3rd SEMESTER ELECTRICAL
DC Machines & Transformer
(2130904)
D.C. Machine
Assignment – I
1)A 4- pole, d.c. shunt
generator with a wave wound armature
having 390 conductors has to supply the load of 500 lamps, each of 100W at 250
V. Allow 10V as voltage drop in the
connecting leads of the wire between generator and a brush contact drop of 1
volt/brush. Calculate the speed of armature at which the generator should be in
driven. The flux/pole is 30mwb.The armature and shunt field resistances are 0.05Ω
and 65 Ω respectively. (Ans : 698rpm) .
2)A shunt generator
deliver 195A at 250V, armature and shunt field resistances are 0.02 Ω and 50 Ω
respectively. The iron and friction losses are 950W.Calculate (1) Emf generated
,(2)Total copper loss,(3)Output of prime-mover,(4)Mechanical efficiency ,
electrical efficiency and total efficiency.(Ans :254V,2050W,50800W,
98%,95.9%,94.2%).
3) A shunt generator has a full load current of 196A at 220V.
Stray losses are 720W and the shunt field resistance is 55 Ω. It has a
full load efficiency of 88%. Find the
armature resistance. Also find the load current at maximum efficiency.(Ans :
0.114 Ω , 122.28A).
4) A 4-pole DC shunt generator with a shunt & armature
resistances of 100 Ω & 1 Ω respectively .It has 378 wave connected
conductors in its armature. The flux/pole is 20mwb.If a load resistance of 10 Ω
is connected across armature terminals
the generator is driven at 1000rpm. Calculate the power absorbs by the
load and terminal voltage. (Ans : 5153.5W,227V).
5)In a DC machine the
total iron loss is 8KW at its rated speed & excitation .If excitation remains same but speed is reduced
by 25% the total Iron loss is found to
be 5KW. Calculate the hysteresis & eddy current losses. (Ans :
2666.67W,5333.33W).
6)Two shunt generators
with armature resistance 0.1 Ω and field resistance of 20 Ω connected in
parallel & supply load current is 4000A. Generated emf of both are 210V and
220V respectively. Calculate the busbar voltage and output of each machine.(Ans
: 14.93V,29.112kW,30.605kW).
7)Two DC shunt
generator connected in parallel to supply at load of 1500A. One generator feed
armature resistance of 0.5 Ω, emf of
400V. And other generator feed armature resistance of 0.04 Ω and emf of
440V. And field resistances are 100 Ω and 80 Ω respectively. Calculate the load
supply by the generator and busbar voltage .(Ans : 12.88kW,558.9kW,381.2A).
8)A 200V 4-pole lap wound DC shunt motor has 800 conductors
of its armature winding. The armature and field winding resistances are 0.5 Ω
and 200 Ω respectively. Motor takes 21A and flux /pole is 30mwb. Find the
torque and speed of the motor .(Ans : 475rpm , 76.4 Nm).
9)A 230V DC shunt motor runs at a 1000rpm and
takes 5A.The armature and shunt field
resistances are 0.25Ω and 230 Ω respectively. Determine the drop in speed when
the motor is loaded & takes 41A from
the mains.(Ans : 39.31rpm).
10)A DC
shunt motor takes an armature current of 120A from 400V supply & runs at 800rpm. Calculate armature
current and motor speed when the magnetic field is reduced to 80% of its
initial value. The armature resistance is 0.25 Ω and the torque remains same in
both case.(Ans : 150A,979.72rpm).
11)A 250V
DC shunt motor takes rated current of 67A. Armature and field resistances are 0.15 Ω and 166.67 Ω
respectively. The motor takes 6.5A when running without load at 1280rpm.
Determine (1) Full load speed of motor
,(2)Speed regulation ,(3) speed when the motor current is 50A,(4)Speed when
motor current is 40A and there is a armature reaction of 3%.(Ans :
1233.40rpm,0.037,1246.5rpm,1216.59rpm).
12) A 500V
DC shunt motor runs at its nominal speed of 250rpm, the armature current
200A.Calculate the speed when a resistance is inserted in the field winding to
reduce shunt field to 80% of normal value. And the
armature current is 100A ,take armature resistance is 0.12 Ω.(Ans : 320.73rpm).
13)A 250V
DC shunt motor has an armature
resistance of 0.5 Ω and field resistance of 250 Ω. When driving a load of constant torque at 600rpm ,the
armature current is 20A.If it is desire to raise the speed from 600rpm to
800rpm ,what resistance should be inserted in the field circuit.(Ans : 5911.68
Ω).
14)A DC
shunt motor drive a centrifugal pump whose torque varies as the squre of the
speed. Motor is feed from 200V DC supply and takes 50A, when running at 1000rpm
.What resistance must be inserted in the armature circuit to reduce the speed
to 800rpm. Take armature and field resistances 0.1Ω and 100 Ω respectively.(Ans
: 1.329 Ω).
15)A DC series motor with un-saturated speed and
negligible armature and series resistances when running at a certain speed on a
given load takes 15A at 460V.If the load torque
vary as the cube of the speed ,calculate the resistance required to
reduce the speed by 25%.(Ans : 7.26 Ω).
16)A
4-pole,110V ,wave wound dc series generator has armature and series field
resistances of 0.5Ω and 0.3 Ω respectively. Calculate the induced emf when the
generator delivers load current of 10A at 110V. Calculate the speed at which it
should be driven if flux per pole is 5mwb and armature conductors are 1200.(Ans
: 118V,590rpm).
17)A 2-pole
long shunt compound generator has shunt field ,series field and armature
resistances of 200 Ω,0.5 Ω and 0.3 Ω respectively. Calculate the induced emf if
the generator delivers load of 4kW at 400V.Allow a total brush drop of 2V.(Ans
: 411.6V).
18) A 10kW DC
shunt generator has the following losses at full load 1)mechanical
loss=290W,2)Iron loss=410W,3)Shunt cu loss=120W,4)Armature cu
loss=595W.Calculate efficiency at 1)full load ,2)at 25% of full load.(Ans :
87.6%,74.47%).
19)A 4-pole DC machine has a lap wound
armature with 90 slots each
containing 6 conductors. If the machine runs at 1500rpm and the flux per
pole is 0.03wb. Calculate the emf generated. If the machine runs as shunt
generator with same field flux .The armature and field resistances being 1 Ω
and 200 Ω respectively. Calculate output current when the armature current is
25A. If due to a fall in speed, emf becames
380V. Calculate load current in a 40 Ω load.(Ans : 405V,23.1A,9.2A).
20) The open circuit characteristic of a d.c.
shunt generator at 300rpm is
Ish
(A)
|
0
|
0.2
|
0.3
|
0.4
|
0.5
|
0.6
|
0.7
|
E0
(V)
|
7.5
|
93
|
135
|
165
|
186
|
202
|
215
|
The field resistance is adjusted to 354.5 Ω
and speed is 300rpm. Determine 1) residual flux graphically the no load
voltage, 2)critical field resistance ,
3) critical speed for the given field resistance, 4) Additional resistance
inserted in the field resistance to reduce the no-load voltage to 175V.(Ans :
195V,462.5 Ω,227rpm,34.4 Ω).
21) The occ of a shunt generator at 750rpm
are given below :
Ish
(A)
|
3.5
|
6
|
8
|
10
|
12
|
14
|
E0
(V)
|
200
|
300
|
360
|
400
|
440
|
464
|
Calculate 1)The maximum
voltage that can be developed at 750rpm when the field resistance is a)50 Ω , b)35 Ω, 2) The
critical field resistance at 750rpm, 3) The field resistance to develop 400V at
750rpm, 4) The critical speed with Rf.(Ans : 1)300V,446V, 2)57.1 Ω,
3)
40 Ω, 4)525rpm).
22)The data
for magnetisation curve of a 6-pole lap connected shunt generator having 940
conductors and driven at a constant speed of 500rpm is as under :
Ish
(A)
|
0
|
2
|
4
|
6
|
8
|
10
|
12
|
14
|
E0
(V)
|
15
|
100
|
200
|
275
|
312
|
335
|
355
|
370
|
Determine
1)The residual flux ,2)The load supplied by the generator when the terminal
voltage is 245V.Armature reaction weakens the field 4%,3)The speed at which the
machine just fail to excite . Armature and shunt field resistances are 0.3Ω and
35 Ω respectively.(Ans : 0.001915wb,117.1A,350rpm).
23)The open
circuit characteristics of a DC shunt excited generator is at a speed of 600rpm
as under :
Ish
(A)
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
E0
(V)
|
22
|
44
|
66
|
84
|
99
|
111
|
120
|
125
|
Determine the load current when (1)Terminal
potential difference is 120V ,the field resistance is 15Ω at a speed of
600rpm,(2)Terminal voltage is 144V, the field resistance is 18 Ω at a speed of
700rpm. Armature resistance is 0.025 Ω.(Ans : 192A,64A).
24)A
DC shunt generator driven at a constant speed of 600rpm has the following occ:
Ish
(A)
|
1.5
|
3
|
4.5
|
6
|
7.5
|
9
|
E0
(V)
|
75
|
150
|
200
|
230
|
250
|
265
|
Determine
1) The voltage that can be developed with a field resistance of 30Ω, 2)When the
field resistance is 30 Ω ,the terminal voltage of the generator is 240V.Find
the voltage drop due to reduction in field current and the load current of the
generator .Armature reaction weakens the field by 2%, 3)Determine the voltage
drop due to armature reaction when the field resistance is 40 Ω ,terminal
voltage is 120V and the armature current is 90A.The armature resistance is 0.07
Ω and brush contact drop =1V per brush.(Ans : 1)265V,2)10V,104.8A,3)21.7V.).
25)A
20kW,450V DC shunt motor has full load current of 85A at 1100rpm.The armature
resistance is 0.18 Ω. Determine 1)The internal torque developed ,2)The internal
torque developed if the field is suddenly reduced to 80%,3)The steady motor
speed in part 2 assembly load torque remaining same.(Ans :
173.2Nm,138.56Nm,1353rpm).
26) A 220V DC shunt motor runs at 500rpm. When
the armature current is 50A. Calculate the speed if the torque is double.
Armature resistance is 0.2Ω. (Ans : 476rpm).
27)A
4-pole , 240V wave connected DC shunt motor gives 11.19kW,when running at
1000rpm and drawing armature and field currents
of 50A and 1A respectively. It
has 540 conductors .Its resistance is 0.1 Ω, assuming a drop of 1V per brush.
Find a) Total torque, b)Useful torque ,c)useful flux per pole ,d)
rotational losses and e) Efficiency.(Ans
: 111Nm,106.9Nm,12.9mwb,460W,91.4%).
28)A 250V DC shunt motor on no-load runs at
1000rpm takes 5A,the field and armature resistances are 250 Ω and 0.25 Ω
respectively. Calculate the speed when the motor is loaded such that it takes
41A if the armature reaction weakens the field by 3%. (Ans : 994rpm).
29)A
6-pole lap wound shunt motor has 500 conductors in the armature. The armature
and field resistances of 0.05 Ω and 25 Ω respectively. Find the speed of the
motor when it takes 120A from the main dc supply of 100V. Flux per pole is 2x
wb.(Ans ; 565rpm).
wb.(Ans ; 565rpm).
30)
The armature of 6-pole DC shunt motor takes 300A at a speed of 400rpm. The flux
per pole is 75mwb.The number of armature turns is 500. The torque lost in friction and windage losses
can be 2.5%.Calculate 1)Armature torque ,2)shunt torque,3)shaft power.(Ans :
3577.5Nm,3488Nm,146kW).
31)
A 200V DC shunt motor running at 1000rpm takes an armature current of 17.5A.It
is required to reduce the speed to 600rpm.What must be the value of resistance
to be inserted in the armature circuit. Armature resistance is 0.4 Ω.(Ans : 4.4
Ω).
32)A
220V DC shunt motor has an armature and field resistances of 0.4Ω and 200Ω
respectively .It takes an armature current of 22A and runs at 600rpm.It drives
at a load whose torque is constant .Suggest a suitable method to raise the
speed to 900rpm and calculate the value of the controllable parameter.(Ans
:106.8 Ω).
33)A
220V shunt motor has an armature resistance of 0.5 Ω and takes a current of 40A
on full load .By how much must the main flux be reduced to raise the speed by
50%, if the developed torque is continuous.(Ans : 37.5%).
34)A
230V DC shunt motor runs at 800rpm and takes armature current of 50A .Find the
resistance to be added to the field circuit to increase speed to 1000rpm at an
armature current of 80A.Assume that flux is proportional to field current and
armature and shunt field resistance are 0.15 Ω and 250 Ω respectively.(Ans : 69
Ω).
35)
A DC series motor having a resistance of 1 Ω drives a fan for which the torque varies as the
squre of the speed. At 220V the set runs at 350rpm and takes 25A. The speed is
to be raised to 500rpm by increasing the voltage .Determine the necessary
voltage and the corresponding current.(Ans : 433.7V).
36)
A separately excited DC shunt motor is driving a fan whose torque is proportional to the
square of the speed. When 100V are applied to the motor , the armature current
taken by the motor is 8A. With the speed being 500rpm. At what applied voltage
does the speed reach 750rpm and then what is the current drawn by armature?
Armature resistance is 1 Ω.(Ans : 18A,156V).
37)
When the running on no-load a 400V shunt motor takes 5A.Armature resistance is
0.5 Ω and field resistance is 200Ω. Find the output of the motor and efficiency
when running at full load and taking 50A. Also find % change in speed.(Ans :
16.8kW,84.26%,5.98%).
38)
The no-load test of a 44.76kW, 220V dc shunt motor gave the following data :
Input current is 13.25A,field current is 2.55A,armature resistance is 0.032 Ω
and brush voltage drop is 2V. Estimate full load current and efficiency .(Ans :
226A,90%).
39)
A 200V dc shunt motor develops an output of 17.158kW,when taking 20.2kW.The
field resistance and armature resistances are 50 Ω and 0.06 Ω respectively. What is the
efficiency and input power if output power is 7.46kW?(Ans : 10kW,74.6%).
40)
A 250V,14.92kW dc shunt motor has a maximum efficiency of 88% and a speed of
700rpm when delivering 80% of its rated output. The resistance of its shunt
field is 100 Ω. Determine the efficiency
and speed when motor draws a current of 78A from mains.(Ans : 86.9%,680rpm).
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