Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce EXERCISE OBJECTIVE When you hve completed this exercise, you will know the effects of vrying the rotor resistnce of three-phse wound-rotor induction mchine on the mchine strting current nd torque, s well s on the mchine rottion speed nd efficiency. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Three-phse wound-rotor induction mchine strting current nd torque versus the rotor resistnce Vrying the speed of three-phse wound-rotor induction mchine through the rotor resistnce DISCUSSION Three-phse wound-rotor induction mchine strting current nd torque versus the rotor resistnce Three-phse wound-rotor induction mchines hve n importnt dvntge over three-phse squirrel-cge induction mchines during strt-up: it is possible to reduce the strting current while incresing the strting torque. The bility of wound-rotor induction mchines to reduce the strting current is crucil for lrgesize induction mchines (0.75 MW, or 1000 hp, nd more) becuse these mchines operte with lrge currents, especilly during strt-up. During this period, the current requirements of lrge squirrel-cge induction mchines cn be s high s four times the mchine nominl current. Such high current requirements put gret strin on the c power network nd mke lrge squirrel-cge induction mchines imprcticl or even impossible to strt. Lrge-size induction mchines re lso required to produce high strting torques s they re generlly used to drive high-inerti lods (e.g., lrge pumps, crnes, grinders). In such cses, the bility of three-phse wound-rotor induction mchines to increse the mchine strting torque is beneficil becuse it reduces the mount of time required for the mchine to rech the nominl speed nd shortens the mchine strt-up time (nd thus, the mount of time during which the current drwn by the mchine is mny times the nominl current). The following two sections explin in more detil the bility of three-phse woundrotor induction mchines to reduce the mchine current nd increse the mchine torque during strt-up. Festo Didctic 86367-00 15
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Discussion Three-phse wound-rotor induction mchine strting current The strting current of three-phse wound-rotor induction mchine cn be lowered by incresing the mchine rotor resistnce. The reltionship between the mount of current drwn by three-phse wound-rotor induction mchine s function of the mchine speed for different rotor resistnce vlues is shown in Figure 4. 500 Mchine current (% of nominl current) 400 300 200 100 Short-circuited rotor Low rotor resistnce Moderte rotor resistnce High rotor resistnce 0 25 50 75 100 Mchine speed (% of synchronous speed) Figure 4. Three-phse wound-rotor induction mchine strting current s function of the mchine speed for different rotor resistnce vlues. As Figure 4 shows, the higher the rotor resistnce of three-phse wound-rotor induction mchine, the lower the mchine strting current. For instnce, pssing from short-circuited rotor condition to high rotor resistnce condition reduces the mchine strting current from bout 400% of the mchine nominl current to bout 200% of the mchine nominl current. As lrge-size mchines require lrge mount of current during strt-up, such gin cn mke n enormous difference. Figure 4 lso shows tht, s the mchine ners the synchronous speed, the effect of the mchine rotor resistnce on the mchine current becomes less nd less importnt until, t the synchronous speed, the mchine current is the sme for ny rotor resistnce. 16 Festo Didctic 86367-00
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Discussion Three-phse wound-rotor induction mchine strting torque The strting torque of three-phse wound-rotor induction mchine, like its strting current, cn be djusted by chnging the rotor resistnce. Incresing the rotor resistnce of three-phse wound-rotor induction mchine hs the effect of shifting towrd the left the brekdown torque region on the torque-versus-speed curve. The reltionship between the torque nd speed of three-phse woundrotor induction mchine for different rotor resistnce vlues is shown in Figure 5. 200 Short-circuited rotor Mchine torque (% of nominl torque) 160 120 80 40 Low rotor resistnce Moderte rotor resistnce High rotor resistnce 0 25 50 75 100 Mchine speed (% of synchronous speed) Figure 5. Three-phse wound-rotor induction mchine torque-versus-speed curves for different rotor resistnce vlues. As Figure 5 shows, the higher the rotor resistnce of the three-phse wound-rotor induction mchine (up to certin vlue), the higher the mchine strting torque. However, s the mchine speed increses, high rotor resistnce cuses the mchine torque to decrese more rpidly thn when lower rotor resistnce vlues re used. For this reson, fter the mchine initil strt-up, the rotor resistnce is usully progressively lowered, or simply short-circuited, in order to reduce the rotor resistnce to minimum nd optimize the mchine torque. Figure 6 shows on the sme grph both the current-versus-speed nd torqueversus-speed curves of three-phse wound-rotor induction mchine for different rotor resistnce vlues. As you cn see, when the mchine rotor resistnce is high, the strting current is miniml nd the strting torque is mximl. This is perfectly Festo Didctic 86367-00 17
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Discussion dpted to pplictions where lrge induction mchines re strted under hevy mechnicl lods. 500 200 Short-circuited rotor Mchine current (% of nominl current) 400 300 200 100 Mchine torque (% of nominl torque) 160 120 80 40 Low rotor resistnce Moderte rotor resistnce High rotor resistnce 0 25 50 75 100 Mchine speed (% of synchronous speed) Figure 6. Three-phse wound-rotor induction mchine current-versus-speed nd torque-versusspeed curves for different rotor resistnce vlues. Vrying the speed of three-phse wound-rotor induction mchine through the rotor resistnce Another dvntge three-phse wound-rotor induction mchines hve over threephse squirrel-cge induction mchines is the bility to djust the mchine speed. Trditionlly, this bility ws used in pplictions requiring the mchine to run over wide rnge of speeds. Nowdys, however, the bility to djust the speed of three-phse wound-rotor induction mchines is less nd less used s the sme results cn be chieved more efficiently by controlling three-phse squirrel-cge induction mchines using vrible-frequency motor drives. It is possible to djust the speed of three-phse wound-rotor induction mchines by vrying the rotor resistnce. For ny given torque (within the norml operting rnge of the mchine), the higher the rotor resistnce, the lower the mchine rottion speed. This reltionship is illustrted in Figure 7. In this exmple, threephse wound-rotor induction mchine is coupled to constnt-torque brke tht produces constnt opposing torque. As you cn see, for given mchine torque during motor opertion, the mchine speed decreses s the mchine rotor resistnce increses. 18 Festo Didctic 86367-00
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Discussion 200 Short-circuited rotor Mchine torque (% of nominl torque) 160 120 80 40 Low rotor resistnce Moderte rotor resistnce Mchine speed vrition for given torque High rotor resistnce 0 25 50 75 100 Mchine speed (% of synchronous speed) Figure 7. Three-phse wound-rotor induction mchine speed vrition for given torque for different rotor resistnce vlues. However, vrying the three-phse wound-rotor induction mchine speed in such wy hs drwbcks. The most importnt is tht it reduces the mchine efficiency. This is due to the fct tht dding resistnce to the mchine rotor increses the ctive power losses ( ) nd thus lowers the mchine efficiency. In ddition, the higher the rotor resistnce of three-phse wound-rotor induction mchine, the greter the vrition in the rottion speed s the torque vries. This reltionship is illustrted in Figure 8. As the figure shows, for given torque vrition, the resulting mchine speed vrition is much more importnt when the mchine rotor resistnce is high thn when the mchine rotor is short-circuited. This mens tht three-phse wound-rotor induction mchines hving high rotor resistnce re more susceptible to speed vritions s the lod torque chnges. Festo Didctic 86367-00 19
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Discussion 200 Mchine torque vrition Short-circuited rotor Mchine torque (% of nominl torque) 160 120 80 40 Low rotor resistnce Moderte rotor resistnce Mchine speed vrition s the mchine torque vries High rotor resistnce 0 25 50 75 100 Mchine speed (% of synchronous speed) Figure 8. Three-phse wound-rotor induction mchine speed vrition s the mchine torque vries for different rotor resistnce vlues. 20 Festo Didctic 86367-00
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up nd connections Effect of the rotor resistnce on the opertion of three-phse woundrotor induction mchine PROCEDURE High voltges re present in this lbortory exercise. Do not mke or modify ny bnn jck connections with the power on unless otherwise specified. Set up nd connections In this section, you will set up circuit contining three-phse wound-rotor induction mchine coupled to prime mover/brke. You will then set the mesuring equipment required to study the three-phse wound-rotor induction mchine opertion when the rotor windings re interconnected through resistors. 1. Refer to the Equipment Utiliztion Chrt in Appendix A to obtin the list of equipment required to perform this exercise. Mke sure tht 32 teeth pulley is instlled on the shft of the Three-Phse Wound-Rotor Induction Mchine. If not, sk your instructor to instll the pulley required on the shft of the mchine. Appendix E shows how to replce the pulley instlled on the shft of mchine. Instll the required equipment in the Worksttion. Mke sure you use the sme Four-Qudrnt Dynmometer/Power Supply module s in Exercise 1 of this mnul by confirming tht the module s seril number is the sme s the seril number you recorded in the first step of Exercise 1. Mechniclly couple the Three-Phse Wound-Rotor Induction Mchine to the Four-Qudrnt Dynmometer/Power Supply using the timing belt (type 341L) supplied with the 32 teeth pulley. 2. Mke sure tht the c nd dc power switches on the Power Supply re set to the O (off) position, then connect the Power Supply to three-phse c power outlet. Mke sure tht the min power switch on the Four-Qudrnt Dynmometer/ Power Supply is set to the O (off) position, then connect its Power Input to n c power wll outlet. Connect the Power Input of the Dt Acquisition nd Control Interfce to 24 V c power supply. Festo Didctic 86367-00 21
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Procedure 3. Connect the USB port of the Dt Acquisition nd Control Interfce to USB port of the host computer. Connect the USB port of the Four-Qudrnt Dynmometer/Power Supply to USB port of the host computer. 4. Connect the equipment s shown in Figure 9. L1 40 A terminl L2 L3 40 A terminl Three-phse wound-rotor induction mchine Prime mover/ brke Locl c power network Voltge (V) Frequency (Hz) Rotor resistnce,, () 120 60 171 220 50 629 240 50 686 220 60 629 Figure 9. Three-phse wound-rotor induction mchine with rotor resistnce coupled to prime mover/brke. 22 Festo Didctic 86367-00
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Procedure 5. Mke the necessry switch settings on the Resistive Lod module in order to obtin the rotor resistnce vlue required. The vlues of certin components used in the circuits of this mnul depend on your locl c power network voltge nd frequency. Whenever necessry, tble below the circuit digrm indictes the vlue of ech component for c power network voltges of 120 V, 220 V, nd 240 V, nd for c power network frequencies of 50 Hz nd 60 Hz. Mke sure to use the component vlues corresponding to your locl c power network voltge nd frequency. Appendix C lists the switch settings required on the Resistive Lod in order to obtin vrious resistnce vlues. 6. Turn the Four-Qudrnt Dynmometer/Power Supply on, then set the Operting Mode switch to Dynmometer. This setting llows the Four- Qudrnt Dynmometer/Power Supply to operte s prime mover, brke, or both, depending on the selected function. 7. Turn the host computer on, then strt the LVDAC-EMS softwre. In the LVDAC-EMS Strt-Up window, mke sure tht the Dt Acquisition nd Control Interfce nd the Four-Qudrnt Dynmometer/Power Supply re detected. Mke sure tht the Computer-Bsed Instrumenttion function for the Dt Acquisition nd Control Interfce is vilble. Select the network voltge nd frequency of your locl c power network, then click the OK button to close the LVDAC-EMS Strt-Up window. 8. In LVDAC-EMS, set the Rnge setting of current inputs I1 nd I2 to High. 9. In LVDAC-EMS, open the Four-Qudrnt Dynmometer/Power Supply window, then mke the following settings: Set the Function prmeter to Speed Sweep. Set the Strt Speed prmeter to 100 r/min bove the synchronous speed of the three-phse wound-rotor induction mchine. The synchronous speed of the Three-Phse Wound-Rotor Induction Mchine is 1500 r/min t locl c power network frequency of 50 Hz nd 1800 r/min t locl c power network frequency of 60 Hz. Set the Finish Speed prmeter to 200 r/min below the synchronous speed of the three-phse induction motor. Set the Number of Steps prmeter to 15 steps. Set the Step Durtion prmeter to 7 s. Set the Record Dt to Tble prmeter to Yes. Set the Pulley Rtio prmeter to 24:32. Festo Didctic 86367-00 23
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Procedure 10. In LVDAC-EMS, strt the Metering ppliction. Mke the required settings in order to mesure the rms vlues (c) of the three-phse wound-rotor induction mchine line current (input I1). Set two other meters to mesure the mchine ctive power nd rective power using the two-wttmeter method (meter function PQS1 PQS2). Finlly, set meter to mesure the mchine power fctor from inputs E1, I1, E2, nd I2 (meter function PF(EI1,EI2)). 11. In LVDAC-EMS, open the Dt Tble window. Set the Dt Tble to record the three-phse wound-rotor induction mchine speed, torque, nd mechnicl power indicted in the Four-Qudrnt Dynmometer/Power Supply window. Also, set the Dt Tble to record the three-phse wound-rotor induction mchine line current (input I1), ctive power, rective power, nd power fctor indicted in the Metering ppliction. Effect of the rotor resistnce on the opertion of three-phse wound-rotor induction mchine In this section, you will mke the three-phse wound-rotor induction mchine speed decrese by step from 100 r/min bove the mchine synchronous speed to 0 r/min, recording t ech step in the Dt Tble the mchine speed, torque, mechnicl power, line current, ctive power, rective power, nd power fctor. You will clculte the mchine efficiency using the recorded mchine mechnicl power nd ctive power vlues. On the sme grph, you will plot the torque versus speed curve when the rotor is short-circuited (using the results you obtined in the previous exercise), s well s when resistnce is dded to the rotor, nd interpret the results. Finlly, you will plot on the sme grph the efficiency versus mechnicl power curve when the rotor is short-circuited (using the results you obtined in the previous exercise), s well s when resistnce is dded to the rotor, nd interpret the results. 12. On the Power Supply, turn the three-phse c power source on. The Three-Phse Wound-Rotor Induction Mchine is fitted with n overvoltge protection circuit preventing the rotor voltge from reching vlues tht could dmge the mchine rotor windings. If the overvoltge protection trips when performing the mnipultion bove, turn the three-phse c power source off, wit few seconds, nd turn the three-phse c power on while pressing nd holding the Protection Override push-button on the Three-Phse Wound-Rotor Induction Mchine. Before strting the prime mover/brke (i.e., before strting the Speed Sweep function), mke sure tht the Three-Phse Wound-Rotor Induction Mchine is rotting in the clockwise direction. If so, proceed directly to the next step. Otherwise, turn the three-phse c power source off, invert the connections t two of the three phse terminls of the mchine sttor windings, then repet this step from the beginning. 24 Festo Didctic 86367-00
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Procedure 13. In the Four-Qudrnt Dynmometer/Power Supply window, strt the Speed Sweep function. 14. Wit for the Speed Sweep function to complete its sweep of the specified speed intervl. Then, in the Four-Qudrnt Dynmometer/Power Supply window, mke the following settings: Set the Strt Speed prmeter to 40 r/min below the speed vlue t which you set the Finish Speed prmeter in step 9. Set the Finish Speed prmeter to 0 r/min. Set the Number of Steps prmeter to vlue between 13 nd 16 steps. Mke sure tht the Step Durtion prmeter is set to 7 s. Mke sure tht the Record Dt to Tble prmeter is set to Yes. Mke sure tht the Pulley Rtio prmeter is set to 24:32. 15. In the Four-Qudrnt Dynmometer/Power Supply window, strt the Speed Sweep function. Wit for the Speed Sweep function to complete its sweep of the specified speed intervl. If the overvoltge protection trips t the end of the speed sweep performed bove (i.e., when the speed of the three-phse wound-rotor induction mchine pproches 0 r/min), stop the Speed Sweep function, turn the three-phse c source power source off, delete the dt recorded to the Dt Tble during this speed sweep, turn the three-phse c power source on, nd strt the Speed Sweep function gin. This time, however, press nd hold the Protection Override push-button on the Three-Phse Wound-Rotor Induction Mchine s soon s the speed decreses below 200 r/min. Relese the push-button once the speed sweep is completed. 16. When ll dt hs been recorded, turn the three-phse c power source in the Power Supply off. Festo Didctic 86367-00 25
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Procedure 17. In the Dt Tble window, sve the recorded dt, then export it to spredsheet ppliction. The mchine torque recorded when the mchine speed is equl to 0 r/min is slightly underestimted due to the wy friction compenstion is implemented in the Four-Qudrnt Dynmometer/Power Supply. In the spredsheet ppliction, dd new prmeter to the results: the threephse wound-rotor induction mchine efficiency. To clculte the mchine efficiency, use the following equtions: When the mchine mechnicl power is of positive polrity (i.e., when the mchine opertes s motor), divide ech mchine mechnicl power vlue by the corresponding mchine ctive power vlue, then multiply the result by 100 to express the efficiency s percentge. When the mchine mechnicl power is of negtive polrity (i.e., when the mchine opertes s genertor), divide ech mchine ctive power vlue by the corresponding mchine mechnicl power vlue, then multiply the result by 100. Do not clculte the mchine efficiency when the mchine mechnicl power is lower thn bout 50 W (positive or negtive). Also, in the spredsheet ppliction, mke sure tht ll power fctor vlues re of positive polrity by inverting the polrity of ll negtive power fctor vlues. 18. Plot on the sme grph curves of the three-phse wound-rotor induction mchine torque s function of the mchine speed when the rotor is shortcircuited, s well s when resistnce is dded to the rotor. Use the results you recorded in the previous exercise nd in this exercise to do so. Observe the grph you plotted. How does dding resistnce to the rotor ffect the strting torque of three-phse wound-rotor induction mchine? Compre the strting current (line current t speed of 0 r/min) of the threephse wound-rotor induction mchine with short-circuited rotor (recorded in the previous exercise) with the strting current of the three-phse wound-rotor induction mchine with rotor resistnce (recorded erlier in this exercise). How does dding resistnce to the rotor ffect the strting current of three-phse wound-rotor induction mchine? 26 Festo Didctic 86367-00
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Conclusion Does this confirm tht three-phse wound-rotor induction mchine with dded resistnce t the rotor is perfectly dpted to pplictions where the mchine hs to strt under hevy mechnicl lods? Yes No 19. Observe the grph you plotted in the previous step. Wht is the effect, if ny, of dding resistnce to the rotor of three-phse wound-rotor induction mchine on the mchine torque versus speed curve? Do your results confirm tht incresing the resistnce t the rotor of threephse wound-rotor induction mchine chnges the mchine speed for given mchine torque? Yes No 20. Plot on the sme grph curves of the three-phse wound-rotor induction mchine efficiency s function of the mchine mechnicl power with short-circuited rotor, s well s with rotor resistnce, when the mchine opertes s motor ( 50 W). Use the results you recorded in the previous exercise nd in this exercise to do so. Do not plot on the grph the points recorded fter the motor mechnicl power strted to decrese fter hving reched mximl vlue. Observe the grph. Wht is the effect, if ny, of dding resistnce to the rotor of three-phse wound-rotor induction mchine on the mchine efficiency during motor opertion? 21. Close LVDAC-EMS, then turn off ll the equipment. Disconnect ll leds nd return them to their storge loction. Ask your instructor to replce the 32 teeth pulley instlled on the Three-Phse Wound-Rotor Induction Mchine with 24 teeth pulley. Appendix E shows how to replce the pulley instlled on the shft of mchine. CONCLUSION In this exercise, you lerned the effects of vrying the rotor resistnce of threephse wound-rotor induction mchine on the mchine strting current nd torque, s well s on the mchine rottion speed nd efficiency. Festo Didctic 86367-00 27
Exercise 2 Three-Phse Wound-Rotor Induction Mchine with Rotor Resistnce Review Questions REVIEW QUESTIONS 1. Describe the effect of incresing the rotor resistnce of three-phse woundrotor induction mchine on the mchine strting current. 2. Describe the effect of incresing the rotor resistnce of three-phse woundrotor induction mchine on the mchine strting torque. 3. Why is the rotor resistnce of three-phse wound-rotor induction mchine usully short-circuited fter the mchine initil strt-up? 4. How is it possible to vry the speed of three-phse wound-rotor induction mchine? 5. Wht is the min drwbck of lowering the speed of three-phse woundrotor induction mchine using the mchine rotor resistnce? 28 Festo Didctic 86367-00