complete analysis of induction motor drive using slip power recovery scheme including the MATLAB simulation model,

1. ANALYSIS OF SLIP POWER RECOVERY
SCHEME 0F INDUCTION MOTOR
INDUCTION MOTOR:AN INTRODUCTION
Induction motors are widely used motors in industrial motion control applications as well as in
different home appliances.
Having simple and rugged design, low-cost, low maintenance and direct connection to an ac
power source are the main advantages of induction motors over other types of motors.
The recent advancement in semiconductor technologies have paved the way for the
development of power electronics based variable speed induction motor drives
The induction machine is basically an a. c. poly phase machine which is connected to an a.c.
power supply. The a. c. power sources are three phase or may also be single phase , depending
upon the requirement . In both cases the connection is done on the part of the machine such that
primary is connected to the supply (the stator in general) producea traveling field in the
machine air gap. This traveling field will induce voltages in conductors onthe part of the
machine not connected to the supply (the rotor, or the mover in general),which is the
secondary. If the windings on the secondary(rotor) are closed, a. c. currents occurin the rotor.
The interaction between the primary field and secondarycurrents or flux produces torque from
zero rotor speed onward. The rotor speed at which the rotor currents are zero is called the ideal
no-load (or synchronous)speed. The rotor winding may be multiphase (wound rotors) or made
of bars short circuited by end rings (cage rotors).
PRINCIPLE OF OPERATION
 This rotating magnetic field cuts the rotor windings and produces an induced voltage in
the rotor windings
 Due to the fact that the rotor windings are short circuited, for both squirrel cage and
wound-rotor, and induced current flows in the rotorwindings
 The rotor current produces another magnetic field
 A torque is produced as a result of the interaction of those two magnetic fields.
ind R skB B  

2. Where ind is the induced torque and BR and BS are the magnetic flux densities of the rotor and
the stator respectively
( ) ( ) ( ) ( )net a b cB t B t B t B t  
sin( ) 0 sin( 120 ) 120 sin( 240) 240M M MB t B t B t           
ˆsin( )
3
ˆ ˆ[0.5 sin( 120 )] [ sin( 120 )]
2
3
ˆ ˆ[0.5 sin( 240 )] [ sin( 240 )]
2
M
M M
M M
B t
B t B t
B t B t

 
 

     
     
x
x y
x y
1 3 1 3
ˆ( ) [ sin( ) sin( ) cos( ) sin( ) cos( )]
4 4 4 4
3 3 3 3
ˆ[ sin( ) cos( ) sin( ) cos( )]
4 4 4 4
net M M M M M
M M M M
B t B t B t B t B t B t
B t B t B t B t
    
   
    
    
x
y
ˆ ˆ[1.5 sin( )] [1.5 cos( )]M MB t B t  x y

3. METHODS OF SPEED CONTROLOF INDUCTION MOTORS
The various techniques of speed control of induction motor have been classified according to
the main action on the motor part i.e.
 From the stator side
 From the rotor side
From the stator side :
 Stator voltage control
 Pole changing control
 Supply Frequency control
From the rotor side :
 Rotor resistance control
 Slip power recovery control
 Slip Power Recovery Method is most prevalent and best method till date to controlthe speed
of the induction motor drives becauseother methods suffer from many limitations such as
significant amount of power loss, generation of noise and harmonics on supply network,
have lower power factor and low efficiency.
SLIP POWER RECOVERYSCHEME
Slip power recovery scheme (SPRS)is a method of speed controlof wound rotor induction
motor (WRIM) in which some amount of rotor recovered power is feedback to main supply
instead of wasting it in the rotor resistance.
SPRS is used for limited speed range operation of induction motor drive, where slip power is
fraction of motor power rating, therefore low power rating of converter and lower cost.

4. A. ConventionalForm of SPRS
M
Induction Motor
Three Phase Supply
S2
S1
Rotor Resistance Starter Diode Bridge
L
Inverter Bridge
Recovery Transformer
D1 D3 D5
D4 D6 D2
T1 T3
T4 T6
T5
T2
Vd1
Vd2
Id
(90° < α < 180°)
R
Y
B
Speed
In slip power recovery scheme shown in Fig., the three phase full-wave diode bridge rectifier
connected to the rotor windings through slip rings, converts a portion of slip power in to DC
which in turn converted into line frequency AC by a three-phase natural commutated inverter and
feedback to the supply mains. This feedback power can be controlled by varying the inverter emf
2dV , which can be varied by changing the firing angle of the three-phase inverter bridge. The DC
link inductor dL has been provided to reduce ripples in DC link current dI and the transformer to
match the voltages 1dV and 2dV by taking a suitable turn’s ratio. Neglecting stator and rotordrops,
1
3 2
1.35d
sV sV
V
n n
    (1)
2
3 2
cos 1.35 cosd
V V
V
m m
 

    (2)
Where 1dV is the output voltage of diode bridge rectifier, 2dV is the output voltage of inverter-
bridge,  is the inverter firing angle, n is the stator to rotor turns ratio, m is source to converter
side turns ratio of the transformer, s is the slip, V is the supply voltage. Maximum value of  has
been restricted to 165 for safe commutation of thyristors. By appropriate choice of , required
speed can be obtained.
Neglecting drop across the inductor,

5. 1 2 0d dV V  (3)
From equations (1)-(3), the value of slip‘s’ will be given by
cos
n
s
m
 (4)
Considering the equivalent circuit of motor referred to rotor side, neglecting magnetizing
branch and approximate DC equivalent circuit ignoring the commutation overlap angle in the
diode bridge, the equations for DC link current, air gap power, and electromagnetic or developed
torque are given by (6), (7) and (11) below .
1 2
'
2( )
d d
d
s r d
V V
I
sR R R


  (5)
Neglecting copper loss
2 1.35 cosg d d d
V
sP V I I
m
    (6)
From equations (4) and (6)
1.35g d
V
P I
n
  (7)
 1m g d rP s P T     (8)
 
2
1m d mP T s
p
    (9)
The electromagnetic or developed torque is given by:
2
g
d
m
Pp
T

 
(10)
From equations (7) and (10)
1.35
2
d d
m
p V
T I
n 
 
  
  (11)
Where dcI is DC link current, '
sR is statorresistance referred to rotor side, rR is rotorresistance, dR
is resistance of DC link inductor, gP is power in the air gap, dT is developed torque, mP is
mechanical power developed, m and r are the synchronous speed and rotor speed in rad/sec, p
is no of poles.

6. Equation (11) imply that the torqueis directly proportionalto DC link current and the magnitude
of this current depends upon the difference between the d1V and 2dV . So for a fixed value of firing
angle ofinverter, torquespeed characteristics ofthedrive are almost linear equivalent to separately
excited DC motor.
TYPES OF SLIP POWER RECOVERY SCHEMES
 There are two types of slip power recovery controlmethods.
StaticKramerdrive used to speed control of wound rotor induction motor below the
synchronous speed.
StaticScherbius drive used to speed control of wound rotor induction above and below
the synchronous speed.
STATIC KRAMER
 Rotorslip power is converted to dc by diode bridge.
 DC power is fed to dc motor mechanically coupled to induction motor.
 Speed control is obtained by controlling field current of dc motor.
 Speed can be controlled upto standstill
STATIC SCHERBIUS
 Provides speed control of wound rotor motor above and below synchronous speed.
 Controlled rectifier working as inverter converts it back to ac and feed back to source.
 Power fed back can be controlled by controlling counter emf.

7. SLIP POWER RECOVERY SCHEME USING MATLAB SIMULINK
EXPLANATION OF DRIVE CIRCUIT
 A portion of rotor power is converted by a three-phase full-wave diode bridge rectifier
and three-phase full-wave fully controlled bridge working as line commutated inverter,
inverts this power and fed back the same to the three-phase ac source.
 This feed-back power can be controlled by varying the inverter. emf Vd2 which can be
varied by changing the firing angle of the three-phase inverter bridge.
 The dc line inductor Ld is provided to reduce ripples in dc link current Id .
 Since, slip power is fed back to the source; the scheme eliminates wastage of energy and
therefore has higher efficiency .

10. APPLICATIONS OF SPRS
Slip-power recovery drives are used in the following applications:
 Large-capacity pumps and fan drives
 Variable-speed wind energy systems
 Shipboard VSCF (variable-speed/constant frequency) systems
 Variable speed hydro-pumps/generators
 Utility system flywheel energy storage systems
LIMITATIONS OF SPRS
The main drawback of SPRS has been found to be
i) poorpower factor of the supply,
ii) requirement of higher reactive power from the supply
iii) high harmonic contents introduce the distortion in the power supply
SPRS WITHOUT CHOPPER
 SPRS operated drives have limitation of high reactive power requirements and lower
power factor, i.e., in the range of 0.4 to 0.6 .
 The performance of the conventional ( SPRS)can be enhanced by inserting a chopper
between the rotor rectifier and inverter in a SPRS .
The controlin SPRS operated drives can be done in two stages. First stage consists of chopper
control from the minimum speed to an intermediate speed and second stage
comprises of inverter control from intermediate to rated speed .
SPRS WITH CHOPPER
M
`
Induction
Motor
Three Phase Supply
S2
S1
Rotor Resistance
Starter
Diode Bridge
Inductor
DC-DC converter
Inverter Bridge
Recovery Transformer

11. REFERENCES
1. Dr. P. S. Bhimbra, “Power Electronics,” Third Edition, 2005, Khanna Publishers .
2. Dubey, G.K,“Fundamentals of Electrical Drives,” 1995, Narosa Public house Delhi
3. Sita Ram,O.P. Rahi, Veena Sharma , Performance Analysis of Slip Power Recovery
Scheme employing two Inverter Topology” Michael Faraday IET International Summit:
4. Sita Ram, O.P. Rahi,Veena Sharma , Analysis of Induction Motor Drive using Buck-
Boost Controlled Slip Power Recovery Scheme”
5. P Pilleyand L. Refoufi, “Calculation of slip energy recovery induction motor drive behavior
using the equivalent circuit,” IEEE Transactions on Industry Applications, January/February
1994, pp. 154-163,vol. 30, no. I.