Lab report 7

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:


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:


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


3. n critical: a) Calculated = 1289.76 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.

Documents

Lab report 7