DC Generator

A DC generator is an electromechanical energy conversion device that converts mechanical

power into DC electrical power through the process of electromagnetic induction.

Working Principle:

According to Faraday’s laws of electromagnetic induction, whenever a conductor is placed in a

varying magnetic field (OR a conductor is moved in a magnetic field), an emf (electromotive
force) gets induced in the conductor. The magnitude of induced emf can be calculated from
the emf equation of dc generator. If the conductor is provided with a closed path, the induced
current will circulate within the path. In a DC generator, field coils produce an electromagnetic
field and the armature conductors are rotated into the field. Thus, an electromagnetically induced
emf is generated in the armature conductors. The direction of induced current is given
by Fleming’s right hand rule.

Construction of a DC Generator:

Here is the schematic diagram of a DC Generator

A DC generator consists of six main parts, which are as follows

Yoke

The outer frame of a DC generator is a hollow cylinder made up of cast steel or rolled steel is
known as yoke. The yoke serves following two purposes
 • It supports the field pole core and acts as a protecting cover to the machine.
• It provides a path for the magnetic flux produced by the field winding.

Magnetic Field System

The magnetic field system of a DC generator is the stationary part of the machine. It produces
the main magnetic flux in the generator. It consists of an even number of pole cores bolted to the
yoke and field winding wound around the pole core. The field system of DC generator has
salient poles i.e. the poles project inwards and each pole core has a pole shoe having a curved
surface. The pole shoe serves two purposes

• It provides support to the field coils.

• It reduces the reluctance of magnetic circuit by increasing the cross-sectional area of it.

The pole cores are made of thin laminations of sheet steel which are insulated from each other to
reduce the eddy current loss.

Armature Core

The armature core of DC generator is mounted on the shaft and rotates between the field poles. It
has slots on its outer surface and the armature conductors are put in these slots. The armature
core is a made up of soft iron laminations which are insulated from each other and tightly
clamped together.

Armature Winding

The insulated conductors are put into the slots of the armature core. The conductors are suitably
connected. This connected arrangement of conductors is known as armature winding.

Commutator

A commutator is a mechanical rectifier which converts the alternating emf generated in the
armature winding into the direct voltage across the load terminals. The commutator is made of
wedge-shaped copper segments insulated from each other and from the shaft by mica sheets

Brushes

The brushes are mounted on the commutator and are used to collect the current from the
armature winding. The brushes are made of carbon.
Applications of DC Generator:
Used in laboratories for testing purpose.
As a source of supply to DC motors.
Lighting purpose
For charging the batteries.
 DC motor

A DC motor is an electromechanical energy conversion device, which converts electrical

energy input into the mechanical energy output.

The operation of the DC motor is based on the principle that when a current carrying conductor
is placed in a magnetic field, a mechanical force acts on the conductor. The magnitude of the
force is given by,

F=BIl Newtons

The direction of this is given by the Fleming's left hand rule.

Construction of a DC Motor:
The construction of a DC motor is same as DC Generator.

Principle of Operation:

Consider a two pole DC motor as shown in the figure. When the DC motor is connected to an
external source of DC supply, the field coils are excited developing alternate N and S poles and a
current flows through the armature windings.

All the armature conductors under N pole carry current in one direction (say into the plane of the
paper), whereas all the conductors under S pole carry current in the opposite direction (say out of
the plane of the paper). As each conductor carrying a current and is placed in a magnetic field,
hence a mechanical force acts on it.
By applying Fleming’s left hand rule, it can be seen that the force on each conductor is tending
to move the armature in anticlockwise direction. The force on all the conductors add together to
exert a torque which make the armature rotating.

Why the Force or Torque on the conductors remain same?

When the conductor moves from one side of a brush to the other, the current in the conductor is
reversed and at the same time it comes under the influence of next pole of opposite polarity. As a
result of this, the direction of force on the conductor remains the same. Therefore, the motor
being rotating in the same direction.

Applications of DC Motor:

Elevators, steel mills, rolling mills, locomotives, and excavators

 Transformer

What is a transformer?
The transformer transfers the electrical power from one circuit to the other circuit without the
change in frequency. The transfer happens based on the mutual induction between the two
circuits that are linked by a magnetic flux.

Working Principle of Transformer

The Transformer works on the principle of Faraday’s Law of Electromagnetic Induction.
When the primary winding is connected to the AC supply, a current starts to flow through the
primary winding.

Since this winding is linked to the magnetic core, the current through the primary coil produces
an alternating flux Φ in the core. Since this flux is alternating in nature and it is also linked with
the secondary winding, so a mutually induced emf is induced in the secondary coil.

When a load is connected to the secondary winding, a current starts to flow in the winding. Thus
the energy is transferred from the primary to the secondary side by means of electromagnetic
induction. This transfer is achieved without the change in frequency.

Construction of a Transformer:

Basically a transformer consists of two inductive windings and a laminated steel core.

The coils are insulated from each other as well as from the steel core.

A transformer may also consist of a container for winding and core assembly (called as tank),
suitable bushings to take out the terminals, oil conservator to provide oil in the transformer tank
for cooling purposes etc.
In all types of transformers, core is constructed by assembling (stacking) laminated sheets of
steel, with minimum air-gap between them (to achieve continuous magnetic path).

The steel used is having high silicon content and sometimes heat treated, to provide high
permeability and low hysteresis loss.

Laminated sheets of steel are used to reduce eddy current loss.

Applications of Transformer:

Transformers are used in a variety of applications, including power generation, transmission and
distribution, lighting, audio systems, and electronic equipment.
 INDUCTION MOTOR/ASYNCHRONOUS MOTOR

Construction of an Induction motor:

An induction motor can come in several shapes and sizes, but it is most commonly a cylindrical
device with an axial shaft protruding from it. The rotatory action of the shaft is carried out by
arranging the following components in a particular manner.

Stator:
The rotating part of a IM is called Stator.
An induction motor’s stator is a hollow cylindrical core comprised of laminated and layered thin
metal sheets. It’s the stationary part with slots for the coil of the motor’s electromagnetic circuit
to wind through. The laminated structure of the stator is employed to prevent eddy current and
hysteresis losses that would otherwise occur with a solid core. The coil of the stator, also known
as stator winding, is made of copper wires insulated with enamel, varnish, or resins, to avoid any
short circuit.
Rotor
A rotor is the rotatory part of an induction motor. It is a cylindrical unit mounted on the shaft that
carries the mechanical load. There are two types of rotors used in the manufacturing of induction
motors.
Squirrel cage Rotor:

A squirrel-cage rotor is one of the most widely used rotors in induction motor manufacturing
because of its exceptional characteristics, such as reliability, robustness, and low manufacturing
cost. It gets its name from its cylindrical cage-like structure that consists of longitudinal
conductive bars, made of aluminum or copper, short-circuited with the rings formed of the same
material on both ends. The rotor bars are slightly skewed to prevent them from locking against
the gaps between the stator coils, ensuring a smooth and noise-free rotation.

Wound Rotor:

A wound rotor, also known as a slip-ring rotor, is a cylindrical unit made of thin laminated steel
sheets stacked together, and it has slots on its periphery to hold the rotary windings. The ends of
the rotary windings are connected to three slip-rings placed around the shaft. The slip-rings are
connected to variable power resistance banks via brushes, which allows the operator to change
the speed of the motor by varying the resistance.
In addition to the above two parts it also consists of the following which are
Bearings
Fan
Casing
Shaft

Working principle of Induction Motor:

An induction motor works on the principle of electromagnetic induction. In a DC motor, supply

is needed to be given for the stator winding as well as the rotor winding. But in an induction
motor only the stator winding is fed with an AC supply.
 ▪ Alternating flux is produced around the stator winding due to AC supply. This alternating
flux revolves with synchronous speed. The revolving flux is called as "Rotating Magnetic
Field" (RMF).
▪ The relative speed between stator RMF and rotor conductors causes an induced emf in the
rotor conductors, according to the Faraday's law of electromagnetic induction. The rotor
conductors are short circuited, and hence rotor current is produced due to induced emf.
That is why such motors are called as induction motors.
(This action is same as that occurs in transformers, hence induction motors can be called
as rotating transformers.)

▪ Now, induced current in rotor will also produce alternating flux around it. This rotor flux
lags behind the stator flux. The direction of induced rotor current, according to Lenz's law,
is such that it will tend to oppose the cause of its production.
▪ As the cause of production of rotor current is the relative velocity between rotating stator
flux and the rotor, the rotor will try to catch up with the stator RMF. Thus the rotor rotates
in the same direction as that of stator flux to minimize the relative velocity. However, the
rotor never succeeds in catching up the synchronous speed. This is the basic working
principle of induction motor of either type, single phase of 3 phase.

Synchronous Speed:
The rotational speed of the rotating magnetic field is called as synchronous speed.

where, f = frequency of the spply

P = number of poles

Slip:
Rotor tries to catch up the synchronous speed of the stator field, and hence it rotates. But in
practice, rotor never succeeds in catching up. If rotor catches up the stator speed, there wont be
any relative speed between the stator flux and the rotor, hence no induced rotor current and no
torque production to maintain the rotation. However, this won't stop the motor, the rotor will
slow down due to lost of torque, the torque will again be exerted due to relative speed. That is
why the rotor rotates at speed which is always less the synchronous speed.

The difference between the synchronous speed (Ns) and actual speed (N) of the rotor is called as
slip.
 Applications of Induction motor:

• Pumps,Compressors.Small fans,Mixers.Toys.High-speed vacuums.Electric shavers.

Drilling machines, conveyors etc.
 Alternator/AC Generator/Synchronous Generator

A synchronous generator is a synchronous machine which converts mechanical power into AC

electric power through the process of electromagnetic induction.

Synchronous generators are also referred to as alternators or AC generators. The term

"alternator" is used since it produces AC power. It is called synchronous generator because it
must be driven at synchronous speed to produce AC power of the desired frequency.

A synchronous generator can be either single-phase or poly-phase (generally 3phase).

Construction of Synchronous Generator or Alternator:

As alternator consists of two main parts viz.

• Stator – The stator is the stationary part of the alternator. It carries the armature winding
in which the voltage is generated. The output of the alternator is taken form the stator.
• Rotor – The rotor is the rotating part of the alternator. The rotor produces the main field
flux.
Stator Construction of Alternator:

The stator of the alternator includes several parts, viz. the frame, stator core, stator or armature
windings, and cooling arrangement.

• The stator frame may be made up of cast iron for small-size machines and of welded steel
for large-size machines.
• The stator core is assembled with high-grade silicon content steel laminations. These
silicon steel laminations reduce the hysteresis and eddy-current losses in the stator core.
• The slots are cut on the inner periphery of the stator core. A 3-phase armature winding is
put in these slots.
• The armature winding of the alternator is star connected. The winding of each phase is
distributed over several slots. When current flows through the distributed armature
winding, it produces an essential sinusoidal space distribution of EMF.
Rotor Construction of Alternator:

The rotor of the alternator carries the field winding which is supplied with direct current through
two slip rings by a separate DC source (also called exciter). The exciter is generally a small DC
shunt generator mounted on the shaft of the alternator.

For the alternator, there are two types of rotor constructions are used viz. the salient-pole
type and the cylindrical rotor type.

Salient Pole Rotor:

The term salient means projecting. Hence, a salient pole rotor consists of poles projecting out
from the surface of the rotor core. This whole arrangement is fixed to the shaft of the alternator
as shown in the figure. The individual field pole windings are connected in series such that when
the field winding is energised by the DC exciter, the adjacent poles have opposite polarities
The salient pole type rotor is used in the low and medium speed (from 120 to 400 RPM)
alternators.

Cylindrical Rotor:

The cylindrical rotors are made from solid forgings of high-grade nickel-chrome-molybdenum
steel.

• The construction of the cylindrical rotor is such that there are no-physical poles to be
seen as in the salient pole rotor.
• In about two-third of the outer periphery of the cylindrical rotor, slots are cut at regular
intervals and parallel to the rotor shaft.
• The field windings are placed in these slots and is excited by DC supply. The field
winding is of distributed type.
• The unslotted portion of the rotor forms the pole faces.
• It is clear from the figure of the cylindrical rotor that the poles formed are non-salient,
i.e., they do not project out from the rotor surface.

The cylindrical type rotor construction is used in the high-speed (1500 to 3000 RPM) alternators
such as those driven by steam turbines because of the following reasons −

• The cylindrical type rotor construction provides a greater mechanical strength and
permits more accurate dynamic balancing.
• It gives noiseless operation at high speeds because of the uniform air gap.
• The flux distribution around the periphery of the rotor is nearly a sine wave and hence a
better EMF waveform is obtained.
A cylindrical rotor alternator has a comparatively small diameter and long axial length. The
cylindrical rotor alternators are called turbo-alternators or turbo-generators. The alternator
with cylindrical rotor have always horizontal configuration installation.

Working Principle and Operation of Alternator:

An alternator or synchronous generator works on the principle of electromagnetic induction, i.e.,

when the flux linking a conductor changes, an EMF is induced in the conductor. When the
armature winding of alternator subjected to the rotating magnetic field, the voltage will be
generated in the armature winding.

When the rotor field winding of the alternator is energised from the DC exciter, the alternate N
and S poles are developed on the rotor. When the rotor is rotated in the anticlockwise direction
by a prime mover, the armature conductors placed on the stator are cut by the magnetic field of
the rotor poles. As a result, the EMF is induced in the armature conductors due to
electromagnetic induction. This induced EMF is alternating one because the N and S poles of the
rotor pass the armature conductors alternatively.

The direction of the generated EMF can be determined by the Fleming’s right rule and the
frequency of it is given by,

𝑓=𝑁𝑠𝑃/120

Where,

• Ns is the synchronous speed in RPM

• P is the number of rotor poles.

Applications of Alternator:
Used in power stations for generating the power, charging purpose, lightning purpose and in
automobiles etc.
 Permanent Magnet Moving Coil Instrument (PMMC)

PMMC Construction:

A Permanent Magnet Moving Coil (PMMC) meter – also known as a D’Arsonval meter or
galvanometer – is an instrument that allows you to measure the current through a coil by
observing the coil’s angular deflection in a uniform magnetic field. It is used only for measuring
the DC current and Voltage.

A PMMC meter (or D’Arsonval meters) is constructed of 5 main components:

• Stationary Part or Magnet System
• Moving Coil
• Control System
• Damping System
 • Meter
Stationary Part or Magnet System
Here we use the permanent magnet to produce a uniform magnetic field.
Moving Coil
The moving coil can freely moves between the two permanent magnets as shown in the figure
given below. The coil is wound with many turns of copper wire and is placed on rectangular
aluminium which is pivoted on jeweled bearings.
Control System
The spring generally acts as control system for PMMC instruments. The spring also serves
another important function by providing the path to lead current in and out of the coil.
Damping System
The damping force hence torque is provided by movement of aluminium former in the magnetic
field created by the permanent magnets.
Meter
Meter of these instruments consists of light weight pointer to have free movement and scale
which is linear or uniform and varies with angle.

Principle of operation:
Let a current I flows through the rectangular coil of n number of turns and a cross-sectional area
A. When this coil is placed in a uniform radial magnetic field B, the coil experiences a torque τ.
Using Fleming’s left-hand rule, we can determine the forces produced on two coil sides. When
equal and opposite forces F acts on the coil sides, it produces a torque. This torque causes the
coil to deflect.

Moving Iron Instrument

Moving iron instruments are normally utilized either as ammeters or voltmeters and for both DC
and AC quantities. Moving iron instruments are of two types:

1. Attraction type
2. Repulsion type

Attraction Type Moving Iron Instrument:

.
Construction - It consists of a hollow cylindrical coil (or solenoid) that is kept fixed. An oval-
shaped soft iron piece is attached to the spindle in such a way that it can move in or out of the
coil. The pointer is attached to the spindle so that it is deflected with the motion of the soft iron
piece. The controlling torque on the moving system is usually provided by the spring control
method while the damping is provided by air friction.

Working Principle - When the instrument is connected in the circuit, the operating current
flows through the coil. This current sets up a magnetic field in the coil. The coil then behaves
like a magnet and it attracts the soft iron piece towards it. The pointer attached to the moving
system moves from zero position across the dial.
If the current in the coil is reversed, the direction of the magnetic field also reverses and so does
the magnetism produced in the soft iron piece. Hence the direction of deflecting torque remains
unchanged. Therefore, such instruments can be used both for dc as well as ac measurement of
current and voltage.

Repulsion type Moving Iron Instruments:

These instruments are based on the principle of repulsion between the two iron pieces
magnetized with the same polarity.
Construction - Any Repulsion Instrument consists of a fixed cylindrical hollow coil that
consists of the operating current. Inside the coil, there are two soft iron pieces of vanes, one of
which is fixed and the other is movable. The fixed iron vane is attached to the coil whereas the
movable vane is attached to the spindle. Under the action of deflection torque, the pointer
attached to the spindle moves over the scale.

The controlling torque is produced by the spring control method and damping torque is provided
by air friction damping in repulsion type instruments.

Working Principle - When the instrument is connected in a circuit and current is flowing
through the circuit, the current sets up a magnetic field in the coil within the instrument. The
magnetic field magnetizes both the iron vanes in the same direction (i.e. both pieces become
magnets with the same polarity) they repel each other. Due to this force of repulsion, only
movable iron vane can move as the other piece is fixed and cannot move. The result is that the
pointer attached to the spindle moves from zero position.
If the current in the coil is reversed, the direction of deflection torque remains unchanged. This is
because both iron vanes are in the same magnetic field and so they will be magnetized similarly
and consequently repel each other irrespective of the direction of the magnetic field. Hence, such
instruments can be used both for ac and dc measurements. The deflection torque is generated due
to the repulsion between the similarly charged iron pieces.
 Wheatstone Bridge

What is Wheatstone Bridge?

Wheatstone bridge, also known as the resistance bridge, calculates the unknown resistance by
balancing two legs of the bridge circuit. One leg includes the component of unknown resistance.

The Wheatstone Bridge Circuit comprises two known resistors, one unknown resistor and one
variable resistor connected in the form of a bridge. This bridge is very reliable as it gives
accurate measurements.

Construction of Wheatstone Bridge

A Wheatstone bridge circuit consists of four arms, of which two arms consist of known
resistances while the other two arms consist of an unknown resistance and a variable resistance.
The circuit also consists of a galvanometer and an electromotive force source. The emf source is
attached between points a and b while the galvanometer is connected between points c and d.
The current that flows through the galvanometer depends on its potential difference.

What is the Wheatstone Bridge Principle?

The Wheatstone bridge works on the principle of null deflection, i.e. the ratio of their resistances
is equal, and no current flows through the circuit. Under normal conditions, the bridge is in an
unbalanced condition where current flows through the galvanometer. The bridge is said to be
balanced when no current flows through the galvanometer. This condition can be achieved by
adjusting the known resistance and variable resistance.

Wheatstone Bridge Derivation

The current enters the galvanometer and divides into two equal magnitude currents as I1 and I2.
The following condition exists when the current through a galvanometer is zero,