Comprehensive Analysis of Slip Power Recovery Scheme Shiv Kumar 1, Ajay Kumar 2, Himanshu Gupta 3 1 Student, M. Tech, E-Max group of Institutions, Ambala, Haryana 2 Associate Professor, Dept. of EE, Baddi University of Emerging Sciences & Technology, Baddi, H.P. 3 Assistant Professor, E-Max group of Institutions, Ambala, Haryana Abstract Induction motors are the most commonly used motors in industrial motion control systems. The slip power recovery scheme (SPRS) provides speed control of slip ring induction motor (SRIM) below synchronous speed. The slip power recovery drives is used in very large-capacity pumps and fan drives, shipboards VSCF (variable-speed/constant-frequency) systems, variable-speed hydro pumps/generators etc. In this paper a comprehensive analysis of slip power recovery scheme is presented. Simulation of the scheme is carried out using MATLAB/SIMULINK environment and experimental set up is prepared in the laboratory for a 2 HP motor. The experimental and simulation results are analyzed. Index Terms Analysis, Feedback Power, Simulation, Slip power recovery scheme, Matlab/Simulink. I. INTRODUCTION To control the speed of SRIM with higher efficiency and better performance, an attempt was made by Scherbius and Kramer to replace the additional resistances from the rotor circuit with the help of recovered rotor slip power using auxiliary machines. This played a significant role in enhancing the development of electrical drive systems using induction motors. Use of static inverter in the rotor circuit was proposed by Lavi et al in 1966 for the speed control of SRIM in sub synchronous range. The scheme comprising of bridge rectifier in rotor circuit, filter inductor, line commutated bridge inverter and recovery transformer as indicated in Fig. 1. Since then numerous modifications in the proposed scheme were made by various authors as stated in the literature. Fig 1. Basic slip power recovery scheme In this paper, a comprehensive analysis of slip power recovery scheme is presented. The scheme is simulated using MATLAB/SIMULINK environment. The results are validated with the experimental set-up results using microcontroller as firing angle controller for the inverter circuit. The advantage of using microcontroller over digital and microprocessor techniques are it is flexible, simple, economical, and consumes less hardware. In this work, steady state relationships between inverter firing angle torque, speed, and inductor current for the SPRS are derived. It has been observed that the drive offers linear torquecurrent relationship like a separately excited DC motor. II. PERFORMANCE ANALYSIS OF SCHEME The basic slip power recovery scheme employing static Kramer drive is shown in Fig. 1. In this scheme a voltage V ir is applied to the slip ring terminals, in phase with the rotor current through recovery transformer & line commutated inverter. Thus the effective equivalent circuit of the drive can be represented as: IJIRT 102378 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 28
If the injected voltage is in phase with the rotor current, then the voltages in the equivalent circuit may be written as: Fig 2. Voltage injection in rotor circuit The injected voltage can be referred to the stator as: Above equation gives the following equivalent circuit: Re-arranging, the slip may be found as Power and Torque The air gap power of the machine may be written as Fig 3. Equivalent circuit refer to Stator Considering the given equivalent circuit, if the injected voltage is V i /s increased, the rotor current I r will be reduced, resulting in a reduction in the available torque generated by the motor. If there is a load applied to the motor, the rotor will slow down, resulting in an increase in slip. As slip increases, the effective voltage seen by the stator will be reduced (the actual voltage physically induced in the rotor, due to the stator, will increase). As a result, rotor current will increase. This allows the machine to find a new steady state position where the induced rotor current produces enough torque to equal the load torque but at a reduced speed. Exploration of operation To simplify the analysis, assuming that the magnetizing reactance is moved to the terminals of the equivalent circuit. (Otherwise, the stator phase voltage, stator impedance and magnetizing reactance can be replaced by a Thevenin equivalent source and impedance.). Breaking this equation into parts, it can be seen that the air gap power is the sum of rotor resistive losses, power recovered through the slip rings and the mechanical power produced. Using the expression for air gap power, the torque may be written as Now, substituting the slip expression into the torque expression gives the result that torque is only a function of rotor current, not slip or injected voltage: The expression above means that for a given torque, the rotor current will always be the same, independent of speed. The voltage at the input to the diode rectifier is given by: Fig 4. Approximate equivalent circuit refer to Stator The dc link voltage can be found from the diode input line-line voltage as: IJIRT 102378 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 29
Considering the thyristor converter, this circuit can be thought of as a thyristor rectifier connected in reverse, and the DC link voltage is related to the line-line inverter voltage as: Substituting the above expressions, the voltage injected into the rotor can be calculated as: In the case that the inverter line-line voltage is connected to the supply through a transformer, as shown in the diagram above, the injected voltage can be related to the supply voltage as: Using this simplified analysis together with the slip power recovery torque equations, the thyristor firing angle required for a particular torque at a particular speed can be found. As the power recovered from the rotor is feedback to the stator again, the drive is having better efficiency as compared to rotor resistance method. III. EXPERIMENTAL SET-UP & IMPLEMENTATION OF SCHEME For developing the experimental set-up, power diodes & thyristors of 16 amperes have been used for the converter and line commutated inverter circuits respectively. A filter inductor of 25 mh has been connected. The experimental set-up of the firing circuit of the drive is microcontroller based. Three number of single-phase; star-star connected step-down transformers have been used to get synchronized reference signals from the supply. The signal available from each step-down transformer has been fed to a high gain operational amplifier LM324 consisting of fourindependent internal channels. The zero crossing of each phase is detected here and the rectangle output signals from LM324 are fed to the microcontroller unit. Low power, high performance 40 pin, CMOS, 8-bit microcontroller Micro-controller ATMEL AT89S52 has been used to produce firing pulses. Fig 5. Torque-speed characteristics of the drive No-Load Condition Consider again the expression for slip: Fig 6. Experimental Set-up of the Scheme If the torque is zero, then the rotor current will also be zero and at zero torque, the slip is given by: Efficiency Since some of the power supplied to the motor is recovered from the rotor circuit, the efficiency cannot be calculated as simply output power over input power. Instead, in a slip power recovery drive the efficiency is: Here, P recovered is the effective recovered power at the stator terminals. IV. SIMULATION OF SCHEME To study the performance of the drive, a simulation block-set in Matlab/Simulink has been implemented as shown in Figure 7. A 2 HP, 400 V, 50 Hz wound rotor induction motor has been used for the simulation. Provision has been made to measure stator current, speed and torque of the motor. The active and reactive power input of the motor, the recovery transformer and the source have been measured using P-Q block. Provision has also been made to measure different voltages and currents of the scheme wherever required. The data has been saved to the workspace for further analysis. Other parameters of the model have been given in Appendix-A. IJIRT 102378 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 30
Fig 7. Simulation circuit for slip power recovery scheme V. SIMULATED & EXPERIMENTAL RESULTS Results & analysis of Simulation work: Simulation results show that by varying firing angle (above 90 degree in small intervals), motor speed can be controlled from zero to nominal speed. The motor speed vs. time characteristics at two different firing angles have been shown in Figure 8. It can be observed that steady state speed for higher firing angle is less as compared to lower firing angle. The upper value of firing angle is restricted to 165 for the safe commutation of thyristors [16]. Fig 9. Reactive power taken by a) motor b) inverter The presence of power electronic converter and inverter circuit in this system, also cause low frequency odd harmonics (3 rd, 5 th, 7 th ) injection to the supply network. Motor torque, feedback current and source current waveforms have been shown in Figure 10. Fig 8. Motor Speed at a) 92 degree b) 100 degree firing angle Moreover, inverter circuit consumes negative active power and positive reactive power from line side as shown in Figure 9. This means that active power is returned to the network but a large amount of reactive power is absorbed from the source. Because of the reactive power consumption, the overall power factor of the scheme becomes low. Fig 10. a) Motor torque b) Feedback current c) Source current It can be observed that torque produced by the motor is not constant but pulsating. It can also be IJIRT 102378 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 31
observed that the feedback current and the source current waveforms are distorted. This leads to increased torque ripple and consequently motor temperature is increased. The THD of the source current has been found to be 11.57%. The nonsinusoidal stator and inverter feed-back current affect line-side current and injects harmonics in the supply network. Fast Fourier Transform (FFT) window of source current wave form has been shown in Figure 11. It can be observed that it consists of sub-harmonic corresponding to dc component, 25 Hz and multiple harmonics corresponding to higher order (third, fifth and seventh etc) harmonics. b) inductor current The input power of the drive with and without slip power recovery has been shown in Figure 13. If we neglect the inverter and transformer losses, we can find that for the same load, the input power taken from the source with slip power recovery scheme is less than that of power consumed without slip power recovery scheme. Fig 13. Input power of drive Fig 11. FFT Analysis of supply current It can be further observed that, fifth and seventh harmonics are the dominant one. Results & analysis of Experimental work: The performance characteristics of the drive have also been analyzed using the experimental set-up. The variation of the rotor speed w.r.t. a) firing angle & b) inductor current has been shown in Figure 12 a) and 12 b) respectively. Fig 12. Variation of speed w.r.t a) firing angle Hence an overall increase in efficiency is obtained. VI. CONCLUSIONS In this paper, the slip power recovery method for the speed control of three-phase slip ring induction motor has been investigated. The performance equations have been drawn and a simulation block-set model in Matlab/Simulink has been implemented. A microcontroller based open-loop speed control experimental set-up has also been developed in the laboratory. The following conclusions have been drawn from the study: 1. The torque of the drive varies linearly with the dc link current. Hence, the drive has similar characteristics as that of separately excited dc motor. 2. The increase in efficiency has been observed as compared to rotor resistance method of speed control. 3. The simulation/experimental results show the overall reduced power factor of the drive. 4. Presence of converter and line commutated inverter results in harmonic injection on the source side. IJIRT 102378 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 32
5. Presence of current harmonics in stator and rotor windings causes additional heating of motor. Thus, de-rating of the drive is required for the same load as compared to rotor resistance control method. APPENDIX-A Slip ring induction motor: 3-phase, 2 HP, 400 volt, 4.5 amps, 50 Hz, 1440 rpm, Y-Y connected. Parameters: Stator resistance = 4.8 Ω; Rotor resistance = 4.2 Ω; Stator leakage reactance = 9.5 Ω; Rotor leakage reactance = 9.5 Ω; Magnetizing reactance =185 Ω; Stator to rotor turn ratio = 5. Magneti Other Parameters of the drive system: Turns ratio of recovery transformer (inverter to line side) = 0.2; Resistance of smoothing inductor = 2 Ω; Inductance of smoothing inductor = 0.025 H REFRENCIES 1. A. Lavi, R.J. Polge, Induction Motor Speed Control with Static Inverters in the Rotor, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-85, No. 1, Jan 1966, pp. 76-84. 2. W Shepherd, J Stanway, Slip Power Recovery in an Induction Motor by the use of a Thyristor Inverter, IEEE Transaction on Industry and General Applications, Vol. IGA-5, No. 1, Jan/Feb 1969, pp. 74-82. 3. W Shepherd and A.Q. Khalil, Capacitive, Compensation of Thyristor Controlled Slip- Energy Recovery System, Proceedings IEEE, Vol. 117, No. 5, May 1970, pp. 948-956. 4. S.K. Pillai, K.M. Desai, A Static Sherbius Drive with Chopper, IEEE Transaction on Industrial Electronics and Control Industrial Instrumentation, Vol. IEC-24, No. 1, Feb 1977, pp. 24-29. 5. V.N. Mittle, K. Venkatesan, S.C. Gupta, Switching Transient in Static Slip-Energy Recovery Drive, IEEE Transaction Power Apparatus and System, Vol. 98, July/August 1979, pp. 1315-1320. 6. K. Taniguchi, H. Mori, Application of a Power Chopper to the Thyristor Scherbius, IEEE Proceedings, Vol. 133, Pt. B, No. 4, July 1986, pp. 225-229. 7. M.S. Hilderbrandt, Reference Frame Theory applied to the Analysis of a Slip-Recovery system, Purudue University 1986. 8. B.A.T. Al Zahawi, B.L. Jones, W. Drury, Effect of Rotor rectifier on motor performance in Slip Energy drives, Canadian Electrical Engineering Journal, Vol. 13, No. 1, 1987. 9. S.R. Doradla, S. Chakravorty, K.E. Hole, A new Slip Power Recovery Scheme with Improved Supply Power Factor, IEEE Transaction on Power Electronics, Vol. 3, No. 2, April 1988, pp. 200-207. 10. Maria G. Ioannides, John A. Tegopoulos, optimal efficiency Slip-Power Recovery Drive, IEEE Transactions on Energy conversion, Vol. 3, No. 2, June 1988. 11. G.D. Marques, Synthesis of active and reactive Power Controllers for the Slip Power Recovery drive, in Proceedings EPE 1989, Vol. 2, Aachen, Germany, Oct. 1989, pp. 829-833. 12. E. Akpinar, P. Pillay, Modeling and Performance of Slip Energy Recovery Induction Motor drive, IEEE Transaction on Energy Conversion, Vol. 5, No. 1, March 1990. 13. Maria G. Ioannides, John A. Tegopoulos, Generalized Optimization Slip Power Recovery drives, IEEE Transaction on Energy Conversion, Vol. 5, No. 1, March 1990. 14. Y. Baghzouz, M. Azam, Harmonic Analysis of Slip Power Recovery Drives, 90/CH 2935-5/90/0000 1990 IEEE. 15. F. Liao, J.I. Sheng, A.L. Thomas, A New Energy Recovery Scheme for Doubly fed, Adjustable-Speed Induction Motor drives, IEEE transaction on Industry Applications, Vol. IA-27, No. 4, July/Aug. 1991, pp. 728-733. 16. Fundamentals of Electrical Drives, G. K. Dubey, 2002 IJIRT 102378 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 33
BIBLOGRAPHICAL NOTES Shiv Kumar is a student of M. Tech Electrical Engineering, E-Max group of Institutions, Ambala. He graduated from Kurukshetra University, Kurukshetra. His research interest includes power electronics, electrical machines and electrical drives systems. Ajay Kumar is an Associate Professor in the Department of Electrical Engineering, Baddi University, Baddi, India. He graduated from Kurukshetra University, Kurukshetra, did his masters in Power Electronics & Drives from M M University, Mullana. His research interest includes power electronics, electrical machines, drives and wind energy systems. He is a life member of ISTE. Himanshu Gupta is an Assistant Professor in the Department of Electrical Engineering, E-Max group of Institutions, Ambala, India. He graduated from Kurukshetra University, Kurukshetra did his masters in Power electronics and Drives from Kurukshetra university, Kurukshetra. His area of interest in electrical machine using drives. IJIRT 102378 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 34