DHANALAKSHMI COLLEGE OF ENGINEERING MANIMANGALAM. TAMBARAM, CHENNAI B.E. ELECTRICAL AND ELECTRONICS ENGINEERING

Transcription

1 DHANALAKSHMI COLLEGE OF ENGINEERING MANIMANGALAM. TAMBARAM, CHENNAI B.E. ELECTRICAL AND ELECTRONICS ENGINEERING V SEMESTER EE2305 ELECTRICAL MACHINES II LABORATORY LABORATORY MANUAL 1

2 CONTENT S. No. Name of the experiment Marks Signature 1 Regulation of a three phase alternator by EMF and MMF methods 2 Regulation of a three phase alternator by ZPF and ASA methods 3 Regulation of a three phase alternator by Slip test 4 Measurement of negative sequence and zero sequence impedance of alternators 5 V curves and inverted V curves of synchronous Motor 6 Load test on a three phase squirrel cage induction motor 7 No load and blocked rotor tests on a three phase induction motor 8 Load test on single phase Induction Motor 9 No load test and blocked rotor on single phase Induction Motor 2

3 REGULATION OF A THREE PHASE ALTRENATOR BY EMF AND MMF METHODS AIM: To conduct OC and SC tests on a given 3-Φ alternator and hence to predetermine the regulation by (i) EMF method (ii) MMF method. APPARATUS REQUIRED: S. No. Apparatus Range &Type Quantity 1 Alternator Set up - I No. 2 Rheostat 200 Ω/3 A 2 Nos. 3 Voltmeter AC (0-600 V), MI I No. 4 Ammeter AC (0-10 A), MI I No. 5 Ammeter DC (0-2 A), MC I No. 6 Tachometer - 1No. 7 DPST Switch and TPST Switch - 1 each 8 Connecting wires & Fuse - As Required THEORY: Loading an alternator causes its terminal voltage to drop or rise depending upon (i) Magnitude of load (ii) Nature of load. For a pure resistive load it drops by 8-12% below no-load value while for a lagging p.f. load the drop is 25-50% below no load value and it is 20-30% higher for leading p.f. loads. The reasons are 1) Armature resistance 2) Armature winding leakage reactance and 3) Armature reaction. Electromotive force (EMF) and Magnetomotive force (MMF) methods are used to predetermine the regulation of non-salient pole alternators. In emf method, the effect of armature reaction is represented as a fictitious reactance X ar for each phase of the alternator. In mmf method effect of armature leakage reactance is replaced by additional armature reaction. MMF method is more accurate. Direct load Test is not preferred due to the absence of large sized loads and the enormous power wastage involved in testing. Voltage regulation is defined as a percentage of rated voltage when load current is reduced to zero suddenly by throwing off the load keeping I f and speed constant. PRECAUTIONS: (Not to be included in the Record) 1. Remove the fuse before starting wiring. 2. Fuse rating calculation: Since this is no load test, the required fuse rating is only 20% of the rated current of the alternator. 3. Keep Motor field Rheostat in minimum resistance Position. 4. Keep the potential divider for alternator field in minimum voltage position. 5. Check that the TPST on alternator side is open. PROCEDURE: 1. The circuit connections are given as shown in the circuit diagram. 2. Keeping the motor field rheostat in the indicated position and with the TPSTS open, the motor supply is switched ON, by closing DPSTS Motor is started using the 3-point starter by moving the handle from OFF to ON position and the motor is brought to its rated speed by adjusting the rheostat in the motor field circuit. 4. Supply is switched on to the field winding of alternator by closing the DPSTS 2. 3

4 O.C. TEST: 1. Using the 200 ohm potential divider, current in field circuit is increased in steps of 0.1A and at each step the alternator induced voltage indicated by voltmeter and the corresponding field current (I f ) are noted in tabular column. 2. This procedure is continued until the alternator voltage is 120% of its rated voltage. 3. After completing O.C. Test, the potential divider and motor field rheostat are brought to its minimum position. 4. After completing the experiment, calculate Synchronous Impedance, Synchronous Reactance & Regulation using the formulae given. 5. Using the data, Plot the graph between E o Vs I f. S.C. TEST: 1. The alternator terminals are short circuited by closing TPST switch through an ammeter. 2. The rated current is made to flow through the armature of the stator windings by carefully adjusting 220 ohms potential divider from the minimum position. 3. After completing the experiment, calculate the Load current, Field Current and Regulation. 4. Using the data, Plot the graph I sc vs I f and % Regulation vs Power Factor for both the EMF and MMF methods. CIRCUIT DIAGRAM: FUSE CALCULATION: MOTOR 4

5 ALTERNATOR TABULATION: O.C. TEST S.C TEST S. No. I f (A) V g(1-1) (V) V g (ph) (V) S.C TEST S. No. I f (A) I SC (A) FORMULAE USED: 1. Synchronous Impedance (ph),z S = (Open circuit voltage/phase) / (Short circuit current/phase) (For the same field current) 2. Synchronous Reactance (Ph), X S = [(Z S ) 2 -(R a ) 2 ] 1/2 EMF Method: E 0 ={(Vcos Φ +I a R a ) 2 +(VSin Φ)±I a X S ) 2 } 1/2 E 0 = Induced EMF per Phase, V=Rated voltage per phase, R a = Armature resistance in Ω, I a = Armature current in A; + for lagging p.f. load. - for leading p.f. load. E 0 V % Reg X100 V MMF Method: From the O.C.C. graph, find (1) I f1 - Field current required to produce rated voltage per phase. (2) I f2 - Field current required to produce rated current per phase during S.C. test. I f = {I f1 2 +I f2 2-2I f1 I f2 cos(90±φ)} 1/2 Where + for lagging p.f. load, - for leading p.f. load. 5

6 Now determine V o corresponding to I from graph. % Reg E 0 V X100 V EMF METHOD: S. No. P.F (lag) E o (V) Reg. (%) P.F (lead) E o (V) Reg. (%) MODEL CALCULATION: (EMF method) MODEL GRAPH: 6

7 MMF METHOD: S. No. P.F. (lag) I f (A) E 0 (V) Reg. (%) P.F. (lead) I f (A) Reg. (%) E 0 (V) MODEL CALCULATION: (MMF method) RESULT: REVIEW QUESTIONS: 1. Define Regulation 2. What is meant by pessimistic method? 3. Which method is called as optimistic method? 4. What are the advantages of EMF and MMF method? 5. List out the various methods used to predetermine the regulation. 7

8 REGULATION OF A THREE PHASE ALTRENATOR BY ZPF AND ASA METHODS AIM: To conduct OC and SC tests on a given 3-Φ alternator and hence to predetermine the regulation by (i) ZPF method (ii) ASA method APPARATUS REQUIRED: S. No. Apparatus Range &Type Quantity 1 Alternator Set up - I No. 2 Rheostat 200 Ω/3 A 2 Nos. 3 Voltmeter AC (0-600 V), MI I No. 4 Ammeter AC (0-10 A), MI I No. 5 Ammeter DC (0-2 A), MC I No. 6 Tachometer - 1No. 7 DPST Switch and TPST Switch - 1 each 8 Connecting wires & Fuse - As Required THEORY: Loading an alternator causes its terminal voltage to drop or rise depending upon (i) Magnitude of load (ii) Nature of load. For a pure resistive load it drops by 8-12% below no-load value while for a lagging p.f. load the drop is 25-50% below no load value and it is 20-30% higher for leading p.f. loads. The reasons are 1) Armature resistance 2) Armature winding leakage reactance and 3) Armature reaction. Electromotive force (EMF) and Magnetomotive force (MMF) methods are used to predetermine the regulation of non-salient pole alternators. In emf method, the effect of armature reaction is represented as a fictitious reactance X ar for each phase of the alternator. In mmf method effect of armature leakage reactance is replaced by additional armature reaction. MMF method is more accurate. Direct load Test is not preferred due to the absence of large sized loads and the enormous power wastage involved in testing. Voltage regulation is defined as a percentage of rated voltage when load current is reduced to zero suddenly by throwing off the load keeping I f and speed constant. PRECAUTIONS: (Not to be included in the Record) 1. Remove the fuse before starting wiring. 2. Fuse rating calculation: Since this is no load test, the required fuse rating is only 20% of the rated current of the alternator. 3. Keep Motor field Rheostat in minimum resistance Position. 4. Keep the potential divider for alternator field in minimum voltage position. 5. Check that the TPST on alternator side is open. PROCEDURE: 1. The circuit connections are given as shown in the circuit diagram. 2. Keeping the motor field rheostat in the indicated position and with the TPSTS open, the motor supply is switched ON, by closing DPSTS Motor is started using the 3-point starter by moving the handle from OFF to ON position and the motor is brought to its rated speed by adjusting the rheostat in the motor field circuit. 4. Supply is switched on to the field winding of alternator by closing the DPSTS 2. 8

9 O.C. TEST: 1. Using the 200 ohm potential divider, current in field circuit is increased in steps of 0.1A and at each step the alternator induced voltage indicated by voltmeter and the corresponding field current (I f ) are noted in tabular column. 2. This procedure is continued until the alternator voltage is 120% of its rated voltage. 3. After completing O.C. Test, the potential divider and motor field rheostat are brought to its minimum position. 4. After completing the experiment, calculate Synchronous Impedance, Synchronous Reactance & Regulation using the formulae given. 5. Using the data, Plot the graph between E o Vs I f. S.C. TEST: 1. The alternator terminals are short circuited by closing TPST switch through an ammeter. 2. The rated current is made to flow through the armature of the stator windings by carefully adjusting 220 ohms potential divider from the minimum position. 3. After completing the experiment, calculate the Load current, Field Current and Regulation. 4. Using the data, Plot the graph I sc vs I f and % Regulation vs Power Factor for both the EMF and MMF methods. CIRCUIT DIAGRAM: FUSE CALCULATION: MOTOR 9

10 ALTERNATOR TABULATION: O.C. TEST S.C TEST S. No. I f (A) V g(1-1) (V) V g (ph) (V) S.C TEST S. No. I f (A) I SC (A) FORMULAE USED: 1. Synchronous Impedance (ph),z S = (Open circuit voltage/phase) / (Short circuit current/phase) (For the same field current) 2. Synchronous Reactance (Ph), X S = [(Z S ) 2 -(R a ) 2 ] 1/2 PROCEDURE ZPF TEST 1. Connections are made as per the circuit diagram. 2. Close the D. P. S. T. switch. 3. Start the D. C. motor (prime mover) with the help of three point starter. 4. The field rheostat of the motor should be adjusted to bring the motor speed equivalent to the synchronous speed of the alternator. 5. Close the D. P. S. T. switch in the field circuit of the alternator. 6. The potential divider of the alternator field is varied till the ammeter in the alternator circuit reads rated current of the alternator. 7. Reduce the field current on the alternator side to zero value. 8. Reduce the speed by adjusting the motor field rheostat. 9. Open all the switches. 10

11 TABULAR COLOUMN ZPF test Sl. No Field current, I f (A) Voltage, E 0 (V) FORMULAE TO BE USED At Lagging power factor E 0 = (V cosφ + I a R a ) 2 + (V sinφ + IX L ) 2 Percentage Regulation = ((E 0 V) / V) 100 I f = I f1 2 + I f I f1 I f2 cos(180 (90 - ф)) Percentage regulation = ((E 0 V) / V) 100 POWER FACTOR PERCENTAGE VOTAGE REGULATION ZPF METHOD ASA METHOD 0.2 LAG UNITY LEAD RESULT The predetermination of percentage of voltage regulation of given alternator using ZPF and ASA methods were found. 11

12 REGULATION OF A THREE PHASE ALTRENATOR BY SLIP TEST AIM: To conduct OC and SC tests on a given 3-Φ alternator and hence to predetermine the regulation by Slip test APPARATUS REQUIRED: S. No. Apparatus Range &Type Quantity 1 Alternator Set up - I No. 2 Rheostat 200 Ω/3 A 2 Nos. 3 Voltmeter AC (0-600 V), MI I No. 4 Ammeter AC (0-10 A), MI I No. 5 Ammeter DC (0-2 A), MC I No. 6 Tachometer - 1No. 7 DPST Switch and TPST Switch - 1 each 8 Connecting wires & Fuse - As Required THEORY: Loading an alternator causes its terminal voltage to drop or rise depending upon (i) Magnitude of load (ii) Nature of load. For a pure resistive load it drops by 8-12% below no-load value while for a lagging p.f. load the drop is 25-50% below no load value and it is 20-30% higher for leading p.f. loads. The reasons are 1) Armature resistance 2) Armature winding leakage reactance and 3) Armature reaction. Electromotive force (EMF) and Magnetomotive force (MMF) methods are used to predetermine the regulation of non-salient pole alternators. In emf method, the effect of armature reaction is represented as a fictitious reactance X ar for each phase of the alternator. In mmf method effect of armature leakage reactance is replaced by additional armature reaction. MMF method is more accurate. Direct load Test is not preferred due to the absence of large sized loads and the enormous power wastage involved in testing. Voltage regulation is defined as a percentage of rated voltage when load current is reduced to zero suddenly by throwing off the load keeping I f and speed constant. CIRCUIT DIAGRAM: 12

13 PRECAUTION 1. All the switches should be kept open at the time of starting the experiment. 2. There should be no load at the time of starting the experiment. 3. The motor field rheostat should be kept at minimum resistance position. PROCEDURE 1. Connections are made as per the circuit diagram. 2. The D. P. S. T. switch should be closed. 3. The motor should be started with the help of three point starter. 4. The motor speed should be made equivalent to alternator synchronous speed with the help of field rheostat of the motor. 5. The auto transformer should be varied to get maximum variations in the meter. 6. All the readings should be noted down. TABULAR COLOUMN S. No. MAXIMUM VOLTAGE (V) MINIMUM VOLTAGE (V) MAXIMUM CURRENT (A) MINIMUM CURRENT (A) FORMULAE X d = (Maximum Voltage / 3 ) / Minimum current X q = (Minimum voltage / 3) / Maximum current d direct axis component q quadrature axis component RESULT The direct axis and quadrature axis component of the three phase alternator have been calculated. 13

14 MEASUREMENTS OF NEGATIVE SEQUENCE IMPEDANCE AND ZERO SEQUENCE IMPEDANCE OF ALTERNATORS AIM To obtain the negative sequence and zero sequence reactance of a given three phase alternator. APPARATUS REQUIRED S. No. Name of the apparatus Range Type Quantity 1 Voltmeter (0 300 V) M. I. 1 2 Voltmeter (0 150 V) M. I. 1 3 Ammeter (0 20 A) M. I. 1 4 Ammeter (0 10 A) M. I. 1 5 Ammeter (0 2 A) M. I. 1 6 Wattmeter 300 V, 10 A UPF 1 7 Rheostat 220 Ω, 2 A Single phase Auto Transformer (0 270 V) Tachometer ( rpm) Digital 1 NEGATIVE SEQUENCE PARAMETERS PRECAUTION 1. All the switches should be kept open at the time of starting the experiment. 2. The D. C. motor field rheostat should be kept at minimum resistance position at the time of starting the experiment. 3. The generator field potential divider should be kept at minimum potential position. PROCEDURE 1. Connections are made as per the circuit diagram. 2. The motor should be started with the help of three point starter. 3. The motor speed should be made equivalent to alternator synchronous speed with the help of field rheostat of the motor. 4. The D. P. S. T. switch on the alternator field side should be closed. 5. The alternator field potential divider must be varied in steps. 6. At each step, all the meter readings should be noted down. 7. The above procedure should be repeated till the ammeter reads the rated alternator current. 14

15 TABULAR COLOUMN NEGATIVE NEGATIVE S. No. VOLTAGE V RY (V) CURRENT I SC (A) POWER (W) SEQUENCE IMPEDANCE SEQUENCE REACTANCE AVERAGE (X 2 ) (Z 2 ) (X 2 ) FORMULAE Z 2 = V RY / (3 I SC ) X 2 = Z 2 (W 2 / (V RY I SC )) ZERO SEQUENCE PARAMETERS PRECAUTION 1. All the switches must be kept open at the time of starting the experiment. 2. The auto transformer should be kept at minimum potential position. PROCEDURE 1. Connections are made as per the circuit diagram. 2. The auto transformer should be varied in steps. 3. At each step the meter readings should be noted down. 4. The above procedure should be repeated till the ammeter reads rated current of the alternator. TABULAR COLOUMN S. No OBSERVATION VOLTAGE CURRENT (V) (A) CALCULATION X 0 = (3 V) / I AVERAGE X 0 15

16 RESULT Thus the negative sequence reactance and zero sequence reactance have been determined. 16

17 AIM: V AND INVERTED V CURVES OFTHREE PHASE SYNCHRONOUS MOTOR To draw V and inverted V curves for given three phase synchronous motor APPARATUS REQUIRED: S. No. Name of the apparatus Range Type Quantity 1 Voltmeter (0 600 V) M. I. 1 2 Ammeter (0 10 A) M. I. 1 3 Ammeter (0 2 A) M. C. 1 4 Wattmeter 600 V, 10 A UPF 1 5 Tachometer ( rpm) Digital 1 6 Three phase Auto Transformer (0 470 V) Rheostat 950 Ω, 0.8 A - 1 CIRCUIT DIAGRAM: 17

18 PRECAUTION 1. All the switches should be kept open at the time of starting the experiment. 2. The potential divider in the field circuit of synchronous motor should be kept at minimum potential position. PROCEDURE 1. Connections are made as per the circuit diagram. 2. Close the T. P. S. T. switch. 3. The auto transformer is varied gradually to start the motor. 4. The auto transformer is adjusted till the voltmeter reads the rated voltage of the synchronous motor. 5. Close the D. P. S. T. switch and increase the field current. 6. At no load condition, increase the field current in steps and note down the corresponding armature current. 7. The potential divider is brought to the minimum potential position. 8. Repeat the same procedure for different load conditions. 9. Reduce the load on the motor. 10. Reduce the field current to zero value. 11. Reduce voltage by varying auto transformer. 12. Open all the switches. TABULAR COLOUMN S. No. I a (A) I f (A) WATTMETER READING W 1 (W) W 2 (W) W 1 + W 2 (W) POWER FACTOR 18

19 GRAPH Field current, I f Vs Armature current, I a Field current, I f Vs Power factor, cosф RESULT Thus the V and inverted V curves of the given synchronous motor have been drawn. 19

20 LOAD TEST ON THREE PHASE INDUCTION MOTOR AIM: To conduct the direct load test on a given 3-phase induction motor and plot the performance characteristics of the machine. NAME PLATE DETAILS: Rated voltage = Rated power = Rated current = Frequency = Rated speed = APPARATUS REQUIRED: S. No. Apparatus Range & Type Quantity 1 3 Ф Induction Motor - 1 No. 2 3 Ф Autotransformer 415 V (0 470 V), 12.4 KVA 1 No. 3 Voltmeter AC (0-600 V), MI 1 No. 4 Ammeter AC (0-10 A), MI 1 No. 5 Wattmeter 600 V, 10 A, UPF ( Double Element) 1 No. 6 Tachometer, TPST Switch - 1 each 7 Connecting wires & fuse - As Required THEORY: Squirrel cage induction motors are so called because of the rotor construction, which is the most rugged construction. The rotor conductors are heavy bars of copper, Aluminium that are permanently shortcircuited. The rotor slots are given a slight skew for quieter operation and to prevent the locking tendency of the rotor. The direct load test is conducted on the squirrel cage induction motor to plot its performance characteristics under loading condition. This is more accurate than the predetermination techniques as the latter doesn t take into account the effect of factors such as temperature, which cause significant change in its operation. PRECAUTIONS: (Not to be included in the Record) 1. Remove the fuse carrier before starting wiring 2. Fuse rating calculation: Since this is load test, the required fuse rating is only 120% of the rated current of the motor 3. Before switching on the supply ensure the motor in on no load condition and the autotransformer is in the minimum position 4. Replace the fuse carriers with appropriate fuse wires after the circuit connections are checked by the staff in charge 20

21 PROCEDURE: 1. The connections are given as shown in circuit diagram. 2. The 3Ф ac supply is switched ON to the motor using the starter. 3. Under this load condition, one set of readings of the ammeter (I L ), voltmeter (V L ), wattmeter (W), spring balance and the speed (N) of motor are noted down. 4. Now the mechanical load on motor is increased in regular steps in such a way that the current drawn by the motor increases in steps of 1A. 5. At each step of loading, the entire meter readings are noted down in the tabular column. 6. This procedure is continued until the current drawn by the motor equals 120% of its rated value. 7. After the experiment is completed, the main supply is switched OFF. 8. After completing the experiment, Torque, Output Power, Power Factor, % Slip and % efficiency are calculated by using the given formulae. 9. Using the obtained data, the plot of % efficiency Vs Output power,.% Slip vs Output power, Speed vs Output power, power factor vs Output power, Line current vs Output power and Slip vs torque. CIRCUIT DIAGRAM: FUSE CALCULATION: 21

22 TABULAR COLUMN: S. No. Line voltage V L (V) Line current I L (A) Input power (W) Speed (RPM) Spring balance reading F 1 F 2 F 1 ~ F 2 (Kg) (Kg) (Kg) Torque (Nm) Output power (W) P.F % %Slip MODEL GRAPHS: 22

23 FORMULAE USED: 1. Torque = 9.81*R*(F 1 ~F 2 ) N-m R Radius of the brake drum including belt thickness. (m) F 1, F 2 spring balance readings in kg. 2. Output Power = 2πNT/60 W T Torque in N-m N Speed in rpm. 3. Power Factor = Input Power / 3 V L* I L Watt 4. % Efficiency = (Output/Input)* 100 % 5. % Slip = [(N s -N)/N s ]*100% MODEL CALCULATION: RESULT: REVIEW QUESTIONS: 1. What is Skewing? 2. What is cogging? 3. What is crawling? 4. Define Slip 23

24 AIM: NO LOAD AND BLOCKED ROTOR TESTS ON A THREE PHASE INDUCTION MOTOR To draw the equivalent circuit diagram of the given 3-phase squirrel-cage induction motor by conducting no load and blocked rotor tests. NAME PLATE DETAILS: Rated Voltage (volts) : Rated Speed (R.P.M) : Rated Current (Amps) : Frequency (Hz) : Rated Output (H.P.) : APPARATUS REQUIRED: S. No. Apparatus Type Range Quantity 1 3 Ф Induction Motor Set up No 2 Voltmeter AC MI 3 Ammeter AC MI 4 3 Auto transformer - (0-600 V) (0-150 V) (0-10 A) (0-2 A) kva/ 415 (0-470 V) 1 each 1 each 1 No 5 Double Element Wattmeter V,10 A 1 No 6 Wattmeter LPF 600 V, 10 A 2 No 7 Connecting wires - - As Required THEORY: This is a predetermination technique used to calculate the characteristics of the motor under difference load conditions without actually loading the machine. The disadvantages of direct load test are (i) Absence of loads of large magnitude (ii) Wastage of large amount of power during testing. The results obtained are accurate enough for practical purposes. Therefore we apply the predetermination techniques to obtain the load characteristics of the machine. From the losses obtained in these tests the equivalent circuit of three phase induction motor can be determined and drawn. PRECAUTIONS: (Not to be included in record) 1. Remove the fuse carriers before wiring and start wiring as per the circuit diagram. 2. Fuse Calculations: This being a load test, the required fuse ratings are 120% of rated current. 3. The auto transformer should be kept in minimum position. 4. Replace the fuse carrier with appropriate fuse wires after the circuit connections are checked by the staff-in-charge. 24

25 PROCEDURE: NO LOAD (OPEN CIRCUIT) TEST: 1. The circuit connections are given as per the circuit diagram. 2. With the autotransformer starter in minimum position, the supply is switched ON and the voltage is gradually increased to rated voltage as the motor picks up speed. 3. All the meter readings are noted down for this no-load condition. 4. If anyone of the wattmeter shows a negative deflection, then bring the autotransformer to the minimum position, switch off the supply and reverse the current coil connection. Then apply rated voltage and record this wattmeter reading as a negative power. 5. The autotransformer is brought back to its minimum position and the mains are switched OFF. BLOCKED ROTOR TEST: 1. The circuit connections are given as per the circuit diagram, 2. With the autotransformer starter in minimum position and the rotor in blocked position, the mains are switched ON. 3. By varying the autotransformer, the input voltage is gradually increased such that the ammeter reads rated current. 4. All the meter readings are noted down in this condition. 5. The autotransformer is brought back to its minimum position and the mains are switched OFF. CIRCUIT DIAGRAM: NO LOAD TEST: FUSE CALCULATION: 25

26 BLOCKED ROTOR TEST: FUSE CALCULATION: TABULATION: NO LOAD (OPEN CIRCUIT) TEST: Open circuit Voltage V oc (volts) No load Current I oc (amps) No load Power W oc W 1 W 2 W oc = W 1 + W 2 (watts) BLOCKED ROTOR (SHORT CIRCUIT) TEST: Short circuit Voltage V sc (volts) Short circuit Current I sc (amps) Short circuit Power W sc (watts) 26

27 FORMULAE USED: No load Impedance (Z 0 )= V oc / (I oc / 3) No load Resistance( R 0 )= W oc /I oc 2 No load Reactance( X 0 )= [( Z 0 ) 2 - (R 0 ) 2 ] Power factor angle ( 0 )= cos -1 [W oc / ( 3V oc I oc )] Block rotor resistance (R BR )= W sc / I sc 2 Block rotor impedance( Z BR )= V sc / (I sc / 3) Block rotor reactance ( X BR )= [Z BR 2 R BR 2 ] R iwf Resistance accounting for rotational losses R 1 = 1.2*stator winding resistance (dc) P r = W oc I oc 2 R 1 (Since P r = P 0 3(I oc / 3) 2 R 1 ) R iwf = V oc 2 / P r X m = Magnetic reactance I iwf = V oc / R iwf I m = (I oc 2 - I iwf 2 ) 1/2 X m = V oc / I m EQUIVALENT CIRCUIT: MODEL CALCULATIONS: RESULT: The No-load and blocked rotor test was conducted on the given three-phase induction motor & the equivalent circuit is drawn. REVIEW QUESTIONS: 1. Prove that three phase power can be measure using two watt meters. 2. What is the necessity to have starter for three phase induction motor? 3. How mechanical load is represented in the equivalent circuit of induction motor? 4. Define Synchronous Speed. 5. Why induction motors cannot run at synchronous speed? 27

28 AIM: LOAD TEST ON SINGLE PHASE INDUCTION MOTOR To draw load characteristics of a single phase induction motor by conducting the load test APPARATUS REQUIRED: S. No. Apparatus Range Type Quantity 1 Single phase induction motor Single phase auto transformer (0 270 V) 1 3 Voltmeter (0-300) V MI 1 4 Ammeter (0-10 ) A MI 1 5 Wattmeter 300 V, 10 A, UPF 1 6 Connecting wires - - As required THEORY: Constructional of this motor is more or less similar to a poly phase induction motor, except that its stator is provided with a single phase winding. A centrifugal switch is used in some type of motor in order to cut out a winding, used in some type of motor, in order to cut out a winding, used in some type of motors for starting squirrel cage rotor, when fed from a single phase only alternating one which alternates along one phase axis only. Now, alternating or pulsating flux acting on a stationary squired cage rotor cannot produce rotation that is why a single phase motor is not self starting. FORMULAE 1. Torque ( T )=S*9.81*R Nm 2. Output power (P o ) = 2πNT/60 watts 3. Efficiency (η) = Output power / Input power X 100 % 4. Slip S = (Ns Nr) / Ns * 100 % 5. Synchronous speed Ns = 120 f / P rpm 6. Power factor cos Φ = P in / (V L *I L ) where, R Radius of brake drum. V L Line Voltage 28

29 I L Line current N Speed in rpm Nr - Rated speed in rpm T Torque in Nm CIRCUIT DIAGRAM FUSE RATING NAME PLATE DETAILS PRECAUTIONS: 1. Motor should be started and stopped under no load condition. 2. Brake drum should be cooled with water when it is under load. PROCEDURE: 1. Connections are made as per the circuit diagram. 2. The DPST switch is closed. The autotransformer is adjusted to get rated voltage and corresponding no load readings are noted down. 3. Gradually increase the load upto the rated current and for each load the corresponding meter readings are tabulated 4. Then load is removed and autotransformer reduced to zero. Then DPST switch opened. 29

30 S. No 1 TABULAR COLUMN: Voltage V L (V) Current I L (A) Input Power P in (W) Spring Balance Reading S=S 1 S 2 S 1 (Kg) S 2 (Kg) (Kg) Circumference of the Brake drum = m. Speed N (rpm) Torque T (Nm) Output Power P o (W) Efficiency % SLIP S Power Factor Cos Φ Radius of Brake Drum Circumference = 2 π R = Radius = R=Circumference / (2 π) = meter MODEL CALCULATION 30

31 MODEL GRAPH RESULT: 31

32 NO LOAD AND BLOCKED ROTOR TESTS ON SINGLE PHASE INDUCTION MOTOR AIM To draw the equivalent circuit of the given three phase induction motor by conducting no load and blocked rotor test. APPARATUS REQUIRED S. No. Apparatus Range Type Quantity 1 Single phase induction motor Single phase auto transformer (0 270 V) 1 3 Voltmeter 4 Ammeter 5 Wattmeter (0-300) V (0-150) V (0-10) A (0-10) A 150 V, 5 A, LPF 300 V, 10 A, UPF MI 1 MI 1 1 each 6 Connecting wires - - As required CIRCUIT DIAGRAM NO LOAD TEST 32

33 BLOCKED ROTOR TEST PRECAUTION 1. All the switches should be kept open at the time of starting the experiment. 2. The three phase auto transformer should be kept at minimum potential position at the time of starting the experiment. 3. For the blocked rotor test, the load is applied on the rotor and the rotor is not allowed to rotate. 4. During the blocked rotor test, the three phase auto transformer should be adjusted carefully and the current should not exceed the rated current of the motor. PROCEDURE NO LOAD TEST 1. Connections are made as per the circuit diagram. 2. Close the D. P. S. T. switch. 3. Start the motor by varying the three phase auto transformer gradually. 4. At no load all the meter readings should noted down. 5. The three phase auto transformer should be reduced to zero potential position. 6. Open the D. P. S. T. switch. BLOCKED ROTOR TEST 1. Connections are made as per the circuit diagram. 2. The load applied on the brake drum and the rotor is not allowed to rotate. 3. Close the D. P. S. T. switch. 4. The three phase auto transformer should be adjusted gradually till the ammeter reads the rated current of the motor. 5. All the meter readings should be noted down. 6. The three phase auto transformer should be reduced to zero potential position. 7. Open the D. P. S. T. switch. 33

34 TABULAR COLOUMN OPEN CIRCUIT TEST Sl. No. Voltage, Vo (V) Current, Io (A) Power, Wo (W) SHORT CIRCUIT TEST Sl. No. Voltage, V SC (V) Current, I SC (A) Power, W SC (W) FORMULAE W 0 = V 0 I 0 COS ф 0 COS ф 0 = W 0 / (V 0 I 0 ) I C = I 0 COS ф 0 I m = I 0 SIN ф 0 R 0 = V O / I C X 0 = V 0 / I m X 2 = X eq / 2 R 2 = R eq - R 1 R 0 = V 0 / (I 0 COS ф 0 ) X 0 = V 0 / (I 0 SIN ф 0 ) 2 R 01 = W SC / I SC X 01 = Z R 01 R 2 = R 01 (R m ll el R s ) X 2 = X 01 (X m ll el (X S - X L )) X S = Z 2 2 S R S X m = Z 2 2 m R m Z SC = V SC / I SC 2 R SC = W SC / I SC X SC = Z 2 2 SC R SC Slip = (N s - N r ) / N s R l 2 = R SC R 1 X l 2 = X 1 = X SC / 2 X m = 2 (X 0 X 1 (X l 2 / 2)) 34

35 RESULT Thus the equivalent circuit of single phase induction motor has been drawn using no load and blocked rotor tests. 35