4. Three-phase Induction Motors are the most common and frequently encountered machines in industry Simple design, rugged, low-price, easy maintenance. Wide range of power ratings: fractional horsepower to 10 MW. Run essentially as constant speed from no-load to full load. Its speed depends on the frequency of the power source. Not easy to have variable speed control. Requires a variable-frequency power-electronic drive for optimal speed control.

6. Squirrel cage rotor Wound rotor Notice the slip rings

7. Where s is the slip Notice that : if the rotor runs at synchronous speed s = 0 if the rotor is stationary s = 1 Slip may be expressed as a percentage by multiplying the above eq. by 100, notice that the slip is a ratio and doesn’t have units.

8. It gives high starting torque. Speed of the Motor can be controlled. Slip rings Brushes

9. LOSSES OF INDUCTION MOTOR Fixed Losses 1. Core Loss 2. Bearing Friction loss 3. Brush Friction Loss 4. Windage Loss Variable Losses 1. Stator Ohmic Loss 2. Rotor Ohmic Loss 3. Slip Power Loss 4. Stray Load loss

10. For a 1000 kW fan, with SPRS the energy saving could be around 200 kW or 4800 kWh per day..

11. IT IS REQUIRED TO RECOVER SLIP POWER

12. Where, V r =Voltage Injected into Rotor. Power Developed P g =P m +P r (Where, P r =Power absorbed by V r ) => P m =P g -P r (Where,P m =Mechanical Power Developed) Torque Developed per Phase (Where,P g =Air Gap Power) (Where, =Developed Torque) (Where, =Rotor Speed) When, P r =0, then P m =P g, & motor rotates on natural Speed. When, P r goes to increasing, then P m goes to decreasing & as well as speed also decreasing. When, P r =P g, then P m =0 & motor stops down. By, Controlling P r (Slip Power) Speed also controlled.

13. SLIP POWER RECOVERY SYSTEM OF SLIP RING INDUCTION MOTOR

14. In a wound-field induction motor the slip rings allow easy recovery of the slip power which can be electronically controlled to control the speed of the motor. The oldest and simplest technique to invoke this slip- power recovery induction motor speed control is to mechanically vary the rotor resistance. Instead of wasting the slip power in the rotor circuit resistance, a better approach is to convert it to ac line power and return it back to the line.

15. Slip Power Recovery System (SPRS) is a variable speed drive for slip ring induction motors. It recovers and delivers the slip dependent rotor power from the motor to the grid. At changeover speed, SPRS is connected to the rotor and the rotor resistance is disconnected. The diode rectifier converts the rotor voltage to DC voltage. This rectified rotor voltage is counter- balanced by a line commutated inverter. By controlling the ‘counter-balancing’ inverter voltage, the rotor current, hence rotor speed is regulated. The slip power collected at the slip rings is fed back to the grid through the inverter.

17. SLIP POWER RECOVRY SYSTEM Static Scherbius Drive- allows operation above and below synchronous speed. Static Kramer Drive - only allows operation at sub- synchronous speed.

18. STATIC SCHERBIUS DRIVE

19. The ac slip power is first rectified by the three phase diode bridge, then turned back into ac power at the line frequency by the Thyristor inverter and finally return to the supply network by means of a transformer. Brings the rotor circuit voltage up to the value corresponding to the voltage of the ac supply network. The speed of the induction motor is regulated by the controlling the firing angle of the inverter. The gate pulses are provided by the firing circuit,synchronized with the supply voltage. Both the rectifier and the inverter are line-commutated by the alternating emfs appearing at the slip ring and the supply network respectively.

20. STATIC KRAMER DRIVE

21. A slip ring induction motor’s rotor circuit feeds the slip Power, rectified by means of a diode bridge to the armature of a separately exited dc motor, which is mechanically coupled to induction motor. The system is started by switching on s1 first & then s2,while switches s3 and s4 are off. As soon as the motor attempts steady speed, the dc motor is energized by switching s2 off and s3 and s4 on. speed control is achieved by varying the field current of the motor. An emf proportional to the back emf of the dc motor may be considered to be injected into the rotor circuit of the induction motor to cause variation in speed of the system.

22. HOW SPEED CONTROL IS ACHIEVED??

23.  The air gap flux is constant and the torque is controlled by the dc link current I d (controlled in the inner control loop).  The speed is controlled via the outer control loop (see performance curves below).

25. The average torque developed by the motor = total fundamental air gap power synchronous speed of motor  where P gf ’ = fundamental frequency per-phase air gap power.

26. A torque expression in terms of inverter firing angle may be derived:

27. The torque-speed curves at different firing angles of the inverter are shown below:

29. The rectification of slip-power causes harmonic currents in the rotor which are reflected back into the stator. This results in increased machine losses. The harmonic torque is small compared to average torque and can generally be neglected in practice.

31.  Harmonics and losses are the main obstacle for rapid uses of SPRS system. To overcome these, now Engineers are designed for “12 pulse configuration of SPRS system”.  Typically for ratings above 1000kW, SPRS comes with a 12-pulse series-parallel configuration.

32. Eliminates 5th, 7th, 17th, 19th.... harmonics (which form about 95% of total harmonics) permanently - thus ensuring good quality power without polluting the grid. Better power factor due to optimization of the power circuit. Reduction in SPRS losses.

34.  ADVANTAGES:- SLIP POWER RECOVERY SYSTEM(SPRS) provides:-  Lower operating costs by slashing energy bills.  Enhanced life of mechanical equipment by reducing vibrations.  Accurate process control with excellent speed holding Accuracy.

35.  DISADVANTAGES:- SLIP POWER RECOVERY SYSTEM has the following disadvantages:-  High initial cost.  Technology is not available readily.  The rectification of slip-power causes harmonic currents in the rotor which are reflected back into the stator. This results in increased machine losses. The harmonic torque is small compared to average torque and can generally be neglected in practice.  the static Kramer drive is characterized by poor line PF because of phase controlled inverter.

37. Large Capacity Fan & Pump Drives Variable-speed wind energy systems. Shipboard VSCF (variable-speed/constant frequency) systems. Variable speed hydro- pumps/generators. Utility system flywheel energy storage systems.

39.  Role of SPRS in energy saving can be appreciated vis-à-vis conventional methods of controlling air/gas flow applications.  In process plants, a majority of fans (ID, FD, preheater, ESP, baghouse, etc.) are driven by slip ring induction motors, typical ratings being 400kW to 6000kW.  Conventional methods of flow control are: 1). Damper control, where around 20% of the power is lost. 2). Speed variation by rotor resistance control, where around 10% to 15% of the power is lost as heat, in the rotor resistance. For a 1000 kW fan, with SPRS the energy saving could be around 200 kW or 4800 kWh per day, equivalent to a saving of Rs. 20,000 @ Rs. 4.20 per unit (one kWh) of energy. That is the saving potential!!

40. 1.“CALCULATION OF SLIP ENERGY RECOVERY INDUCTION MOTOR DRIVE USING THE EQUIVALENT CIRCUIT” research paper by “Pragasen Pillay, Senior member,IEEE,and Larbi Refoufi. 2.“STARTING TRANSIENTS IN SLIP ENERGY RECOVERY INDUCTION MOTOR” research by E. A.K pinar, P. Pillay. MIEEE. 3.“ENERGY RECOVERY” by Paul Blaiklock & William Horvath,TMEICGE,USA. 4.“POWER ELECTRONICS AND VARIABLE FREQUENCY DRIVES” by B.K BOSE, UNIVERSITY OF TENNESSEE, KNOXVILLE. 5.“FUNDAMENTALS OF ELECTRICAL DRIVES” by Gopal K. Dubey, Department of Electrical Engineering, IIT,Kanpur. 6.“A FIRST COURSE ON ELECTRICAL DRIVES” by S.K Pillai, Department of Electrical Engineering, IIT,Bombay. 7.“SLIP POWER RECOVERY SYSTEM” by

41. We are greatly in debt to our guide Mr.Sanjit Kumar Chaudhuri Department of Electrical Engineering, Meghnad Saha Institute of Technology, Kolkata for his unending support and guidance. His invaluable suggestion has helped a lot in completing this presentation.

42. A.SouviK mondal. B.rajat Kumar Bose. C.subrata mondAL. D.ABHIJIT NATH. E.DEBESWAR MISHRA. F.SRIRUP DAS. G.ARUNAVA SANYAL. STUDENT OF ELECTRICAL ENGINEERING DEPARTMENT (2008-2012) MEGHNAD SAHA INSTITUTE OF TECHNOLOGY WEST BENGAL UNIVERSITY OF TECHNOLOGY