Dummy Coils

The dummy coils (also called as Idle Coils) are used in some of the electrical machines wounded with wave winding, to have mechanical balance.

When a machine is wound with simplex wave winding the following equation must be satisfied.

Coil Pitch or Coil Span

Coil Pitch also called coil span is a measure of distance (in terms of armature slots) between the two either sides of a coil.

Full Pitched Winding

If the coil pitch is equal to the pole pitch then such a type of winding is said to be full pitched winding. For such a winding, the coil pitch is 180 electrical degrees and the either sides

Pole Pitch

In an electrical machine, pole pitch is the distance between two adjacent poles. It can also be defined as the periphery of the armature divided by the number of poles in the machine.

Poles pitch gives the number of armature conductors per pole.

For example, if in an electrical machine there are 4 poles and 32 conductors then the poles pitch is 8. Since, \[\frac{armature conductors}{number of poles}= frac{32}{4}=8\]

Star connection | Y connection

The star connection also called Y - connection or wye connection is a type of circuit configuration in electrical circuits. If three elements in a circuit viz. aa’, bb’ and cc’ are connected in such a way that their ends either a, b and c or a’, b’ and c’ connected all together while

Power Electronics

Power Electronics is a technology associated with flexible and efficient way of conversion and control of electric power. It is the application of electronic principles at the power level instead of at the signal level as used in electronic circuits. There are various power semiconductor

Yoke in an electrical machine

The outer frame of an electrical machine either motor or a generator is called yoke.

The yoke provide two functions. First is providing the mechanical support and protection cover to the machine assembly. The second is it provides low reluctance path to carry the magnetic flux. The below figure show the diagram of an electrical machine with the location of Yoke, poles and rotor.

Electrical Machine diagram showing Yoke and the Poles

Yoke is generally made of cast steel or rolled steel for large electrical machines whereas for small machines cast iron may be used.

Magnetostriction

Magnetostriction is a property of ferromagnetic materials that causes changes in physical dimensions and shape during magnetization of these materials. As the applied magnetic field changes, the dimensions of these material also

Diversity Factor

In power systems, diversity factor is defined as the ratio of sum of individual maximum demands of all the consumers to the maximum demand of the overall load on the system.

\[Diversity factor=\frac{Sum of individual maximum demands}{Maximum demand of the system}\]

Diversity factor helps in deciding

System

A system is said to be meaningful interconnection of group of independent but interrelated components working coherently to fulfill an objective. For example, an electric fan can be observed as system with its components

Active Elements and Passive Elements

The elements or components in electrical circuits can be classified in to either active or passive elements or components. Active elements are the energy sources which deliver power to the network whereas passive elements are the elements in the network which consume energy.

Active Elements

Power supply elements such as voltage sources and current sources either independent or dependent which deliver power are called active elements. For an active element the ratio of the voltage across it to the current flowing is negative i.e. \[\frac{V}{I}<0\]. For Non-linear elements \[\frac{dV}{dI}<0\].

The graphical representations of active elements can be as follows.

characteristics of active elements

Passive Elements

The elements which consume energy are called passive elements. These elements either convert energy into another form or store energy in electric or magnetic field. The elements such as a resistor, inductor and capacitor are called passive elements. For linear passive elements the ratio of voltage to current is positive i.e \[\frac{V}{I}>0\] and for Non-Linear elements \[\frac{dV}{dI}>0\]

Characteristics of passive element

Construction of a Transformer

The basic parts of a transformer consist of two coils having mutual inductance and a core material. Apart from these there are other parts such as the outer container, insulating and cooling medium, bushings to obtain the terminals of the transformers out. During the construction of transformer first the windings are wound and then the core laminations are pushed through openings of coil and the core is prepared. Then the laminations are tightened with clamps and bolts.

Core of a transformer

The core is generally made of silicon steel laminations insulated from each other by a light coating of varnish. The most popular material for core is cold-rolled grain oriented sheet steel (C.R.G.O.). It reduces the core loss and has high permeability when magnetized in rolling direction. The laminations are used to reduce the eddy currents. For a 50 Hz transformer the thickness of laminations is around 0.35 mm while for a 25 Hz transformer it is 0.5 mm. The laminations are cut in L, T and I shapes and are assembled to form a core. The vertical parts of the core are usually called limbs or legs and the horizontal parts are called yoke. The core is assembled in such a manner that adjacent layers are staggered. It avoids continuous air gap, provide low reluctance and provide good mechanical strength.

For high frequency transformers which are employed in communication circuits the core is made of powdered ferromagnetic alloy.

Types of Core construction:

Generally there are two types of construction of core based on the manner in which the winding and the core are placed. They are

1. Core type
2. Shell type

Core type Transformer:

Core type transformer

In core type, the windings surround a considerable part of the core and require more conductor material but less iron material. For a single phase transformer core type has two legs. To reduce leakage flux half of the both low voltage (LV) and high voltage (HV) windings together are placed on one leg and the remaining halves are placed on the other leg. To minimise the insulation requirement LV windings which are easy to insulate are placed near the core and the HV winging is placed outside.

Shell type Transformer:

shell type transformer construction

In shell type transformer, the core surrounds the major part of the windings and require more core material and less conductor material. For a single phase shell type transformer there are three limbs. Both the LV and HV windings are wound around the central limb and are sandwiched over one another.

Air-core transformers:

This type of transformer is used in radio devices and certain measuring instruments. The core in these transformers is made on non-magnetic materials.

Cruciform of Core:

Two stepped core

For small transformers the limbs of the core are rectangular but it is not preferred for large transformers. In large sized transformers, circular cylindrical coils are used; for such transformers the cruciform core is used with multiple steps. It gives better space factor. To form the steps different sizes of the lamination are required. To form a single step core, it require to different sized laminations. Generally three stepped core is the most common.

Transformer tank:

The assembly of the power transformer is placed in a suitable container or casing and it can be called as a transformer tank.

Cooling:

Low power transformers are generally air cooled whereas large power transformers are immersed in oil for cooling. The circulating oil in the transformer apart from cooling the transformer provides better insulation. In some transformers the sides of the transformer are corrugated with radiators mounted on the sides which provide a path for better cooling.

Also Read:

Transformer
Different Core materials used in transformer

Transformer

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Transformer is a static device or a machine that transfers electrical energy from one electrical circuit to another through medium of magnetic field without change in frequency. It is an electromagnetic energy conversion device and can raise or lower the voltage with a corresponding decrease or increase in current. A transformer consists of two conducting coils (or windings) having a mutual inductance. These coils are wound on a laminated core made of high permeability magnetic material. The coil which receives electrical energy from supply terminals is called

Alternating Current AC

An electric current in which the periodic reversal of direction of flow of charge particles takes place is called alternating current, in contrast to DC where the flow of electric charge is only in one direction. The waveform of alternating current is usually sinusoidal wave and below figure shows its graphical representation of single phase alternating current.

alternating current

Where T=is the time period

            i=current

            I=peak value of current

This is the form of current used throughout the world in electric power system. The main advantage of an alternating current transmission is the AC voltage can be increased or decreased easily and efficiently with the help of a device called transformer.

With increase in voltage of transmission the power loss reduces whereas the consumer loads at the distribution end are of different voltage levels and also such a high voltage is not safe. This necessitates the changes in the voltage levels at different parts of power system. This can be easily achieved alternating current system with the help of a transformer. Whereas it not possible in system.

Three phase system:

Instead of a single phase system three phase AC system is used has more power transfer capability. In three phase system each single phase is placed at 120 electrical degrees with each other.

Frequency:

Frequency of inverse of time period is depends on the poles in the machine and the speed of the rotation of machine. In a power system all the generators are interconnected and hence their frequencies should be same and they should be run in synchronism with each other throughout the system. The frequency of power system varies from country to country usually 50 Hz or 60 Hz are used. In USA the frequency is 60 Hz, and in India it is 50 Hz. In some countries like Japan mixture of 50 Hz and 60 Hz also used.

Sources of Generation of AC:

Alternator: alternator is a synchronous machine which is the most widely used to generate alternating current and hence its name alternator.

Induction generator: Though induction generators can generate alternating voltages and current, it not used generally as the performance of induction generator is poor. But it is used widely used in wind energy conversion.

Inverter: inverter converts direct current DC to AC. The DC from various sources such as batteries and solar cells can be converted to AC with the help of inverter.

Advantages and disadvantages of AC:

Advantages of AC:

1.         The principle advantage of an AC system is that the level of voltage can easily altered with the help of simple transformer which offers wide range of voltages.

2.         The cost of generation of AC is cheaper than that of an equivalent DC.

3.         The initial cost and maintenance cost of AC machines are cheaper.

4.         AC machines are robust and can almost work in any kind of environment.

5.         Ease of conversion to DC with a rectifier to supply DC loads.

Disadvantages of AC:

1.         Skin effect, in which the charge concentration is more near the surface of the conductor and less at the center of the conductor. This non uniformity in the charge concentration in conductor decreases the effective area of cross section of the conductor thereby the increase in effective resistance.

2.         Analysis and design of AC systems involve complex numbers and is harder when compared with DC analysis which requires simple mathematics.

Direct Current (DC)

The unidirectional flow of electric charge particles is called Direct current or DC. That is the electric current flows in a single direction. When a direct current is passed through a conductor the charge across the area of cross-section is uniform. Direct current is usually abbreviated as DC.

The value of the Direct Current is always constant with respect to time and the below figure shows its graphical representation. Hence in DC circuits the voltage and current are independent on time.

Direct Current (DC)

At the initial stages of development of electrical systems DC is used being used as means of transmission and distribution. Later with the adoption AC with its capability of easy transformation DC system is rarely used.

Sources of Direct Current:

Batteries: batteries are electrochemical energy conversion devices which produce Direct currents.

Solar cells: solar cells or photo voltaic cells convert light energy into electrical energy and they can only generate direct current.

DC Generators: Though DC generators generate DC but they do not generate pure DC instead they generate pulsating voltages. These have to be filtered with the help of filter circuits to get pure DC

Rectifiers: Rectifiers are the devices convert AC to DC

Applications:

Electronics: Most of the electronic devices like mobile phones, computers, radios, televisions, Audio/Video Players, LED lights, electronic watches and calculators etc. work on DC. As the supplies system is alternating current, these devices are powered with the help of rectifiers or run over batteries.

HVDC: For bulk power transmission of electric power over long distance High Voltage Direct Current (HVDC) transmission is used. DC has advantage of less losses and no skin effect.

Electric Vehicles: Electric Vehicles are battery powered vehicles which works on DC.

Rechargeable Batteries: Rechargeable batteries are energy storage devices which require DC input.

Voltage

Description

The electric potential difference between two points is called voltage. The voltage is equal to the work done per unit charge to move a test charge between two points against static electric field.

Electric potential is given by

Ohm's law

Ohms’s law was proposed by German physicist George Ohm and the law named after him. It is one of the basic equations used in the analysis of electrical circuits.

Definition:

Ohms law states that the current flowing in a circuit is directly proportional to the applied voltage and inversely proportional to the resistance across the circuit. Provided the temperature remains constant.

Mathematical representation:

I α V

⇒ I=V/R          (or)      V=IR

Where,I is the current passing through the circuit in amperes,

V is the voltage measured across the circuit in units of volts, and

R is the resistance across the circuit in ohms.

Ohms law assumes that R is a constant

Ohms law can also be defined as at constant temperature and conductivity of material, the current density is directly proportional to electric field intensity

J= σE where

Where, J is the current density,

E is the electric field, and

σ is a conductivity of the material.

Ohms law is not valid on all cases and can be applied only to linear circuits which have linear v-i characteristics.

Electrical Resistance

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The property of a material that reduces the flow of current through it is called electrical resistance. Every material will have some amount of resistance except super conductors which have zero resistance. The materials which have very low resistance are called conductors. The inverse of resistance is called conductance.  Resistors are used to control the amount of current passing through conductors. But they dissipate the energy in

Battery

A battery is a device which contains electrochemical cells which converts chemical energy into electrical energy. Batteries supplies DC power with the help to two electrode terminals called anode and cathode. The terminal marked negative is the source of electrons. Alessandro Volta built and described the first electrochemical battery.

Basically there are two types of batteries called primary and secondary,

Primary batteries

These can only be used for single time. The chemical process in the primary batteries is irreversible and hence once the charge exhaust they have to be discarded.

Example: Alkaline batteries, Zinc-carbon batteries etc

General Applications: wristwatches, torches, toys etc

Secondary batteries

These can be reused and recharged multiple times. Here the process is reversible and can be recharged by sending reverse current.

Examples: Lithium-ion, Nickle-MH, Nickle-Cadmium etc

General Applications: Toys, Battery banks, Uninterruptable power supplies-UPS, in Space applications to power artificial satellites, electric vehicles etc.

Symbol of a Battery

Battery symbol