# Chapter 21: D.c. machines

### Example 1, page no. 354

In [1]:
from __future__ import division
import math
#initializing  the  variables:
Z  =  600;#  no.  of  conductors
c  =  2;#  for  a  wave  winding
p  =  4;#  no.  of  pairs
n  =  625/60;#  in  rev/sec
Phi  =  20E-3;#  in  Wb

#calculation:
#Generated  e.m.f.,  E  =  2*p*Phi*n*Z/c
E  =  2*p*Phi*n*Z/c

#Results
print  "\n\n  Result  \n\n"
print  "\n  the  generated  e.m.f  is  ",round(E,2),"  V  "


Result

the  generated  e.m.f  is   500.0   V  

### Example 2, page no. 354

In [2]:
from __future__ import division
import math
#initializing  the  variables:
Z  =  50*16;#  no.  of  conductors
p  =  1;#  let  no.  of  pairs
c  =  2*p;#  for  a  lap  winding
Phi  =  30E-3;#  in  Wb
E  =  240;#  in  Volts

#calculation:
#Generated  e.m.f.,  E  =  2*p*Phi*n*Z/c
#Rearranging  gives,  speed
n  =  E*c/(2*p*Phi*Z)

#Results
print  "\n\n  Result  \n\n"
print  "\n  the  speed  at  which  the  machine  must  be  driven  is  ",round(n,2),"  rev/sec  "


Result

the  speed  at  which  the  machine  must  be  driven  is   10.0   rev/sec  

### Example 3, page no. 354

In [3]:
from __future__ import division
import math
#initializing  the  variables:
Z  =  1200;#  no.  of  conductors
p  =  1;#  let,  no.  of  pairs
c  =  2*p;#  for  a  lap  winding
Phi  =  30E-3;#  in  Wb
n  =  500/60;#  in  rev/sec

#calculation:
#Generated  e.m.f.,  E  =  2*p*Phi*n*Z/c
E  =  2*p*Phi*n*Z/c

#Results
print  "\n\n  Result  \n\n"
print  "\n  Generated  e.m.f.  is  ",round(E,2),"  V  "


Result

Generated  e.m.f.  is   300.0   V  

### Example 4, page no. 355

In [4]:
from __future__ import division
import math
#initializing  the  variables:
Z  =  1200;#  no.  of  conductors
p  =  4;#  let,  no.  of  pairs
c  =  2;#  for  a  wave  winding
Phi  =  30E-3;#  in  Wb
n  =  500/60;#  in  rev/sec

#calculation:
#Generated  e.m.f.,  E  =  2*p*Phi*n*Z/c
E  =  2*p*Phi*n*Z/c

#Results
print  "\n\n  Result  \n\n"
print  "\n  Generated  e.m.f.  is  ",round(E,2),"  V  "


Result

Generated  e.m.f.  is   1200.0   V  

### Example 5, page no. 355

In [1]:
from __future__ import division
import math
#initializing  the  variables:
E1 = 150; # in Volts
x = 0.2;

#calculation:
E2  =  E1*(1- x)

#Results
print  "\n\n  Result  \n\n"
print  "\n  Generated  e.m.f.  is  ",round(E2,2),"  V  "


Result

Generated  e.m.f.  is   120.0   V  

### Example 6, page no. 356

In [5]:
from __future__ import division
import math
#initializing  the  variables:
n1  =  30;#  in  rev/sec
E1  =  200;#  in  Volts
n2  =  20;#  in  rev/sec
E2  =  250;#  in  Volts

#calculation:
#generated  e.m.f.,  E  proportional  to  phi*w  and  since  w  =  2*pi*n,  then
#  E  proportional  to  phi*n
#  E1/E2  =  Phi1*n1/(Phi2*n2)
#  let  Phi2/Phi1  =  Phi
Phi  =  E2*n1/(E1*n2)
Phi_inc  =  (Phi  -  1)*100#/in  percent

#Results
print  "\n\n  Result  \n\n"
print  "\n  percentage  increase  in  the  flux  per  pole  is  ",round(Phi_inc,2),"  percent  "


Result

percentage  increase  in  the  flux  per  pole  is   87.5   percent  

### Example 7, page no. 357

In [6]:
from __future__ import division
import math
#initializing  the  variables:
Ra  =  0.30;#  in  ohms
Ia  =  30;#  in  Amperes
E  =  200;#  in  Volts

#calculation:
#terminal  voltage,
#V  =  E  -  Ia*Ra
V  =  E  -  Ia*Ra

#Results
print  "\n\n  Result  \n\n"
print  "\n  terminal  voltage  of  a  generator  is  ",round(V,2),"  V  "


Result

terminal  voltage  of  a  generator  is   191.0   V  

### Example 8, page no. 357

In [7]:
from __future__ import division
import math
#initializing  the  variables:
RL  =  60;#  in  ohms
Ia  =  8;#  in  Amperes
Ra  =  1;#  in  ohms

#calculation:
#terminal  voltage,
#V  =  Ia*RL
V  =  Ia*RL
#Generated  e.m.f.,  E
E  =  V  +  Ia*Ra

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)terminal  voltage  is  ",round(V,2),"  V  "
print  "\n  (b)generated  e.m.f.  is  ",round(E,2),"  V  "


Result

(a)terminal  voltage  is   480.0   V

(b)generated  e.m.f.  is   488.0   V  

### Example 9, page no. 357

In [8]:
from __future__ import division
import math
#initializing  the  variables:
E1  =  150;#  in  Volts
n1  =  20;#  in  rev/sec
Phi1  =  0.10;#  in  Wb
n2  =  25;#  in  rev/sec
Phi2  =  0.10;#  in  Wb
n3  =  20;#  in  rev/sec
Phi3  =  0.08;#  in  Wb
n4  =  24;#  in  rev/sec
Phi4  =  0.07;#  in  Wb

#calculation:
#generated  e.m.f.,  E  proportional  to  phi*w  and  since  w  =  2*pi*n,  then
#  E  proportional  to  phi*n
#  E1/E2  =  Phi1*n1/(Phi2*n2)
E2  =  E1*Phi2*n2/(Phi1*n1)
E3  =  E1*Phi3*n3/(Phi1*n1)
E4  =  E1*Phi4*n4/(Phi1*n1)

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)the  generated  e.m.f  is  ",round(E2,2),"  V  "
print  "\n  (b)generated  e.m.f.  is  ",round(E3,2),"  V  "
print  "\n  (c)generated  e.m.f.  is  ",round(E4,2),"  V  "


Result

(a)the  generated  e.m.f  is   187.5   V

(b)generated  e.m.f.  is   120.0   V

(c)generated  e.m.f.  is   126.0   V  

### Example 10, page no. 359

In [9]:
from __future__ import division
import math
#initializing  the  variables:
Ps  =  20000;#  in  Watts
Vs  =  200;#  in  Volts
Rs  =  0.1;#  in  ohms
Rf  =  50;#  in  ohms
Ra  =  0.04;#  in  ohms

#calculation:
Is  =  Ps/Vs
#Volt  drop  in  the  cables  to  the  load
Vd  =  Is*Rs
#Hence  terminal  voltage,
V  =  Vs  +  Vd
#Field  current,  If
If  =  V/Rf
#Armature  current
Ia  =  If  +  Is
#Generated  e.m.f.  E
E  =  V  +  Ia*Ra

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)terminal  voltage  is  ",round(V,2),"  V  "
print  "\n  (b)generated  e.m.f.  is  ",round(E,2),"  V  "


Result

(a)terminal  voltage  is   210.0   V

(b)generated  e.m.f.  is   214.17   V  

### Example 11, page no. 361

In [12]:
from __future__ import division
import math
#initializing  the  variables:
Is  =  80;#  in  amperes
Vs  =  200;#  in  Volts
Rf  =  40;#  in  ohms
Rse  =  0.02;#  in  ohms
Ra  =  0.04;#  in  ohms

#calculation:
#Volt  drop  in  series  winding
Vse  =  Is*Rse
#P.d.  across  the  field  winding  =  p.d.  across  armature
V1  =  Vs  +  Vse
#Field  current,  If
If  =  V1/Rf
#Armature  current
Ia  =  If  +  Is
#Generated  e.m.f.  E
E  =  V1  +  Ia*Ra

#Results
print  "\n\n  Result  \n\n"
print  "\n  generated  e.m.f.  is  ",round(E,2),"  V  "


Result

generated  e.m.f.  is   205.0   V  

### Example 12, page no. 363

In [13]:
from __future__ import division
import math
#initializing  the  variables:
Ps  =  10000;#  in  Watt
Pl  =  600;#  in  Watt
Ra  =  0.75;#  in  ohms
Rf  =  125;#  in  ohms
V  =  250;#  in  Volts

#calculation:
#Output  power  Ps  =  V*I
I  =  Ps/V
#Field  current,  If
If  =  V/Rf
#Armature  current
Ia  =  If  +  I
#Efficiency,
eff  =  Ps*100/((V*I)  +  (Ia*Ia*Ra)  +  (If*V)  +  (Pl))#  in  Percent

#Results
print  "\n\n  Result  \n\n"
print  "\n  Efficiency  is  ",round(eff,2),"  percent  "


Result

Efficiency  is   80.5   percent  

### Example 13, page no. 364

In [14]:
from __future__ import division
import math
#initializing  the  variables:
Ra  =  0.2;#  in  ohms
V  =  240;#  in  Volts
Ia  =  50;#  in  Amperes

#calculation:
#For  a  motor,  V  =  E  +  Ia*Ra
E  =  V  -  Ia*Ra

#Results
print  "\n\n  Result  \n\n"
print  "\n  back  e.m.f.  is  ",round(E,2),"  V  "


Result

back  e.m.f.  is   230.0   V  

### Example 14, page no. 365

In [15]:
from __future__ import division
import math
#initializing  the  variables:
Ra  =  0.25;#  in  ohms
V  =  300;#  in  Volts
Ig  =  100;#  in  Amperes
Im  =  80;#  in  Amperes

#calculation:
#As  a  generator,  generated  e.m.f.,
#  E  =  V  +  Ia*Ra
Eg  =  V  +  Ig*Ra
#For  a  motor,  generated  e.m.f.  (or  back  e.m.f.),
#  E  =  V  -  Ia*Ra
E  =  V  -  Im*Ra

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)As  a  generator,  generated  e.m.f.  is  ",round(Eg,2),"  V  "
print  "\n  (b)back  e.m.f.  is  ",round(E,2),"  V  "


Result

(a)As  a  generator,  generated  e.m.f.  is   325.0   V

(b)back  e.m.f.  is   280.0   V  

### Example 15, page no. 366

In [16]:
from __future__ import division
import math
#initializing  the  variables:
p  =  4;
c  =  2;#  for  a  wave  winding
Phi  =  25E-3;#  Wb
Z  =  900;
Ia  =  30;#  in  Amperes

#calculation:
#torque  T  =  p*Phi*Z*Ia/(pi*c)
T  =  p*Phi*Z*Ia/(1*math.pi*c)

#Results
print  "\n\n  Result  \n\n"
print  "\n  the  torque  exerted  is  ",round(T,2),"  Nm  "


Result

the  torque  exerted  is   429.72   Nm  

### Example 16, page no. 366

In [17]:
from __future__ import division
import math
#initializing  the  variables:
V  =  350;#  in  Volts
Ra  =  0.5;#  in  ohms
n  =  15;#  in  rev/sec
Ia  =  60;#  in  Amperes

#calculation:
#Back  e.m.f.  E  =  V  -  Ia*Ra
E  =  V  -  Ia*Ra
#torque  T  =  E*Ia/(2*n*pi)
T  =  E*Ia/(2*n*math.pi)

#Results
print  "\n\n  Result  \n\n"
print  "\n  the  torque  exerted  is  ",round(T,2),"  Nm  "


Result

the  torque  exerted  is   203.72   Nm  

### Example 17, page no. 366

In [18]:
from __future__ import division
import math
#initializing  the  variables:
p  =  1;#  let
c  =  2*p;#  for  a  lap  winding
Phi  =  20E-3;#  Wb
Z  =  500;
V  =  250;#  in  Volts
Ra  =  1;#  in  ohms
Ia  =  40;#  in  Amperes

#calculation:
#Back  e.m.f.  E  =  V  -  Ia*Ra
E  =  V  -  Ia*Ra
#E.m.f.  E  =  2*p*Phi*n*Z/c
#  rearrange,
n  =  E*c/(2*p*Phi*Z)
#torque  T  =  E*Ia/(2*n*pi)
T  =  E*Ia/(2*n*math.pi)

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)speed  n  is  ",round(n,2),"  rev/sec  "
print  "\n  (b)the  torque  exerted  is  ",round(T,2),"  Nm  "


Result

(a)speed  n  is   21.0   rev/sec

(b)the  torque  exerted  is   63.66   Nm  

### Example 18, page no. 367

In [19]:
from __future__ import division
import math
#initializing  the  variables:
T1  =  25;#  in  Nm
T2  =  35;#  in  Nm
Ia1  =  16;#  in  Amperes
V  =  100;#  in  Volts
x  =  0.15;

#calculation:
#the  shaft  torque  T  of  a  generator  is  proportional  to  (phi*Ia),
#where  Phi  is  the  flux  and  Ia  is  the  armature  current.  Thus,  T  =  k*Phi*Ia,  where  k  is  a  constant.
#The  torque  at  flux  phi1  and  armature  current  Ia1  is  T1  =  k*Phi1*Ia1.
#similarly  T2  =  k*Phi2*Ia2

Ia2  =  T2*Ia1/(0.85*T1)

#Results
print  "\n\n  Result  \n\n"
print  "\n  armature  current  at  the  new  value  of  torque  is  ",round(Ia2,2),"  A  "


Result

armature  current  at  the  new  value  of  torque  is   26.35   A  

### Example 19, page no. 367

In [20]:
from __future__ import division
import math
#initializing  the  variables:
T  =  12;#  in  Nm
I  =  15;#  in  Amperes
V  =  100;#  in  Volts
n  =  1500/60;#  in  rev/sec

#calculation:
#the  efficiency  of  a  generator  =  (output  power/input  power)*100  %
#The  output  power  is  the  electrical  output,  i.e.  VI  watts.
#The  input  power  to  a  generator  is  the  mechanical  power  in  the  shaft  driving  the  generator,
#i.e.  T*w  or  T(2*pi*n)  watts,  where  T  is  the  torque  in  Nm  and  n  is  speed  of  rotation  in  rev/s.  Hence,  for  a  generator
#efficiency  =  V*I*100/(T*2*pi*n) %
eff  =  V*I*100/(T*2*math.pi*n)#  in    Percent
#The  input  power  =  output  power  +  losses
#  hence,  T*2*math.pi*n  =  V*I  +  losses
Pl  =  T*2*math.pi*n  -  V*I

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)  efficiency  is  ",round(eff,2)," % "
print  "\n  (b)  power  loss  is  ",round(Pl,2),"  W  "


Result

(a)  efficiency  is   79.58  %

(b)  power  loss  is   384.96   W  

### Example 20, page no. 368

In [21]:
from __future__ import division
import math
#initializing  the  variables:
Rf  =  150;#  in  Ohms
Ra  =  0.4;#  in  Ohms
I  =  30;#  in  Amperes
V  =  240;#  in  Volts

#calculation:
#Field  current  If
If  =  V/Rf
#Supply  current  I  =  Ia  +  If
#Hence  armature  current,  Ia
Ia  =  I  -  If
#Back  e.m.f.  E  =  V  -  Ia*Ra
E  =  V  -  (Ia*Ra)

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)    current  in  the  armature  is  ",round(Ia,2),"  A  "
print  "\n  (b)  Back  e.m.f.  E  is  ",round(E,2),"  V  "


Result

(a)    current  in  the  armature  is   28.4   A

(b)  Back  e.m.f.  E  is   228.64   V  

### Example 21, page no. 370

In [22]:
from __future__ import division
import math
#initializing  the  variables:
Ia1  =  30;#  in  Amperes
Ia2  =  45;#  in  Amperes
Ra  =  0.4;#  in  ohm
n1  =  1350/60;#  in  Rev/sec
V  =  200;#  in  Volts

#calculation:
#The  relationship  E  proportional  to  (Phi*n)  applies  to  both  generators  and  motors.  For  a  motor,
#E  =  V  -  (Ia*Ra)
E1  =    V  -  (Ia1*Ra)
E2  =    V  -  (Ia2*Ra)
#The  relationship,  E1/E2  =  Phi1*n1/Phi2*n2,    applies  to  both  generators  and  motors.
#Since  the  flux  is  constant,  Phi1  =  Phi2
n2  =  E2*n1/(E1)

#Results
print  "\n\n  Result  \n\n"
print  "\n  the  speed  of  the  motor  is  ",round(n2,2),"  rev/sec  "


Result

the  speed  of  the  motor  is   21.78   rev/sec  

### Example 22, page no. 370

In [23]:
from __future__ import division
import math
#initializing  the  variables:
Ia1  =  30;#  in  Amperes
Ra  =  0.4;#  in  ohm
n  =  800/60;#  in  Rev/sec
V  =  220;#  in  Volts
x=  0.1;

#calculation:
#For  a  d.c.  shunt-wound  motor,  E  =  V  -  (Ia*Ra),Hence  initial  generated  e.m.f.,
E1  =    V  -  (Ia1*Ra)
#The  generated  e.m.f.  is  also  such  that  E  proportional  to  (Phi*n)
#so  at  the  instant  the  flux  is  reduced,  the  speed  has  not  had  time  to  change,  and
E  =  E1*(1-x)
#Hence,  the  voltage  drop  due  to  the  armature  resistance  is
Vd  =  V  -  E
#The  instantaneous  value  of  the  current  is
Ia  =  Vd/Ra
#T  proportional  to  (Phi*Ia),  since  the  torque  is  constant,
#Phi1*Ia1  =  Phi2*Ia2,    The  flux  8  is  reduced  by  10%,  hence
Ia2  =  Ia1/0.9

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)instantaneous  value  of  the  current  ",round(Ia,2),"  A  "
print  "\n  (b)steady  state  value  of  armature  current,  ",round(Ia2,2),"  A  "


Result

(a)instantaneous  value  of  the  current   82.0   A

(b)steady  state  value  of  armature  current,   33.33   A  

### Example 23, page no. 372

In [3]:
from __future__ import division
import math
#initializing  the  variables:
Ia1  =  15;#  in  Amperes
Ia2  =  30;#  in  Amperes
Rf  =  0.3;#  in  ohms
Ra  =  0.2;#  in  ohm
n1  =  24;#  in  Rev/sec
V  =  240;#  in  Volts
x=  2;

#calculation:
#generated  e.m.f.,  E,  at  initial  load,  is  given  by
E1  =    V  -  Ia1*(Ra  +  Rf)
#When  the  current  is  increased  to  30  A,  the  generated  e.m.f.  is  given  by:
E2  =    V  -  Ia2*(Ra  +  Rf)
#E  proportional  to  (Phi*n)
#E1/E2  =  Phi1*n1/Phi2*n2
n2  =  E2*n1/(2*E1)

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)generated  e.m.f.,  E  is  ",round(E1,2),"  V  "
print  "\n  (b)speed  of  motor,  n2,  ",round(n2,2),"  rev/sec  "


Result

(a)generated  e.m.f.,  E  is   232.5   V

(b)speed  of  motor,  n2,   11.61   rev/sec  

### Example 24, page no. 374

In [25]:
from __future__ import division
import math
#initializing  the  variables:
I  =  80;#  in  Amperes
C  =  1500;#  in  Watt
Rf  =  40;#  in  ohms
Ra  =  0.2;#  in  ohm
n  =  1000/60;#  in  Rev/sec
V  =  320;#  in  Volts

#calculation:
#Field  current,  If
If  =  V/Rf
#Armature  current  Ia
Ia  =  I  -  If
#Efficiency
eff = ((V*I - (Ia*Ia*Ra) - (If*V) - C)/(V*I))*100 # in percent

#Results
print "\n\n Result \n\n"
print "\n efficiency is",round(eff,2),"%"


Result

efficiency is 80.09 %

### Example 25, page no. 374

In [25]:
from __future__ import division
import math
#initializing  the  variables:
I  =  40;#  in  Amperes
Rf  =  0.05;#  in  ohms
Ra  =  0.15;#  in  ohm
V  =  250;#  in  Volts

#calculation:
#However  for  a  series  motor,  If  =  0  and  the  Ia*Ia*Ra  loss  needs  to  be  I*I*(Ra  +  Rf)
#For  maximum  efficiency  I*I*(Ra  +  Rf)  =  C
#Efficiency
eff = ((V*I - (2*I*I*(Ra + Rf)))/(V*I))*100 # in percent

#Results
print "\n\n Result \n\n"
print "\n efficiency is",round(eff,2)


Result

efficiency is 93.6

### Example 26, page no. 375

In [26]:
from __future__ import division
import math
#initializing  the  variables:
T  =  15;#  in  Nm
n  =  1200/60;#  in  rev/sec
eff  =  0.8;
V  =  200;#  in  Volts

#calculation:
I = T*2*math.pi*n/(V*eff)

#Results
print "\n\n Result \n\n"
print "\n current supplied, I is ",round(I,2),"A"


Result

current supplied, I is  11.78 A

### Example 27, page no. 376

In [27]:
from __future__ import division
import math
#initializing  the  variables:
R  =  2;#  in  ohm
n  =  30;#  in  rev/sec
I  =  10;#  in  A
C  =  300;#  in  Watt
V  =  400;#  in  Volts

#calculation:
#Efficiency
eff = ((V*I - (I*I*R) - C)/(V*I))*100 # in percent

#Results
print "\n\n Result \n\n"
print "\n efficiency is",round(eff,2),"%"


Result

efficiency is 87.5 %

### Example 28, page no. 378

In [2]:
from __future__ import division
import math
#initializing  the  variables:
Ia1  =  120;#  in  A
Ia2  =  60;#  in  A
Ra  =  0.2;#  in  ohm
n1  =  10;#  in  rev/sec
R  =  0.5;#  in  ohm
x  =  0.8;
V  =  500;#  in  Volts

#calculation:
#back  e.m.f.  at  Ia1
E1  =  V  -  Ia1*Ra
#at  Ia2
E2  =  V  -  Ia2*(Ra  +  R)
#E1/E2  =  Phi1*n1/Phi2*n2
n2  = n1*E2/E1
#Back  e.m.f.  when  Ia2
E3  =  V  -  Ia2*Ra
n3  =  n1*E3/(x*E1)

#Results
print  "\n\n  Result  \n\n"
print  "\n  (a)speed  n2  is  ",round(n2,2),"  rev/sec"
print  "\n  (b)speed  n3  is  ",round(n3,2),"  rev/sec"


Result

(a)speed  n2  is   9.62   rev/sec

(b)speed  n3  is   12.82   rev/sec

### Example 29, page no. 379

In [29]:
from __future__ import division
import math
#initializing  the  variables:
Ia1  =  90;#  in  Amperes
Ra  =  0.1;#  in  ohm
Rse  =  0.05;#  in  ohm
Rd  =  0.2;#  in  Ohm
n1  =  15;#  in  rev/sec
V  =  300;#  in  Volts

#calculation:
#e.m.f.  E1
E1  =  V  -  Ia1*(Ra  +  Rse)
#With  the  Rd  diverter  in  parallel  with  Rse
#equivalent  resistance,  Re
Re  =  Rd*Rse/(Rd+Rse)
#Torque,  T  proprtional  to  Ia*Phi  and  for  full  load  torque,  Ia1*Phi1  =  Ia2*Phi2
#Since  flux  is  proportional  to  field  current  Phi1  proportional  to  Ta1  and  Phi2  Proportional  to  I1
I1  =    (Ia1*Ia1*0.8)**0.5
#By  current  division,  current  I1
Ia2  =  I1/(Rd/(Rd  +  Rse))
#Hence  e.m.f.  E2
E2  =  V  -  Ia2*(Ra  +  Re)
#E1/E2  =  Phi1*n1/Phi2*n2
n2  =  E2*Ia1*n1/(I1*E1)

#Results
print  "\n\n  Result  \n\n"
print  "\n  speed  n2  is  ",round(n2,2),"  rev/sec"


Result

speed  n2  is   16.74   rev/sec

### Example 30, page no. 380

In [30]:
from __future__ import division
import math
#initializing  the  variables:
Ia1  =  25;#  in  Amperes
Ra  =  0.4;#  in  ohm
Rse  =  0.2;#  in  ohm
n1  =  800/60;#  in  rev/sec
n2  =  600/60;#  in  rev/sec
V  =  400;#  in  Volts

#calculation:
#e.m.f.  E1
E1  =  V  -  Ia1*(Ra  +  Rse)
#At  n2,  since  the  current  is  unchanged,  the  flux  is  unchanged.
#E1/E2  =  n1/n2
E2  =  E1*n2/n1
#and  E2  =  V  -  Ia1(Ra  +  Rse  +  R)
R  =  (V  -  E2)/Ia1  -  Ra  -  Rse

#Results
print  "\n\n  Result  \n\n"
print  "\n  Resistance  is  ",round(R,2),"  ohm"


Result

Resistance  is   3.85   ohm