Tutorial 8 DC Machines 1. A 4 pole, lap wound dc generator has a useful flux of 0.07 Wb per pole. Calculate the generated emf when it is rotated at a speed of 900 rpm with the help of prime mover. Armature consists of 440 numbers of conductors. Also calculate the generated emf if lap wound armature is replaced by wave wound armature. Ans: Lap winding: E = 462V, Wave = 924V. 2. A DC shunt generator has shunt field winding resistance of 100. It is supplying a load of 5 KW at a voltage of 250 V. If its armature resistance is 0.22, calculate the induced emf of generator. Ans: E = 254.95V 3. A 250 V, 10 KW, separately excited generator has an induced emf of 255V at full load. If the brush drop is 2V per brush, calculate the armature resistance of the generator. Ans: Ra = 0.025 . 4. A DC series generator has armature resistance of 0.5 and series field resistance of 0.03. It drives a load of 50A. If it has 6 turns/coil and total 540 coils on the armature and is driven at 1500 rpm, calculate the terminal voltage at the load. Assume 4 poles, lap type winding, flux per pole as 2 mWb and total brush drop as 2V.Ans: Vt = 295.5V 5. A short shunt compound Dc generator supplies a current of 75A at a voltage of 225V. calculate the generated voltage if the resistance of armature , shunt field and series field winding are 0.04,90 and 0.02 respectively.Ans :E = 229.6V. 6. A 4 pole DC generator has a wave wound armature with 792 conductors. The flux per pole is 0.12 Wb. Determine the speed at which it should be run to generate 240V on no load. Ans : N = 757.576 rpm 7. Calculate the generated EMF of a DC generator which has 4 poles, 256 total conductors connected in lap fashion and running at 2000 rpm. The useful flux per pole is 0.2Wb. Ans : Eg = 1706.067V 8. A 4 Pole, wave connected shunt generator is supplying a load of 50 KW at 400V. The armature and field resistances are 0.05 and 100 respectively, Calculate the field, armature currents and induced voltage on open circuit. Take brush contact drop as 1V per brush. Ans : Eg = 408.25 V 9. A shunt generator has an induced voltage an open circuit of 127V. When the machine is on load, the terminal voltage is 120V. Find the load current if the field circuit resistance is 15 and the armature resistance of 0.02. Ignore armature reaction. (emf under no load is same as in load incase to neglect the armature reaction) Ans: Il = 342A. 10. A 4 pole, lap wound 750 rpm dc shunt generator has an armature resistance of 0.4 and field resistance of 200. The armature has 720 conductors and the flux per pole is 30 mWb. If the load resistance is 15, determine the terminal voltage. Ans Vt = 262.4757V.
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DC Motors A 220V DC motor has an armature resistance of 0.75. It is drawing an armature current of 30A, driving a certain load. Calculate the induced emf in the motor under this condition. Ans:Eb = 197.5 Volts A 4 pole DC motor has a lap connected armature winding. The flux per pole is 30 mWb. The number os armature conductor is 250. When connected to 230V dc supply it draws an armature current of 40A. Calculate the back emf and the speed with which motor is running. Assume armature resistance is 0.6. Ans : n = 1648 rpm. A 4 pole , lap wound dc motor has 540 conductors. Its speed is found to be 1000 rpm when it is made run light. The flux per pole is 25 mWb. It is connected to 230 V dc supply. The armature resistance is 0.8. Calculate (i) Induced emf ,(ii) Armature current. Ans:225V, 6.25A A 4 pole, 250 V DC series motor has a wave connected armature with 200 conductors. The flux per pole is 25mWb when motor is drawing 60A from the supply. Armature resistance is 0.15 while series field winding resistance is 0.2. Calculate the speed under this condition. Ans : N = 1374 rpm. A 250 V dc shunt motor takes a line current of 20 A. Resistance of shunt field winding is 200 and resistance of the armature is 0.3. Find the armature current and the back emf. Ans : 18.75A, 244.375V. A dc shunt motor runs at a speed of 1000 rpm on no load ta king a current of 6 A from the supply, when connected to 220 V dc supply. Its full load current is 50A. Calculate its speed on full load. Assume Ra = 0.3 and Rsh = 110. A DC series motor is running with a speed of 800rpm while taking a current of 20A from the supply. If the load is changed such that the current drawn by the motor is increased to 50A, calculate the speed of the motor on new load. The armature and series field winding resistances are 0.2 and 0.3 respectively. Assume the flux produced is proportional to the current. Assume supply voltage as 250V. Ans N2 = 300 rpm. A DC series motor runs at 500 rpm on 220V supply drawing a current of 50A. The total resistance of the machine is 0.15, calculate the value of the extra resistance to be connected in series with the motor circuit that will reduce the speed to 300 rpm. The load torque being then half of the previous value. Assume flux proportional to the current. Ans:Rx = 3.5225 .
