Introduction
D. C. machines are seldom used in ordinary applications because all electric supply
companies furnish alternating current However, for special applications such as in steel mills,
mines and electric trains, it is advantageous to convert alternating current into direct current in
order to use D.C. motors. The reason is that speed/torque characteristics of D.C. motors are much
more superior to that of a.c. motors. Therefore, it is not surprising to note that for industrial
drives, D.C. motors are as popular as 3-phase induction motors. Like D.C. generators, D.C. motors
are also of three types viz., series-wound, shunt-wound and compoundwound. The use of a
particular motor depends upon the mechanical load it has to drive.
In DC Machines the field poles are present on the stator called as YOKE. Armature windings
and Commutator are on the Rotor. The Figure gives the cross-sectional view of a 4-pole DC
machine. Brushes press on to the Commutator view for collecting the power from a dc generator
or for feeding the DC Power to the armature of a DC Motor. DC Machines are of three types:
1. Series (A)
2. Shunt (B)
3. Compound (C)
DC Series machines has the field winding in series with armature circuit, DC Shunt Machine
has field winding across the armature circuit. Where as DC Compound machine has two Field
Windings. One across the armature and the other in series with the armature. DC Machines has
inter poles. Large DC machines have also compensating windings embedded in the pole faces of
the main poles.
DC Machine Posses very versatile characteristics. DC motor is easily adaptable for drives requiring wide range speed control and maintenance. It is highly versatile energy conservation device. It can meet the demand of loads requiring high starting torques, high accelerating and decelerating torques. In view of these outstanding features, DC Machines are widely used for the industrial purpose particularly for tough jobs as are in steel mills. There are two types of DC Machines depending on their field system employed, they are:
· HOMO-POLAR Machines
· HETERO-POLAR Machines
HOMO-POLAR Machines
These types of machines are used where low Voltage and High Currents are required and
the fields system is unusual as in the Faraday Disc Dynamo, which is an example of this type of
machine.
HETERO-POLAR Machines
These types of machines are most commonly used in Practice. The magnetic poles are mounted
as shown in figure to form alternate south and north poles when traversed along the
circumference of an armature. These machines are used for moderate voltage and high output
power.
Construction of a DC machine
Basically the construction of a DC machine includes mainly 7 parts. They are:
i. Magnetic Frame (or) Yoke
ii. Poles
a. Pole Face (or) Pole Core
b. Pole Shoe
iii. Pole Windings (or) Field Windings
iv. Armature Core
v. Commutator
vi. Armature Windings
vii. Brushes and Bearings
i. Magnetic Frame (or) Yoke
· It is cylindrical in shape and made out of cast steel or cast iron.
· It is the outer most part of the DC machine on which the poles
are located on the inner part of the Yoke.
PURPOSE:
It gives mechanical support to the poles situated on the inner
part of the Yoke.
It acts as a protecting layer to the entire DC machine and protects from the atmospheric
ailments.
It acts as Magnetic Flux carriers in the DC machine.
ii. Poles (Pole Core and Pole Shoe)
Pole Shoe:
The field magnet consists of mainly pole shoe and pole core. The pole shoe serves mainly for two purposes.
In spreading the magnetic flux in the air gap and being of larger cross-section, reduce the reluctance of the magnetic path.
It supports the exciting coils (or field coils) shown in figure.
Pole Core:
The pole core itself maybe a solid piece made up of either cast iron or cast steel. But
the pole shoe is laminated.
Construction:
Method: 1
1. In this method the pole core is a simple solid, made out of cast
iron or cast steel.
2. Pole shoe is laminated and is fastened to the pole face by means
of “COUNTER SUNK SCREW” as shown in above figure.
Method: 2:
1. In this method both the pole shoe and pole core are laminated
and are riveted permanently under hydraulic pressure.
2. The thickness of each laminated sheet varies from 1 mm to
0.25 mm.
All the poles should be fitted on to the Yoke by means of screws inside
the pole such that it should enter the Yoke and gets fixed. This is shown
in the above figure.
iii. Pole Windings (or) Field Windings
The field coils or pole coils which consists of
copper wire or strip, are former- wound for thecorrect dimension shown in the left figure. Then
the former is removed and wound coil is put into
place over the core as shown in the right figure.
iv. Armature Core
1. It is the main part of the DC machine which houses or holds the armature coils or conductors.
2. It is basically cylindrical in shape built by several circular steel disks.
3. Every disk is punched, the punched portion is called as “SLOTS” and the unpunched portion is called as “TEETH”.
4. The slot must be covered with the insulating material MICA.
5. The laminations are perforated for air duct which permits the axial flow of air through the armature for cooling purpose.
6. The inner peripheral consists of “KEYWAYS” where as the outer peripheral consists of
“SLOTS and TEETH”.
7. Keyways are used to fix the shaft, which appears as a “DOVE- Tailed” or “Wedge –Shaped”.
v. Commutator
1. The shape of a Commutator appears to be wedge-shaped and cylindrical structure.
2. The function of the Commutator is to facilitate collection of current from the armature
conductors or coils.
3. It rectifies i.e., it converts the alternating current induced in the armature conductors
into unidirectional current in the external load circuit.
4. The wedge-shaped segments are insulated from each other with the help of thin layers
MICA.
5. Each Commutator segment is connected to the armature conductor by means of a copper
lug or strip (riser).
6. To prevent them from flying out under the action of centrifugal forces, the segments
have V-shaped grooves, these grooves being insulated by conical micanite rings. This is
shown in the figure.
vi. Armature Windings
1. The armature windings are done by using high grade copper wires or strips.
2. The armature windings includes any other factors such as
a. Pole –Pitch
b. Conductor
c. Pitch of a Winding (Y)
d. Back Pitch (YB)
e. Front Pitch (YF)
f. Resultant Pitch (YR)
g. Commutator – Pitch (YG)
h. Single Layer Winding
i. Two –Layer Winding
j. Multiplex Winding etc…….
vii. Brushes and Bearings
1. The function of Brushes is to collect the current from the Commutator, is usually made of carbon or graphite and is in the shape of a rectangular block.
2. These brushes are housed in brush-holders usually of the box-type variety.
3. The brushes are made to bear down on the commutator by a spring whose tension can be adjusted by changing the position of the lever in the notches.
4. A flexible copper wire is mounted at the top of the brush conveys current from the brushes to the holder.
5. The number of brushes per spindle depends on the magnitude of the current to be collected from the Commutator.
6. Because of the reliability, ball-bearings are frequently employed, though for heavy duties. Roller bearings are most preferred.
7. Sleeve bearings are used which are lubricated by ring oilers fed from oil reservoir in the bearing bracket. This is shown in the figure.
Operation of a DC Machine as a Generator
Principle:
An Electric Generator is a machine which converts mechanical energy or Power in to
Electrical energy or power. The energy conversion is based on the principle of the production of
dynamically induced emf. Whenever a conductor cuts the magnetic flux dynamically, induced emf
is produced in it according to the Faraday’s laws of Electromagnetic induction. This emf causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are 1. Magnetic Field 2. Conductor or Conductors which can move as to cut the flux.
Construction of Generators
The generator mainly consists of magnetic poles which are mounted on the stator and a armature core in between the magnetic field. The armature conductors are placed within the slots in the armature core. Initially the armature conductors are wounded in rectangular shape flat coils. Let us consider a single turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent or electromagnets. The two ends of the coil are joined to two slip rings which are insulated from each other and from the central shaft as shown in the figure. The collecting brushes are made up of copper or carbon and are pressed against the split rings. Their function is to collect the current induced in the coil and to convey it to the external load resistance ‘R’. The rotating coil may be called as ‘Armature’ and the magnets as ‘Field Magnets’.
Working (Generator)
Assume the coil to be rotating in clock-wise direction as shown. As the coil assumes
successive position in the field, the flux linked wit it changes. Hence an emf is induces in it which
is proportional to the rate of change of flux linkage
E = Nd ะค
dt
When the plane coil is at right angle to the lines of flux. Then the flux linked with the coil is
maximum, but rate of change of flux linkage is minimum. As a result, the coil sides do not cut the
flux, rather they move along them parallel. Hence there is no emf induced in the coil. As a result
coil continues rotating further, rate of change of flux linkage increases when it makes an angle of
90°. Here, the coil will be in horizontal, the flux linked with the coil is minimum but rate of change
of flux is maximum. Hence maximum emf is induced in the coil at 90°.
In the next quarter revolution (i.e. from 90° to 180°), flux linked with the coil gradually
increases but the rate of change of flux linkages decreases. Hence the induced emf decreases
gradually till the position, it is reduced to zero. In the first half revolution, emf is induced at the
initial position, maximum when it is at 90° and no emf at 180°. The direction of this induced emf
can be found by applying Fleming’s right-Hand Rule which gives its direction. Hence the current
flows.
In the next cycle (i.e. from 180° to 360°) the variations in the magnitude of emf are just
like to that of the first half revolution. Therefore for every half revolution, the current gets
reversed in its direction which is known as alternating current. It is later converted in to
unidirectional current (DC) by replacing the slip ring with the split rings. Split rings are made out
of a conducting cylinder cut into two halves or segments insulated from each other by a thin sheet
of mica.
As before the coil ends are joined to the segments on which rest the copper or
carbon brushes. It is seen that in the first half revolution current flows i.e. the brush will be in contact with the segment acts as a positive end of the supply and other as negative. In the next half revolution, the direction of induced current in the coil has reversed. But at the same time the position of the segments are also reversed with the result that the initial brush comes in contact with the segment which is positive.
Hence the current is purely unidirectional but not continuous.
