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Experiments performed in LAB:
# Circuits diagrams used in lab:
Observations:
Part 1:
Speed of rotor = 1480 rpm
S No. E (Volts) If ( Amp ) 1 164 0.15
2 204 0.2 3 242 0.25
4 248 0.26 5 254 0.27
6 260 0.28
Part 2:
Speed of rotor = 1480 rpm
S No. Load (W) VM (V) IM (A) VG (V) IG (A)
1 40 220 0.90 218 0.20 2 60 222 1.02 210 0.27
3 100 220 1.16 196 0.42 4 140 220 1.27 182 0.58
5 160 220 1.35 174 0.66 6 200 222 1.50 160 0.76
Part 3:
1. Rf critical = 920 ohm
2. Rf = 876 ohm
3. Speed Critical ( ncritical ) = 1314 rpm
Part 4:
Speed of rotor = 1480 rpm
S No. Load (W) VM (V) IM (A) VG (V) IG (A)
1 40 224 1.14 200 0.18 2 60 220 1.18 198 0.19 3 100 220 1.26 180 0.175
4 140 220 1.34 168 0.165 5 160 222 1.34 154 0.15
6 200 220 1.32 134 0.13
Calculation:
1. Calculation of Rf critical from E Vs If plot:-
Rf critical = Slope of initial linear curve of E Vs If plot
= (205-165)/(0.2-0.15625)
= 914.3 ohm
Rf critical calculated = 914.3 ohm
Rf critical observed = 920.0 ohm
Rf critical calculated ≈ Rf critical observed
2. Calculation of ng
ng = VGIG / VMIMnM
Calculation table:
S No. VM (V) IM (A) nM VG (V) IG (A) ng %
1 220 0.90 0.51 218 0.20 43.18 2 222 1.02 0.53 210 0.27 47.24 3 220 1.16 0.55 196 0.42 58.65
4 220 1.27 0.55 182 0.58 68.70 5 220 1.35 0.55 174 0.66 70.30
6 222 1.50 0.54 160 0.76 67.62
3. Calculation of field circuit resistance from E Vs If plot
Field Circuit Resistance = slope of the joining plot(220) to origin
= ( 220-0 ) / ( 0.2475 -0 )
= 888.89 ohm
Rf calculated = 888.89 ohm
Rf observed = 876 ohm
Rf calculated ≈ Rf observed
4. Calculation of critical speed from E Vs If plot
Rf = 888.89 ohm calculated from E Vs If plot
At If = 0.2 A
Ef , from Rf = 888.89*0.2 = 177.78 V
Ea , from E Vs If plot = 204 V
As E is proportional to ∅
Speed critical / speed for E Vs If plot = Ef / Ea
Speed critical = 1480*177.78 / 204
= 1289.76 rpm
Speed critical observed = 1314 rpm
Speed critical observed ≈ Speed critical calculated
Result:
1. Field resistance Rf : a) Calculated = 888.89 ohm
b) Observed = 876 ohm
2. Rf critical: a) Calculated = 914.30 ohm
b) Observed = 920 ohm
3. n critical: a) Calculated = 1289.76 ohm
b) Observed = 1314 ohm
4. Residual E i.e. V at If = 0 : is present
5. VG at IG = 0 : a) For separate excited
b) For shunt excited
Inference:
1. Rf critical and nf critical estimated from no load characteristic of
DC generator are approximately equal to thrie observed values.
2. No load characteristic indicated that residual magnetism in
field exists.
3. Load characteristics of DC Generator for separately excited
mode decreases with increase in load current.
4. Load characteristic curve of DC generator for shunt mode first
decrease with increase in load current and afterwards, it
decreases with decrease in load current.
5. Efficieny of a DC Generator. ng first increases with increases in
IG and reached its maximum. For a particular value of IG and
after that it starts decreasing with further increase in IG.
Precautions:
1. Readings of Voltage and currents for determination of “no load
” and “ load ” characteristics amust be taken at a constant
speed.
2. Before switching on, circuit should be checked by the
instructor.
3. Never reduces the field current when you are determining the
“No Load” characteristics by increasing the current in the field
and vice – versa.
4. Measurements of voltage currents and speeds should be taken
carefully.
5. Wear shoes while doing the experiment.