Sheet No. Answeres

Sheet No. 3

GUST Gulf University f or Science and Technology

Military Education Departement Depar tement Specialized Training institute Technical school

DC and Synchronous Machines For Electrical Machinery Major First Year Second Semester –

Complied with Public Authority for Technical Education and Training (PATET) Syllabus

Prepared & Edited by:

Eng. Mohammed M. El-Adeeb 2014-2015 DC and Synchronous Machines…. Prepared Prepared & Edited By: Eng. Mohammed M. El -Adeeb

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Sheet No. 3

First Part: Theoretical

1. Explain with aid of drawing the theory of operation of DC motor. 2. Explain with aid of drawing the production of developed torque in DC motors. 3. Explain with aid of drawing the dynamic behavior of a DC motor when loading and unloading it. 4. What are the types of DC motors? or classify DC motors according to their connections. 5. What are the practical applications of DC shunt, series and compound motors? 6. What is the difference between cumulative, level and differential DC compound motors? 7. What is the purpose of using DC motor starter? 8. Why the starting current in DC motor is too high? 9. How can we limit the starting current in DC motors? 10.Explain 10. Explain with aid of drawing the three point starter, mention the protection schemes used in it. 11.Voltage 11. Voltage control is a method of speed control in DC motors. Mention two techniques used in it. Explain your answer with drawing.





Sheet No. 3

Second Part: Problems Notes: 1.

Developed torque is gross torque is armature torque

2.

Neglect Armature reaction and brush voltage brush unless otherwise mentioned

1. A 220-V 220-V d.c. machine has an armature resistance of 0.5 Ω. If the full-load full-load armature current is 20 A, find the induced e.m.f. when the machine acts as : (i) Generator Ans. 230 V (ii) Motor. Ans. 210 V Answer

2. A separately excited D.C. generator has armature circuit resistance of 0.1 ohm and the total brush-drop is 2 V. When running at 1000 r.p.m., it delivers a current of 100 A at 250 V to a load of constant resistance. If the generator speed drop to 700 r.p.m., with field-current unaltered, find the current curr ent delivered to load . Ans. 96.77 A Answer

3. A 440-V, 440-V, shunt motor has armature resistance of 0.8 Ω and field resistance of 200 200 Ω. Determine the back e.m.f. e.m.f. when giving an output output of 7.46 kW at 85 percent efficiency. Ans. 425 .8 V Answer





Sheet No. 3

4. A 250-V, 4-pole, wave-wound d.c. series motor has 782 conductors on its armature. It has armature and series field resistance resistance of 0.75 ohm. The motor takes a current of 40 A. Estimate its speed and gross torque developed if it it has a flux per pole of 25 mWb. Ans. 249 N.m Answer

5. Determine developed torque and shaft torque of 220-V, 4-pole series motor with 800 conductors wave-connected supplying a load of 8.2 kW by taking 45 A from the mains. The flux per pole is 25 mWb mWb and its armature circuit circuit resistance is 0.6 Ω. 286.2 N.m, 270.5 N.m

Answer

6. A 500-V, 37.3 kW, 1000 r.p.m. DC shunt motor has on full-load an efficiency of 90 percent. The armature circuit resistance is 0.24 0.24 Ω and there there is total voltage drop of 2 V at the brushes. The field current is 1.8 A. Determine (i) full-load full-load line current (ii) full load shaft torque in N.m and (iii) total resistance in motor starter to limit the starting current to 1.5 times the full -load current. 82.9 A, 356 N.m , 3.825 Ω

Answer





Sheet No. 3

7. A 4-pole, 220-V shunt motor has 540 lap-wound conductor. It takes 32 A from the supply mains mains and develops output output power of 5.595 kW. The field winding winding takes 1 A. The armature resistance is 0.09 Ω and the flux per pole is 30 mWb. Calculate (i) the speed and (ii) the torque developed in newton-meter . Ans. 804.4 rpm, 66.5 N.m

Answer

8. Determine the torque established by the armature of a four-pole D.C. motor having 774 conductors, two paths in parallel, 24 milli-webers of pole-flux and the armature current is 50 Amps. Ans. 295.36 N.m

Answer

9. A DC series motor takes 40 A at 220 V and runs at 800 r.p.m. r.p.m. If the armature and field resistance are 0.2 Ω and 0.1 Ω respectively and a nd the iron and friction losses are 0.5 kW, find the torque developed developed in the armature. What will be the





Sheet No. 3

Answer

10.The 10.The armature winding winding of a 200-V, 4-pole, series motor is lap-connected. There are 280 slots and each slot has 4 conductors. conductors. The current is 45 A and the the flux per pole is 18 mWb. The field resistance is 0.3 Ω; the armature armatur e resistance 0.5 Ω and the iron and friction losses total 800 W. The pulley diameter is 0.41 m. Find the pull in newton at the the rim of the pulley. Ans. 634 N

Answer

11.A 11. A 4-pole, 240 V, wave connected shunt motor gives 11.19 kW when running at 1000 r.p.m. and drawing armature and field currents of 50 A and 1.0 A respectively. It has 540 conductors. Its resistance resistance is 0.1 Ω. Assuming a drop of 1 volt per brush, find (a) total torque (b) useful torque (c) useful flux / pole (d) rotational losses and (e) efficiency. Ans. 111 N.m , 106.9 N.m , 12.9 mWb , 460 W , 91.4 %







Sheet No. 3

12.A 12.A DC shunt machine machine generates 250-V on open circuit at 1000 r.p.m. Effective armature resistance is 0.5 Ω, field resistance is 250 Ω, input to machine running as a motor on no load is 4 A at 250 V. Calculate speed of machine as a motor motor taking 40 A at 250 V. Armature reaction weakens field by 4%. Ans. 960 rpm

Answer





Sheet No. 3

Answer

14.A 14. A DC compound long-shunt motor draws a current of 50 A at a voltage of 220 V and rotates at 1200 rpm. Calculate the back induced emf if the armature resistance is 0.04 ohms, series field resistance is 0.01 ohms and shunt field resistance is 125 ohms. Calculate also the developed torque. Ans. 217.588 V, 83.53 N.m

15.A 15. A DC compound long-shunt motor draws a current of 50 A at a voltage of 220 V. if the armature resistance is 0.04 ohms , series field resistance is 0.01 ohms and shunt field resistance is 125 ohms. Knowing that the motor is rotating at 1200 rpm and has an efficiency of 85% calculate: a) back induced emf. b) Input power. c) Output power. d) Shaft torque. e) Copper losses. f) Iron losses. Ans. 217.588 V, 11 kW, 9.35 kW, 74.41 N.m, 503.55 W, 1146.45 W

16.A 16. A DC compound short-shunt motor draws a current of 50 A at a voltage of 220 V. if the armature resistance is 0.04 ohms , series field resistance is 0.01 ohms and shunt field resistance is 125 ohms. Knowing that the motor is rotating at 1200 rpm and has an efficiency of 90% calculate: a) back induced emf. b) Input power.

Sheet No. Answeres

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