Proc. of Int. Conf. on Advances in Electrical & Electronics 2010

Improved Dynamic Performance and Stability of a Three Phase Induction Motor fed by a Tri-State CSI with Reduced Total Harmonic Distortion on the Input side of the Motor 1

B. Jaganathan, 2C.Anuradha, 3N.Kiran, 4S.Archana, 5Pallavi Murthy 1,2,3,4,5

EEE Department, SRM University jagana78@gma il.com anuradhac@ktr.srmuniv.ac.in n.kiran234@gmail.com radhika_mickey@yahoo.co.in pllv.murthy@gmail.com

Abstract :The conventional current source inverters (CSI) feeding induction motors generally have an RHP in the control to output transfer function. This in turn leads to a voltage d i p in the inverter’s output voltage, which also makes the motor to operate in the unstable region. Apart from these, this leads to a poor dynamic response of the induction motor connected to. To attain a better dynamic response, this paper proposes the idea of a tristate current source inverter with only an additional semiconductor switch, which introduces unique freewheeling states apart from the traditional six active states and two null states. The appropriate insertion of freewheeling states into the inverter state sequence enhances the dynamic performance of the system. The designed inverter is controlled by the Space Vector Modulation technique. The resulting IM drive is tested by simulation using MATLAB/Simulink for demonstrating the dynamic performances and the stability analysis. The input voltage and current waveforms for these are also obtained along with the THD graphs. It has been observed that the proposed method gives much improved dynamic performance and stability and also reduced THD when compared with the other method. The order of the THD with the conventional approach is about 47.24% and with the proposed method it is only 9.7%. The transfer function of the overall system is also obtained and checked for stability by using Nyquist plot and it is found to be stable. The various waveforms and the Nyquist plot are also shown. Keywords—Current s ource inverters (CSI), S pace vector modul ati on (SVM) techni que, Inducti on motor (IM) dynamic res pons e, Stability, Nyquis t plot, Closed Loop Contr ol (CLC).

presence of RHP ze ro in its control-to-output transfer function. This RHP zero [1], according to classical control theory causes a non-minimal phase effect which results in fall of inverter output before r i s e when a step reference command is given. Apart from this effect, the RHP zero makes the design of wide bandwidth controller with fast dynamic response difficult [2], since RHP effect varies with changes in system parameters and operating conditions . Various methods have been proposed to eliminate the RHP zero in the control-to-output transfer function earlier, but these methods have some limitations such as small boost inductance (res ulting in large ripple current), large equivalent series res istance in the output capacitor. Though a tri-state concept [1] has been proposed earlier to completely eliminate RHP zero for a “single-input single-output” converter, this paper extends the concept to a “s ingle-input multi-output” CSI..,This paper uses only a single semiconductor switch to introduce the freewheeling states to the inverter state sequence. For controlling the designed inverter, the Space Vector Modulation (SVM) [5] technique is adhered. The input voltage and current waveforms for both the circuits are obtained and compared. The output speed is also considered and is taken for feedback. The overall transfer function of the system is then found and checked for stability by using Nyquist stability criterion. II. S IM UL I N K CIRCUITS

I. INT RODUCTION The three phase current source inverter has found wide applications in ac motor drives and other areas due to its ruggedness , implicit output short circuit protection and direct current control ability.Despite these advantages , CSI inverters have poor dynamic response due to the 123 © 2010 ACEEE DOI: 02.AEE.2010.01.108

Three inverters ,the conventional Current source inverter, SVM controlled inverter and the proposed Tri-state Current source inverter feeding a three phase induction motor are s imulated and the simulation results are obtained. The following sections explain the individual simulink circuit.

Proc. of Int. Conf. on Advances in Electrical & Electronics 2010

meas ured by us ing a machines measurement de mu x b lock, where the waveforms are s een with the help of a s cope.

Conventional CSI feeding a three phase IM

III. THE PROPOSED INVERTER

Fig. 1 Conventional CSI feeding a three phas e induction Motor.

Fig.1 s hows the conventional CSI feeding a three phase induction motor. The inverter is operated with the normal gating pulses i.e.., T 1 triggered at 0o , T 2 triggered at 60o , T3 at 120o and s o on. The diodes are connected in s eries with the thyristors to block the revers e voltages on the trans is tors . For which , there should be a continuous current flow from the source a n d two s witches must always conduct and the conduction sequence is 61,12,23,34,45,56,61 and s o on. Each device o conducts for 120 .The current flows through V+ T 1 - phas e A - phas e B - T6 – V- .A similar s equence follows for the conduction of other switches . The pulse generators connected to the various s witches trigger the m according to the above des cription. The rotor s peed and the rotor currents of the induction motor are meas ured by machines meas ure ment demu x block available in the MAT LAB Simu link library bro ws er and the waveforms are obs erved with the s cope.

Fig 3 Proposed Tri-state CSI feeding a three phas e IM The proposed inverter circuit is s hown in Fig 3 and its Simulation diagram is shown in Fig 4.

The inverter section and the SV generation are divided into subsystems

Fig. 4 The proposed s ys tem with inverter and the gating circuit are s hown as s ubs ys tems

B. SVM controlled CSI feeding a three phas e IM

Fig. 5 Simulink diagram of the proposed Tri-state CSI Feeding a three phas e induction motor Fig. 2 SVM controlled CSI feeding a three phas e Induction motor.

A SVM controlled CSI is s hown in Fig. 2. A three phas e univers al bridge is fed by a current s ource. The gating puls e given to this bridge are space vector modulated s ignals generated by a gating circuit as s hown in fig 4,the des cription of which is expla ined in the following s ection. The CSI controlled by the SVM s ignals , feeds a three phas e induction motor. The output of the IM i.e., the rotor currents in d-q frame and the rotor s peed are 124 © 2010 ACEEE DOI: 02.AEE.2010.01.108

The SVM s ignal generating s ection is already e xplained in s ection-B. The inverter s ubsystem consists of a CSI with the series combination of the additional s witch SW0 and diode connected in parallel with the inductor. This s witch SW0, is given with gating puls es , depending upon the state of the inverter, i.e., the gating pulses to the switch SW0 is given, when the inverter is at null s tate (i.e., 111 or 000 ). Th is makes the dc current to free wheel through the clos ed circuit formed with ‘L’, ‘SW0”

Proc. of Int. Conf. on Advances in Electrical & Electronics 2010

and the diode. The output current of the inverter is s ens ed and given to a s witch which cons is ts of three input ports 1, 2 & 3.Input port 2 called control port is s et to a thres hold value u2 = 0,which makes the s witch to select port 1, and if the thres hold equal to value is other than ‘0’, i.e., for u2 not zero, the input port 3 is chos en. Further, output port of the selector switch is connected to the gate of the semiconductor s witch SW0. The puls e generator is connected to port ‘1’.Thus whenever the output current of the inverter is zero, port 1 of the s witch is s elected so that SW0 is triggered ON and if the inverter current is not zero input port 3 is selected, i.e., SW0 is left untriggered. Again this inverter is als o connected to a three phas e IM and the motor’s performances s uch as the rotor currents in d-q fra me are obtained through a machine meas ure ment demu x connected to a s cope. Therefore, when u2 (i.e.., the output current of the inverter is ze ro), input port ‘1’is s elected and hence SW0 is turned ON and if u2 is not equal to 0, input port ‘3’ is s elected and SW0 is turned OFF. This freewheeling increase the energy s tored in the inductor. And the moment the inverter regains the active s tates (other than 111 or 000), the gating puls e to SW0 is withdrawn and the IM is s upplied with a boos ted energy (fro m the inductor).

From this figure, it can be obs erved that these two are exactly oriented at 90 with each other i.e., orthogonal s patial orientation of d-q fluxes (and hence rotor currents ) is achieved. This is an important case obtained, which can be extended to the vector control of IM S.

Fig. 6 ‘ω ’ vs . T ime o f conventional CSI fed induction Motor

Inverter parameters PARAMETERS

VALUES

Inp ut voltage

350 V

Inductance

10 H

Fig. 7 Rotor d-q currents of the induction motor fed by the conventional CSI

Inductio n Motor parameters PARAMET ERS

VALUES

Nominal Power

3730 W

Voltage

415 V

Frequency

50 Hz

Stator Resistance

1.115 ohms

Stator Inductance

5.974 mH

Rotor Resistance

1.083 ohms

Rotor Inductance

5.974 mH

M agnetizing Inductance Pole Pairs

Fig. 8 ‘ω ’ vs . T ime o f SVM controlled CSI fed induction motor.

0.2037 H 2

IV. SIMULATION RESULTS As the torque is proportional to the square of the voltage, the dynamic res pons e is improved i.e., the s teady state is reached at a very much lesser time when compared to the other two approaches i.e., IM fed with conventional CSI and CSI controlled by SVM. The rotor fluxes in d-q axes are also measured.

Fig. 9 Rotor d-q currents of the induction motor fed by the SVM controlled CSI

125 © 2010 ACEEE DOI: 02.AEE.2010.01.108

Proc. of Int. Conf. on Advances in Electrical & Electronics 2010

Fig.13 Input current waveform of the convent ional syst em.

The input voltage and current waveforms as obtained with the conventional approach circuit and with the propos ed circuit are pres ented and obs ervations are made. Als o the THD obtained with both the approaches are pres ented and the corres ponding obs ervations are made. The input voltage waveform with the conventional circuit is s hown in fig. 12.It can be obs erved that the wave form is not proper and als o with richer a mount of harmonics . Als o the current waveforms are s hown in fig. 9.Als o here it can be obs erved that the current has richer content of harmonics .

Fig. 10 ‘ω ’ vs . T ime of the proposed Tri-s tate CSI fed induction motor.

Fig. 11 Rotor d-q currents of the induction motor fed by the tri-s tate CSI fed induction motor

The rotor speed waveforms of the three phas e IM for the above mentioned s i m u l i n k circuits are shown in figs . 6, 8 and 10 res pectively. In fig. 6, it can be obs erved that the steady state speed is reached s econds and fro m fig. 8, the s ame at about 1.41 is reached at about 0.73 s econds . Finally, fig. 10, the s peed curve obtained with the propos ed method, reveals that the s teady state s peed is reached at about 0.28 s econds itself. Therefore the proposed method improves the dynamic res pons e of the induction method. Also in figs . 7, 9 and 11 the rotor currents in d-q fra me are s hown. The idea of obs erving the rotor currents is to extend this propos ed method, with ease, to vector control of CSI fed induction motor. From the above waveforms 9 and 11, it can a ls o be obs erved that the proposed also ma kes the rotor current to be an ‘atleast’ s ine wave along with an almost orthogonal spatial orientation of these two currents ,i.e.,the torque producing component of the rotor current and the flux producing component of the rotor current.

Fig. 14 THD graph obt ained with the convent ional Syst em

The THD g raph s hown in figure 10 depicts that the input current harmon ic dis tortion is about 47.24% . But as per the s tandards, the tolerable T HD leve l can be only 5%. Hence the conventional approach can be cons idered as the one that has a higher level of THD.

Fig. 15 Input voltage waveform of the propos ed s ys tem

Fig. 12 Input voltage waveform of the convent ional syst em

Fig. 16 Input current waveform of the proposed system.

126 © 2010 ACEEE DOI: 02.AEE.2010.01.108

Proc. of Int. Conf. on Advances in Electrical & Electronics 2010

The input voltage and current waveforms obtained with the proposed method are s hown in figures 15 and 16 res pectively. It can be obs erved that the wave forms obtained are much better than that of the conventional approach. Also the harmonic content of the above waveforms particula rly, the current waveform has a reduced amount of harmonic content. This can be further proved by the obs ervation of the THD graph obtained with the propos ed s ys tem as s hown in figure 17 below.

Fig. 17 T HD graph of the input current of the propos ed Syste m.

T rans fer function of the s ys tem inc luding the inner current loop and outer speed loop is ..,

The Nyquis t plot of the same is given below. Fro m the above plot, it is obs erved that, there are no right plane zeros and poles . Also from Nyquis t plot, it is obs erved that the critical point (-1+j0) is encircled NIL (zero) times . Hence, it can be concluded that the system is s table.

Fig. 18 Nyquis t plot pertaining to Equation (1)

CONCLUSION The operation of induction motors when fed by conventional CSI and even with SVM controlled CSI gives a poor dynamic response of the induction

127 © 2010 ACEEE DOI: 02.AEE.2010.01.108

motor connected to the inverter. To overcome this , a tri-state CSI is proposed, and it is made to feed a three phase IM. All the three circuits are simulated and compared and it is observed that the proposed method gives a much better dynamic response, i.e.., the steady state of the induction motor is reached at a much earlier time and also a better stability of the system is obtained. Nyquist Stability analysis i s done and the plot is also shown. Also the rotor currents of the induction motor, in d-q fra me are obtained and are compared. These rotor currents are obs erved to be nearly sine wave with the proposed method than when compared with the other two methods . This makes the vector control of CSI fed IM much easier, because of the orthogonal spatial orientation also. Thus this proposed method improves the dynamic response of the induction motor and can be extended for the imp le mentation of vector control of CSI fed induction motors and on the load side the THD was found to be 9.7% . REFERENCES [1] P. C. Loh, F. Blaabjerg, C. P. Wong, and P . C. Tan, “Tri-stat e current source inverter with improved dynamic performance,” in Proc. IEEE PESC’06, 2006, pp. 2144–2150. [2] K. Viswanathan, R. Orugant i, and D. Srinivasan, “A novel t ri-stat e boost convert er with fast dynamics,” IEEE Trans. Power Electron., vol. 17, no. 5, pp. 677–683, Sep. 2002. [3] G. Joos, G. Moschopoulos, and P. D. Ziogas, “ A high performance current source invert er,” IEEE Trans. Power Electron., vol. 8, no. 4, pp. 571–579, Oct . 1993. [4] D. N. Zmood and D. G. Holmes, “A generalized approach t o the modulat ion of current source invert ers,” in Proc. IEEE PESC’98, 1998, pp. 739–745. [5] Y-S Lai; Bowes, S.R., “ A universal Space Vector modulat ion st rat egy based on regular-sampled pulse-widt h modulat ion [invertors]”, Proc. of t he 1996 IEEE IECON 22nd, Vol.1 , pp. 120 - 126 [6] R. D. Middlebrook and S. Cuk, “ A general unified approach to modeling swit ching convert er power st ages,” in Proc. IEEE PESC’76, 1976, pp. 18–34. [7] J. R. Espinoza and G. Joos, “ Current-source converter on -line patt ern generator swit ching frequency minimizat ion,” IEEE Trans. Ind. Electron.,vol. 44, no. 2, pp. 198–206, Apr. 1997