Question

3. A 20kW, 250V, 1000rpm shunt excited dc motor has armature and field resistances of 0.2 ohm and 2002. When the motor takes 80A rated current at rated conditions. Draw the equivalent circuit and calculate induced torque at 1200rpm. Explain the starting current limiting methods for DC motors. (30 pts) 4. Explain the conditions for parallel operation of synchronous generators. A three phase, wye connected, synchronous generator is rated 150 MW, 0.85 lagging power factor, 12.6 kV, 60 Hz, and 1800 rpm. Each winding has an armature resistance of 0.05N and a synchronous reactance of 0.612. Draw the phasor diagram with values, show torque angle and determine the induced voltage for the condition of rated load. (30 pts)

Answer 1

solution وله مل asov lotoo I Rata Ra= oar Rg=20016 1 given 1.25 is 80 A 250 goo cia 80 - 1.25 = 78.75A X = Eat jara 250 = Eat 78.75 X 0.2 Ea a 234.25V Pout = Ea sa = 234.25x78.75 Pout a Torque a 18447, 187 Pout D 1200 X QIT 60 Tonque a 18447.187 1200X QT 60 Torque 146.798 Nom To linit Starting current we use resistances in Series cwith armature and cut portions in steps such that maxim value of off some

armatore current remains in limit, Rsh Si Se S3 Rz Ea, Ra * Rz RE hat to-si closed itg- So closed i tz - Szclosed ig ny nat- et de tu ts initially 6 -o RI = ولا N- Ealno) again v-falno R2 Ru R2 i ig current is always in between i, & is

at all time sustant of switching Re RK-1 Ro Rz Rk ول a ) Rs XS da + sut Specification 150 mw 0.85 Pt lagging at 12. 6 ka, 60Hz toge connected out 1800 RPM.. B + 5x = 0.05 tjo.6 Vt = 12.6 1x V 53 Is S da cue know, 3 V ia cosa a 150x106 138 12.6x10²x0.85

ja = 8086. 1382 A os PA is 0.85 lagging & cor10.85 808613221-31883 A 31.7883° vt in (esejxs) (pen Phase) fy 12:6x103+ (8086.1382 /-31.7883). 13 (0.05+30.6) -> fj= 10899.84626 / 21,022 v . Ele = 18879.0875 L21.027° v and tonque angle 21, 022 Paralll operation of synchronous generators After the two synchronous generators brought to their respective synchronous speed and the in terminal voltage made almost Same by controlling treld current, All the three phases and frequency of both generators are synchronized and then both are connected to same bus sing orrcuit breaker. The active power sharing of the two is dependent on their draup or frequency (speed) - power characterstich.