Electrical Machines-I (EE-241) For S.E (EE)

Transcription

1 PRACTICAL WORK BOOK For Academic Session 2013 Electrical Machines-I (EE-241) For S.E (EE) Name: Roll Number: Class: Batch: Department : Semester/Term: NED University of Engineer ing & Technology

2 Electrical Machines-I Contents CO NT EN TS Lab. No. Da te d List of Experiments Pa ge No. Re ma rk s Energization of Benches installed in electrical machines lab, through main panel. Reading and explanation of the name plate data of DC & AC rotating machines To draw the magnetization curve of self exited DC shunt generator (open circuit characteristics curve O.C.C). To draw the load characteristic curve of self excited D.C shunt generator. To draw the external and internal characteristics of separately excited DC generator Speed control of a DC shunt motor by flux variation method Speed control of a D.C. Shunt Motor by armature or rheostatic control method. To study rotors of electric machines To study parallel operation of two dc generators and shift of load on one another To find out the Cu losses of a single phase transformer by short circuit test. 23 Revised 2012

3 Electrical Machines-I Contents To find out the Core losses of a single phase transformer by open circuit test. To observe the effect of increasing load on DC shunt motor s speed, armature current, and field current. To observe the starting of synchronous motor Revised 2012

4 Electrical Machines-I Lab Session 01 LAB SESSION 01 OBJECT Energization of Benches installed in electrical machines lab, through main panel. APPARATUS 1. Bench 2. Main Panel THEORY Every motor-generator set bench located here in electrical machines laboratory is not directly connected to KESC supply. Instead of connecting bench with KESC supply, we have main panel which is connected with KESC supply and benches are connected with main panel. Every bench has DC supply, 3-Φ supply, 1-Φsupply and supply given to bench for services. Services supply is used for energization of any equipment through that specific bench for instance if we want to connect tachometer to a bench, it is energized through service supply etc. Main panel has three major portions, namely 1. Main supply-services-fix DC lines 2. Main supply-single phase AC fix lines-three phase AC fix lines 3. Main supply-interconnections 1. Main supply-services-fix DC lines This is first portion and it includes a knife switch, three ammeters connected through CTs measuring phase current and a voltmeter connected through PT measuring Line voltage. Upon energization of this portion fans installed inside every bench start running and services switches of every bench are also energized. Except this one line diagram of fix DC supply is also on this portion. 1

5 Electrical Machines-I Lab Session Main supply-single phase AC fix lines-three phase AC fix lines As the name suggest, one line diagram of single and three phase supply is located on this portion, mean you can connect three phase and single phase supply to any bench through this portion. 3. Main supply-interconnections The function of this portion is to interconnect different benches. If we are generating three phase AC supply or DC supply with the help of any motor-generator set and intend to give our generated supply to any DC or three phase AC motor located on any other bench, in this situation we can connect both benches through this portion. PROCEDURE 1. Select any bench, to which you intend to energize 2. With the help of single line diagram drawn on main panel, connect dc, single phase ac and three phase ac supply RESULT 2

6 Electrical Machines-I Lab Session 02 LAB SESSION 02 OBJECT Reading and explanation of the name plate data of DC & AC rotating machines. APPARATUS DC Motor DC Generator 3-ΦInduction Motor 3-ΦSynchronous Synchronous Motor 3-ΦSynchronous Synchronous Generator CIRCUIT DIAGRAM THEORY Name plate, is a sheet fixed on every electrical machine, shows the rated parameters. Rated parameters are the parameters on which machine perform at best efficiency. Therefore it is of immense importance to know about the rated parameters of any machines before putting it in operation. In addition to this, these parameters are also necessary for the further analysis like designing any controlling circuitry for that machine. Name plate data includes voltage, current, ambient temperature, number of poles, operating frequency, enclosure type, cooling employed, field current and voltage (in case of doubly excited machines/generator) etc. 3

7 Electrical Machines-I Lab Session 02 PROCEDURE Check out name plate data of machines given below, installed at different benches. 1. NAME PLATE DATA OF DC MOTOR: 2. NAME PLATE DATA OF DC GENERATOR: 3. NAME PLATE DATA OF 3-ΦINDUCTION MOTOR: 4

8 Electrical Machines-I Lab Session NAME PLATE DATA OF 3-ΦSYNCHRONOUS MOTOR: 5. NAME PLATE DATA OF 3-ΦSYNCHRONOUS GENERATOR: 5

9 Electrical Machines-I LAB SESSION 03 LAB SESSION 03 OBJECT To draw the magnetization curve of self exited DC shunt generator (open circuit characteristics curve O.C.C). APPARATUS 1. Bench 10-ES/EV or Bench 14-ES/EV 2. DC multi-range ammeter 3. DC multi-range voltmeter CIRCUIT DIAGRAM THEORY The magnetization characteristics also known as No load or Open circuit characteristics is the relation between emf generated and field current at a given speed. Due to residual magnetism in the poles, some emf is generated even when filed current is zero. Hence the curve starts a little way up. It is seen that the first part of the curve is practically straight. This is due the fact that at low flux densities reluctance of iron path is being negligible, total reluctance is given by air gap reluctance which is constant. Hence the flux and consequently the generated emf is directly proportional to exciting current. However at high flux densities iron - 6 -

10 Electrical Machines-I LAB SESSION 03 path reluctance is being appreciable and straight relation between emf and field current no longer holds good. In other words saturation of poles starts. PROCEDURE 1. Connect the shunt field to armature terminal through the ammeter, switch and rheostat. 2. Connect the multi-range voltmeter across the terminals of armature. 3. Press yellow switch on and increase AC voltage of induction motor (prime mover) by the help of 3-phase autotransformer until it reaches at normal speed. 4. Note the reading of voltmeter which indicates the voltage due to residual magnetism. 5. Close field switch and excite the field at low current. 6. Increase the field current in steps and note the voltage each time. 7. Take at least readings. 8. Tabulate the reading and draw the curve between armature induced e.m.f and exciting current OBSERVATIONS S.No. FIELD CURRENT I F (A) TERMINAL VOLTAGE V T (volts)

11 Electrical Machines-I LAB SESSION 03 RESULT 1. The curve starts somewhat above the origin. The voltage at zero excitation is due to residual magnetism of the field, which is necessary for building up the voltage of selfexcitation generator. 2. The voltage increases rapidly at first and then changes a little in value at higher excitations indicating the effect of the poles saturation

12 Electrical Machines-I LAB SESSION 04 LAB SESSION 04 OBJECT To draw the load characteristic curve of self excited D.C shunt generator. APPARATUS 1. Bench 10-ES/EV or Bench 14-ES/EV 2. DC multi-range ammeter 3. DC multi-range voltmeter CIRCUIT DIAGRAM - 9 -

13 Electrical Machines-I LAB SESSION 04 THEORY Load characteristic curve is the graphical representation which shows change in terminal voltage with respect to change in load. After building up of voltage, if a shunt generator is loaded then terminal voltage drops with increase in load current. There are three main reasons for the drop of terminal voltage for a shunt generator under load. i) Armature Reaction Armature reaction is the effect of magnetic field set up by the armature current on the distribution of flux under main poles of a generator. Due to demagnetizing effect of armature reaction, pole flux is weakened and so induced e.m.f in the armature is decresed. ii) Armature Resistance As the load current increases, more voltage is consumed in ohmic resistance of armature circuit. Hence the terminal voltage (V t =E IaRa) is decreased where E is the e.m.f induced in armature under load condition. iii) Drop In Terminal Voltage The drop in terminal voltage (V t ) due to armature resistance and armature reaction results in decreased field current, which further reduces e.m.f induced. For a shunt generator Ia = I L + I f E = V t + IaRa PROCEDURE 1. Make the connections as shown in circuit diagram. 2. Press yellow switch on and increase AC voltage of induction motor (prime mover) by the help of 3-phase autotransformer until it reaches at normal speed. 3. When motor reaches rated speed, close the shunt field switch. 4. Increase field current by changing the field resistance until the terminal voltage reaches to 220 volt. 5. Close the switch of load and vary the load current by means of load rheostat. 6. Note down the meter readings from all meters carefully

14 Electrical Machines-I LAB SESSION 04 OBSERVATIONS S.No I f (A) I L (A) V T (V) I a =I f +I L V d =I a R a Ra=0. 5 ohm RESULT The terminal voltage of a D.C. generator is maximum at no load, which decreases with increasing load

15 Electrical Machines-I LAB SESSION 05 LAB SESSION 05 OBJECT To draw the external and internal characteristics of separately excited DC generator. APPARATUS 1. Bench 10-ES/EV or Bench 14-ES/EV 2. DC multi-range ammeter 3. DC multi-range voltmeter CIRCUIT DIAGRAM THEORY The load or external characteristic of a generator is the relation between the terminal voltage and load current. The characteristic expressed the manner in which the voltage across the load varies with I, the value of load current. The internal or total characteristic of a generator is the relation between the e.m.f actually induced in the generator E a and the armature current I a. The internal characteristic of the generator, which is separately excited, can be obtained as below: Let: Then, V t = Terminal voltage, I a = Armature current, R a = Armature resistance E a = V t + I a R a

16 Electrical Machines-I LAB SESSION 05 I a = I L Therefore if we add drop of armature (I a R a ) to terminal voltage V t we get actually induced e.m.f (E a ). PROCEDURE 1. Make the circuit as shown in circuit diagram. 2. Press yellow switch on and increase AC voltage of induction motor (prime mover) by the help of 3-phase autotransformer until it reaches at normal speed. 3. When motor reaches rated speed, close the shunt field switch. 4. Increase field current by changing the field resistance until the terminal voltage reaches to 220 volt. 5. Close the switch of load and vary the load current by means of load rheostat. 6. Note down the meter readings from all meters carefully. OBSERVATIONS S.No I L (A) I f (A) V T (V) E a = V t + I a R a (V) RESULT From the graph it is observed that the terminal voltage across generator decreases as the load increases

17 Electrical Machines-I LAB SESSION 06 OBJECT LAB SESSION 06 Speed control of a DC shunt motor by flux variation method. APPARATUS 1. Bench 13-ES/EV or Bench 15-ES/EV 2. DC multi-range ammeter 3. DC multi range voltmeters 4. Digital tachometer CIRCUIT DIAGRAM THEORY This method is used to increase speed of DC motor above base speed.to understand what happens when the field resistance of dc motor is changed, assume that the field resistance is increased then the following sequence of cause and effect will take place 1. Increasing R f causes I f to decrease 2. Decreasing I f Decreases φ 3. Decreasing φlowers Ea 4. Decreasing Ea Increases Ia 5. Increasing Ia increases T ind 6. Increasing T ind makes T ind > T load, hence speed increases

18 Electrical Machines-I LAB SESSION Increasing speed increases Ea 8. Increasing Ea decreases Ia 9. Decreasing Ia decrease T ind until T ind = T load at higher speed. Naturally decreasing R f would reverse the whole process and speed of motor will decrease. It is important to bear in mind, changing field resistance does not effect torque induced,at the end its magnitude remains same but at higher or lower speed depending upon change in resistance. PROCEDURE 1. Make connections as shown in the circuit. 2. Keep the motor starting rheostat at its maximum position and field rheostat at its minimum position while starting motor. 3. Start the motor by pressing yellow switch "ON" without load. 4. Adjust the motor start rheostat to its minimum value. 5. Decrease field current by the help of field rheostat step by step and take readings of field current and speed from digital tachometer at every step. Adjust the field rheostat to give maximum speed at which it is safe to operate the motor. OBSERVATIONS S. No Field Current If(A) Speed N (RPM) RESULT Speed increases as the field excitation decreases

19 Electrical Machines-I LAB SESSION 07 LAB SESSION 07 OBJECT Speed control of a D.C. Shunt Motor by armature or rheostatic control method. APPARATUS 1. Bench 13-ES/EV or Bench 15-ES/EV 2. DC multi-range ammeter 3. Voltmeters 4. Digital tachometer CIRCUIT DIAGRAM THEORY This method is used to decrease speed of DC motor below base speed. To understand what happens when the armature resistance of DC motor is changed, assume that the armature resistance is increased then the following sequence of cause and effect will take place 1. Increasing R a causes Ia to decrease

20 Electrical Machines-I LAB SESSION Decreasing Ia deccreases T ind 3. Decreasing T ind makes T ind < T load, hence speed decreases. 4. Decreasing speed decreases Ea 5. Decreasing Ea increases Ia again. 6. Increasing Ia increases T ind until T ind = T load at lower speed. Naturally decreasing Ra would reverse the whole process and speed of motor will increase. It is important to bear in mind, changing armature resistance does not effect torque induced,at the end its magnitude remains same but at higher or lower speed depending upon change in resistance. PROCEDURE 1. Make connections as shown in the circuit. 2. Keep the motor starting rheostat at its maximum position and field rheostat at its minimum position while starting motor. 3. Start the motor by pressing yellow switch "ON" without load. 4. Adjust the motor start rheostat to its minimum value. 5. Increase the value of starting resistance by the help of motor start rheostat step by step and take readings of voltage across armature and speed from digital tachometer at every step. OBSERVATIONS S. No Armature Voltage Va(V) Speed N (RPM) RESULT Speed is very nearly proportional to the applied voltage in the case of armature control method

21 Electrical Machines-I LAB SESSION 08 OBJECT To study rotors of electric machines APPARATUS 1. Rotor of DC Machine 2. Rotors of Asynchronous AC Machine CIRCUIT DIAGRAM LAB SESSION

22 Electrical Machines-I LAB SESSION 08 THEORY One of the classifications of parts of electric motors is according to the state of component i.e. stationary or rotating. If it is rotating, it is called rotor otherwise it is known as stator. Construction of rotor in DC motor is different from rotor used in 3-phase induction motor. Therefore here we will discuss them individually. ROTOR OF DC MOTOR commutator. It is also known as armature, it consists of armature core, armature winding and (i) (ii) (iii) Armature core: It is cylindrical in shape and made up of silicon steel sheets. It carries armature winding, causes them to rotate and hence cut the magnetic flux produced by field winding- located in stator core. It is important to bear in mind, function of armature core is to provide very low reluctance path to the flux through the armature from N-pole to S-pole. Armature winding: There are two types of armature winding (i) Lap winding (ii) Wave winding Lap winding is used in high current and low voltage machines where as use of wave winding is for low current and high voltage machines. Commutator: Function of commutator in DC motor is to produce unidirectional torque. In DC motors we are providing DC supply through commutator. It is of cylindrical structure and built up of segments of hard drawn copper. These segments are insulated from each other by layer of mica. The number of segments is equal to the number of armature coil. Rotors of 3- Phase Induction Motors: Process of energy conversion in induction motor took place by induction principal hence rotor is not electrically connected either with stator or with external supply. There are two types of rotors used in induction motor, discussed below: Squirrel Cage Rotor: Almost 90 percent of induction motors are squirrel-cage type, because this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible. The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors which, it should be noted clearly, are not wires but consists of heavy bars of copper, aluminum or alloys. One bar is placed in each slot; rather the bars are inserted from the end when semi-closed slots are used. The rotor bars are brazed or electrically welded or bolted to two heavy and stout short-circuiting end-rings, thus giving us, what is so picturesquely called, a squirrel-case construction

23 Electrical Machines-I LAB SESSION 08 It should be noted that the rotor bars are permanently short-circuited by themselves; hence it is not possible to add any external resistance in series with the rotor circuit for starting purposes. The rotor slots are usually not quite parallel to the shaft but are purposely given a slight skew. This is useful in two ways: (1) It helps to make the motor run quietly by reducing the magnetic hum and (2) It helps in reducing the locking tendency of the rotor i.e. the tendency of the rotor teeth to remain under the stator teeth due to direct magnetic attraction between the two. Phase-wound Rotor This type of rotor is provided with 3-phase,double-layer, distributed winding consisting of coils as used in alternator. The rotor is wound for as many poles as the number o stator poles and is always wound 3-phase even when the stator is wound two-phase `. The three phases are starred internally; the other winding terminals are brought out and connected to three insulated slip-rings mounted on the shaft with brushes resting on them. These three brushes are further externally connected to a three-phase star-connected rheostat, this makes possible the introduction of additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor and for changing its speedtorque/current characteristics. When running under normal conditions, the slip-rings are automatically short-circuited by means of a metal collar, which is pushed along the shaft and connects all the rings together. Next, the brushes are automatically lifted from the slip-rings to reduce the frictional losses and the wear and tear. Hence, it is seen that under normal running condition, the wound rotor is short-circuited on itself just like the squirrel-case rotor

24 Electrical Machines-I LAB SESSION 09 LAB SESSION 09 OBJECT To study parallel operation of two dc generators and shift of load on one another. APPARATUS 1. Two Voltmeters (0 600V) 2. Two Ammeters (0 15A) THEORY: This arrangement of operation in parallel can be made to meet the load demand easily and work them near to their maximum efficiency. It also helps to prevent the complete shut down in case any generator fails. 1. If two or more generators run at full load then it is more economical and also improves efficiency. 2. Periodical over halving and general repairs can be carried out without shut down to total supply; only one generator can be shut down. 3. If load on the power station increases, additional generators can be added to the already working generators. For proper synchronizing of generators the following condition must be achieved. PROCEDURE 1. The terminal voltage of incoming generator must be the same as that of the running generator. 2. Polarity of the incoming generator should be the same as line polarity. 1. Connect the generators and meters as shown in the diagram and check connections. 2. Run first generator at its normal speed and connect to bus bar. 3. Adjust the voltage at 220V with the help of field excitation. 4. Now put he load on first generator and increase the load slowly. 5. Run second generator at its normal speed and regulate the voltage till it equals the bus bar voltage. 6. Check the polarity of second generator and connect it to the bus bar. 7. Note readings of ammeters of both generators and that of load ammeter. 8. Shift the load of first on second by weakening the field strength of first generator but at the same time increasing field strength of second generator. 9. When all the load of first generator has shifted to second generator, disconnect first from bus bar.note readings of all ammeters as before

25 Electrical Machines-I LAB SESSION 09 OBSERVATIONS For first generator S.No I L (A) I f (A) V T (V) For second generator S.No I L (A) I f (A) V T (V)

26 Electrical Machines-I LAB SESSION 10 LAB SESSION 10 OBJECT APPRATUS To find out the iron core losses of single phase transformer (open circuit test). CIRCUIT DIAGRAM 1. Voltmeter (0 300V ) 2. Ammeter ( 0 2A ) 3. Wattmeter ( W ) THEORY The purpose of this test is to determine no load loss or core loss and no load current Io which is helpful in finding Xo and Ro. One winding of the transformer which ever is convenient but usually high voltage winding is left open and the other is connected to its supply of normal volt and frequency. A wattmeter, voltmeter and ammeter are connected in low voltage winding i.e. Primary winding in the present case. Normal voltage is applied to primary normal flux will be set up in the core hence normal iron loss will occur which are recorded by the wattmeter. As the primary no load I o is small usually

27 Electrical Machines-I LAB SESSION % of rated load current Cu losses is negligible small in primary I will in secondary b/c it is open. Therefore the wattmeter reading will show practically the core loss under no load condition. OBSERVATIONS S.No W (watts) V (Volts) I o (Ampere) RESULT The iron losses of single phase transformer are watt

28 Electrical Machines-I Lab Session 11 OBJECT APPARATUS LAB SESSION 11 To find out the Cu losses of a single phase transformer by short circuit test. 1. Voltmeter (0-15V) 2. Wattmeter (0-750) 3. Ammeter (0-15A) THEORY In this test one winding (usually low voltage winding) is short circuited by a thick conductor or by means of ammeter (Which may serve an additional purpose of indicating rated load). A low voltage (5-10% of the normal voltage) at normal frequency is applied to the primary and gradually increased, till full load current is flowing in both primary and secondary. Since in this test the applied voltage is a small percentage of the normal voltage the mutual flux produced is also a small percentage of its normal value. Hence core losses are very small with the result that the wattmeter reading represents the full load copper loss. PROCEDURE 1. Make connections according to the given circuit. 2. Connect primary of transformer with variable ac voltage supply. 3. Note down transformer rated current from name plate data and keep on increasing voltage until you get rated current read by Ammeter connected. -25-

29 Electrical Machines-I Lab Session Once you get rated current at any specific voltage level, note down reading of instruments connected and calculate different parameters. OBSERVATION S.No W (watts) Vsc (Volts) I (Ampere) CALCULATIONS RESULT The copper losses of single phase transformer are Watts -26-

30 Electrical Machines-I LAB SESSION 12 OBJECT LAB SESSION 12 To observe the effect of increasing load on DC shunt motor s speed, armature current, and field current. APPARATUS 1. Bench 13-ES/EV 2. DC multi-range ammeter 3. DC multi-range voltmeter 4. Tachometer CIRCUIT DIAGRAM THEORY Here AC generator is used as load on dc shunt motor. As we know generator has counter torque which opposes input power given by dc shunt motor and counter torque is dependant load current. Hence on increasing load on generator, it will develop more counter torque, thus more load will be reflected on dc shunt motor. N = (Speed Equation of DC Motors) I a = (Armature current equation) -27-

31 Electrical Machines-I LAB SESSION 12 I f = (Field current equation) In shunt dc motor field is connected in parallel with armature. From its speed equation it is vivid, on increasing load speed will drop due to increased armature drop. Decrease in speed will decrease back e.m.f. and consequently armature current will increase. As for as field current is concern, it will remain constant until and unless terminal voltage remains constant. PROCEDURE 11. Make the circuit as shown in figure. 12. Keep the motor starting rheostat at its maximum position and field rheostat at its minimum position while starting motor. 13. Start the motor by pressing yellow switch "ON" without load. 14. Adjust the motor start rheostat to its minimum value 15. Note down the readings of instrument connected. 16. Now connect electrical load on generator and start increasing load in steps. 17. After every step, note down readings of instrument connected. 18. Draw curves between armature current and load current and between speed and load current OBSERVATIONS S.No I Load I Armature I Field N rpm I 1 I 2 I 3 RESULT 1. Speed decreases and armature current increase with increase in load but field current remains constant

32 Electrical Machines-I Lab Session 13 OBJECT APPARATUS LAB SESSION 13 To observe the starting of synchronous motor 1. 3-ΦSynchronous motor 2. Variable 3-ΦAC supply 3. DC Supply 4. Techometer CIRCUIT DIAGRAM THEORY To understand how the synchronous motor works, assume that the application of threephase ac power to the stator causes a rotating magnetic field to be set up around the rotor. The rotor is energized with dc (it acts like a bar magnet). The strong rotating magnetic field attracts the strong rotor field activated by the dc. This results in a strong turning force on the rotor shaft. The rotor is therefore able to turn a load as it rotates in step with the rotating magnetic field. It works this way once it s started. However, one of the disadvantages of a synchronous motor is that it cannot be started from a standstill by applying three-phase ac power to the stator and dc to its rotor. When ac is applied to the stator, a high-speed rotating magnetic field appears immediately. This rotating field rushes past the rotor poles so quickly that the rotor does not have a chance to get started. In effect, the rotor is repelled first in one direction and then the other. A synchronous motor in its purest form has no starting torque. It has torque only when it is running at synchronous speed. A squirrelcage type of winding is added to the rotor of a synchronous motor to cause it to start. The -29-

33 Electrical Machines-I Lab Session 13 squirrel cage is shown as the outer part of the rotor in figure. It is so named because it is shaped and looks something like a turn able squirrel cage. Simply, the windings are heavy copper bars shorted. Hence, three phase synchronous motor is not self started. At the starting time, it behaves as induction motor and gets accelerated. Once it approaches speed near to synchronous speed, its rotor winding is excited then synchronous motor start rotating at synchronous speed. If we have given rotor supply at start, motor will just produce humming sound. PROCEDURE 19. Make the circuit and switch on both ac and dc supply and observe the performance. 20. Disconnect dc supply, switch on ac supply and observe the performance. 21. When motor run near to synchronous speed, which already calculated, switch on dc supply also and observe the behavior. OBSERVATIONS: Starting of Synchronous Motor Applied Voltage to Induction Motor Stator Current R.P.M Synchronous Motor Applied Voltage Stator Current Field Current D.C Volts R.P.M -30-