Dc machines

2
DC GENERATORS:
• Constructional details – emf equation – Methods of excitation
– Self and separately excited generators – Characteristics of
series, shunt and compound generators

Wednesday, January 27, 2016 Ch. 14 Alternators and Synchronous Motors 5
Electro-mechanical Energy-conversion
(EMEC) Machines
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AC and DC
Generators and Motors
• When the electrical system is characterized by direct
current, the machines are called dc motors and dc
generators.
• Similarly, if the electrical system is characterized by
alternating current, the machines are called ac motors
and ac generators.
• Basically, the ac machines are not different from the dc
machines.
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Electrical Rotary Machine
• It has two parts :
(1) the fixed part, called the stator, and
(2) the moving part, called the rotor.
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Generator Action
• Whenever the conductors of a coil cut across the
magnetic flux (or are cut by it), an emf e is induced in
it.
• This emf can supply a current i to an electrical load.
• Thus, an electrical power ei is generated.
• This is the Generator Action.
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Motor Action
• Whenever current-carrying conductors of a coil are
placed (perpendicularly) in a magnetic flux, a force is
experienced by each conductor.
• This gives rise to an electromagnetic torque e.
• This torque can rotate a mechanical load at an angular
speed ω.
• Thus, a mechanical power e ω is generated.
• This is the motor action.
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Power Considerations for a Generator
Large-size generators are usually called
alternators.
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Wednesday, January 27, 2016 Ch. 16 DC Machines 11
Importance
• DC machines were the first electrical machines invented.
• It is still the best motor to drive trains and cranes.
• Because semiconductor rectifiers can easily convert ac into dc, dc
generators are not needed.
• Even in the automobiles, the dc generator has been replaced by
the alternator plus diodes for rectification.
• Portable devices powered by batteries require dc motors, such as
portable tape players, walkman, window-lifters, etc.
• The dc motor is readily controlled in speed and torque and hence
is useful for control systems.
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DC Machine
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Construction
• The main parts of a DC machine are
– Yoke
– Poles, pole shoes.
– Field Coils.
– Armature.
– Commutator.
– Brushes.
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YOKE
• Gives mechanical support for poles
• Protects whole machine as a protecting cover
• Provides path for the circulation of magnetic flux
• Small generators – cast iron
• Large machines – cast steel
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Pole cores & Pole shoes
• Field magnets has two parts
– Pole cores
– Pole shoes
• Pole shoes: spread out the flux in the air gap & reduce the reluctance
– Support the exciting coils
• Pole cores: solid piece made of cast iron & cast steel
• Pole shoes are laminated to the pole face by screws.
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Field Winding
• Function : To carry current due to which pole core behaves as an
ELECTROMAGNET, producing necessary flux.
• It helps in producing magnetic flux.
• Field winding is divided into various coils called field coils. These are
connected in series with each other and wound in such a direction that an
alternated ‘N’ and ‘S’ poles are created.
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Two Types of Winding
(1) Lap Winding : The sides of the successive coils
overlap each other.
(2) Wave Winding : One side of a coil under one
pole is connected to the other side of a coil
which occupies approximately the same
position under the next pole.
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Lap Winding:
Wave Winding:
Armature winding
1
3
2
4
5
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(a) Lap winding. (b) Wave winding.
(for lap winding)
2 (for wave winding)
A P
A


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The simplest DC machine
segments
brushes

Example of a commutator…
• Function :Commutator
A commutator is a mechanical rectifier which converts the
alternating voltage generated in the armature winding
into direct voltage across the brushes. The commutator is
made of copper segments insulated from each other by
micasheets and mounted on the shaft of the machine .The
armature conductors are soldered to the commutator
segments in a suitable manner to give rise to the armature
winding. Depending upon the manner in which the
armature conductors are connected to the commutator
segments, there are two types of armature winding in a
d.c. machine viz., (a) lap winding (b) wave winding. Great
care is taken in building the commutator because any
eccentricity will cause the brushes to bounce, producing
unacceptable sparking. The sparks may bum the brushes
and overheat and carbonise the commutator.

Example of a commutator…
• Function :Brushes
The purpose of brushes is to ensure electrical
connections between the rotating
commutator and stationary external load
circuit. The brushes are made of carbon
and rest on the commutator.
The brush pressure is adjusted by means of
adjustable springs . If the brush pressure is
very large, the friction produces heating of the
commutator and the brushes. On the other
hand, if it is too weak, the imperfect contact
with the commutator may produce sparking.

Dc machines

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