Chapter 4

DC Machines

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 Outline
1. Introduction
2. Classification of DC Motors
3. Motor Characteristics
4. Efficiency
5. Speed Control
6. Starting of DC Motor
7. Classification of DC Generators
8. Voltage Build Up Process
9. Generator Characteristics
10. Load Flow and Efficiency
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 Introduction
• extensively used in industry.
• In these machines, conversion of energy from electrical
to mechanical form or vice versa results from the following
two electromagnetic phenomena:
1. When a conductor moves in a magnetic field, voltage
is induced in the conductor.
2. When a current-carrying conductor is placed in a
magnetic field, the conductor experiences a
mechanical force.
• A wide variety of volt–ampere or torque-speed characteristics can be
obtained from various connections of the field windings.
• field winding is placed on the stator and the armature winding on the
rotor.
• A simple DC Machine shown in the following Fig.
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Simple DC Machine

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Animated Simple DC Machine

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Construction Of DC Machine

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 Magnetization Curve
 If we plot the generated voltage on the Y axis and field current on the X
axis then the magnetization curve will be as shown in figure below.

• Magnetization curve of a DC
Machine has a great
importance because it
represents the saturation of
the magnetic circuit.
• For this reason this curve is
also called saturation curve.

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 Magnetization curve of a DC machine
 The internal generated voltage EA is directly proportional to the flux in the
machine and the speed of its rotation.
 The field current in a DC machine produces a field mmf F = NFIF, which
produces a flux in the machine according to the magnetization curve.

or in terms
of internal
voltage vs.
field current
for a given
speed.

To get the maximum possible power per weight out of the machine,
most motors and generators are operating near the saturation point
on the magnetization curve. Therefore, when operating at full load,
often a large increase in current IF may be needed for small increases
in the generated voltage EA.
 Classsification of DC Machines
• Each DC machine can
act as a generator or a
motor.
• Hence, this
classification is valid for
both: DC generators
and DC motors.
• DC machines are
usually classified on
the basis of their field
excitation method.
• This makes two broad
categories of dc
machines;
1. Separately excited
and
2. Self-excited.

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 Classsification of DC Machines
 Separately excited: In separately excited dc machines, the field winding is
supplied from a separate power source. That means the field winding is
electrically separated from the armature circuit. Separately excited DC
generators are not commonly used because they are relatively expensive
due to the requirement of an additional power source or circuitry. They are
used in laboratories for research work, for accurate speed control of DC
motors with Ward-Leonard system and in few other applications where self-
excited DC generators are unsatisfactory. In this type, the stator field flux
may also be provided with the help of permanent magnets (such as in the
case of a permanent magnet DC motors). A PMDC motor may be used in a
small toy car.
 Self-excited: In this type, field winding and armature winding are
interconnected in various ways to achieve a wide range of performance
characteristics (for example, field winding in series or parallel with the
armature winding).In self-excited type of DC generator, the field winding is
energized by the current produced by themselves. A small amount of flux is
always present in the poles due to the residual magnetism. So, initially,
current induces in the armature conductors of a dc generator only due to the
residual magnetism. The field flux gradually increases as the induced
current starts flowing through the field winding. 10
 Classsification of DC Machines
Self-excited machines can be further classified as –

 Series wound – In this type, field winding is connected in series with the
armature winding. Therefore, the field winding carries whole load current
(armature current). That is why series winding is designed with few turns of
thick wire and the resistance is kept very low (about 0.5 Ohm).
 Shunt wound – Here, field winding is connected in parallel with the
armature winding. Hence, the full voltage is applied across the field
winding. Shunt winding is made with a large number of turns and the
resistance is kept very high (about 100 Ohm). It takes only small current
which is less than 5% of the rated armature current.
 Compound wound – In this type, there are two sets of field winding. One
is connected in series and the other is connected in parallel with the
armature winding. Compound wound machines are further divided as -
• Short shunt – field winding is connected in parallel with only the
armature winding
• Long shunt – field winding is connected in parallel with the
combination of series field winding and armature winding

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 Classsification of DC Machines

Fig: Different connections of dc machines. (a) Separately excited dc

machine. (b) Series dc machine. (c) Shunt dc machine. (d) Compound dc
machine. 12
Separately Excited DC Motors

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Series DC Motors

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Cmpound DC Motors

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Example

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Power Flow and Efficiency

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Example

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Speed Control Techniques

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Speed Control Techniques

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Speed Control Techniques

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Speed Control Techniques

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Speed Control Techniques

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Speed Control Techniques

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Speed Control Techniques

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Starting of DC Motor

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 DC Generators
• The dc machine operating as a generator
is driven by a prime mover at a constant
speed and the armature terminals are
connected to a load.
• In many applications of dc generators,
knowledge of the variation of the terminal
voltage with load current, known as the
external or (terminal) characteristic, is
essential.

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 1. Separately excited DC Generator

• The defining equations are the following:

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1. Separately excited DC Generator

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 Armature Reaction
• In a DC machine, the main field is produced by field coils.
• In both the generating and motoring modes, the armature carries
current and a magnetic field is established, which is called the
armature flux.
• The effect of armature flux on the main field is called the armature
reaction.

• Much of the armature mmf can be

neutralized by using a compensating
winding, which is fitted in slots cut on
the main pole faces.
• MMF produced by compensating
winding opposes the armature mmf.
• The compensating winding is
connected in series with the armature
winding so that its mmf is
proportional to armature mmf.
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2. Shunt (Self-Excited) Generator

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Voltage buildup in a self-excited dc generator.

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Effect of field Resistance

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External Characteristic Curve

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3. Series Generator

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4. Compound Machine

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External Characteristic Curve

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Power Flow and Efficiency

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