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DC and Synchronous Machines

For

Electrical Machinery Major

First

Year

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

heet

Prepared & Edited by:

Eng. Mohammed M. El-Adeeb

2014-2015

Military Education Departement

Specialized Training institute

Technical school

GUST

Gulf University

or Science and Technologyf

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1DC and Synchronous Machines. Prepared & Edited By: Eng. Mohammed M. El-Adeeb

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 with aid of drawing the three point starter, mention the protection schemes

used in it.

11.Voltage control is a method of speed control in DC motors. Mention two techniques

used in it. Explain your answer with drawing.

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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 d.c. machine has an armature resistance of 0.5 . If the 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 delivered to load.

Ans. 96.77 A

Answer

3. A 440-V, shunt motor has armature resistance of 0.8 and field resistance of200

. Determine the back e.m.f. when giving an output of 7.46 kW at 85 percent

efficiency.

Ans. 425 .8 V

Answer

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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 of 0.75 ohm. The motor takes

a current of 40 A. Estimate its speed and gross torque developed if 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 fromthe mains. The flux per pole is 25 mWb and its armature circuit resistance is 0.6

.

286.2N.m, 270.5N.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 and there is total voltage

drop of 2 V at the brushes. The field current is 1.8 A. Determine (i) full-load line

current (ii) full load shaft torque in N.m and (iii) total resistance in motor starterto limit the starting current to 1.5 times the full-load current.

82.9 A, 356 N.m , 3.825

Answer

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7. A 4-pole, 220-V shunt motor has 540 lap-wound conductor. It takes 32 A from

the supply mains and develops output power of 5.595 kW. The field winding

takes 1 A. The armature resistance is 0.09 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 andthe 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. If the armature

and field resistance are 0.2 and 0.1 respectively and the iron and friction

losses are 0.5 kW, find the torque developed in the armature. What will be the

output of the motor?

Ans. 99.3 N.m , 7.82 kW

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Answer

10.The armature winding of a 200-V, 4-pole, series motor is lap-connected. There

are 280 slots and each slot has 4 conductors. The current is 45 A and the flux per

pole is 18 mWb. The field resistance is 0.3 ; the armature resistance 0.5 and

the iron and friction losses total 800 W. The pulley diameter is 0.41 m. Find the

pull in newton at the rim of the pulley.

Ans. 634 N

Answer

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 is 0.1 . 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 %

Answer

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12.A DC shunt machine generates 250-V on open circuit at 1000 r.p.m. Effective

armature resistance is 0.5 , field resistance is 250 , input to machine running

as a motor on no load is 4 A at 250 V. Calculate speed of machine as a motor

taking 40 A at 250 V. Armature reaction weakens field by 4%.

Ans. 960 rpm

Answer

13.

A 220-V, DC shunt motor takes 4 A at no-load when running at 700 r.p.m. The

field resistance is 100 .The resistance of armature at standstill gives a drop of

6 volts across armature terminals when 10 A were passed through it. Calculate:

(a) speed on load (b) torque in N-m and (c) efficiency. The normal input of the

motor is 8 kW.

Ans. 623.9 rpm, 103 N.m,79.2 %

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Answer

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 resistanceis 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 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 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.

c) Output power. d) Shaft torque. e) Copper losses. f) Iron losses.

Ans. 217.57V, 11 kW , 9.9 kW , 78.78 N.m , 503.54 W , 769.46 W