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2012

UNIVERSITY OF JOHANNESBURG

ISHMAEL FARAI CHIREMBA 200907705

Contents

AIM..............................................................................................................................................................3

APPARATUS.................................................................................................................................................3

SAFETY MEASURES......................................................................................................................................3

EXPERIMENTAL SETUP.................................................................................................................................3

FIGURE 1: SEPERATELY EXCITED DC MOTOR.......................................................................................4

FIGURE 2: SIMPLIFIED DIAGRAM OF A WARD LEONARD SYSTEM.......................................................4

FIGURE 3: WIRING DIAGRAM OF A WARD LEONARD SYSTEM.............................................................5

PICTURE 1: EXPERIMENTAL SETUP ON THE DAY OF THE PRACTICAL...................................................6

EXPERIMENTAL PROCEDURE.......................................................................................................................6

OPEN CIRCUIT TEST.................................................................................................................................6

SHORT CIRCUIT TEST................................................................................................................................6

WARD LEONARD SYSTEM TEST................................................................................................................6

RESULTS & CALCULATIONS..........................................................................................................................7

TABLE 1: OPEN CIRCUIT TEST...................................................................................................................7

PLOT 1: OPEN CIRCTUIT TEST..................................................................................................................7

PLOT 1a: OPEN CIRCUIT...........................................................................................................................8

TABLE 2: SHORT CIRCUIT TEST.................................................................................................................8

PLOT 2: SHORT CIRCUIT TEST...................................................................................................................8

TABLE 3: WARD LEONARD SYSTEM TEST.................................................................................................9

PLOT 3: WARD LEONARD SYSTEM TEST.................................................................................................10

PLOT 4: COMBINED GRAPH FOR WARD LEONARD SYSTEM TEST..........................................................10

TABLE 4: DEVELOPED POWER TABLE.....................................................................................................11

PLOT 5: POWER vs SPEED......................................................................................................................12

PLOT 5: TOQUE vs SPEED.......................................................................................................................12

DISCUSSION...............................................................................................................................................13

CONCLUSION.............................................................................................................................................13

AIMThe aim of this practical is to show two DC machines connected in a Ward Leonard System configuration (connected back to back) can be used to control power flow by adjusting the field winding currents.

APPARATUS 3 X Multimeters 2 X DC machine coupled with and induction machine 2 X Tachometers 3 Phase power supply

SAFETY MEASURES1. Whenever the DC machine has to be turned off firstly precaution must be noted that the power

supply voltage has been reduced to the 0V point otherwise the machines will runaway.

2. For the second part of the experiment where the two machines have to be synchronized, caution must be exercised to ensure that the two machines are connected correctly. The cable used to draw power from the 3 phase supply on one machine has to match the connections made on the other machine i.e when the live terminal of the supply is connected to the terminal V1 on one of the machines that connection has to match even on the other machines otherwise this will cause the two machines to rotate in opposite direction resulting in disaster.

3. Precaution should be taken when working around the machine whilst it is operating , make sure that you do not have any loose things on you that can be caught on the rotating shaft as these will cause the shaft to catch onto these and draw you into the machine and hurting you in the process.

4. Make sure that all connecting wires have no broken or burnt insulation as these will cause fires or electric shock.

5. Ensure that in the short circuit test the current passing through the armature does not exceed 10A.

6. For the Ward Leonard System test ensure that both machines are switched on at the same time at the 3 phase power source.

EXPERIMENTAL SETUP

FIGURE 1: SEPERATELY EXCITED DC MOTOR

FIGURE 2: SIMPLIFIED DIAGRAM OF A WARD LEONARD SYSTEM

FIGURE 3: WIRING DIAGRAM OF A WARD LEONARD SYSTEM

PICTURE 1: EXPERIMENTAL SETUP ON THE DAY OF THE PRACTICAL

EXPERIMENTAL PROCEDURE

OPEN CIRCUIT TESTFor the open circuit test use only one of the machines, connect the DC variac to the field side of the machine then connect one multimeter in parallel to the armature. Connect the other multimeter to the DC outlet of the variac because it is much easier to read off digital data from the multimeter than analog data as there is a higher probability of parallax errors on the analog scale of the variac than on data displayed digitally. Increase the field voltage from 0V in increments of 10V to a maximum voltage of 110V whilst measuring the DC voltage on the field using the other ammeter and the speed at each of the increments using the tachometer. Repeat the same procedure but now decrement the input from the variac from 110V to 0V.

SHORT CIRCUIT TESTFor the short circuit test use only one of the machines, connect the DC variac to the field side of the machine then connect one multimeter in series with the armature. Connect the other multimeter to the DC outlet of the variac because it is much easier to read off digital data from the multimeter than analog data as there is a higher probability of parallax errors on the analog scale of the variac than on data displayed digitally. Increase the field voltage from 0V in increments of 10V to a maximum voltage of 110V whilst measuring the current passing through the armature using the other ammeter and the speed at each of the increments using the tachometer. Repeat the same procedure but now decrement the input from the variac from 110V to 0V.

WARD LEONARD SYSTEM TESTFor the Ward Leonard System test couple the two machines together by connecting the armature of one DC machine to the armature of the other DC machine. Then connect one variac to the field of one machine and take another variac and connect it to the field of the other machine, now connect one multimeter to each of the two DC outlets of the two variacs. Connect another multimeter in series with the connection joining the armatures of the machines together, increase the field excitation of both machines simultaneously from 0V to 80V in steps of 10V whilst noting down the armature current and the speed of the shaft at each of the increments. When 80V is reached keep the field excitation of one of the machines constant at 80V but increase the other in steps of 10V till 110V whilst noting down the armature current and the speed of rotation of the shaft, in this case the machine whose field excitation is varying is driving the other machine and acting as a generator while the other is acting as a motor. Now when 110V is reached take down the field excitation of the machine that acted as a generator to 80V and now keep it constant then use the other machine as a generator and repeat the same procedure.

RESULTS & CALCULATIONS

TABLE 1: OPEN CIRCUIT TEST OPEN CIRCUIT TEST FORWARD PATH REVERSE PATHFIELD VOLTAGE (V) ARMATURE VOLATGE(V) FIELD VOLTAGE (V) ARMATURE VOLATGE(V)0 6 100 22710 33 90 21820 62 80 20830 95 70 19440 125 60 17850 150 50 15860 171 40 13270 190 30 10580 203 20 7290 216 10 36100 224 0 7110 233

PLOT 1: OPEN CIRCTUIT TEST

0 50 100 150 200 2500

20

40

60

80

100

120

OPEN CIRCUIT TEST

OPEN CIRCUIT TEST

ARMATURE VOLTAGE/V

FIELD

VOL

TAGE

/V

PLOT 1a: OPEN CIRCUIT

0.0 50.0 100.0 150.0 200.0 250.00

50

100

150

200

250

OPEN CIRCUIT CHARACTERISTIC

OPEN CIRCUIT CHARACTERISTIC

FIELD CURRENT/mA

emf/

Ea(V

)

TABLE 2: SHORT CIRCUIT TEST SHORT CIRCUIT TEST FORWARD PATH REVERSE PATHFIELD VOLTAGE (V) ARMATURE

CURRENT(I/mA)FIELD VOLTAGE (V) ARMATURE CURRENT(I/mA)

0 0.08 100 6.6810 1.02 90 6.3220 1.84 80 5.8530 2.80 70 5.2840 4.28 60 4.6950 5.06 50 4.1260 5.10 40 3.5170 5.65 30 2.8080 6.15 20 1.9290 6.63 10 1.07100 7.01 0 0.07110 7.28

PLOT 2: SHORT CIRCUIT TEST

0 1 2 3 4 5 6 7 80

20

40

60

80

100

120

SHORT CIRCUIT TEST

SHORT CIRCUIT TEST

CURRENT/A

FIELD

VOL

TAGE

/V

TABLE 3: WARD LEONARD SYSTEM TEST

FIELD VOLTAGE (V) ARMATURE CURRENT(mA) SPEED (rpm)10 2.1 149420 3.9 149530 5.9 149440 7.8 149150 11.8 149060 13.6 149070 12.9 148980 11.2 1483 LEFT MACHINE (GENERATOR)90 8.9 1480100 8.8 1478110 9.1 1476 RIGHT MACHINE (GENERATOR)90 4.2 1484100 5.8 1483110 6.4 1481

PLOT 3: WARD LEONARD SYSTEM TEST

0 2 4 6 8 10 12 14 160

20

40

60

80

100

120

WARD LEONARD SYSTEMLEFT MACHINE GENERATORRIGHT MACHINE GENERATOR

ARMATURE CURRENT/mA

FIEL

D VO

LTAG

E/V

PLOT 4: COMBINED GRAPH FOR WARD LEONARD SYSTEM TEST

0 2 4 6 8 10 12 14 160

20

40

60

80

100

120

LEFT MACHINE GENERATORRIGHT MACHINE GENERATOR

ARMATURE CURRENT (I/Ma)

FIELD

VOL

TGAE

/V

¿Field Resistance=547Ω

¿Armature Resistance=7.6Ω

V a=Ea+ I aRa…………. (1)

-We need to determine the value of Ea so that we can use equation (2a) to determine the value of the voltage constant since it was not given prior to the conduction of the practical.

Ea=V a−I a Ra………... (1a)

Ea=K f I fω …………. (2)

K f=EaI fω

………….. (2a)

¿where I f=V f

R f…………. (3)

T d=K f I a I f …………. (4)

V a=I a Ra+K f I fω ………….. (5)

Pd=T dω…………. (6)

≫Pd=K f I a I fω ………… (7)

-Using equation (7) the developed power of a single machine can be calculated, but however for this particular case of the Ward Leonard System connection we need to obtain the power flow equations in order to determine the resultant power one machine (generator) is supplying to the other (motor) at any given instant.

PR=PG−PM ………… (8)

PR−resultant power

PG−generator power

PM−motor power

PG=K f G I aG I f GωG

PM=K f M I aM I f MωM

PR=K f GI aG I f GωG−K f M I aM I f MωM

but I aG=I aM=I a ¿K f G=K fM=K f

∴PR=K f I a(I f GωG−I f MωM) ………… (9)

-incorporating losses in our equations the power flow equation will look as below:

PL=I a2Ra

PR=(PG−PM )−PL

PR=K f I a ( I f GωG−I fMωM )−I a2Ra ………… (10)

TABLE 4: DEVELOPED POWER TABLE

ARMATURE CURRENT(I/mA)

FIELD CURRENT (mA)

ω(rev/min) ARMATURE VOLATGE(V)

BACK EMF (Ea)

Kf DEVELOPED POWER (Pd)/WATTS

0.08 0.0 1492 6 5.999392 UNDEFINED 0.0000001.02 18.3 1494 33 32.992248 1.206730309 0.0336520931.84 36.5 1495 62 61.986016 1.13595118 0.114054269

2.8 54.8 1494 95 94.97872 1.160099278 0.265940416

4.28 73.1 1491 125 124.967472 1.146573669 0.534860785.06 91.4 1490 150 149.961544 1.101152424 0.758805413

5.1 109.7 1490 171 170.96124 1.045935162 0.8719023245.65 128.0 1489 190 189.95706 0.99666859 1.0732573896.15 146.3 1483 203 202.95326 0.935428407 1.2481625496.63 164.5 1480 216 215.949612 0.887002432 1.4317459287.01 182.8 1478 224 223.946724 0.828884633 1.5698665357.28 201.1 1476 233 232.944672 0.784791614 1.6958372126.68 182.8 1480 227 226.949232 0.838862558 1.516020876.32 164.5 1485 218 217.951968 0.892212778 1.3774564385.85 146.3 1488 208 207.95554 0.955263617 1.2165399095.28 128.0 1488 194 193.959872 1.018354503 1.0241081244.69 109.7 1489 178 177.964356 1.089511207 0.834652834.12 91.4 1491 158 157.968688 1.159170105 0.6508309953.51 73.1 1492 132 131.973324 1.210040636 0.463226367

2.8 54.8 1492 105 104.97872 1.283961175 0.2939404161.92 36.6 1492 72 71.985408 1.318240232 0.1382119831.07 18.3 1493 36 35.991868 1.317326687 0.0385112990.07 0.0 1493 7 6.999468 UNDEFINED 0.000000

PLOT 5: POWER vs SPEED

1475 1480 1485 1490 1495 15000.000000

0.200000

0.400000

0.600000

0.800000

1.000000

1.200000

1.400000

1.600000

1.800000

POWER vs SPEED

POWER vs SPEED

SPEED(rev/min)

POW

ER/W

PLOT 5: TOQUE vs SPEED

1475 1480 1485 1490 1495 15000.00000

0.00020

0.00040

0.00060

0.00080

0.00100

0.00120

0.00140

TOQUE vs SPEED

TOQUE vs SPEED

SPEED(rev/min)

TOQ

UE/N

m

DISCUSSION

CONCLUSION