3. A 240-V DC shunt DC motor has an armature resistance of 0.25 2 and a field resistance of 120 2. At full load the armature draws a current of 40 A and the speed is 1100 rpm. Find the developed torque The field rheostat is adjusted so that the field resistance is 150 2. Find the new operating speed if the torque and the armature current remain constant. (i) (ii)

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1. A DC motor develops a torque of 30 N-m. Determine the torque when the armature
current is increased by 50% and the flux is reduced by 10%.
2. A 10-hp 230-V DC series motor has a line current of 37 A and a rated speed of 1200 rpm.
The armature and series field resistances are 0.4 22 and 0.2 2 respectively. The total brush
voltage drop is 2 V. Calculate the following:
a. Speed at a line current of 20 A.
b. No-load speed when the line current is 1 A.
c. Speed at 150% of the rated load when the line current is 60 A and the series field flux is
125% of the full load flux.
3. A 240-V DC shunt DC motor has an armature resistance of 0.25 2 and a field resistance
of 120 2. At full load the armature draws a current of 40 A and the speed is 1100 rpm.
(i)
Find the developed torque
(ii)
The field rheostat is adjusted so that the field resistance is 150 2. Find the new
operating speed if the torque and the armature current remain constant.
4. Compute the efficiency of 100 kW, 250-V, 6-pole, 1000 rpm long shunt compound
generator. The generator has the following parameters
Armature resistance R₁ = 0.015
Series field resistance R, = 0.0102
Shunt field resistance R₁ = 100.0 2
Total rotational losses = 4000 W
Assume stray losses to be 1% of the output and a brush drop of 2 volts.
5. A 100 V de series motor has an armature resistance of 0.2 2 and series field resistance of
0.250. When its pulley exerts a torque of 27.58 N.m it runs at a speed of 600 rpm, iron
and friction losses at this speed are 300 W. Determine
lost torque
copper losses and
(i)
(ii)
(iii) efficiency.
Transcribed Image Text:1. A DC motor develops a torque of 30 N-m. Determine the torque when the armature current is increased by 50% and the flux is reduced by 10%. 2. A 10-hp 230-V DC series motor has a line current of 37 A and a rated speed of 1200 rpm. The armature and series field resistances are 0.4 22 and 0.2 2 respectively. The total brush voltage drop is 2 V. Calculate the following: a. Speed at a line current of 20 A. b. No-load speed when the line current is 1 A. c. Speed at 150% of the rated load when the line current is 60 A and the series field flux is 125% of the full load flux. 3. A 240-V DC shunt DC motor has an armature resistance of 0.25 2 and a field resistance of 120 2. At full load the armature draws a current of 40 A and the speed is 1100 rpm. (i) Find the developed torque (ii) The field rheostat is adjusted so that the field resistance is 150 2. Find the new operating speed if the torque and the armature current remain constant. 4. Compute the efficiency of 100 kW, 250-V, 6-pole, 1000 rpm long shunt compound generator. The generator has the following parameters Armature resistance R₁ = 0.015 Series field resistance R, = 0.0102 Shunt field resistance R₁ = 100.0 2 Total rotational losses = 4000 W Assume stray losses to be 1% of the output and a brush drop of 2 volts. 5. A 100 V de series motor has an armature resistance of 0.2 2 and series field resistance of 0.250. When its pulley exerts a torque of 27.58 N.m it runs at a speed of 600 rpm, iron and friction losses at this speed are 300 W. Determine lost torque copper losses and (i) (ii) (iii) efficiency.
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