Three-Phase Wound-Rotor Induction Machine with a Short- Circuited Rotor

Transcription

1 Exercise 1 Three-Phse Wound-Rotor Induction Mchine with Short- Circuited Rotor EXERCISE OBJECTIVE When you hve completed this exercise, you will know how three-phse woundrotor induction mchine cn operte s three-phse squirrel-cge induction mchine. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Three-phse wound-rotor induction mchine with short-circuited rotor windings DISCUSSION Three-phse wound-rotor induction mchine with short-circuited rotor windings As previously mentioned, three-phse wound-rotor induction mchines re generlly used insted of three-phse squirrel-cge induction mchines in pplictions requiring mchines with high power rting (bout 0.75 MW, or 1000 hp, nd more). This is due to the fct tht the construction design of the squirrel cge in lrge induction mchines renders them imprcticl to build. Threephse wound-rotor induction mchines re not subject to such limittion, s their rotor is primrily mde of wire wound over the lminted rotor core. Mking threephse wound-rotor induction mchine operte s three-phse squirrel-cge induction mchine is chieved simply by short-circuiting the rotor windings of the three-phse wound-rotor induction mchine. The primry difference between squirrel-cge rotors nd wound rotors is tht the min components of squirrel-cge rotors (i.e., the conducting brs) re permnently short-circuited while the min components of wound rotors (i.e., the windings) re not. This is why it is possible to ccess ech winding of wound rotors seprtely nd impossible to do so for squirrel-cge rotors. If the rotor windings of three-phse wound-rotor induction mchine re short-circuited, however, the primry difference between squirrel-cge rotors nd wound rotors is eliminted. Thus, when the rotor windings of three-phse wound-rotor induction mchine re short-circuited, the mchine opertes exctly s squirrel-cge induction mchine. This mens tht the rotting mgnetic field creted in the sttor induces three-phse voltges in the rotor windings. The induced voltges then cuse current to flow in the rotor windings (s in the conducting brs of squirrel-cge rotors). The currents flowing in the rotor windings crete strong mgnetic field. The interction between the rotting mgnetic field in the sttor nd the mgnetic field creted by the currents in the rotor cretes force between the sttor nd the rotor. This force cuses the rotor to be pulled long in the sme direction s the rotting mgnetic field in the sttor. The torque produced is proportionl to the strength of the sttor rotting mgnetic field nd the strength of the rotor mgnetic field. Festo Didctic

2 Exercise 1 Three-Phse Wound-Rotor Induction Mchine with Short-Circuited Rotor Procedure Outline When the rotor windings of three-phse wound-rotor induction mchine re short-circuited, the mchine chrcteristics (e.g., the torque versus speed curve, the efficiency versus mechnicl power curve, the ctive power nd rective power versus mechnicl power curves) re very similr to the chrcteristics of squirrel-cge induction mchine, whether the mchine opertes s motor or s genertor. PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up nd connections Three-phse wound-rotor induction mchine opertion with shortcircuited rotors 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 is short-circuited. 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. Record below the seril number of the Four-Qudrnt Dynmometer/Power Supply tht you re using. Seril number: 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. 6 Festo Didctic

3 Exercise 1 Three-Phse Wound-Rotor Induction Mchine with Short-Circuited Rotor Procedure 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. Turn the 24 V c power supply on. 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 3. L1 40 A terminl L2 L3 40 A terminl Three-phse wound-rotor induction mchine Prime mover/ brke Figure 3. Three-phse wound-rotor induction mchine with short-circuited rotor coupled to prime mover/brke. Appendix D shows in more detil the equipment nd the connections tht re required for ech circuit digrm symbol used in this mnul. 5. 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. Festo Didctic

4 Exercise 1 Three-Phse Wound-Rotor Induction Mchine with Short-Circuited Rotor Procedure 6. 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. 7. In LVDAC-EMS, set the Rnge setting of current inputs I1 nd I2 to High. 8. In LVDAC-EMS, open the Four-Qudrnt Dynmometer/Power Supply window, then mke the following settings: Set the Function prmeter to Speed Sweep. This function mkes the Four-Qudrnt Dynmometer/Power Supply operte s constntspeed prime mover/brke whose speed vries over rnge defined by the Strt Speed nd Finish Speed prmeters in specified number of steps (determined by the Number of Steps prmeter) of equl time durtion. The function lso llows recording of the motor prmeters in the Dt Tble t ech step of the speed sweep. Set the Strt Speed prmeter to 100 r/min bove the synchronous speed of the three-phse wound-rotor induction mchine. This sets the speed t which the constnt-speed prime mover/brke mkes the three-phse induction motor rotte during the first step of the speed sweep to the synchronous speed. 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. This setting determines the speed t which the constnt-speed prime mover/brke mkes the three-phse induction motor rotte during the lst step of the speed sweep. Set the Number of Steps prmeter to 15 steps. This setting determines the number of steps tht the constnt-speed prime mover/brke tkes while vrying the speed t which it mkes the three-phse induction motor rotte during the speed sweep. Set the Step Durtion prmeter to 7 s. This setting determines the time durtion of ech step of the speed sweep. Set the Record Dt to Tble prmeter to Yes. This settings mkes the Dt Tble record the vrious prmeters (determined by the Record Settings of the Dt Tble) of the three-phse induction motor t the end of ech step of the speed sweep. Set the Pulley Rtio prmeter to 24:32. 8 Festo Didctic

5 Exercise 1 Three-Phse Wound-Rotor Induction Mchine with Short-Circuited Rotor Procedure 9. 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)). 10. 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. Three-phse wound-rotor induction mchine opertion with short-circuited rotors 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 motor speed, torque, mechnicl power, line current, ctive power, rective power, nd power fctor. You will clculte the mchine efficiency using the mchine mechnicl power nd ctive power vlues you recorded. You will plot grph of the three-phse woundrotor induction mchine torque s function of the mchine speed, nd interpret the results. You will then plot grph of the three-phse wound-rotor induction mchine line current, ctive power, rective power, power fctor, nd efficiency s function of the mchine mechnicl power, nd interpret the results. 11. On the Power Supply, turn the three-phse c power source on. 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. Festo Didctic

6 Exercise 1 Three-Phse Wound-Rotor Induction Mchine with Short-Circuited Rotor Procedure 12. In the Four-Qudrnt Dynmometer/Power Supply window, strt the Speed Sweep function. 13. 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 8. 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. 14. In the Four-Qudrnt Dynmometer/Power Supply window, strt the Speed Sweep function. 15. Wit for the Speed Sweep function to complete its sweep of the specified speed intervl. Then, when ll dt hs been recorded, turn the three-phse c power source in the Power Supply off. 10 Festo Didctic

7 Exercise 1 Three-Phse Wound-Rotor Induction Mchine with Short-Circuited Rotor Procedure 16. 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. Festo Didctic

8 Exercise 1 Three-Phse Wound-Rotor Induction Mchine with Short-Circuited Rotor Conclusion 17. Plot grph of the three-phse wound-rotor induction mchine torque s function of the mchine speed using the results you imported from the Dt Tble. 18. Plot grph of the three-phse wound-rotor induction mchine ctive power nd rective power s function of the mchine mechnicl power using the results you imported from the Dt Tble. Do not plot on the grph the points recorded fter the motor mechnicl power strts to decrese fter hving reched mximl vlue. 19. Plot grph of the three-phse wound-rotor induction mchine line current s function of the mchine mechnicl power using the results you imported from the Dt Tble. Do not plot on the grph the points recorded fter the mchine mechnicl power strts to decrese fter hving reched mximl vlue. 20. Plot grph of the three-phse wound-rotor induction mchine power fctor s function of the mchine mechnicl power using the results you imported from the Dt Tble. Do not plot on the grph the points recorded fter the motor mechnicl power strts to decrese fter hving reched mximl vlue. 21. Plot grph of the three-phse wound-rotor induction mchine efficiency s function of the mchine mechnicl power when the mchine opertes s motor ( 50 W) using the results you recorded in the spredsheet ppliction. Do not plot on the grph the points obtined fter the motor mechnicl power strts to decrese fter hving reched mximl vlue. 22. Do the observtions you hve mde in this exercise confirm tht three-phse wound-rotor induction mchine whose rotor is short-circuited opertes s squirrel-cge induction mchine? Yes No 23. 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 tht three-phse wound-rotor induction mchine opertes s three-phse squirrel-cge induction mchine when the mchine rotor windings re short-circuited. 12 Festo Didctic

9 Exercise 1 Three-Phse Wound-Rotor Induction Mchine with Short-Circuited Rotor Review Questions REVIEW QUESTIONS 1. Wht re the three min dvntges of three-phse wound-rotor induction mchines over three-phse squirrel-cge induction mchines? 2. Wht is the primry difference between the min components of squirrel-cge rotors (i.e., the conducting brs) nd the min components of wound rotors (i.e., the windings)? 3. Wht is the effect of short-circuiting the rotor windings of three-phse woundrotor induction mchine on the mchine opertion? 4. Wht hppens to the ctive power of three-phse wound-rotor induction mchine whose rotor windings re short-circuited s the mchine mechnicl power increses? 5. Wht hppens to the rective power of three-phse wound-rotor induction mchine whose rotor windings re short-circuited s the mchine mechnicl power increses? Festo Didctic