Module
9
DC Machines
Version 2 EE IIT, Kharagpur

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Lesson
41
Problem Solving on D.C
Machines
Version 2 EE IIT, Kharagpur

Contents
41 Problem solving on D.C machines (Lesson-41)
4
41.1
Introduction
...............................................................................................................
4
41.2
Shunt motor problems
...............................................................................................
4
41.3
Problems on Series Motor
………………………………………………………….
8
41.4
D.C generator problems
............................................................................................
12
41.5
Operation of d.c machine connected to d.c bus (fixed d.c voltage) ………………...
13
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Introduction
In this lecture some typical problems on D.C machines are worked out not only to solve the
problems only but also to bring out important features of the motors involving its performance.
To begin with few problems on d.c motors have been solved and then problems on generator are
taken up.
For a beginner, it is suggested to follow the following guidelines for successful completion of
the problem.
1.
Read the problem and note down the informations provided about the motor.
2.
In general, some initial steady operating conditions are given in terms of armature
current, field current, speed etc.
3.
Draw a circuit diagram showing the initial variable values.
4.
Write down the
back emf
equation and the
torque
equation for the initial operating point,
it is reiterated here that these two equations hold the key for the gateway of correct
solution.
5.
In the statement of the problem, with respect to the initial operating point, some variable
(like, armature resistance, field resistance, load torque, armature applied voltage etc.) will
be changed and you will be asked perhaps to calculate new armature current, speed etc.
6.
Assume the variables to be calculated and write down once again the
back emf
equation
and the
torque
equation for the new operating point.
7.
You have now four equations – 2 corresponding to initial operating point and the other 2
corresponding to the new operating point.
8.
Take the ratio of the torque equations and the ratio of back emf equations.
9.
Solve from the above equations the unknown.
Let us now solve some problems on shunt, separately excited and series motors.
41.2 Shunt motor problems
1.
A 220 V shunt motor has armature and field resistance of 0.2
Ω
and 220
Ω
respectively.
The motor is driving a constant load torque and running at 1000 rpm drawing 10 A
current from the supply.
Calculate the new speed and armature current if an external
armature resistance of value 5
Ω
is inserted in the armature circuit.
Neglect armature
reaction and saturation.
Version 2 EE IIT, Kharagpur