Figs. 5 and 14. It also can be observed from Fig. 14 showing the total losses in the proposed GEPC during pole transition is much lesser than compared to instantaneous EPC. This can be attributed to forced pole formation in case of instantaneous EPC. Further, the maximum magnitude of the losses never exceeds its starting, whereas same is not true for the instantaneous EPC. Apart from this due to higher settling time of instantaneous EPC further increases its over-all losses when compared with proposed GEPC.  To justify and support the simulation studies, experimental waveforms are recorded by a 415 V, 36 slots, 3.0 HP IM. The Table III gives the parameters of IM used in experimental setup. The experimental prototype of the EPC IM by 9-ϕ inverter is shown in Fig. 15. The experimental results are presented in Figs. 16 and 17. The subplots (a) and (b) of Fig. 16 show the experimental waveforms of voltage for instantaneous EPC and proposed GEPC technique (Ref: phase-'a') respectively. From the subplots (a) of Fig. 16, it can be observed that a transition from 3-ϕ to 9-ϕ happened instantaneously. While, in case of GEPC, the transition from 3-ϕ to 9-ϕ happened gradually as seen in subplot (b) of Fig. 16. The subplots (c) and (d) of Fig. 16 show the experimental waveforms of stator current along with the zoomed view for instantaneous EPC and proposed GEPC technique respectively. From the subplot (c) of Fig. 16 it can be    observed that, by using the instantaneous EPC the stator current waveform suffers with oscillations and DC offset. Whereas, by using proposed GEPC technique the stator current waveform is smooth and free from DC offset. The presented experimental waveforms exactly match with simulation waveforms shown in Figs. 2, 3 and 6. In addition to the motor input voltage and stator current waveforms the experimental waveforms of speed and torque are also captured. The subplots (a) and (b) of Fig. 17 shows experimental waveforms of speed-torque along with their zoomed view for instantaneous EPC and proposed GEPC technique respectively. In Fig. 17(a) and (b) for torque waveform plot 1V/div is 1Nm/div and for speed waveform 1V/div is 1rpm/div. The absence of negative torque and reduced torque distortions can be clearly observed in proposed GEPC compared instantaneous EPC. Also, the presented experimental waveforms of speed-torque exactly match with simulation waveforms shown in Fig. 13. This further justifies the proposed GEPC for IMD.

V. CONCLUSION
A new proposed GEPC switching technique is proposed for induction motor drive (IMD) for a smooth transition from 3-ϕ mode to 9-ϕ mode. The proposed GEPC technique linearly demagnetizes and progressively magnetizes from 3-ϕ to 9-ϕ mode. By gradual demagnetizing and magnetizing of windings, the circulating currents during the mode transition are minimized. During the mode transition period the proposed GEPC maintains interlocking between stator and rotor fields, eliminates the oscillations in the flux between 12-pole to 4-pole, avoids the strengthening and weakening nature of stator flux. The proposed GPEC has a benefit of progressive change in pole during mode transition. This eliminates the negative torque and reduces torque-speed ripple during mode transition. Thus, by the proposed GEPC technique the performance of IMD during EPC is enhanced compared to existing techniques. The proposed concept is further justified with simulation and experimental results. Therefore, the proposed GEPC-IMD is effective for EV application by smooth pole changeover and also by adopting flexible wide speed-torque characteristics.