1- The following particulars refer to a specific dc machine:
Number of slots = 18
Number of coil sides/slot = 2
Number of poles = 6
Design appropriate winding’s pitches and draw a schematic diagram showing brush
positions and parallel circuits.
2- The armature of a dc machine in wound in wave such that commutator pitch is 7
commutator segments (retrogressive) while number of poles is 6.
a. What is the number of coils and commutator segments?
b. What is the total number of conductors, if the coil is made of 25 turns?
c. What is the armature voltage collected across brushes assuming an average value
for conductor voltage of 0.8-V?
d. Draw a schematic diagram showing appropriate brush position and number of
parallel paths.
3- Design a suitable armature winding for a dc machine having 360 coils, 18 poles,
1000-A, so that the current in the conductor does not exceed 250-A (it is sufficient to
show the connection of 3 successive coils).
4- The armature of a 4-pole wave wound dc shunt generator has 144 slots and 4
conductors/slot. The armature is rotated with a speed of 1200-rpm in a field of 25
mWb/pole, calculate the emf generated.
5- An 8-poles generator has an output of 200-A at 500-V, the lap connected armature
has 1280 conductors, 160 commutator segments. If the brushes are advanced 4
commutator segments from the no-load neutral axis, estimate the armature
demagnetizing and cross magnetizing ampere-turns per pole.
6- A 4-pole wave wound generator armature has 880 conductors and delivers 120-A.
The brushes have been displaced through 3o from the geometrical axis. Calculate:
a. Demagnetizing ampere-turns/pole.
b. Cross-magnetizing ampere-turns/pole.
c. The additional field current for neutralizing the demagnetization if the field
winding has 1100 turns/pole.
7- A 4-pole generator supplies a current of 162-A. It has 400 armature conductors, lap
wound. When delivering full-load, the brushes are given an actual lead of 10o.
Calculate the demagnetizing ampere-turns/pole. This field is shunt connected and
takes 10-A. Find the number of extra shunt field turns necessary to neutralize this
demagnetization.
8- A 4-pole, 50-kW, 250-V, wave wound shunt generator has 400 armature conductors;
the coils are single turn coils. Brushes are given a lead of 5 commutator segments.
Calculate the demagnetizing ampere-turns/pole if shunt field resistance is 50-Ω. Also,
calculate extra shunt field turns per pole to neutralize the demagnetization.
9- In an 8-poles shunt dc generator has 960 conductors connected in wave. It delivers an
output of 11-kW at a voltage of 220-V. If the field winding of such machine has 750
turns with a field current of 4-A flowing in it, plot to scale the mmf distribution along
armature periphery. Pole arc/pole pitch = 0.75.
10- A 6-pole, 600-kW, 600-V dc generator has a lap winding with 696 armature
conductors. There are 6 commutating poles. Calculate the number of turns in the
commutating pole winding if the mmf of the commutating poles 1.25 times that of the
armature.
11- If the pole face of the generator in problem (10) covers 70% of the pole span;
a. Calculate for the compensating winding the number of conductors in each pole
face.
b. What should be the number of turns per pole in the commutating pole winding
when the compensating winding of part (a) is in the circuit?
12- A long shunt compound generator delivers a load of 50-A at 500-V and has armature,
series field and shunt field resistances of 0.05-Ω, 0.03-Ω, and 250-Ω respectively.
Calculate the generated voltage and the armature current.
13- An 8-poles dc shunt generator with 778 wave-wound armature conductors and
running at 500-rpm supplies a load of 12.5-Ω resistance at terminal voltage of 250-V.
The armature resistance is 0.24-Ω and the field resistance is 250-Ω. Find the armature
current, the induced emf and the flux per pole.
14- A 4-poles, lap wound dc shunt generator has a useful flux per pole of 70-mWb. The
armature winding consists of 220 turns each of 0.004-Ωresistance. Calculate the
terminal voltage when running at 900-rpm if the armature current is 50-A.
15- A 1500-kW, 550-V, 16-poles shunt dc generator runs at 150-rpm. The shunt field
current is 1% of rated load current. What must be the useful flux per pole if there are
2500 conductors lap connected and the full-load copper losses are 100-kW? Calculate
the area of the pole shoe if the gap flux density is 1.2-Tesla.
16- A 10-kW, 250-V, self excited generator, when delivering rated load, has an armature
circuit voltage drop 5% of the terminal voltage and a shunt field current equal to 5%
of rated load current. Calculate the resistance of the armature circuit and that of the
field circuit.
17- The OCC of a dc shunt generator driven at 400-rpm is as follows:
If (A)
2
3
4
5
6
7
8
9
E (V)
110
155
186
212
230
246
260
271
Find:
a. Voltage to which the machine will excite when running at 400-rpm with shunt
field resistance of 34-Ω.
b. Resistance of shunt field to reduce the open-circuit voltage to 220-V.
c. The critical value of the shunt field circuit resistance.
d. The critical speed when the field circuit resistance is 34-Ω.
18- In a 110-V compound generator the resistance of the armature, shunt field, and series
field windings resistances are 0.06-Ω, 25-Ω, and 0.04-Ω respectively. The load
consists of 200 lamps each rated at 55-W, 110-V. Find the emf and armature current
when the machine is connected (a): long-shunt, (b): short-shunt. How will the
ampere-turn of the series winding be changed if in (a), a diverter of resistance 0.1-Ω
be connected in parallel with the series field winding? Ignore armature reaction.
19- A 120-V, 1800-rpm, cumulatively long-compounded dc generator has the following
particulars: Ra + Rs = 0.21-Ω, Rsh = 20-Ω, Nsh = 1000 turns, Ns = 20 turns. The
machine has the following magnetization curve:
If (A)
1
2
3
4
5
6
7
E (V)
40
80
106
120
128
135
140
Answer the following questions about this machine, assuming no armature reaction:
a. If the generator is operating at no-load, what is its terminal voltage?
b. If the generator has an armature current of 50-A, what is its terminal voltage?
c. Repeat (b) if the series winding is connected as a differential excitation.
20- A 20-kW, 200-V, 1800-rpm, dc shunt generator has Ra = 0.1-Ω, Rsh= 150-Ω. Data for
magnetization curve at 1800-rpm are:
If (A)
0.125
0.25
0.5
0.625
0.75
0.875
1.0
1.25
1.5
E (V)
33.5
67
134
160
175
190
200
214
223
The shunt generator is now connected as long shunt compound generator.
a. Show the generator connection.
b. Determine the number of turns per pole of the series field winding required to
make the no-load and full-load terminal voltage equal 200-V. The series field
resistance is 0.04-Ω, shunt field turns Nsh= 1200 turns/pole.
21- The magnetization curve for a separately excited dc generator at 1800 is as follows:
E(V)
40
80
106
120
128
135
141
If (A)
1
2
3
4
5
6
7
The generator is rated at 6-kW, 120-V, 50-A, and 1800-rpm. Its field circuit is rated
at 5-A. The following are known about the machine: Ra = 0.2-Ω, Rf = 24-Ω, Nf =
1000 turns. The machine has a field rheostat that can vary from 0 to 30-Ω. Assume
no armature reaction.
a. If the generator is running at 1800-rpm, and has no-load terminal voltage of 120V, what is the setting of the field rheostat? What will the no-load terminal
voltage become if field rheostat is decreased by 5-Ω?
b. If the generator has a demagnetizing armature reaction at full load equivalent to
500-AT, what will the terminal voltage be if If = 5-A, Ia = 40-A, at 1800-rpm.
22- A 6-poles, lap wound, shunt generator has to deliver 250-kW at a terminal voltage of
500-V. The armature and field resistances are 0.1-Ω and 100-Ω respectively. There
are 1260 armature conductors and the open circuit characteristics at normal speed is:
Field AT/pole
0
2000
3500
5000
6500
8500
Induced EMF (V)
0
275
420
500
560
615
The brushes are given a forward lead of 6 mechanical degrees. Assume
demagnetizing effect armature reaction AT to be 10% of cross magnetizing armature
reaction.
a- Calculate the number of turns of the field winding per pole.
b- What is the value of the no load voltage?
c- Calculate the terminal voltage and efficiency of the generator when giving the
maximum output current if the iron and mechanical losses equal 9.5-kW. Neglect
stray losses.
23- A 50-hp, 250-V, 1200-rpm shunt motor with compensating windings has an
armature resistance (including the brushes, compensating windings and interpoles)
of 0.06-Ω. Its field circuit has a total resistance of 50-Ω, which produces a no load
speed of 1200-rpm. There are 1200 turns per pole on the shunt field winding.
a- Find the speed of this motor when its input current is 100-A.
b- Find the speed of this motor when its input current is 200-A.
c- Find the speed of this motor when its input current is 300-A.
d- Use these three points to plot the torque-speed characteristic of this motor.
24- The magnetization curve of the motor described in problem (23) at a speed of 1200rpm is as follows:
E (V) 62.5 125 178 219 250 265 276 283 287
289
If (A) 1
2
3
4
5
6
7
8
9
10
The machine has no compensating windings and the demagnetizing effect of
armature reaction is 600 AT at a load of 200-A.
a- Find the speed of this motor when its input current is 200-A.
b- How does this speed compare to that of the previous problem at the same load
current?
25- A 100-hp, 250-V, 1200-rpm, dc shunt motor has an armature resistance of 0.03-Ω and
a field resistance of 41.67-Ω. The motor has compensating winding, so that armature
reaction can be neglected. Mechanical and core losses may be assumed to be
negligible for the purpose of this problem. The field current of the motor is 6-A. The
motor is assumed to be driving a constant torque load with a line current of 126-A
and an initial speed of 1103-rpm.
a- If the motor's magnetization curve is unknown and linearity is assumed, what is the
motor's speed if the field current is reduced to 5-A?
c- If the machine's magnetization curve is the one given in problem (24), what is the
motor's speed if the field current is reduced to 5-A?
26- The motor of problem (25) is now connected separately excited. The motor is initially
running with 250-V, Ia =120-A and 1103-rpm while supplying a constant torque load.
What will the speed of this motor be if supply voltage is reduced to 200-V?
27- The magnetization curve of 250-V series motor with compensating windings and a
total series resistance of 0.08-Ω at a speed of 1200-rpm is as follows:
E (V)
62.5 125 178
219 250 265
276 283 287
289
AT/pole 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
The series field consists of 25turns/pole.
a- Find the speed and the induced torque of this motor for Ia = 25, 50, 75, 100, 200,
300, and 400-A.
b- Plot the resulting torque/speed characteristics of this motor.
28- A 100-hp, 250-V compounded dc motor has an internal resistance, including the
series winding, of 0.04-Ω. There is 1000 turns/pole on the shunt field and 3 turns/pole
on the series winding. The magnetization curve for this machine is the one given for
problem (24) at a speed of 1200-rpm. At no load, the shunt field resistor has been
adjusted to make the motor run at 1200-rpm. The core, mechanical, and stray losses
can be neglected.
a- What is the shunt field current in this machine at no load?
b- If the motor is cumulatively compounded, find its speed when Ia= 200-A.
c- Repeat part (b) if the machine is differentially compounded.
29- A 10-hp, 250-V, shunt motor takes a line current of 5-A when running light. Stray
losses may be neglected. Calculate the efficiency of this motor when delivering full
load output, if the armature and field resistances are 0.5-Ω and 250-Ω respectively. At
what horse-power output will the efficiency be maximum? Is it possible to obtain this
output from the machine?