2009 IEEE Symposium on Industrial Electronics and Applications (ISIEA 2009), October 4-6, 2009, Kuala Lumpur, Malaysia
Decoupled Random modulation technique for an open-end winding induction motor based 3-level Inverter David Solomon George Department of Electronics & Communication College of Engineering Trivandrum, Kerala, India Email:
[email protected]
M. R. Baiju Department of Electronics & Communication College of Engineering Trivandrum, Kerala, India Email:
[email protected]
Abstract-This paper presents a random pulse width modulation (RPWM) technique for 3-level inverter based on open-end winding induction motor. A 3-level inverter can be realized by feeding an induction motor in open-end winding configuration with two 2-level inverters from both sides. A decoupled random PWM strategy for an open-end winding induction motor drive is proposed in this paper, in which, the switching signals for the individual inverters are derived separately. In the proposed scheme, the instantaneous switching states for the inverters are independently generated by comparing the decoupled reference voltage of the individual inverters with a random carrier wave. The proposed scheme is implemented for 2hp induction motor drive and experimental results are presented. Experimental results showing the comparison of the performance of the proposed scheme with that of the sine-triangle PWM is also presented.
In the present paper, a carrier based random PWM technique with decoupled strategy for a 3-level open-end winding induction motor is presented. The proposed scheme is implemented on a 2hp open-end winding induction motor drive and experimental results are presented.
I.
INTRODUCTION
The multilevel inverters have the advantages of low output voltage distortion, reduced stress on power switches and lower common mode voltage [1]. As the output voltage is synthesized from multiple sources of DC supply, the output spectrum of the multilevel inverters has lower harmonics and hence multilevel inverters can be switched at a lower frequency compared to the 2-level Inverter. Multilevel inverters can be realized in different topologies like Neutral Point Clamped, Flying capacitor and cascaded inverters with different dc sources [1]. The open-end winding induction motor based multilevel inverter structure is realized by opening the neutral point of the conventional induction motor. The motor is fed by inverters from both the ends [2-7]]. As in the case of other multilevel inverter topologies, Sine Triangle PWM and Space Vector PWM have been implemented for the open-end winding induction motor based multilevel inverters [2-7]. Random Pulse Width Modulation (RPWM) techniques have been used to achieve spread spectrum characteristics in the output voltage spectrum of voltage source inverters [10-14]. RPWM based multi-level inverters have less acoustic noise and reduced electromagnetic interference [10, 12].
978-1-4244-4683-4/09/$25.00 ©2009 IEEE
II. OPEN-END WINDING INDUCTION MOTOR BASED 3LEVEL INVERTER Fig. 1 shows a 3-level inverter structure realized by feeding an open-end winding induction motor with two 2level inverters from both the ends [3-8]. Each of the inverter can separately assume any of the eight switching states from 000 to 111. The switching states for the individual 2-level inverters are shown in Fig. 2. The switching vectors of 2-level inverter when applied from both the ends of an open-end winding induction motor, produces space vector locations equivalent to that of a 3-level inverter [4-5]. The space vector locations of the 3-level inverter are shown in Fig.3. Fig. 4 shows the combinations of voltage vectors of Inverter - I and Inverter - II to realize these vectors.
1022
Fig. 1. Open-end winding motor based 3-level inverter
2009 IEEE Symposium on Industrial Electronics and Applications (ISIEA 2009), October 4-6, 2009, Kuala Lumpur, Malaysia
a 3-level inverter is generated by comparing two level shifted triangular carriers with a sinusoidal modulating signal as shown in Fig. 5 [15]. Fig. 6 (a) - 6 (b) shows the PWM signals generated by comparing the reference with the positively shifted and with the negatively shifted triangular carrier signals. The resultant is a 3-level PWM signal. In an open-end winding induction motor based 3-level inverter, 3-level inversion is achieved by feeding the motor with two 2-level inverters from both the ends. The basic principle behind decoupled strategy is that the PWM for the individual 2-level inverters are derived separately [9]. The PWM signals for the individual inverters are so derived that the resultant PWM corresponds to that of a 3-level inverter. The resultant switching vector for the open-end winding configuration is obtained by subtracting the switching vector applied to inverter - II from the switching vector applied to inverter - I. The PWM generated for inverter - I correspond to the PWM generated by the positively shifted triangular carrier in Fig.6 (a). The PWM generated for Inverter - II is the inverted version of PWM generated by the lower triangular carrier shown in Fig.6 (b).
Fig. 2. Switching vectors of inverter - I and inverter - II
A. Generation of the Reference Signals for the proposed scheme The reference signals for the individual 2-level inverters are obtained from the sinusoidal reference signal corresponding to the 3-level inverter. The reference signal for the 3-level inverter is split into two signals. Both the signals are having amplitudes half the amplitude as that of the reference signal for the 3level inverter and are opposite in phase. These signals are used as the references for the individual inverters. The signal which is in same phase as that of the 3-level reference is used as the reference for inverter - I and the signal which is in opposite phase is used as the reference for inverter - II. The reference signal used for the generation PWM for inverter - II when subtracted from the reference signal used for the generation of PWM for inverter - I gives the reference for the 3-level inverter.
Fig. 3. Space vector locations of 3-level inverter
B. Generation of PWM for the proposed scheme Fig. 4. Space vector locations of the open-end winding induction motor based 3-level inverter and the combinations of voltage vectors of 2-level inverters
The resultant vector of the 3-level inverter generated for any combination of 2-level vectors can be obtained by subtracting the vector applied to inverter - II from the vector applied by inverter - I. It may be noted that any vector for the 3-level inverter can be realized by properly switching the individual 2-level inverters. III. THE PROPOSED METHOD In the Sine Triangle PWM method, the PWM signal for
The PWM for the individual inverters for the decoupled scheme is generated by comparing the individual references with positively shifted triangular carrier signals. Fig. 7(a) and Fig. 7(b) show the PWM generation for A phase for inverter - I and inverter - II respectively. For the generation of inverted PWM for inverter - II, both the reference and the carrier signals are chosen 180º out of phase with the reference and carrier signal for inverter - I. From Fig. 6 and Fig. 7, it is evident that the PWM pattern generated in Fig. 6 (a) and that generated for inverter - I in Fig. 7 (a) are the same. The PWM pattern generated for inverter - II is the inverted one of the pattern in Fig. 6 (b).
1023
2009 IEEE Symposium on Industrial Electronics and Applications (ISIEA 2009), October 4-6, 2009, Kuala Lumpur, Malaysia
Fig. 5. Reference signal and two level shifted carriers for the generation of PWM for A phase of a 3-level inverter
inverter - II switches for the remaining 180º. Thus the proposed scheme ensures equal switching of the individual inverters. In the proposed decoupled Random PWM, instead of a triangular signal, a random signal is used as the carrier. The random carrier signal is generated from uniformly distributed random numbers. This random carrier, when compared with the 3-phase sinusoidal reference, generates Random PWM, which is having spread spectrum characteristics. Fig. 8(a) - 8(b) shows the PWM generation for the individual inverters for the proposed decoupled Random PWM scheme. The switching of the inverters take place depending on the amplitudes of the reference and the random carrier. The maximum possible switching frequency is the frequency of generation of random number. Thus the maximum switching frequency can be limited. C. Switching vectors generated in the proposed scheme
a
Fig. 9(a) - 9(c) show the instantaneous switching
b Fig. 6 PWM generation for A phase of a 3-level inverter (a) by comparing the reference and the positively shifted carrier signal (b) by comparing the reference and the negatively shifted carrier signal
Fig. 7 (a). Reference signal, level shifted carrier and the PWM for Aphase of inverter - I for the decoupled strategy
Fig.8 (a). The reference signal, level shifted random carrier and the PWM for inverter - I for the proposed decoupled Random PWM scheme
Fig. 7 (b). Reference signal, level shifted carrier and the PWM for Aphase of inverter - II for the decoupled strategy
It may be noted that when the reference is positive, the PWM signal switches between 0 and 1. When the reference is negative, there is no intersection between the reference and the carrier and the PWM remains at 0 level. So for any phase, the inverter - I switches for 180º and
1024
Fig. 8 (b). The reference signal, level shifted random carrier and the PWM for inverter - II for the proposed decoupled Random PWM scheme
2009 IEEE Symposium on Industrial Electronics and Applications (ISIEA 2009), October 4-6, 2009, Kuala Lumpur, Malaysia
vectors generated by the proposed scheme for the individual inverters and the resultant vector for the 3level inverter. Only a few switching vectors are shown for brevity. It may be noted that the individual inverters instantaneously generate vectors the resultant of which corresponds to the switching vector of a 3-level inverter. For example, when the inverter - I is at the switching state of 5 and the inverter - II is at the state of 3, the resultant vector generated is 16 which corresponds to the 2-level vector combination 53’ as shown in Fig. 4. Table 1 shows a few combinations of the instantaneous switching vectors of the 2-level inverters and the resultant vector of the 3-level inverter. Thus in the proposed scheme the individual inverters are properly switched and the switching vectors for the 3-level inverter are generated.
TABLE I INSTANTANEOUS SWITCHING VECTORS OF INDIVIDUAL INVERTERS, THE RESULTANT 3-LEVEL SWITCHING VECTORS AND THEIR SPACE VECTOR LOCATIONS Vector Resultant vector of combinations of 2the 3-level inverter level inverters [Refer Fig. 3] [Refer Fig. 4]
Switching vector of Inverter - I
Switching vector of Inverter - II
5
0
5
5
3
16
53’
0
3
6
83’
6
0
6
68’
6
3
17
63’
1
3
18
13’
58’
IV. EXPERIMENTAL RESULTS The proposed scheme is simulated in Matlab / Simulink and implemented for a 2 hp induction motor drive. The control signals are generated using dSPACE DS1104 platform and the experimental results are presented. The inverter gating signals for A-phase for a modulation index of 0.8 are shown in Fig.10. Upper trace shows the gating signal for inverter - I, and middle trace that of inverter - II. Equal switching of both the inverters may be noted. The phase voltage and phase current for the proposed scheme for a modulation index of 0.8 is shown in Fig. 11.
a
The spectrum of phase voltage for the proposed scheme for a modulation index of 0.8 is shown in Fig. 12 (a). The magnitude of all the spectral components are at least 40 db below the fundamental component. Fig. 12 (b) shows spectrum of the phase voltage of the Sine triangle PWM with a fixed switching frequency. The 5 KHz component is almost 15 db and the 10 KHz component is about 20 db less than the fundamental. In comparison with the Sine triangle PWM scheme, the proposed method has much lower amplitudes for its harmonic components. Thus the experimental results confirm the spreading of the spectrum achieved through random PWM.
b
Fig. 9. Instantaneous switching vectors generated in the proposed c scheme; (a): switching vector of Inverter - I; (b): switching vector of Inverter - II; (c): resultant switching vector for the 3-level inverter
Fig. 13 shows the spectra of the phase voltage for the proposed scheme and spectra for the sine triangle PWM for a modulation index of 0.4. For the proposed scheme, the spectrum is spread and the amplitude of all the frequency components are at least 30 db below the fundamental component. And the spreading of the frequency components is achieved. For the sine triangle PWM, the frequency components at 5 KHz is about 15 db less than the fundamental and about 20 db less at 10 KHz.
1025
Voltage (V)
2009 IEEE Symposium on Industrial Electronics and Applications (ISIEA 2009), October 4-6, 2009, Kuala Lumpur, Malaysia
Time Fig. 10: Gating signals for m=0.8; Scale: X-axis:10ms/div; Y-axis: 5V/div
Fig. 12 (b). Phase voltage spectrum for Sine Triangle PWM with m = 0.8; Scale: X-axis: 1.25 KHz/div. Y-axis: 10db/div
Fig. 11. Phase voltage and phase current; for the proposed scheme with m=0.8; Scale: X-axis: 5ms/div; Y-axis: 40V/div, 500mA/div
Fig. 13. (a) Phase voltage spectrum for the proposed scheme with m = 0.4; Scale: X-axis: 1.25 KHz/div. Y-axis: 10db/div.
Fig. 13 (b). Phase voltage spectrum for Sine Triangle PWM with m = 0.4; Scale: X-axis: 1.25 KHz/div. Y-axis: 10db/div
Fig. 12. (a) Phase voltage spectrum for the proposed scheme with m = 0.8; Scale: X-axis: 1.25 KHz/div. Y-axis: 10db/div
1026
2009 IEEE Symposium on Industrial Electronics and Applications (ISIEA 2009), October 4-6, 2009, Kuala Lumpur, Malaysia
V.
[5]
CONCLUSION
In this paper a new scheme of carrier based decoupled random PWM method for open-end winding induction motor is proposed. The 3-level inverter is obtained by switching the 2-level inverters independently using the decoupled strategy. Experimental results are presented for different modulation indices which show the spreading of the voltage spectrum. The spectrum of the phase voltage of the proposed scheme is compared with the spectrum of phase voltage of the fixed frequency sine triangle PWM and the spreading of the spectrum achieved is verified. REFERENCES [1]
[2] [3]
[4]
José Rodrígue; Jih-Sheng Lai; Fang Zheng Peng “Multilevel Inverters: A Survey of Topologies, Controls, and Applications,” IEEE transactions on industrial electronics, vol. 49, no. 4, August 2002 Stemmler H, Guggenbach P. “Configurations of High Power Voltage Source Inverter drives”, EPE Conf. Brighton UK 1993 pp 7-12 Shivakumar, E.G., Gopakumar, K., Sinha, S.K., Pittet, A., Ranganathan, V.T, “Space vector PWM control of dual inverter fed open-end winding induction motor drive” EPEJ. 2002 12, (1) pp. 9-18 Baiju, M.R., Mohapatra, K.K., Gopakumar, K., “PWM signal generation for dual inverter fed open-end winding induction motor drive using only the instantaneous reference phase amplitudes,” The Fifth International Conference on Power Electronics and Drive Systems, 2003 Volume 1, pp. 450 - 455
1027
Kawabata Y, Nasu M. , Nomoto T., Eijiogu E.C., Kawabata T. “High efficiency and low acoustic noise drive using open winding ac motor and two Space Vector Modulated Inverters,” IEEE Trans. Ind. Electron. Vol. 49 pp 783 – 789 Aug. 2002 [6] Baiju M. R., Mohapatra, K.K., Kanchan R S., Gopakumar, K, “A dual two-level inverter scheme with common mode voltage elimination for an Induction motor drive,” IEEE Trans. Power Elec. Vol 19 No. 3 May 2004 [7] Somasekhar V. T, Gopakumar, K, Baiju M. R., Mohapatra, K.K., Umanand L, “A multilevel Inverter system for an Induction Motor with Open-end Windings,” IEEE Trans. Ind. Elec. Vol. 52, No.3, June 2005 [8] Somasekhar V. T., Srinivas S, K. K. Kumar “Effect of Zero Vector Placement in a dual inverter fed open-end winding induction motor drive with a Decoupled Space Vector PWM strategy,” IEEE Transactions on Industrial Electronics Vol. 55, No.6, June 2008 [9] Andrzej M. Trzynadlowski, Frede Blaabjerg, John K. Pedersen, R Lynn Kirlin, and Stanislaw Legowski, “Random Pulse Width Modulation Techniques for Converter-Fed Drive Systems- A review,” IEEE Transactions on Industry Applications, vol. 30, pp. 11661175, September / October 1994 [10] M. M. Bech, Frede Blaabjerg, John K. Pedersen, “ Random Modulation Techniques with Fixed Switching Frequency for Three Phase Power converters,” IEEE Transactions on Power Electronics, Vol. 15, No. 4, July 2000. [11] Ki-Seon Kim; Young-Gook Jung; Young-Cheol Lim, “A New Hybrid Random PWM Scheme”, IEEE Transactions on Power Electronics Volume 24, Issue 1, Jan. 2009 [12] Andrzej M. Trzynadlowski, Konstantin Borisov, Y. Li, Ling Quin, “A novel Random PWM technique with Low Computational Overhead and Constant Sampling Frequency for High Volume, Low cost Applications,” IEEE Transactions on Power Electronics, Volume 20No.1, Jan.2005
