Content 1 - Principle of DC Generator Action and Generation of EMF.pptx

DC MACHINE
A DC machine is an electro-mechanical energy
conversion device. When it converts mechanical power
(ωT) into DC electrical power (EI), it is known as a DC
generator.
On the other hand, when it converts DC electrical power
into mechanical power it is known as a DC motor.

DC GENERATOR
An electro-mechanical energy conversion device (or electrical machine) that
converts mechanical energy (or power) into electrical energy (or power).

DC GENERATOR
It works on the following principle:
“When a conductor cuts magnetic flux,
dynamically induced e.m.f. is produced in it
according to Faraday’s Law of
Electromagnetic Induction”

DC GENERATOR
Consider the figure on the right.
When a conductor is moved vertically upward or
downward, the deflection in the galvanometer will
show that an emf is induced in the conductor since
flux is cut by the conductor.

DC GENERATOR
When it is moved horizontally (left or right), there is
no deflection in the galvanometer which shows that
no emf is induced in the conductor since flux cut is
zero and conductor moves just parallel to the
magnetic lines of force.

DC GENERATOR
In a generator, a coil is rotated at a constant speed
of ω radians per second in a strong magnetic field of
constant magnitude, as shown in the figure on the
right.

DC GENERATOR
Stationary Part – designed mainly
for producing magnetic flux.
Rotating Part – called armature,
where mechanical energy is
converted into electrical (electric
generator) or, conversely, electrical
energy into mechanical (electric motor)

DC GENERATOR
An emf is induced in the coil by the phenomenon
of dynamically induced emf, mathematically
represented as:
e= βlvsinθ

DC GENERATOR
The magnitude and direction of induced emf
changes periodically depending upon sine of angle
θ.
The wave shape of the induced emf is shown on the
lower right, which is AC for internal as well as
external load.

DC GENERATOR
This AC is converted into DC with the help of
commutator.
Thus, the working principle of a DC generator is
electro-magnetic induction.

MAIN CONSTRUCTIONAL FEATURES
The complete assembly of various parts in a
scattered form of a DC machine is shown on
the right.

The outer cylindrical frame to which main poles and inter poles
are fixed is called yoke. It also helps to fix the machine on the
foundation. It serves two purposes:
(i) It provides mechanical protection to the inner parts of the
machine.
(ii) It provides a low reluctance path for the magnetic flux
 Magnetic Frame or Yoke

The yoke is made of cast iron for smaller machines and for
larger machines, it is made of cast steel or fabricated rolled
steel since these materials have better magnetic properties as
compared to cast iron.
 Magnetic Frame or Yoke

The pole core and pole shoes are fixed to the magnetic
frame or yoke by bolts. They serve the following purposes:
(i) They support the field or exciting coils.
(ii) They spread out the magnetic flux over the armature
periphery more uniformly.
(iii) Since pole shoes have larger X-section, the reluctance
of magnetic path is reduced.
 Pole Core and Pole Shoes

Usually, the pole core and pole shoes are made of thin cast
steel or wrought iron laminations which are riveted together
under hydraulic pressure.
 Pole Core and Pole Shoes

Enameled copper wire is used for the construction of field
or exciting coils.
The coils are wound on the former, see figure on the right,
and then placed around the pole core.
 Field or Exciting Coils

When direct current is passed through the field winding, it
magnetizes the poles which produce the required flux.
The field coils of all the poles are connected in series in
such a way that when current flows through them, the
adjacent poles attain opposite polarity as shown in the next
discussion.
 Field or Exciting Coils

It is cylindrical is shape and keyed to the rotating shaft. At the
outer periphery slots are cut, which accommodate the armature
winding. The armature core serves the following purposes:
(i) It houses the conductors in the slots.
(ii) It provides an easy path for magnetic flux.
 Armature Core

Since armature is a rotating part of the machine, reversal of flux
takes place in the core, hence hysteresis losses are produced.
To minimize these losses, silicon steel material is used for its
construction. When it rotates, it cuts the magnetic field and an
emf is induced in it.
 Armature Core

This emf circulates eddy currents which results in eddy
current loss in it. To reduce these losses, armature core is
laminated, in other words we can say that about 0.3 to 0.5
mm thick stampings are used for its construction.
Each lamination or stamping is insulated from the other by
varnish layer, as shown on the right.
 Armature Core

The insulated conductors housed in the armature slots are
suitably connected. This is known as armature winding.
The armature winding acts as the heart of a DC machine.
 Armature Winding

It is a place where one form of power is converted to the
other form i.e., in case of generator, mechanical power is
converted into electrical power and in case of motor,
electrical power is converted into mechanical power.
On the basis of connections, there are two types of
armature windings named (i) Lap winding and (ii) Wave
winding (detailed discussions in the coming articles).

On the basis of connections, there are two types of
armature windings named,
(i) Lap winding and
(ii) Wave winding

Lap winding
In this winding, the connections are such that the number
of parallel paths is equal to number of poles.
Thus, if machine has P poles and Z armature conductors,
then there will be P parallel, paths, each path will have
Z/P conductors in series.

Lap winding
In this case, the number of brushes is equal to the number
parallel paths. Out of which half the brushes are positive
and the remaining (half) are negative.

Wave winding
In this winding, the connections are such that the numbers
of parallel paths are only two irrespective of the number of
poles.
Thus, if machine has Z armature conductors, there will be
only two parallel paths each having Z/2 conductors in
series.

Wave winding
In this case, the number of brushes is equal to two i.e.,
number of parallel paths.

It is an important part of a DC machine and serves the
following purposes:
(i) It connects the rotating armature conductors to the
stationary external circuit through brushes.
 Commutator

(ii) It converts the alternating current induced in the
armature conductors into unidirectional current in the
external load circuit in generator action, whereas, it
converts the alternating torque into unidirectional
(continuous) torque produced in the armature in
motor action.
 Commutator

The brushes are pressed upon the commutator and form the connecting link between
the armature winding and the external circuit.
They are usually made of high grade carbon because carbon is conducting material and
at the same time in powdered form provides lubricating effect on the commutator
surface.
The brushes are held in particular position around the commutator by brush holders and
rocker.
 Brushes

It holds the spindles of the brush holders.
It is fitted on to the stationary frame of the machine with nut and bolts.
By adjusting its position, the position of the brushes over the commutator can be
adjusted to minimize the sparking at the brushes.
 Brush Rocker

End housings are attached to the ends of the main frame and support bearings.
The front housing supports the bearing and the brush assemblies whereas the rear
housing usually supports the bearing only.
 End Housings

The bearings may be ball or roller bearings these are fitted in the end housings.
Their function is to reduce friction between the rotating and stationary parts of the
machine.
Mostly high carbon steel is used for the construction of bearings as it is very hard
material.
 Bearings

The shaft is made of mild steel with a maximum breaking strength.
The shaft is used to transfer mechanical power from or to the machine.
The rotating parts like armature core, commutator, cooling fan etc. are keyed to the
shaft.
 Shaft

SIMPLE LOOP GENERATOR AND FUNCTION OF GENERATOR
For simplicity, consider only one coil AB placed in the strong magnetic field. The two
ends of the coil are joined to slip rings A’ and B’ respectively. Two brushes rest on these
slip rings as shown below:

When this coil is rotated in counter clockwise direction at an angular velocity of ω
radians per second, the magnetic flux is cut by the coil and an emf is induced in it.

The induced emf is alternating and the current flowing through the external resistance
is also alternating i.e., at second instant current flows in external resistance from M to L,
whereas, at fourth instant it flows from L to M as shown below:

Consider that the two ends of the coil are connected to only one slip ring split into two
parts (segment) i.e., A’’ and B’’,
 Commutator Action

Each part is insulated from the other by a mica layer. Two brushes rest on these parts of
the ring as shown,

In this case when the coil is rotated is counter clockwise direction at an angular velocity
of ω radians per second, the magnetic flux is cut by the coil and an emf is induced in it.
The magnitude of emf induced in the coil at various instants will remain the same.

However, the flow of current in the external resistor or circuit will become unidirectional
i.e., at second instant the flow of current in the external resistor is from M to L as well as
the flow of current in the external resistor is from M to L in the fourth instant.

Hence, an alternating current is converted into unidirectional current in the external
circuit with the help of a split ring (i.e., commutator).

TYPES OF ARMATURE WINDINGS
According to the degree of closure produced by winding:
1. Open coil winding
2. Closed coil winding
Closed coil windings are of two types:
1. Ring winding
2. Drum winding

TYPES OF ARMATURE WINDINGS
Drum windings are of two types:
1. Lap winding – suitable for comparatively low voltage but high current
generators
2. Wave winding – suitable for high voltage, low current generators

EMF EQUATION OF A GENERATOR
The emf equation of a generator is given as follows:
Eg =
𝑷𝑵𝒁∅
𝟔𝟎𝒂
where: Eg = generated emf (volt)
P = number of poles
∅ = flux/pole (Wb)
Z = total number of conductors (slots x cond/slot)
N = rotational speed of armature (rpm)
a = number of parallel paths in the armature

EMF EQUATION OF A GENERATOR
For simplex lap winding: a = P
For simplex wave winding: a = 2
Let m = multiplexing of the winding, then
For a lap winding: a = mP
For a wave winding: a = 2m

SAMPLE PROBLEM # 1:
Determine the emf generated by a 4-pole simplex lap wound armature having
300 conductors running at 900 rpm. The flux per pole is 50 mWb.

SAMPLE PROBLEM # 2:
A 4-pole duplex lap wound armature has 120 slots and four conductors per
slot. Determine its speed if the flux per pole is 50 mWb and the emf
generated is 200 volts.

SAMPLE PROBLEM # 3:
The armature of a four-pole shunt generator is lap wound and generates 216
volts when running at 600 rpm. The armature has 144 slots, with six
conductors per slot. If this armature is rewound, wave connected, find the emf
generated at the same speed and flux per pole.

Content 1 - Principle of DC Generator Action and Generation of EMF.pptx

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