© June 2015 | IJIRT | Volume 2 Issue 1 | ISSN: 2349-6002
Comprehensive Analysis of Slip Power Recovery
Scheme
Shiv Kumar1, Ajay Kumar2, Himanshu Gupta3
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.
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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 ViR 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:
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© June 2015 | IJIRT | Volume 2 Issue 1 | ISSN: 2349-6002
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 Vi/s increased, the rotor current Ir
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:
The dc link voltage can be found from the diode
input line-line voltage as:
Fig 4. Approximate equivalent circuit refer to
Stator
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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:
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
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.
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
IV. SIMULATION OF 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, Precovered is the effective recovered power at the
stator terminals.
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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.
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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 (3rd, 5th, 7th …) 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.
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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
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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.
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.
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.
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.
Hence an overall increase in efficiency is
obtained.
VI. CONCLUSIONS
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.
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.
Fig 12. Variation of speed w.r.t a) firing angle
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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 SlipEnergy 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
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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. 829833.
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
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13. Maria G. Ioannides, John A. Tegopoulos,
“Generalized Optimization Slip Power
Recovery drives”, IEEE Transaction on
Energy Conversion, Vol. 5, No. 1, March
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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,
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IEEE transaction on Industry Applications,
Vol. IA-27, No. 4, July/Aug. 1991, pp. 728733.
16. Fundamentals of Electrical Drives, G. K.
Dubey, 2002
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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.
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