The resistance of the armature and field windings is 0.2
Ω
and 150
Ω
respectively. (a) If
the terminal voltage and armature current remain unchanged, what will be the speed of
the machine if it is run as a motor with the flux per pole reduced by 20%? (b) Calculate
the percent change in flux per pole if the machine, has to run as a motor, at the speed of
the generator.
[(a) 953 rpm (b) 4.6 % reduction]
Solution
Data: Power output
P
= 10 kW;
V
L
= 300 V;
N
= 800 rpm;
r
a
= 0.2
Ω
;
R
sh
= 150
Ω
;
rpm
953
;
934
.
292
066
.
307
N
0.8
1000
Hence,
8
.
0
934
.
292
motor
a
as
operating
is
machine
hen the
w
1000
066
.
307
generator
a
as
operating
is
machine
hen the
w
;
Since
(a)
V
934
.
292
2
.
0
33
.
35
300
motor,
a
as
operating
is
machine
When the
V
066
.
307
2
.
0
33
.
35
300
emf
Generated
33
.
35
0
.
2
33
.
33
A
0
.
2
150
300
A;
33
.
33
300
10000
current
Load
b
sh
N
N
E
E
N
E
r
I
V
E
r
I
V
E
A
I
I
V
P
I
b
a
a
a
L
a
a
L
a
a
L
L
Nagsarkar & Sukhija
Basic Electrical Engineering,2/E
Copyright © Oxford University Press , 2011

4.6
flux
in
change
Percentage
954
.
0
or
;
934
.
292
066
.
307
800
800
.
rpm
800
at
running
is
motor
when the
,
is
gap
air
in the
flux
that the
Assume
(b)
x
x
x
9.19(b)
A separately excited dc motor has a full load rated output of 300 HP (metric) at
500 rpm at a rated voltage of 500V. The armature has a lap connected winding with 1000
conductors. If the I
2
R loss in the armature winding is 10 kW, calculate the flux per pole.
Assume that the motor has an efficiency of 90%.
[0.0576 Wb]
Solution
Data: Rated output = 300 metric hp;
N
= 500 rpm;
V
L
= 500 V;
Z
= 1000; Armature
I
2
R
loss = 10 kW, motor efficiency
η
= 90%
A
HP
33
.
490
500
5
.
735
33
.
333
V
Input
current
Armature
33
.
333
9
.
0
300
Output
Input
042
.
0
33
.
490
10000
,
1000
10
2
2
a
a
r
r
I
E
b
= 500
490.33
0.042 = 479.41.
Assume that the machine has
P
poles and the flux per pole is
.
Since the machine is lap wound,
a = P
Wb
0.0575
or
41
.
479
P
60
1000
500
P
9.20
A separately excited dc motor delivers 50 hp (metric) at full load and 1000 rpm. Its
no load speed is 1100 and the armature voltage is 250 V. If the field current is held
constant calculate (a) power at full load, (b) full load current. What is the resistance of the
armature? If the motor terminals are now connected to a supply source of magnitude 500
Nagsarkar & Sukhija
Basic Electrical Engineering,2/E
Copyright © Oxford University Press , 2011