620 ROTATING MACHINES13.4.4When delivering rated load a 10-kW, 230-V self-
excited shunt generator has an armature-circuit
voltage drop that is 6% of the terminal voltage
and a shunt-field current equal to 4% of the rated
load current. Calculate the resistance of the ar-
mature circuit and the field circuit.
13.4.5A 20-hp, 250-V shunt motor has a total armature-
circuit resistance of 0.25and a field-circuit
resistance of 200. At no load and rated voltage,
the speed is 1200 r/min, and the line current is
4.5 A. At full load and rated voltage, the line
current is 65 A. Assume the field flux to be
reduced by 6% from its value at no load, due
to the demagnetizing effect of armature reaction.
Compute the full-load speed.
13.4.6A dc series motor is connected to a load. The
torque varies as the square of the speed. With the
diverter-circuit open, the motor takes 20 A and
runs at 500 r/min. Determine the motor current
and speed when the diverter-circuit resistance is
made equal to the series-field resistance. Neglect
saturation and the voltage drops across the series-
field resistance as well as the armature resistance.
13.4.7A 50-kW, 230-V compound generator has the fol-
lowing data: armature-circuit resistance 0.05,
series-field circuit resistance 0.05, and shunt-
field circuit resistance 125. Assuming the total
brush-contact drop to be 2 V, find the induced
armature voltage at rated load and rated terminal
voltage for: (a) short-shunt, and (b) long-shunt
compound connection.
*13.4.8A 50-kW, 250-V, short-shunt compound genera-
tor has the following data:Ra= 0. 06 ,RS=
0. 04 , andRf= 125 . Calculate the induced
armature voltage at rated load and terminal volt-
age. Take2Vasthetotal brush-contact drop.
13.4.9Repeat the calculations of Problem 13.4.8 for a
machine that is a long-shunt compound genera-
tor.
13.4.10A 10-kW, 230-V shunt generator, with an
armature-circuit resistance of 0.1and a field-
circuit resistance of 230, delivers full load at
rated voltage and 1000 r/min. If the machine is
run as a motor while absorbing 10 kW from 230-
V mains, find the speed of the motor. Neglect the
brush-contact drop.
13.4.11The magnetization curve taken at 1000 r/min on
a 200-V dc series motor has the following data:
Field current, A: 5 10 15 20 25 30
Voltage, A: 80 160 202 222 236 244The armature-circuit resistance is 0.25and the
series-field resistance is 0.25. Calculate the
speed of the motor (a) when the armature current
is 25 A, and (b) when the electromagnetic torque
is 36 N·m. Neglect the armature reaction.
*13.4.12A dc series motor operates at 750 r/min with
a line current of 100 A from the 250-V mains.
Its armature-circuit resistance is 0.15and its
series-field resistance is 0.1. Assuming that
the flux corresponding to a current of 25 A is
40% of that corresponding to a current of 100 A,
determine the motor speed at a line current of 25
Aat250V.
13.4.13A 7.5-hp, 250-V, 1800-r/min shunt motor, having
a full-load line current of 26 A, is started with a
four-point starter. The resistance of the armature
circuit, including the interpole winding, is 0.48
; and the resistance of the shunt-field circuit,
including the field rheostat, is 350. The resis-
tances of the steps in the starting resistor are 2.24,
1.47, 0.95, 0.62, 0.40, and 0.26, in the order
in which they are successively cut out. When the
armature current is dropped to its rated value,
the starting box is switched to the next point,
thus eliminating a step in the starting resistance.
Neglecting field-current changes, armature reac-
tion, and armature inductance, find the initial and
final values of the armature current and speed
corresponding to each step.
13.4.14Two shunt generators operate in parallel to supply
a total load current of 3000 A. Each machine has
an armature resistance of 0.05and a field resis-
tance of 100. If the generated emfs are 200 and
210 V, respectively, determine the load voltage
and the armature current of each machine.
13.4.15Three dc generators are operating in parallel with
excitations such that their external characteristics
are almost straight lines over the working range
with the following pairs of data points:Terminal Voltage (V)
Load Current (A) Generator I Generator II Generator III0 492.5 510 525
2000 482.5 470 475Compute the terminal voltage and current of each
generator.
(a) When the total load current is 4350 A.