Determining the terminals of a three
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UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE II NATIONAL DIPLOMA IN ELECTRICAL ENGINEERING TECHNOLOGY Prony brake L1 A Tach U1 L2 V1 Supply 3Ø 0-415V Ac 0-1A dc U2 A V2 V W2 + Supply 0-250V dc _ W1 L3 ELECTRICAL MACHIENS I I COURSE CODE: EEC233 YEAR II- SEMESTER III PRACTICAL Version 1: December 2008 ELECTRICAL ENGINEERING PROGRAM COURSE CONTENT Subject Machine II Year 2 Semester 3 Course Code EEC233 Credit Hours 3 Theoretical 1 Practical 2 CHAPTER 1 : Three-phase Synchronous Generator Assessment CHAPTER 1 2 : Three-phase Synchronous Motor Assessment CHAPTER 3 Assessment CHAPTER Week 1-2 2 Weeks 3 - 4 : Induction motor Weeks 5 - 8 3 4 : Protective and Control Device Assessment Weeks 9 – 10 4 Final Exam Contents Chapter1: Three-phase Synchronous Generator.................Week1 1.1 Determining the terminals of a three - phase synchronous generator………………………… ……………………………2 1.1.1 Star connection of three-phase synchronous generator ............. 6 1..1.2 Delta connection of three-phase synchronous generator ............ 6 1.2 Dismantle and Re-assemble three-phase synchronous generator ............ 7 1.3 No-Load Characteristic of Three-phase Synchronous Generator .......... 17 1.3.1Relationship between excitation current and output terminal voltage 11 1.3.2Relationship between speed and output terminal voltage ............... 02 Chapter2: Three-phase Synchronous Motor.......................Week3 0.3 Torque – Speed Characteristic ................................................................ 02 .04 Torque – Current Characteristic ............................................................. 02 Chapter3: Induction motor…………………. ....................Week5 3.1Dismantle and Re-assemble three-phase Induction Motor………….… 31 3.2Determine the three phase induction motor terminals…..………………34 3.3Three-phase Induction motor under load……………………………..…39 3.4Measuring the electrical quantities of 3-phase induction motor….… 42 3.5Connecting the 3-phase dual speed squirrel cage induction motor……..47 Chapter4: Protective and Control Device ...........................Week9 4.1 Circuit breaker…………...……………………………………………. 20 4.1.1Disassembling and re-assembling Circuit breaker ........................ 20 4.1.2Use the Circuit breaker in the protected circuit ............................ 25 4.2 Contactor………………………………………………………………. 26 4.2.1 Disassembling and re-assembling the contactor ........................................ 26 4.2.2 Troubleshooting and maintenance ............................................................. 25 4.2.3 Use the contactor in the control circuit................................................... 61 Week 1 Three-phase Synchronous Generator Introduction Alternators or three phase synchronous generator are designed to run at a specific speed (known as synchronous speed) to produce voltage at a specific frequency. That's why they are referred to as "synchronous alternators". Alternators are driven at synchronous speed by a prime mover. Typical prime movers are diesel engines, jet engines, steam turbines, hydro turbines, and DC motors. The field coil of a synchronous alternator is wound on a rotor, which has salient poles. The laminated iron core is called the "spider". The armature coils are imbedded in slots on the stator. As the rotor (field) is driven, its magnetic field sweeps around inside the housing. This moving field is cut by the turns of the armature coil of the stator. This induces a voltage into the armature coils. The amount of voltage induced depends on two things: (1) the speed of the rotor and (2) the strength of the magnetic field. Magnetic field strength, in turn, depends on the amount of current passing through the field coil. As the current increases, so does the field strength up to a point. That point we call saturation. When the spider becomes magnetically saturated, further increase of field current produce little or no further increase of field strength. Thus, the alternator's terminal voltage levels off. Determining the terminals of a three - phase synchronous generator Introduction A three - phase synchronous generator consists of two types of windings, armature winding ( coils ) and field coil. Armature coils are consists of three separate coils equally in resistance. The two leads from each coil are brought out to the generator terminals box which is usually present on the alternator enclosure as shown in figure (1-1). The armature terminals are marked U1 V1 W1 F1 W2 U2 V2 Terminal Box F2 E Figure (1-1) : Terminal Box of three phase synchronous generator - Ul and U2 represent the first and the second terminals of the first coil. - V1 and V2 represent the first-and the second terminals of the second coil. - W1 and W2 represent the first and the second terminals of the third coil. - Also F1 and F2 represents the first and the second terminals of the field coil (rotor coil), Which are brought out to the generator terminals box via a two slip rings and two brushes. - E represents the earthing connection. Apparatus 1 Three-phase synchronous generator 2. Tool Box 3. Ohmmeter Procedure 1. Open the terminals box of the synchronous generator. 2. Connect one lead from the ohmmeter to the first terminal (U1) from the armature terminals, and connect the second lead from the ohmmeter with another terminals from the armature, One terminal only (U2) from them, which will cause the ohmmeter indicates resistance with U1 as shown figure (1-2) Three-phase synchronous generator Rotor Stator Field coil Slip Ring U1 U2 V1 V2 Terminal Box W1 U1 V1 W1 W2 U2 V2 F1 F2 W2 E Ohmeter ∞ _ 0 + Ω Figure (1-2) 3. Record the reading of the ohmmeter in your work book , table (1-1) 4. Repeat step2 to find the second coil (V1 and V2) as shown in figure ( 1-3 ) Three-phase synchronous generator Rotor Stator Field coil Slip Ring U1 U2 V1 V2 Terminal Box W1 U1 V1 W1 W2 U2 V2 F1 F2 W2 E Ohmeter ∞ _ 0 Ω + Figure (1-3) 5. Record the reading of the ohmmeter in your work book , table (1-1) 6. Repeat step2 to find the third coil (W1 and W2) as shown in figure (1-4) Three-phase synchronous generator Rotor Stator Field coil Slip Ring U1 U2 V1 V2 Terminal Box W1 U1 V1 W1 W2 U2 V2 F1 F2 W2 E Ohmeter ∞ _ 0 Ω + Figure (1-4) 7. Record the reading of the ohmmeter in your work book , table (1-1) 8. To find the field coil terminals (F1 and F2), connect the ohmmeter lead. to ((F1 and F2) as shown in figure ( 1-5 ), The ohmmeter will indicates resistance . Then remove the carbon brush holders, If the ohmmeter indicates an open circuit . These leads represent the field coil terminals. Three-phase synchronous generator Rotor Stator Field coil Slip Ring U1 U2 V1 V2 Terminal Box W1 U1 V1 W1 W2 U2 V2 F1 F2 W2 E Ohmeter ∞ _ 0 Ω + Figure (1-5) 9. Record the reading of the ohmmeter in your work book , table (1-1) Star connection of three-phase synchronous generator 10. Connect the armature terminals in star connection as shown in figure (1-6), measure the resistance between (L1 and L2), (L2 and L3) and (L1 and L3). L1 L1 L3 Terminal Box Three-phase synchronous generator Stator Rotor U1 L2 V1 Field coil Slip Ring U1 U2 V2 L3 L2 W2 W1 Star connection V1 W1 + _ F1 + _ F2 W2 V2 U2 Terminal Box E Figure (1-6) : Star connection of three-phase synchronous generator 11. Record the reading of the ohmmeter in your work book , table (1-2) Delta connection of three-phase synchronous generator 12. Connect the armature terminals in star connection as shown in figure (1-7), measure the resistance between (L1 and L2), (L2 and L3) and (L1 and L3). L1 L1 L2 L3 Terminal Box Three-phase synchronous generator Stator Rotor L2 Field coil U1 U2 V1 Slip Ring U1 V1 W1 F1 W2 + _ + _ V2 L3 F2 W2 W1 U2 V2 E Terminal Box Delta connection Figure (1-7): Delta connection of three-phase synchronous generator 13. Record the reading of the ohmmeter in your work book , table (1-2) Dismantle 1.2 and Re-assemble three-phase synchronous generator Introduction Dismantle and reassemble three phase synchronous generator or called alternator is an essential operation to know the main parts and be familiarize with them for repairing and connection purposes. Apparatus Screw driver set Hammer Tools box Puller Rubber hammer Squirrel cage three-phase induction motor Grease Procedure : a) Dismantle three-phase synchronous generator 1. Get alternator from auto section in a good condition. mark on both halves of the alternator case to mark its' position. 2. Loosen three screws and remove rear plastic cover with pushing three lock tabs on the side of the cover. 3. Loosen two screws of the IC regulator and remove the IC regulator. Next, loosen three screws of rectifier. 4. Crimped terminal of the stator coil wire.. 5. Break the terminal by using the diagonal cutter pliers at six terminals. Work carefully so as not to break stator coil wires. 6. Check slip rings and contact terminal of IC regulator. A slip rings is quite worn. 7. Removed rectifier. 8. Loosen the four Phillips bolt head screws located around the outer perimeter of the rear case. 9. Using a flat head screwdriver and turn a screwdriver carefully at four sides. 10. After remove 11. Shaved and clean up the slip rings and remove any oxidation of the voltage regulator contacts face. I shaved the slip ring by the flat file. 12. Replace the fitting ring to new one for new bearing. 13. Remove pulley lock nut by the impact wrench. 14. Four screws can be accessed when removing a pulley. The screw is fixing the cover of the front bearing. This screw removes using the impact screwdriver because it is difficult for it to loosen. 15. Rotor. a little bit rusty. 16. The bearing retaining plate fixing the front bearing to the front casing. 17. Remove Be the the rear most bearing careful of by puller. this work. Attach the push bolt of the puller to the steel shaft of the rotor. Never attach it to the slip ring. Because the slip ring part made by the resin, 18. I guess that it's break easily. If you need the slip ring repair parts, check this out. 19. All parts. 20. The bearing retaining plate fixing the front bearing to the front casing. b) Re-assemble three-phase synchronous generator 21. Install bearing by the rubber hammer. 22. Install the bearing . However front bearing didn't need it. Because the bearing should be inserted in the front case like this. 23. Install the bearing retaining plate. 24. Install the rectifier. 25. Crimp the stator coil wire and the rectifier terminal. 26. Carbon brush of the IC regulator 27. Brush wire is crimped and soldered. 28. Reassemble carbon brush to the IC regulator. 29. Soldered. 30. Installed. 31. Re-installed the pulley and the thrust washer. The reassembling complete. Worksheet 01 Solve the following questions: 1- Dismantle and Re-assemble three-phase synchronous generator and write each step ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... 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Week 2 No-Load 1.3 Characteristic of Three-phase Synchronous Generator Introduction Un like DC generator, synchronous must be excited through their rotor in order to induce a voltage in the stator. A synchronous must be driven at definite constant speed because the frequency of the generator voltage is determined by that rotor speed. The amplitude of the generated voltage is proportional to the rotor speed and to value of the DC flux. Before a synchronous is ready to deliver voltage to a load: a) It must be brought to synchronous speed. b) It must be excited from a separate DC source c) Its terminal voltage must be adjusted to the correct value with the field rheostat. Apparatus 4. One Dc machine 5. One synchronous machine 6. One power supply 7. One AC instrumentation group 8. One DC instrumentation group 9. One tachometer 1.3.1 Relationship between excitation current and output terminal voltage Procedure 1. Couple the synchronous generator to Dc motor as prime mover. 2. Connect the dc motor as in figure 1-9 0-1A dc A Supply 0-250V dc F1 F2 S1 S2 A1 A2 L1 Tach U1 L2 V1 0-1A dc U2 A V2 N V z 0-300V AC W2 + Supply 0-250V dc _ W1 L3 Figure (1-9) 3. Connect the DC excitation supply to the field coil of the synchronous generator as shown in Figure (1-9). Do not turn the power ON yet. 4. Turn the field rheostat knob on the motor fully counterclockwise to its minimum resistance position. Turn the voltage control knobs of the two DC power supplies fully counterclockwise to their zero output position. 5. Turn ON the main AC circuit breaker; and run the motor. 6. Slowly increase the output of the DC supply to the rated volts to start the motor. Be sure the alternator's switch is in the SYNC RUN position. Then turn ON the DC excitation supply. 7. Set the Tachometer for 1500 RPM. Use the motor's field rheostat to adjust motor speed to 1500 RPM. 8. Adjust the output of the DC supply until the alternator's field current is approximately 0.1 ampere, and take your readings there. 9. Record the exact value of field current and terminal voltage in TABLE 1-3. that the voltage across each of the three armature coils is the same, since the being produced by the same field. 10. Repeat steps 8 and 9 for the following values of alternator field current: 0.2 0.4, and 0.5 amps. 11. Slowly decrease the excitation voltage until the ammeter reads approximate 0.4 amps. Record the exact value of field current and terminal voltage in TABLE 1-3 12. Repeat step 11 for the following approximate values of field current: 0.3, 0.2, 0.1 and 0 amperes. 13. Turn OFF all circuit breaker switches. Disconnect all leads. 1.3.2 Relationship between speed and output terminal voltage Procedures 1. Couple the synchronous generator to Dc motor as prime mover. 2. Connect the dc motor as in figure 1-9 Do not turn the power ON yet. 3. Connect the DC excitation supply to the field coil of the alternator as shown in figure 1-9. Do not turn the power ON yet. 4. Turn the field rheostat knob on the motor fully counterclockwise to its minimum resistance position. Turn the voltage control knobs of the two DC power supplies fully counterclockwise to their zero output positions. 5. Turn ON the main AC circuit breaker; DC circuit breaker and the motor. 6. Slowly increase the output of the DC excitation supply to the rated voltage to start the motor. 7. Increase the speed to the motor by turning the field rheostat clockwise until speed is 1500 RPM. 8. Be sure the alternator's switch is in the SYNC RUN position. Then turn ON the DC excitation supply. 9. Adjust the output of the supply until the alternator is generating 240 volts. 10. Adjust the motor's field rheostat until the motor is running at 1400 RPM. Record the terminal voltage in TABLE 1-4. Repeat Steps 8 and 9 for 1300, 1200 and 1000 RPM. 11. Turn OFF all circuit breaker switches. Disconnect all leads Worksheet 02 Solve the following questions: 1. Connect the circuit as shown in figure 1-9, Operate, record your results in Table 1-3 and plot the graph of terminal voltage versus Field current (If VS Vph) Table(1-3) If Vph (Amp) (Volts) Graph 1-1 : the relationship (If VS Vph) 2. Connect the circuit as shown in figure 1-9, Operate, record your results in table (1-4) and plot the graph of Terminal voltage versus prime mover speed (N VS Vph) Table(1-4) Speed (N) Vph (RPM) (Volts) Graph 1-2 : The relationship (N VS Vph) 3. What effect did saturation have on terminal voltage? ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... 4. From your observations would you regard residual magnetism in the rotor core an important or unimportant factor? ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... 5. What led you to this conclusion? ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... 6. Using the equation f = S x P where f is the frequency in hertz; S is speed in revolutions per second; and P is the number of pairs of poles, compute the frequency of the generated voltage at 1500 RPM. (if 4 pole machine). ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... 7. Compute the frequency at 1400 RPM. ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... 8. Compute the frequency at 1300 RPM ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... Week 3 Three-phase Synchronous Motor Introduction When three-phase is applied to the stator of a three-phase motor, a revolving stator magnetic field is created. This field revolves at synchronous speed, which is a speed determined by the number of poles per phase of the motor and the frequency of the incoming power. The rotor of a synchronous motor becomes "locked in" on the revolving stator field. It then rotates at synchronous speed. To accomplish this, the rotor contains a DC field winding. The problem is in starting. If you have DC applied to the field coil, while the rotor is standing still, the revolving field passes the stationary field much too fast to be locked onto. First the DC field coils on the rotor must be made to rotate almost as fast as the revolving stator field. Then, when you apply DC to it, the rotor is pulled into synchronism. That means that the rotor turns at synchronous speed. To get the rotor turning in the first place, a squirrel-cage winding is used. The bars are imbedded in the rotor core. When power is applied to the stator, the revolving field induces voltage into these windings. In other words, a synchronous motor starts as an induction motor. When the rotor reaches 95% of synchronous speed, DC is switched into the rotor field winding. Now, during the start process, there will also be voltage induced into the DC field winding as the rotor turns. Rather than have a charged-up field coil, its terminals are shorted through a resistor while the squirrel-cage winding is getting the rotor started. Because synchronous motors must achieve 95% synchronous speed before being synchronized, they are rarely started under load. Load is applied after it is running as a synchronous motor. The rotor, however, continues to turn at synchronous speed. It is possible to load a synchronous motor beyond its ability to stay in synchronism. The counter-torque of a load overcomes the torque (pull) on the rotor from the revolving stator field. When that happens, the motor pulls out of step with the stator field. It will not simply fall back to running smoothly as an induction motor, however. Induced currents, added to the excitation current in the DC field winding, make the rotor pulsate. Therefore, field excitation should be removed as soon as possible after the rotor pulls out of synchronism. Then, if you want to re-synchronize the motor, you must first remove the overload. The speed of the synchronous motor stays constant under load. However when the load increases, we notice: d) That the line current increase. e) That the speed changes momentarily during load changes. f) That the motor can fall out of synchronism if the load is higher than twice the rated capacity. Torque – Speed Characteristic Apparatus 10. One synchronous machine 11. One power supply 12. One Prony Brake 13. One AC instrumentation group 14. One DC instrumentation group 15. One tachometer Procedure : 1. Couple the Prony Brake to the synchronous motor. 2. Connect the circuit shown in figure 2-1 Prony brake L1 A Tach U1 L2 V1 Supply 3Ø 0-415V Ac 0-1A dc U2 A V2 V W2 W1 L3 Figure (2-1) + Supply 0-250V dc _ 3. Start the motor and make sure its rotation is in the "right direction for the scale of the Prony brake. 4. Adjust the field rheostat for nominal speed . 5. Load the motor by prony brake in five steps to 125% of rated capacity. 6. Record your readings in Table 4-1 for each step. 7. Turn off the power supply. Worksheet 03 Torque (T) Speed (N) 1. Connect the circuit as shown in figure (2-1), (N.m) (RPM) Operate, record your results in Table 2-1 and plot 0 Solve the following questions: the graph of motor speed versus applied torque (T VS N) Table(2-1) Graph 2-1 Week 4 Torque – Current Characteristic Apparatus 1. One synchronous machine 2. One power supply 3. One Prony Brake 4. One AC instrumentation group 5. One DC instrumentation group Procedure 1. Couple the Prony Brake to the synchronous motor. 2. Connect the circuit shown in figure 2-2 Prony brake L1 A U1 L2 V1 Supply 3Ø 0-415V Ac 0-1A dc U2 A V2 W2 V + Supply 0-250V dc _ W1 L3 Figure (2-2) 3. Start the motor and make sure its rotation is in the "right direction for the scale of the Prony brake. 4. Adjust the field rheostat for nominal speed . 5. Load the motor by prony brake in five steps to 125% of rated capacity. 6. Record your readings in Table 4-1 for each step. 7. Turn off the power supply. Worksheet 04 Solve the following questions: Connect the circuit as shown in figure (2-2), Operate, record your results in Table 2-2 and plot the graph of line current versus applied torque (T VS I) Torque (T) Current (I) (N.m) (A) 0 Table (2-2) Graph 2-2 Week 5 3. Induction motor Three-phase motors vary from fractional-horsepower size to several thousand horsepower. These motors have a fairly constant speed characteristic and are made in designs giving a variety of torque characteristics. Some three-phase motors have a high starting torque; others, low starting torque. Some are designed to draw a normal starting current while others, 'high starting current. They are made for practically every standard voltage and frequency and are very often dual-voltage motors. Three-phase motors are used in different applications such as: to drive machine tools, pumps, elevators, fans, cranes, hoists, blowers, and many other machines. 3.1 Dismantle and Re-assemble three-phase Induction Motor Introduction Dismantle and reassemble three phase induction motor is an essential operation to know the main parts and be familiarize with them for repairing and connection purposes Apparatus Screw driver set Hammer Tools box Puller Rubber hammer Squirrel cage three-phase induction motor Grease Procedure : 1. Record the information of the motor from the nameplate in your worksheet. 2. Remove the pulley or the coupling by using a suitable puller as shown in figure (3-1) Figure (3-1) : bearing and puller 3. Mark the end plates and the motor hosing of the machine with a file or a marking pen as shown in figure (3-2). This will help in identifying the matching parts and reassembling them correctly. Figure (3-2) : End plats marking 4. remove the rear cover 5. Slightly loose the cooling fan and remove it, which shown in figure (3-3) 6. Remove the screws, nuts or bolts that secure the rear end plate, and then tap the driving shaft with rubber hammer to free the rear end plate. 7. Remove the nuts or screws securing the font end plate, lightly move the end plate using rubber hammer and remove it . 8. Withdraw the rotor with the end plate. 9. Check and grease the motor bearing or replace them if they were worn out 10. The procedure for re-assembling is reverse of dismantling. Figure (3-3) : 3.2 Determine the three phase induction motor terminal sIntroduction The stator of the three phase induction motor is connected either in star or delta connection, whereas the wound rotor is always star connected, these terminals need to be recognized in order to connect the motor circuit correctly. Apparatus 1. AVO meter 2. Tool box 3. Screwdriver set 4. Squirrel cage three-phase induction motor Procedure: 1. Set the AVO meter to ohm position and adjust the zero of the scale. 2. Remove any connectors from the motor terminal box. 3. Place one test lead at the lower left terminal on the terminal box and other test lead on each upper terminal until the meter read a value of resistance (small value) as shown in fig. (3-4). Terminal Box Ohmeter E ∞ _ 0 Ω + Figure (3-4): Identify phase A 4. Record the reading in table in your workbook. 5. Mark the first lower terminal by letter (U2) and the upper one by letter (U1). The (U2) and (U1) terminals are the start and end of phase A as shown in fig.(3-5) Terminal Box U1 U2 E Figure (3-5): Mark start and end of phase A 6. Place one test lead at the middle lower terminal on the terminal box and other test lead on each upper terminal until the meter read a value of resistance (small value) as shown in fig. (3-6). Terminal Box U1 Ohmeter U2 ∞ _ E 0 Ω + Figure (3-6): Identify phase B 7. Record the reading in table in your workbook. 8. Mark the middle lower terminal by letter (V2) and the upper one by letter (V1). The (V2) and (V1) terminals are the start and end of phase B as shown in fig. (3-7). Terminal Box U1 V1 U2 V2 E Figure (3-7): Mark start and end of phase B 9. The last lower and upper terminal is the phase C and marked by the letter (W2) and (W1) respectively as shown in fig. (3-8). Terminal Box U1 V1 W1 W2 U2 V2 Ohmeter ∞ _ E 0 Ω + Figure (3-8): Mark start and end of phase C 10. Measure the resistance and record the reading in table in your workbook 11. To connect the stator in star connection link the three lower terminals (W2, U2, V2) by a three copper jumpers and connect the supply circuit to the lower terminals (U1, V1, W1) respectively as shown in fig. (3-9). Terminal Box U1 V1 W1 W2 U2 V2 E Figure (3-9): Star connection motor 12. To connect the stator in delta connection, connect the three copper jumpers in parallel as shown in fig. (3-10). Terminal Box U1 V1 W1 W2 U2 V2 E Figure (3-10): Delta connection motor Worksheet 05 Solve the following questions: 1. determine the terminal of the induction motor and record the resistance of each phase in the following table(3-1): coil X-U reading condition Y-V Z-W Table(3-1) 2. connect the motor as star, run the motor and reverse the motor direction 3. connect the motor as delta, run the motor and reverse the motor direction Week 6 3.3 Three-phase Induction motor under load Introduction The speed at which the stator's field revolves is the synchronous speed. As this field is cut by the bars of the squirrel-cage rotor winding, current is induced into the bars. The rotor's magnetic field (caused by this current) interacts with the stator's field to produce torque on the rotor. Apparatus 16. One Squirrel cage three phase induction motor 17. One prony brak 18. One power supply 19. One AC instrumentation group 20. One tachometer Procedure 8. Couple and clamp the machine with tachometer and prony brake 9. Connect the motor and instruments as shown in figure 3- 11 10. Start the motor under no load and make sure its rotation is in the "right direction for the scale of the Prony brake. 11. Load the motor in six steps to 125% of rated capacity. 12. Record your readings in Table in your worksheet for each step. 13. Turn off the power supply. L1 Supply L2 3Ø 0-415V Ac N Prony brake A Tach Stator V1 U1 U2 V2 V W2 Rotor W1 L3 Figure (3-11): 3-phase induction motor circuit diagram Worksheet 06 Solve the following questions: 1. Connect the circuit as shown in figure 3-11, Operate, record your results in Table 3-4 Torque (T) (N.m) Line voltage Line (VL) current(IL ) (Volts) (Ampere) Speed (N) (RPM) Table 3-4 2.plot the graph (3-4) of the torque versus motor speed (4-3) Graph 3-4 : the relationship (T VS Motor speed) Week 7 3.4 Measuring the electrical quantities of 3-phase induction motor Introduction The torque is directly proportional to rotor current, IR, and the cosine of the phase angle between the rotor and stator fields (cos θ). Another way of expressing this relationship is that torque is directly proportional to the in-phase component of rotor current, IR cos θ. At the instant of start, IR is high but the in-phase components is low because of the poor power factor (cos θ). As the rotor picks up speed, both the induced rotor voltage and the inductive reactance decrease. Basically IR is going down while cos θ is going up. You can see that there is not very much difference in the value of cos θ when θ is 0° (cos 0 = 1) and when θ is 20° (cos 20 = 0.94). Therefore, over the operating range of the motor, the rotor power factor does not play an important part in the torque output. More important is rotor current. Rotor current falls off sharply as the rotor approaches synchronous speed (i.e., slip approaches zero). Speed doesn't have to drop back very much to increase the rotor current, stator power factor, and torque. When you are running an induction motor without load, it draws almost as much current as it does fully loaded. This no-load current, however, is made up of two components. The inphase component supplies electrical and mechanical losses. The quadrature (90 degrees out of phase) component is the magnetizing current. It is quite large in comparison with the inphase part. As the motor is loaded, it is like putting a resistive load on the secondary of a transformer. The in-phase component gets larger. The stator's power factor improves accordingly. The increased rotor current does not necessarily add to the total current being drawn by the motor. It simply uses more of that current for useful work. Apparatus 21. One Squirrel cage three phase induction motor 22. One prony brak 23. One power supply 24. One AC instrumentation group 25. Two wattmeter 26. One tachometer Procedure 14. Couple and clamp the machine with tachometer and prony brake 15. Connect the motor and instruments as shown in figure 3- 12 16. Start the motor under no load and make sure its rotation is in the "right direction for the scale of the Prony brake. 17. Load the motor in six steps to 125% of rated capacity. 18. Record your readings in Table in your worksheet for each step. 19. Turn off the power supply. A L1 Supply L2 3Ø 0-415V Ac N Prony brake W1 Stator W2 V1 Tach U1 U2 V2 V W2 Rotor W1 L3 Figure (3-12): 3-phase induction motor circuit diagram Worksheet 07 Solve the following questions: 1. Connect the circuit as shown in figure 3-12, Operate, record your results in Table 3-5 Torque (T) (N.m) Line voltage (VL) (Volts) Line current(IL ) (Ampere) Speed (N) (RPM) Power(P) P =W1 +W2 (KW) Table 3-5 2. Calculate the power factor and record your results in Table 3-6 Torque (T) (N.m) Table 3-6 3.plot the graph (3-5) of the torque versus motor current Power Factor (P.F) Graph 3-5 : the relationship (T VS Motor Current) 4.plot the graph (3-6) of the torque versus power factor Graph 3-6 : the relationship (T VS p.f.) Week 8 3.5 Connecting the 3-phase dual speed squirrel cage induction motor Introduction Until the advent of modern solid-state drives, induction motors in general were not good machines for applications requiring considerable speed control. There, are induction .really motor only can be two techniques controlled. One is by to which vary the the speed synchronous of an speed, which is the speed of the stator and rotor magnetic fields, since the rotor speed always remains near synchronous speed. The other technique is tovary the slip of the motor for a given load. Each of these approaches will be taken up in more detail below. The synchronous speed of an induction motor is given by: NSyn = 120 f P So the only ways in which the synchronous speed of the machine can be varied are (1) by changing the electrical frequency and (2) by changing the number of poles on the machine. Apparatus 1. One Connection leads 2. One 2- Dual speed motor 3. Three AVO-meter Procedure 1. Use the Ohmmeter to determine the starts and ends of coil groups of each phase. 2. Number the starts and the ends of the coils by 1,2.3,4 respectively as shown in fig (3-13 a) 3. For 4 poles motor Join the terminals 3,4 of each phase as shown in fig.(3-13 b, c) 4. Connect the motor to a suitable supply and measure the speed 5. switch off the supply and interchange any two phases to change the direction of rotation and put the switch on and observe at the direction of rotation 6. Switch off the supply. 7. For 8 poles motor Join Terminals for one phase the terminals 2,3 of each phase as shown in fig(3-14 a, b and c) 8. Connect the motor to suitable supply and measure the speed 9. Switch off the supply and interchange any two phases by the aid of the diagram shown in table in your workbook to change the direction of the rotation 10. Switch on and observe at the direction of rotation 11. Turn off the motor 12. Comment the results in your worksheet 2 1 3 4 a) Terminals for one phase N S 1 N S 3 2 b) 4-poles connection for one phase L1 L2 L3 1 2 3 4 Terminal Box C) 4-poles connection for three phase Figure (3-13): 4-poles motor 4 2 1 3 4 a) Terminals for one phase N S N S 1 N S N 3 2 S 4 b) 8-poles connection for one phase L1 L2 L3 1 2 3 4 Terminal Box C) 8-poles connection for three phase Figure (3-14): 8-poles motor Worksheet 08 Solve the following questions: 1. Connect the motor as 4-poles motor, Operate, measure speed, record your results in Table 3-6 2. Reverse the direction of rotation. 3. Connect the motor as 8-poles motor, Operate, measure speed, record your results in Table 3-6 4. Reverse the direction of rotation. Speed (N) (RPM) Connection 4-poles connection 8-poles connection Table 3-6 Week 9 4. Protective and Control Device We will choose the circuit breaker as protective device and the contactor as control device. 4.1 Circuit breaker Circuit breaker is used to protect any electrical installation from the danger of over currents. 4.1.1 Disassembling and re-assembling Circuit breaker Introduction Circuit breaker is a piece of equipment which is designed to protect an electrical apparatus from damage caused by overload or short circuit. Unlike a fuse which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers implemented with (electromagnet) are often a solenoid whose strength increases as the current increases and eventually trips the circuit 1. lever 5. Bimetallic strip 2. lever 6. adjusting screw mechanism 7. Solenoid 3. Contacts 8. Arc divider 4. Terminals Figure(4-1) : Construction of the circuit breaker breaker. Alternatively a bimetallic strip may be used which heats and bends with increased current. Some circuit breakers incorporate both techniques. This allows the properties of the circuit breaker to be tailored to suit the application, with the electromagnet generally responding to short, large surges in current (short circuit) and the bimetallic strip responding to smaller but longer-term (overload) over current conditions, the construction of circuit breaker is shown in figure (5-1) Apparatus 1. One circuit breaker. 2. Tools box Procedure : o Disassembling the circuit breaker o Identify the all parts of the circuit breaker as shown in figure (5-1). o re-assembling the circuit breaker 4.1.2 Use the Circuit breaker in the protected circuit Introduction To discriminate between the current the circuit breaker can handle and the current it has to interrupt. Apparatus 3. 15 rating circuit breaker. 4. 0 to 100 Ω variable resistors (high power rating). 5. 240 V ac power supply. 6. Connecting wires. 7. Ammeter. Procedure : 1. Connect the circuit as shown in fig. (4-2). supply I> Circuit breaker Figure (4-2): circuit diagram of the protected circuit 2. Put the resistance (R) at its maximum value, and then turn on the power supply. 3. Perform the adequate variation of the circuit breaker rating before any alteration of the load resistance, according to the above table. Close the circuit breaker. 4. Start from full resistance to zero resistance, decreasing it in 10 steps. 5. Fill, in table in your worksheet, the circuit breaker tripping time, until you reach zero resistive load. 6. Turn off the system, by opening the circuit breaker. 7. Put the load resistance 'to zero, then turn "ON" the circuit breaker and note the tripping time. Current (Amps) Resistance (Ohms) Tripping Time (seconds) Worksheet 09 Solve the following questions: 1. Calculate the various values of current (I) if we are to start from full resistance to zero resistance, decreasing it in 10 steps. Tabulate the values obtained in table (5-1). Table (4-1) 2.Write your own conclusion regarding the mechanisms that tripped the circuit breaker in step 5 and in step 7. ................................................................................................................................ ............................................................ ................................................................................................................................ ............................................................ ................................................................................................................................ ............................................................ ................................................................................................................................ ............................................................ ................................................................................................................................ ............................................................ ................................................................................................................................ ............................................................ ................................................................................................................................ ............................................................ ................................................................................................................................ ............................................................ ................................................................................................................................ ............................................................ ................................................................................................................................ ........................................................... Week 10 4.2 Contactor A contactor is activated by a control input which is a lower voltage / current than that which the contactor is switching. 4.2.1 Disassembling and re-assembling the contactor Introduction Contactors are magnetic switches. They are used for switching a power circuit remotely. Contactors come in many forms with varying capacities and features. Unlike a circuit breaker a contactor is not intended to interrupt a short circuit current. Contactors are used to control electric motors, lighting, heating, capacitor banks, and other electrical loads. Even though the contactors are rugged and reliable because they do not contain delicate mechanisms, they are in fact subjected to many dangers which should be avoided in order to ensure a proper functioning. In this section, we will try to investigate the various troubles which may be encountered when dealing with contactors. These troubles occur in the various parts constituting the contactor as shown in figure (4-3). Line L1 Line Load T2 L3 Stationary contact L3 T1 Movable contact T1 L2 L1 Line L2 T2 T3 T3 Core Coil Pushbutton open Pushbutton open Armature Armature OFF POSITION ON POSITION Figure (4-3): The various parts constituting the contactor. The main parts of a contactor are the electromagnet used to obtain the electromagnetic forces, the poles used to supply the power circuits with the required energy and the auxiliary contacts used in the control circuits. An exploded view of a contactor is shown in figure (4-4). Contactor Overview 2 3 1 4 9 8 5 7 1. Contactors up to 75 KW 2. Suppressors 3. Amplifier modules 6 5. Overload relays 6,7,9. Auxiliary contact 8. Pneumatic timer th 4. 4 pole Figure (4-4): An exploded view of a contactor Apparatus 8. One contactor 9. Tools box Procedure : 1. Disassembling the contactor 2. Identify the all parts of the contactor as shown in figure (4-3) and (4-4). 3. Re-assembling the contactor 4.2.2 Troubleshooting and maintenance a. Electromagnet If the magnetic circuit vibrates, check: The voltage of the main supply. An electromagnet vibrates when it is operated at a voltage lower than that for which it is designed. That no foreign matter is lodged between the moving and the fixed parts of the armature. The condition of the armature surfaces. These should never be painted, scraped or filed. If they are particularly dirty, clean them with trichloroethylene. b. Coil If it becomes necessary to replace a coil, in case the control circuit voltage is changed for example, the new coil must be chosen in relation to the actual supply voltage. c. Contacts The knowledge of the controlled power circuit and its application is used to determine the service life of the contacts or to select a contactor as a function of the required number of switching operations The number of switching operations is determined by the type of application according to the manufacturer's specifications. A periodic check of the compression travel is necessary to reveal the degree of wear. During the period of use, never try to adjust the compression travel. On the other hand, when the compression travel becomes below 50 % of the initial value when the contactor is new, replace the contacts. After every change of contacts: - The contacts must be aligned, adjusting the initial compression value (according to the specification data sheet). - It is preferable to check the pressure of each contact. - It is essential to check the tightness of the setting screws and nuts. d. Auxiliary contacts The auxiliary contacts generally require no maintenance. Do not modify the travel of auxiliary contacts. This has been determined by the manufacturer according to the contact function (hold in, sequence control, economy resistor switching, signalling, etc ...). e. Enclosures Periodically, lubricate hinges and locking devices. On airtight enclosures, check the condition of the sealing devices (compression glands, cable box ...). Check also the condition of the gaskets. 4.2.3 Use the contactor in the control circuit Introduction Contactors are extensively used in applications which require switching of high power circuits from a low power circuit. There is a number of ways to accomplish this activity; in this practical work, we will investigate some of the many techniques used to connect contactors to enable the switching activity to be carried out properly and safely. The most used techniques are: 1- Remote control by two spring push buttons. 2- Remote control by several spring push buttons. Apparatus 1. Thermal overload relay. 2. Spring return push buttons. 3. Triple pole contactor. 4.2.3.1 Remote control by two spring push buttons : Procedure 1. Connect the circuit as shown in fig. (4-5). When push button S2 is pressed, the coil of C1 is energized and the normally open auxiliary contact C11 (13-14) closes ensuring self-holding of the coil of contactor C1, The poles of the contactor close. If push button S2 is pressed, the contactor is de-energized and its poles and contacts return to their initial states. 2. Connect the control circuit to the power supply. 3. Press on S2 and release. 4. Make sure that the poles of the contactor "have closed. Q1 95 96 11 S1 12 13 13 S2 14 C11 14 A1 C1 A2 Q2 (1) (2) (1) - Control circuit (2) - Poles of the contactor with the thermal overload relay Figure (4-5) : Circuit of two spring return push buttons 5. Press on S1 and release. Make sure that the poles of the contactor have opened If the circuit does not operate, several preliminary checks should be carried out: Check for the presence of voltage at the incoming terminals using a test lamp or a voltmeter. Check to see that the switch Q1 used to connect the circuit to the power supply closes correctly. Make sure that the control circuit fuses have not blown. Check that the overload relay is set (the relay is set by pressing the red push button located on the relay). When all preliminary checks have been carried out and the contactor still does not operate, then the following troubleshooting procedure should be carried out: Switch the power supply off. Check the tightness of all terminals and the continuity of all devices (the start push button should be pressed when investigating its continuity). If the contactor does not close while all the other parts function properly, connect a voltmeter across the coil terminals then connect the control circuit to the power supply. Two cases are possible : There is no voltage: Check again the continuity of the control circuit. The voltage is normal : - Check that the voltage indicated on the coil is the same as that measured by the voltmeter. - Make sure that the coil connections are tight. - Change the coil if the above two checks pass, then re-operate the circuit. 4.2.3.2 Remote control by several spring push buttons Circuit operation, fig (4-6): In the circuit diagram, two push buttons S3 and S4 are connected in parallel for the energization, and two push buttons S1 and S2 are connected in series for the contactor de-energization. Usually, one of the energization push buttons and one of the de-energization push buttons are Used for the remote control operation. Repeat the previous procedure for the two spring return push buttons circuit, except that in this case, four push buttons should be checked. Q1 95 96 11 S1 12 11 S2 12 13 S4 14 13 S3 14 13 C11 14 A1 C1 A2 Q2 (2) (1) (1) - Control circuit (2) - Poles of the contactor with the thermal overload relay Figure (5-6): Circuit of several spring return push buttons Worksheet 10 Solve the following questions: 1. Connect the circuit as shown in fig. (4-5) with supply, and answer the following. a) What happened if you press on S1. ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... b) If the circuit does not operate, write down the preliminary checks should be carried out: ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... c) What happened if you press button S2 ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... 2. Connect the circuit as shown in fig. (4-6) with supply, and answer the following. a) What happened if you press on S1. ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... b) When all preliminary checks have been carried out and the contactor still does not operate, then state the troubleshooting procedure should be carried out: ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... c) What happened if you press button S2 ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... d) What happened if you press button S3 ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... e) What happened if you press button S4 ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... ...................................................................................................................................................... ...................................... Week 11 Week 12 Objective: 1. To study the synchronisation of the given synchronous generator with the main busbars by Three-Dark-lamp method. 2. To study the effect of the change in input to the alternator (under constant excitation) on output power, power angle and power factor. Theory: The synchronous generator (FIG.1) can be connected to the busbars (represented an equivalent generator) only when each of the voltages between R1 and R2, between Y1 and Y2, and between B1 and B2 is zero at every instant of time. This condition is fulfilled when the line voltages on the generator side are equal, at all instants of time, to the corresponding voltages on the busbar side. This is possible only if the following conditions are fulfilled: a. The voltages Vg and Vb are equal in magnitude and are in phase. b. Their frequencies are the same. c. The generator and the busbars have the same phase sequence. When these conditions are fulfilled, the synchronising switch between the genera and the bus can be switched on. Fulfilment of these conditions is checked by the following method: I. Synchronisation by three dark lamp method Connect the D.C. motor -synchronous generator as shown in FIG.2. Start the D.C motor by switching on S1 and bring its speed to the synchronous speed of the generator (1500-rpm). Adjust the field excitation of the generator using Rf2 and R so that about rated voltage (400V, L-L) is obtained. Switch on the a.c mains switc S2 and adjust the variac so that Vb is 400 V. Let the phase sequence of the generator terminals RYB be the same as that of the respective terminals of the mains, RYB. The voltage phasors for this condition are shown in FIG.3.If the generator frequency is slightly more than that of the bus, then the phasors R1, Y1 and B1 move anti-clockwise relative to R2, Y2, and B2. The voltages across the lamps L1, L2, L3 (which are indicated by the phasors R1R2, Y1Y2, and B1B2) w increase & decrease simultaneously and therefore, the three lamps will brighten u and darken at the same time. If the phase sequences are R1Y1B1 and R2 B2Y2, the phase diagram of voltages will be as shown in FIG.4. For this condition the voltages across lamps given by phasors R1R2, Y1Y2 and B1B2 are not equal to each other at the instant shown. Therefore the lamps go through their zero voltage one after the other. The phase sequences are thus different and can be corrected by interchanging any two terminals either on the generator side or on the bus side. When such a change is made both the three-phase main switch S2 and he D.C. main switch S1 should be switched off. With the phase sequence corrected, if there is a large difference between the frequency of the generator and that of the bus, the lamps will brighten & darken i quick succession. By adjusting the speed of the generator, this rapidity can be reduced, which indicates that the frequencies are coming closer and the lamps will brighten up & darken slowly. The correct moment of synchronisation in this method is when all the lamps are completely dark, at which time all the voltages of her bus are exactly in phase with the corresponding voltages of the generator. At this moment the synchronising switch S3 is closed and the generator is synchronised with the mains. After synchronisation do not allow the synchronous machine to run as a motor, i.e. do not allow the wattmeter to read negative. If it reads negative it means that the machine receives power from the a.c mains. In such a case, reduce the excitation of the D.C. motor so that the wattmeter reads a few positive watts. II. Study of the influence of the change in input power of the synchronous generator After synchronisation If is kept constant and the prime -mover excitation Ifpm is slowly decreased taking care that the positive power is shown by the wattmeter which indicates that the machine is only generating. For each value of Ifpm, Ia, W, V, and the power angle are noted. The power angle may be noted using a stroboscope. The generator may become unstable for higher values of current; care should be taken to switch off the a.c mains then. Load of suitable values is connected to the D.C. busbar to absorb the D.C. power in the event the synchronous machine operates as a motor. This load is switched on before synchronisation. Report: Power output, P= 1. Calculate the power factor in each case, Cos f = P/(1.73 Via) 2. Plot power P against d (on X-axis) for different excitations. 3. Plot p.f against P (on X-axis) for different excitations. 4. Plot Ia against P (on X-axis) for different excitations. 5. Suppose lamps 2 & 3 were cross-connected as shown in FIG.5, how will the lamps glow for • Correct phase sequence • Incorrect phase sequence? Draw phasor diagrams to justify your results. FIG.1 R1 R2 Vgp Vbp Vg Vb Y1 B1 Synchronous Generator Y2 B2 Busbar FIG.2 Ifpm A Rf1 S1 Ra Ia Starter 220 V D.C Supply A1 F1 F1 M Vt A2 F2 F2 Star-connected alternator Rf2 Alternator connections are Shown below L1 W R1 S2 A Vg Vb Y1 B1 Alternator L2 L3 400V,50Hz Supply FIG.3 R1 L1 R2 B2 L3 Y1 B1 Y2 FIG.4 R1 L2 L1 R2 Y2 Y1 B1 L3 B2 L2 FIG.5 L1 R1 R2 Y1 Y2 B1 B2 L3 L2 Week 13 D.C.GENERATOR CHARACTERISTICS Objective: To determine and compare the characteristics of different D.C. generators. Theory: A D.C. generator is an energy converter. Mechanical power input received from a prime mover (D.C. motor in this case) is converted electromagnetically into electrical energy. This electrical energy can be changed into heat as is done by connecting load resistors across its armature terminals. It is essential that the field windings of the generator be excited with D.C. curren The two main fields are the shunt & series. If the shunt field alone is used then th generator is called a shunt generator; while if the series field alone is used, it is called a series generator. When both windings are used, we have the compound generator. As the name indicates, the shunt field is connected in parallel with the armature, whereas the series field is connected in series with the armature or the load. The shunt field is of larger number of turns and of higher resistance than the series fie The shunt field can be excited either from a separate source (separately excited generator) or from the armature of the same generator (self-excited shunt generator0. It is possible to have two types of compound generators; one is for the cumulativ operation and the other for the differential operation; in the former the series field assists the shunt-field, while in the latter it opposes it. This will be seen from the results of he experiment. Procedure: A. Shunt Generator The connections are shown in FIG.1 (a). The motor is started keeping Ra1 maximum and Rf1 zero. Cut out step by step Ra1 fully and adjust Rf1 to bring th machine to the rated speed of the generator. This speed is held constant througho Rf2 is adjusted to bring the voltage Vt to the rated value with no load on the generator. Rf2 is not altered afterwards. Switch SW2 is closed and for various loa IL, Vt and If are noted. (Maximum allowable load current depends on the rating o the generator.) B. Compound Generator-Cumulative & differential operations The connections are done as in FIG.1 (b) with series winding connected. The mo is started as before and brought to the rated speed of generator and Rf2 adjusted t get rated voltage on no load. The load is changed and VT, IL and If a re noted. If connection gives the cumulative operation, the differential operation is obtained b interchanging the leads S1 and S2. If the original connection gives differential operation, then cumulative operation is obtained by interchanging the leads S1 and S2.The experiment is done both for cumulative as well as differential operations. The extreme load of short circuit should be attempted only for differential operation and that too after the voltage is brought to a low value by switching on all the sections of load L. C. Separately excited generator The connections are done as in FIG.1 (c) The motor is started and run up to rated speed of the generator and If is adjusted to the rated voltage of the generator at no load. This current is kept constant and the generator is loaded. Vt and IL are noted. Report: (For all graphs the origin should be 0 volt and 0 amp with voltage on the Y-axis and current on the X-axis.) 1. Plots of Vt and IL are called External Characteristics. Plot all the external characteristics (4 curves) on the same sheet). 2. Note the armature resistance, and calculate the induced e.m.f. E= Vt + Ia Ra for each reading of the experiments. Plot E against Ia for all cases. 3. Explain briefly the reasons for the fall of terminal voltage in all the 4 cases. FIG.1 (a) SHUNT GENERATOR Rf1 SW1 Ra L1 SW2 A1 F1 If A A F1 220V No Volt DC Coil Supply of Starter L2 G Vt M F2 A2 F2 IL Load FIG.1 (b) COMPOUND GENERATOR Rf1 SW1 Ra L1 SW2 S2 A1 F1 If A S1 F1 220V No Volt DC Coil Supply of Starter L2 G Vt M F2 A2 F2 A IL Load FIG.1 (c) Separately Excited GENERATOR Rf1 SW1 Ra If L1 220V No Volt DC Coil Supply of Starter L2 A1 F1 S1 A F1 F2 A2 IL G Vt M F2 SW2 S2 A Week 14 Objective: a. To study the construction of a 3-phase induction motor b. To study the different starting methods of 3-phase induction motors c. To study how to reverse the direction of rotation in a 3-phase induction motor. Theory: Construction: The induction motor essentially consists of two parts: • Stator • Rotor. The supply is connected to the stator and the rotor received power by induction caused by the stator rotating flux, hence the motor obtains its name induction motor. The stator consists of a cylindrical laminated & slotted core placed in a frame of rolled or cast steel. The frame provides mechanical protection and carries the • The squirrel-cage rotor, terminal box and the ring) end covers • The wound (or slip rotor. with bearings. In the slots of a 3-phase winding In the squirrel-cage rotor, the rotor winding consists of single copper or of insulatedbars copper wireinisthe distributed can be wound for 2,4,6onetc. aluminium placed slots andwhich short-circuited by end-rings bot poles. sides The rotor consists of a laminated and slotted core tightly pressed on the of the rotor. shaft. In the wound rotor, an insulated 3-phasewinding similar to the stator There are two general types of rotors: winding and for the same number of poles is placed in the rotor slots. The ends of the Methods of Starting: star-connected rotor winding are brought to three slip rings on the shaft so theta The most usual methods of starting 3-phase induction motors are: connection can be made to it for starting or speed control. 1. For slip-ring motors- rotor resistance starting 2. For squirrel-cage motors - direct-on -line starting - star-delta starting - Autotransformer starting. There are two important factors to be considered in starting of induction motors: the starting current drawn from the supply, and The starting torque. The starting current should be kept low to avoid overheating of motor and excessive voltage drops in the supply network. The starting torque must be about 50 to 100% more than the expected load torque t ensure that the motor runs up in a reasonably short time. a. Rotor resistance starting By adding eternal resistance to the rotor circuit any starting torque up to the maximum torque can be achieved; and by gradually cutting out the resistance a high torque can be maintained throughout the starting period. The added resistance also reduces the starting current, so that a starting torque in the range of 2 to 2.5 times the full load torque can be obtained at a starting current of 1 to 1.5 times the full load current. b. Direct-on-line starting This is the most simple and inexpensive method of starting a squirrel cage induction motor. The motor is switched on directly to full supply voltage. The initial starting current is large, normally about 5 to 7 times the rated current but the starting torque is likely to be 0.75 to 2 times the full load torque. To avoid excessive supply voltage drops because of large starting currents the method is restricted to small motors only. To decrease the starting current cage motors of medium and larger sizes are started at a reduced supply voltage. The reduced supply voltage starting is applied in the next two methods. c. Star-Delta starting This is applicable to motors designed for delta connection in normal running conditions. Both ends of each phase of the stator winding are brought out and connected to a 3-phase change -over switch. For starting, the stator windings are connected in star and when the machine is running the switch is thrown quickly to the running position, thus connecting the motor in delta for normal operation. The phase voltages & the phase currents of the motor in star connection are reduced to 1/v3 of the direct -on -line values in delta. The line current is 1/3 of the value in delta. A disadvantage of this method is that the starting torque (which is proportional to the square of the applied voltage) is also reduced to 1/3 of its delta value. d. Auto-transformer starting This method also reduces the initial voltage applied to the motor and therefore the starting current and torque. The motor, which can be connected permanently in delta or in star, is switched first on reduced voltage from a 3-phase tapped auto -transformer and when it has accelerated sufficiently, it is switched to the running (full voltage) position. The principle is similar to star/delta starting and has similar limitations. The advantage of the method is that the current and torque can be adjusted to the required value, by taking the correct tapping on the autotransformer. This method is more expensive because of the additional autotransformer. Reversing: Reversing the connections to any two of the three motor terminals can reverse the direction of rotation of 3-phase induction motor Procedure 1. Study the construction and the various parts of the 3-phase induction motor. 2. For rotor resistance starting, connect the slip-ring motor as shown in FIG.1. Start the motor with full starting resistance and then decrease the resistance in steps down to zero. Take observations of the stator & rotor currents 3. For direct-on -line starting , connect the cage motor as shown in FIG.2 4. For star-delta starting , connect the cage motor to the terminals of the stardelta switch (FIG.3) 5. For autotransformer starting, connect the cage motor as shown in FIG.4. Take care at starting that the "Run" switch is open and that it is not closed before the "Start" switch is opened. 6. In each case observe the starting currents by quickly reading the maximum indication of the ammeters in the stator circuit. 7. Reverse the direction of rotation of the motor by reversing of two phases at the terminal box. The reversal has to be made when the motor is stopped and the supply switched off. Report: 1. Explain the difference between a slip ring and a squirrel -cage motor. 2. Discuss the merits & demerits of the various starting methods. FIG.1 Stator Rotor Starter A A FIG.2 3-phase Supply A 3-phase Induction Motor FIG.3 3 phase Supply A STATOR DELTA RUN STAR START ROTOR FIG.4 3-phase Supply Autotransformer A Motor RUN STA A Week 15 Objective: To separate the various losses occurring in an induction motor. Theory: Apart from the calculable I2R loss, the losses occurring in an induction motor include the following: Pb = brush friction loss Pe = eddy current loss in iron Pf = mechanical losses in windage and bearing friction Ph = hysterisis loss Pp =pulsation loss The suffices 1 and 2 refer to stator & rotor quantities respectively. At standstill the rotor core loss is pe2 +ph2, when the rotor is open-circuited. At a slip s, this loss becomes s2pe2 +sph2. The pulsation loss is a high-frequency tooth loss in stator & rotor, produced by variations of gap reluctance as the tooth tips pass each other. The friction loss can be separated by the no-load test. Procedure: The following procedure separates all losses for the slip-ring motor: No-load Test 1. Measure the power supplied to the stator at normal voltage & at rest. The stato input corrected for I2R loss is the iron losses P1 = pe1 +ph1 + pe2 +ph2 2. Measure the power supplied to the stator at normal voltage and frequency with the rotor short-circuited and running on no-load. The corrected stator input is P2 = pe1 + ph1 + pp + pf + pb. The rotor eddy current loss is small since it is proportional to s. ph2 is also suppli by the stator, but practically the whole of it is returned as a driving torque, partly providing for (pf + pb) 3. With machine running as in (2), the rotor circuit is suddenly opened and the stator input measured. The stator input falls to P4 = pe1 +ph1 +ph2. Transformation Ratio test 4. Apply a voltage to the rotor such that the stator voltage is (V1+v1)/2 on open circuit, and the mutual flux is normal where V1 is the normal stator applied voltage and v1 is the measured stator voltage when the normal voltage V2 is applied to the rotor. Measure the rotor input when running on no-load with the stator short-circuited. This corresponds to the condition of (2), except that the functions of the rotor & stator are reversed. The corrected rotor power input is P5 = pe2 + ph2 + pp + pf + pb. 5. With the machine running as in (4), the stator is suddenly open-circuited and the rotor power input falls to P6 = pe2 +ph2 +ph1. This test corresponds to Test3 The mechanical losses P7 (= pf + pb ) are evaluated by the no-load test Locked Rotor Test 7. Measure the power supplied to the stator at reduced voltage and full-load stator current, with the rotor short-circuited & locked. This test gives the I2R loss of the machine. Observations: Induction Motor Specifications: 110 V, 36A, 3-phase, 50Hz, Y-connected, 1440 RPM 1. Transformation Ratio test Rotor voltage = Stator voltage = Therefore, Turns Ratio = 2. No-load test Wattmeter constant C.T Ratio = No: V(L-L) W1 (Volts) (Watts) W2 (Watts) W1+W2 Power (Watts) (Watts) W1 (Watts) W2 (Watts) V(L-L)2 (Volts) 2 3. Locked Rotor test Wattmeter constant C.T Ratio = No: V(L-L) I (Volts) (amps) Stator resistance per phase = Calculations: From no-load test P7= pf +pb Watts pe1 =P1-P6 Watts pe2 =--- Watts ph1 =P +(P2-P5-P1)/2 Watts W1+W2 (Watts) Corrected power (Watts) ph2 =P4+(P6-P1-P2)/2 Watts pp = ((P5+P2-P1) /2) - P7 Watts Assuming pb = pf = P7 Watts From locked rotor test I f.l = -- amps V1= Reduced Voltage, Volts ISC1=--amps P SC1=--Watts Corresponding Values for normal voltage V will be: ISC=V ISC1 /V1 amps PSC =P SC1 (V/V1)2 Watts Stator I2R loss = 3(ISC )2 R Watts Rotor I2R loss = P SC -3(ISC )2 R Watts Results: 1. Eddy loss in stator =-- Watts 2. Eddy loss in rotor =-- Watts 3. Hysterisis loss in stator =-- Watts 4. Hysterisis loss in stator =-- Watts 5. Pulsation loss =-- Watts 6. Friction + Windage loss =-- Watts Discussion: 1. The brush friction loss cannot be measured if there is no internal short-circuiting device. In that case, assume pb = 0. 2. The rotor eddy current loss is very small, because it is proportional to (slip)2 and the slip itself is very small. 3. To get accurate results, the voltage should be maintained constant NO LOAD TEST Watts Friction Loss V2 LOCKED ROTOR TEST PSC Current,A ISC Power,W VOLTAGE,V 93
