DC MACHINES AND SYNCHRONOUS MACHINES - QUESTIONS FOR PRACTICES

1. Explain construction and working principle of dc machines with the help of a neat diagram. Drive an emf equation for the same. What is back emf?
2. What are the different efficiency associated with dc generator and dc motors explain?
3. Drive an expression for torque in case of dc machine.
4. A d.c. generator has an armature emf of 100 V when the useful flux per pole is 20 mWb, and the speed is 800 rpm. Calculate the generated emf (i) with the same flux and a speed of 1000 rpm, (ii) with a flux per pole of 24 mWb and a speed of 90 rpm. (Ans.:- (i) 125 V, (ii) 135 V)
5. An 8 pole dc generator has 500 armature conductors and a useful flux of 0.05 Wb. What will be the emf generated, if it lap connected and runs at 1200 rpm? What must be the speed at which it is to be driven to produce the same emf, if it is wave wound? (Ans.:- 500V, 300 rpm)
6. A 4 pole dc shunt generator with lap connected armature has field and armature resistance of 80 Ω and 0.1 Ω respectively. It supplies power to 50 lamps rated for 100 volts, 60 watts each. Calculate the total armature current and the generated emf by allowing a contact drop of 1 V per brush. (Ans.:- 26 A, 104.6 V)
7.  A d.c. shunt machine connected to 230 V supply has resistance of armature as 0.115 Ω and of field winding as 115 Ω. Find the ratio of the speed as a generator to the speed as a motor with the line current in each case being 100 A. (Ans.:- 1.1052:1)
8. A d.c. shunt motor draws 10 A at no load from 230 V mains and runs at 1500 rpm. At full load, armature current is 100 A and speed is 1470 rpm. Armature resistance of the motor is 0.1 Ω and field current negligible. Find (a) back emf at no – load and full load (b) speed at which armature should be run to make it deliver 100 A at 220 V as a generator. Assume same flux as with motor operation at full load of 10 A. (Ans.:- (a) 229 V , 220 V (b) 1536.8 rpm)
9. A 200 V dc series motor runs at 500 rpm when taking a current of 25 A. The resistance of the armature is 0.5 Ω and that of the field is 0.3 Ω. If the current remains constant, calculate the resistance necessary to reduce the speed to 250 rpm. (Ans.:- 3.6 Ω)
10. A d.c. series motor has an armature resistance of 0.12 Ω and field resistance of 0.08 Ω. The supply voltage across the motor terminals is 230 V. Determine the back emf and power developed by the motor when line current drawn by motor is 30 A. (Ans.:- 224 V, 6.72 kW)

11. What are the different types of synchornous machines ? Explain the principle of operation of a 3-phase synchronus motor. Why it is not self starting.

12. Why the field winding of synchronus generators is placed on rotor.

13. Draw and explain V-curve for synchronous motor