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 drivesThe 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) produce a traveling field in the machine air gap. This traveling field will induce voltages in conductors on the 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 occur in the rotor. The interaction between the primary field and secondary currents 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 rotor windings

The rotor current produces another magnetic field

A torque is produced as a result of the interaction of those two magnetic fields.

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( )

3ˆ ˆ[0.5 sin( 120 )] [ sin( 120 )]23ˆ 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

METHODS OF SPEED CONTROL OF 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 control the speed of the induction motor drives because other 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 RECOVERY SCHEME

Slip power recovery scheme (SPRS) is a method of speed control of 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.

A. Conventional Form of SPRS

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 , which can be varied by changing the firing angle of the three-phase inverter

bridge. The DC link inductor has been provided to reduce ripples in DC link current and

the transformer to match the voltages and by taking a suitable turn’s ratio. Neglecting stator and rotor drops,

(1)

(2)

Where is the output voltage of diode bridge rectifier, is the output voltage of inverter-bridge, is the inverter firing angle, is the stator to rotor turns ratio, is source to converter side turns ratio of the transformer, is the slip, 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,

(3)

From equations (1)-(3), the value of slip‘s’ will be given by

(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 .

(5)

Neglecting copper loss

(6)

From equations (4) and (6)

(7)

(8)

(9)

The electromagnetic or developed torque is given by:

(10)

From equations (7) and (10)

(11)

Where is DC link current, is stator resistance referred to rotor side, is rotor resistance, is resistance of DC link inductor, is power in the air gap, is developed torque, is

mechanical power developed, and are the synchronous speed and rotor speed in rad/sec, is no of poles.

Equation (11) imply that the torque is directly proportional to DC link current and the magnitude of this current depends upon the difference between the and . So for a fixed value of firing angle of inverter, torque speed characteristics of the drive are almost linear equivalent to separately excited DC motor.

TYPES OF SLIP POWER RECOVERY SCHEMES

There are two types of slip power recovery control methods.

Static Kramer drive used to speed control of wound rotor induction motor below the synchronous speed.

Static Scherbius drive used to speed control of wound rotor induction above and below the synchronous speed.

STATIC KRAMER

Rotor slip 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.

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 .

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) poor power factor of the supply,ii) requirement of higher reactive power from the supplyiii) 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 control in 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 stagecomprises of inverter control from intermediate to rated speed .

SPRS WITH CHOPPER

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 Pilley and 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.