training material - electrical

7/28/2019 Training Material - Electrical

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TRANSFORMER

What is a Transformer:

A transformer is an electrical device that transfers Alternating Current (AC) electrical energy

from one circuit to another through inductively coupled conductors (the transformer's coils),without change of frequency.

Principle of operation:

The electric circuit which receives energy from the supply mains is called primary winding and

the other circuit which delivers electric energy to the load is called the secondary winding.An

AC current in the primary winding creates a varying magnetic flux in the transformer's core, andthus a varying magnetic field through the secondary winding. This varying magnetic field

induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect

is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the secondary winding and

electrical energy will be transferred from the primary circuit through the transformer to the load.In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the

primary voltage (VP), and is given by the ratio of the number of turns in the secondary (NS) to

the number of turns in the primary (NP) as follows:

VS/ VP = Ns/Np = Ip/Is

The above ratio is named as voltage ratio or turn ratio. Ip & Is are primary current & secondary

current respectively.

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current

(AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NS

less than NP. If the voltage is increased, then the current is decreased by the same factor.

In the vast majority of transformers, the coils are wound around a ferromagnetic core,air-core

transformers being a notable exception.
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Energy losses of Transformer:

Transformer losses are divided into losses in the windings, termed copper loss, and those in themagnetic circuit, termed iron loss.

Copper Loss

Current flowing through the windings causes resistive heating of the conductors due to resistance

of winding.

Core Loss

This is again divided into following sub parts.

Hysteresis loss

Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis

within the core. For a given core material, the loss is proportional to the frequency, and is a

function of the peak flux density to which it is subjected.

Eddy current loss

Eddy currents circulate within the core in a plane normal to the flux by electromagneticinduction due to core material in magnetic flux, and are responsible for resistive heating of the

core material. The eddy current loss is a complex function of the square of supply frequency and

inverse square of the material thickness.
http://en.wikipedia.org/wiki/Copper_losshttp://en.wikipedia.org/wiki/Iron_losshttp://en.wikipedia.org/wiki/Resistive_heatinghttp://en.wikipedia.org/wiki/Hysteresishttp://en.wikipedia.org/wiki/Eddy_currenthttp://en.wikipedia.org/wiki/Resistive_heatinghttp://en.wikipedia.org/wiki/File:Transformer_flux.gifhttp://en.wikipedia.org/wiki/Resistive_heatinghttp://en.wikipedia.org/wiki/Eddy_currenthttp://en.wikipedia.org/wiki/Hysteresishttp://en.wikipedia.org/wiki/Resistive_heatinghttp://en.wikipedia.org/wiki/Iron_losshttp://en.wikipedia.org/wiki/Copper_loss


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Magnetostriction

Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand andcontract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This

produces the buzzing/humming sound commonly associated with transformers and in turn causes

losses due to frictional heating in susceptible cores.

Mechanical losses

In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic

forces between the primary and secondary windings. These incite vibrations within nearbymetalwork, adding to the buzzing noise and consuming a small amount of power.

Stray losses

Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is

returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearbyconductive materials such as the transformer's support structure will give rise to eddy currentsand be converted to heat. There are also radiative losses due to the oscillating magnetic field, but

these are usually small.

Polarity of Transformer:

It is common in transformer schematic symbols for there to be a dot at the end of each coil

within a transformer, particularly for transformers with multiple windings on either or both of theprimary and secondary sides. The purpose of the dots is to indicate the direction of each winding

relative to the other windings in the transformer. Voltages at the dot end of each winding are in

phase, while current flowing into the dot end of a primary coil will result in current flowing outof the dot end of a secondary coil.

Types:

Autotransformer

An autotransformer has only a single winding with two end terminals, plus a third at anintermediate tap point. The primary voltage is applied across two of the terminals, and the

secondary voltage taken from one of these and the third terminal. The primary and secondary

circuits therefore have a number of windings turns in common.

Poly phase transformers

Forthree-phase supplies, a bank of three individual single-phase transformers can be used, or all

three phases can be incorporated as a single three-phase transformer. In this case, the magnetic

circuits are connected together, the core thus containing a three-phase flow of flux. A number ofwinding configurations are possible, giving rise to different attributes and phase shifts.
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Instrument transformers

Instrument transformers are used for measuring voltage and current in electrical power systems,and forpower system protection and control. Where a voltage or current is too large to be

conveniently used by an instrument, it can be scaled down to a standardized, low value.

Instrument transformers isolate measurement, protection and control circuitry from the highcurrents or voltages present on the circuits being measured or controlled.

A current transformer is a transformer designed to provide a current in its secondary coilproportional to the current flowing in its primary coil.

Voltage transformers (VTs), also referred to as "potential transformers" (PTs), are designed to

have an accurately-known transformation ratio in both magnitude and phase, over a range of

measuring circuit impedances. A voltage transformer is intended to present a negligible load to

the supply being measured. The low secondary voltage allows protective relay equipment andmeasuring instruments to be operated at a lower voltages.

Both current and voltage instrument transformers are designed to have predictable characteristicson overloads. Proper operation of over-current protection relays requires that current

transformers provide a predictable transformation ratio even during a short-circuit.

Transformer Construction:

Cores (Magnetic Flux path)

This is made of high permeability silicon steel to reduce hysteresis loss & laminated to reduce

eddy-current losses. Each lamination is insulated from its neighbors by a thin non-conducting

layer of insulation.

Windings (Current path)

This is made from copper or aluminum to reduce copper loss. Windings are usually arranged

concentrically to minimize flux leakage.

Both the primary and secondary windings on power transformers may have external connections,called taps, to intermediate points on the winding to allow selection of the voltage ratio. The taps

may be connected to an automatic/manual on/off-load tap changer for voltage regulation of

distribution circuits.

Coolant (Heat path)

Refined Mineral Oil is used as coolant & insulation of the transformer.
http://en.wikipedia.org/wiki/Power_system_protectionhttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Transformer_types#Voltage_transformershttp://en.wikipedia.org/wiki/Permeability_(electromagnetism)http://en.wikipedia.org/wiki/Silicon_steelhttp://en.wikipedia.org/wiki/Tap_(transformer)http://en.wikipedia.org/wiki/Tap_changerhttp://en.wikipedia.org/wiki/Tap_changerhttp://en.wikipedia.org/wiki/Tap_(transformer)http://en.wikipedia.org/wiki/Silicon_steelhttp://en.wikipedia.org/wiki/Permeability_(electromagnetism)http://en.wikipedia.org/wiki/Transformer_types#Voltage_transformershttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Power_system_protection


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Other important parts of a transformer are

(a)Conservator Tank: Takes care the expansion & contraction of transformer oil withtemperature.

(b)Breather: Isolates conservator oil from air, so that moisture ingress in oil is avoided.(c)Buchholtz Relay : This gas operated relay indicates whether there is an internal fault of

the transformer or not.

(d)Winding & Oil Temperature Indicator(e)Pressure Relief Device(f) Oil Surge relay for on load tap changer transformers.(g)Radiator/Heat Exchanger: For cooling of oil.

ELECTRIC MOTOR

What is an electric motor:

An electric motor is an electrical device that uses electrical energy to produce mechanical

energy, usually through the interaction ofmagnetic fields and current-carrying conductors. Thereverse process, producing electrical energy from mechanical energy, is accomplished by a

GeneratororDynamo. Electric motors can be run as generators and vice versa, although this is

not always practical. An electric motor converts electrical power to mechanical power in its rotor(rotating part). There are several ways to supply power to the rotor. In a DC motor this power issupplied to the armature directly from a DC source, while in an induction motor this power is

induced in the rotating device.

Categorization of electric motors:
http://en.wikipedia.org/wiki/Electrical_energyhttp://en.wikipedia.org/wiki/Mechanical_energyhttp://en.wikipedia.org/wiki/Mechanical_energyhttp://en.wikipedia.org/wiki/Magnetic_fieldshttp://en.wikipedia.org/wiki/Electrical_conductorhttp://en.wikipedia.org/wiki/Electrical_generatorhttp://en.wikipedia.org/wiki/Electrical_generatorhttp://en.wikipedia.org/wiki/Dynamohttp://en.wikipedia.org/wiki/Dynamohttp://en.wikipedia.org/wiki/Dynamohttp://en.wikipedia.org/wiki/File:Drehstromtransformater_im_Schnitt_Hochspannung.jpghttp://en.wikipedia.org/wiki/Dynamohttp://en.wikipedia.org/wiki/Electrical_generatorhttp://en.wikipedia.org/wiki/Electrical_conductorhttp://en.wikipedia.org/wiki/Magnetic_fieldshttp://en.wikipedia.org/wiki/Mechanical_energyhttp://en.wikipedia.org/wiki/Mechanical_energyhttp://en.wikipedia.org/wiki/Electrical_energy


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Alternating Current (AC) types and Direct Current (DC) types. Many classic DC motors run on

AC power, these motors being referred to as universal motors.

AC motors are divided into two types. Asynchronous motor or Induction Motor & synchronous

motors. Induction motors contribute 99% of motors in the world. Induction motors are the

preferred choice for industrial motors due to their rugged construction, absence of brushes.

Induction motor

An induction motor (IM) is a type of asynchronous (Speed less than synchronous speed) AC

motor where power is transferred to the rotor winding from stator winding by means ofelectromagnetic induction. Stationary stator having coils supplied with AC current produces a

constant amplitude rotating magnetic field. This magnetic field interacts with the magnetic field

produced by the Rotor (both the fields same rotational speed) to develop torque on the rotor.

There are three types of rotor:

Squirrel-cage rotorThe most common rotor is a squirrel-cage rotor. In this the rotor bars with short circuit ringsresemble a squirrel cage (hamster wheel). The bars are either solid copper (most common) or

aluminum that span the length of the rotor. The rotor bars in squirrel-cage induction motors are

not straight, but have some skew to reduce noise and harmonics.

Slip ring rotorA slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to

slip rings. When these slip rings are shorted, the rotor behaves similarly to a squirrel-cage rotor;they can also be connected to external resistors to produce a high-resistance rotor circuit. Bychanging the resistance connected to the rotor circuit, the speed/current and speed/torque curves

can be altered. The slip ring motor is used primarily to start a high inertia load or a load that

requires a very high starting torque across the full speed range. By correctly selecting the

resistors used in the secondary resistance or slip ring starter, the motor is able to producemaximum torque at a relatively low supply current from zero speed to full speed.

Solid core rotorA rotor can be made from a solid mild steel.

Slip

The difference between the speed of the rotor and speed of the rotating magnetic field in the

stator is called slip. It is unitless and is the ratio between the relative speed of the magnetic field

as seen by the rotor (the slip speed) to the speed of the rotating stator field. Due to this aninduction motor is sometimes referred to as an asynchronous machine.
http://en.wikipedia.org/wiki/Alternating_Currenthttp://en.wikipedia.org/wiki/Direct_Currenthttp://en.wikipedia.org/wiki/Electric_motor#Universal_motorshttp://en.wikipedia.org/wiki/Squirrel-cage_rotorhttp://en.wikipedia.org/wiki/Slip_ringhttp://en.wikipedia.org/wiki/Slip_ringshttp://en.wikipedia.org/wiki/Slip_ringshttp://en.wikipedia.org/wiki/Slip_ringhttp://en.wikipedia.org/wiki/Squirrel-cage_rotorhttp://en.wikipedia.org/wiki/Electric_motor#Universal_motorshttp://en.wikipedia.org/wiki/Direct_Currenthttp://en.wikipedia.org/wiki/Alternating_Current


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Construction

Major parts are Stator, Rotor, Bearings, End covers etc. for Squirrel cage induction motor.

Major parts are Stator, Rotor, Bearings, End covers, slip rings etc. for Slip Ring Induction Motor

The stator consists of wound 'poles' that carry the supply current to induce a magnetic field that

penetrates the rotor. The number of 'poles' can vary between motor types but the poles are alwaysin pairs (i.e. 2, 4, 6, etc.).

Induction motors are most commonly built to run on single-phase orthree-phasepower, but two-phase motors also exist. In theory, two-phase and more than three phase induction motors are

possible; many single-phase motors having two windings and requiring a capacitor can actually

be viewed as two-phase motors, since the capacitor generates a second power phase 90 degrees

from the single-phase supply and feeds it to a separate motor winding. Single-phase power ismore widely available in residential buildings, but cannot produce a rotating field in the motor

(the field merely oscillates back and forth), so single-phase induction motors must incorporatesome kind of starting mechanism to produce a rotating field. They would, using the simplifiedanalogy of salient poles, have one salient pole per pole number; a four-pole motor would have

four salient poles. Three-phase motors have three salient poles per pole number, so a four-pole

motor would have twelve salient poles. This allows the motor to produce a rotating field,allowing the motor to start with no extra equipment and run more efficiently than a similarsingle-phase motor.

Starting of three phase induction motors

Direct-on-line starting

The simplest way to start a three-phase squirrel cage induction motor is to connect its terminals

to the line. This method is often called "direct on line" and abbreviated DOL. When an inductionmotor starts in DOL, a very high current is drawn by the stator, in the order of 5 to 9 times the

full load current. This high current can, in some motors, damage the windings; in addition,

because it causes heavy line voltage drop, other appliances connected to the same line may beaffected by the voltage fluctuation.

Star-delta starters (For squirrel cage induction motors)

A three phase induction motor's windings can be connected to a 3-phase AC line in two different

ways:

wye (star) & delta (mesh)

A delta connection results in a higher voltage to the windings than a wye connection. A star-delta

starter initially connects the motor in wye, which produces a lower starting current than delta,then switches to delta when the motor has reached a set speed. Disadvantages of this method

overDOL starting are:
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Lower starting torque, which may be a serious issue with pumps or any devices withsignificant breakaway torque

Increased complexity, as more contactors and some sort of speed switch or timers areneeded

Two shocks to the motor (one for the initial start and another when the motor switchesfrom wye to delta)

Autotransformer starters (For squirrel cage induction motors)

Such starters are called as auto starters or compensators, consists of an auto-transformer, which

reduces the applied voltage to stator during starting.

Variable Voltage Variable frequency drives (VVVFD) (For squirrel cage induction motors)

VVVFD can be of considerable use in starting as well as running motors. It can easily start amotor at a lower frequency than the AC line, as well as a lower voltage, so that the motor starts

with full rated torque and with no inrush of current. The rotor circuit's impedance increases withslip frequency, which is equal to supply frequency for a stationary rotor, so running at a lowerfrequency actually increases torque.

Resistance starters

This method is used with slip ring motors where variable power resistors are connected in serieswith the rotor winding. During start-up the resistance is large and then reduced to zero at full

speed. As a result, the inrush current is reduced. Another important advantage is higher start-up

torque.

Series Reactor starters/ Series Liquid Resistance Starters (For squirrel cage inductionmotors)

In these starter technology, an impedance in the form of a reactor or resistor is introduced in

series with the motor terminals, which as a result reduces the motor terminal voltage resulting in

a reduction of the starting current; the impedance of the reactor, a function of the current passingthrough it, gradually reduces as the motor accelerates, and at 95 % speed the reactors are

bypassed by a suitable bypass method which enables the motor to run at full voltage and full

speed. Air core series reactor starters or a series reactor soft starter is the most common andrecommended method for fixed speed motor starting. For series liquid resistance starters, the

resistance is lowered continuously as the rotor speeds up & finally at 90% of speed the

resistances are bypassed by a suitable bypass method which enables the motor to run at fullvoltage and full speed.

Synchronous electric motor

A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils

passing magnets at the same rate as the alternating current and resulting magnetic field whichdrives it. Another way of saying this is that it has zero slip under usual operating conditions.
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Contrast this with an induction motor, which must slip to produce torque. The basic difference

between an induction motor and a synchronous AC motoris that in the latter a current is supplied

onto the rotor. This then creates a magnetic field which, through magnetic interaction, links tothe rotating magnetic field in the stator which in turn causes the rotor to turn. It is called

synchronous because at steady state the speed of the rotor is the same as the speed of the rotating

magnetic field in the stator.

These machines find numerous applications where constant speed is necessary. The speed of a

synchronous motor is given by the expression below:

Speed ( in rpm ) = 120 * Supply frequency in Hz / Number of poles.

Parts

A synchronous motor is composed of the following parts:

The stator is the outer shell of the motor, which carries the armature winding. Thiswinding is spatially distributed for poly-phase AC current. This armature creates arotating magnetic field inside the motor like an poly phase induction motor.

The rotor is the rotating portion of the motor. it carries field winding, which is suppliedby a DC source. On excitation, this field winding behaves as a permanent magnet.

The slip rings in the rotor, to supply the DC to the field winding.Operation

The operation of a synchronous motor is simple to imagine. The armature winding, when excited

by a poly-phase (usually 3-phase) winding, creates a rotating magnetic field inside the motor.

The field winding, which acts as a permanent magnet, simply locks in with the rotating magneticfield and rotates along with it. During operation, as the field locks in with the rotating magnetic

field, the motor is said to be in synchronization.

Once the motor is in operation, the speed of the motor is dependent only on the supply

frequency. When the motor load is increased beyond the break down load, the motor falls out ofsynchronization i.e., the applied load is large enough to pull out the field winding from following

the rotating magnetic field. The motor immediately stalls after it falls out of synchronization.

Starting methods

Synchronous motors are not self-starting motors. This property is due to the inertia of the rotor.When the power supply is switched on, the armature winding and field windings are excited.

Instantaneously, the armature winding creates a rotating magnetic field, which revolves at the

designated motor speed. The rotor, due to inertia, will not follow the revolving magnetic field. Inpractice, the rotor should be rotated by some other means near to the motor's synchronous speed

to overcome the inertia. Once the rotor nears the synchronous speed, the field winding is excited,

and the motor pulls into synchronization.
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The following techniques are employed to start a synchronous motor:

A separate motor (called pony motor) is used to drive the rotor before it locks in intosynchronization.

The field winding is shunted or induction motor like arrangements are made so that thesynchronous motor starts as an induction motor and locks in to synchronization once itreaches speeds near its synchronous speed.

Special Properties

Synchronous motors show some interesting properties, which finds applications in power factorcorrection. The synchronous motor can be run at lagging, unity or leading power factor. The

control is with the field excitation, as described below:

When the field excitation voltage is decreased, the motor runs in lagging power factor.The power factor by which the motor lags varies directly with the drop in excitation

voltage. This condition is called under-excitation When the field excitation voltage is made equal to the rated voltage, the motor runs at

unity power factor.

When the field excitation voltage is increased above the rated voltage, the motor runs atleading power factor. And the power factor by which the motor leads varies directly withthe increase in field excitation voltage. This condition is called over-excitation.

The leading power factor operation of synchronous motor finds application in power factorcorrection. Normally, all the loads connected to the power supply grid run in lagging power

factor, which increases reactive power consumption in the grid, thus contributing to additional

losses. In such cases, a synchronous motor with no load is connected to the grid and is run over-

excited, so that the leading power factor created by synchronous motor compensates the existinglagging power factor in the grid and the overall power factor is brought close to 1 ( unity power

factor ). If unity power factor is maintained in a grid, reactive power losses diminish to zero,

increasing the efficiency of the grid. This operation of synchronous motor in over-excited modeto correct the power factor is sometimes called as Synchronous condenser.

Uses

(a)Synchronous motors find applications in all industrial applications where constant speedis necessary

(b)Improving the power factor as Synchronous condensers.(c)Electrical power plants almost always use synchronous generators because it is important

to keep the frequency constant at which the generator is connected.

Advantages

Synchronous motors have the following advantages over asynchronous motors:

Speed is independent of the load, provided an adequate field current is applied.
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Accurate control in speed Theirpower factorcan be adjusted to unity by using a proper field current relative to the

load. Also, a "capacitive" power factor, (current phase leads voltage phase), can beobtained by increasing this current slightly, which can help achieve a betterpower factor

correction for the whole installation.

Their construction allows for increased electrical efficiency when a low speed is required.

Synchronous Generator/ Alternator & Asynchronous Generator/Induction Generator

A synchronous generator is called synchronous because the waveform of the generated voltage

is synchronized with the rotation of the generator. Each peak of the sinusoidal waveform

corresponds to a physical position of the rotor. The frequency is exactly determined by theformula f = RPM x p / 120 where f is the frequency (Hz), RPM is the rotor speed (revolutions

per minute) and p is the number of poles formed by the stator windings. A synchronous

generator is essentially the same machine as a synchronous motor. The magnetic field of the

rotor is supplied by direct current or permanent magnets.

An asynchronous generator is essentially the same machine as an asynchronous or induction

motor. The magnetic field of the rotor is supplied by the stator through electromagneticinduction.

The output frequency of a synchronous generator can be more easily regulated to remain at a

constant value. Synchronous generators (large ones at least) are more efficient thanasynchronous generators. Synchronous generators can more easily accommodate load power

factor variations. Synchronous generators can be started by supplying the rotor field excitation

from a battery. Permanent magnet synchronous generators require no rotor field excitation.

The construction of asynchronous generators is less complicated than the construction of

synchronous generators. Asynchronous generators require no brushes and thus no brush

maintenance. Asynchronous generators require relatively complicated electronic controllers.They are usually not started without an energized connection to an electric power grid. With an

asynchronous generator and an electronic controller, the speed of the generator can be allowed to

vary with the speed of the mechanical prime mover. The cost and performance of such a systemis generally more attractive than the alternative systems using a synchronous generator.

DC Motors

A DC motor is designed to run on DC electric power. The internal configuration of a DC motor

is designed to harness the magnetic interaction between a current-carrying conductor and anexternal magnetic field to generate rotational motion. DC motors can meet demand of loads

requiring high starting torques, high accelerating & decelerating torques, loads requiring wide

range speed control and quick reversal.
http://en.wikipedia.org/wiki/Power_factorhttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://encyclobeamia.solarbotics.net/articles/dc.htmlhttp://encyclobeamia.solarbotics.net/articles/current.htmlhttp://encyclobeamia.solarbotics.net/articles/current.htmlhttp://encyclobeamia.solarbotics.net/articles/dc.htmlhttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://en.wikipedia.org/wiki/Power_factor


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Brushed DC motors

The classic DC motor design involves generation of an oscillating current in a wound rotor, orarmature, with a split ring commutator, and either a wound or permanent magnet stator. A rotor

consists of one or more coils of wire wound around a core on a shaft; an electrical power source

is connected to the rotor coil through the commutator and its brushes, causing current to flow init, producing electromagnetism. The commutator causes the current in the coils to be switched as

the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the

magnetic poles of the stator field, so that the rotor never stops (like a compass needle does) butrather keeps rotating indefinitely (as long as power is applied and is sufficient for the motor to

overcome the shaft torque load and internal losses due to friction, etc.) The geometry of the

brushes, commutator contacts, and rotor windings are such that when power is applied, the

polarities of the energized rotor winding and the stator magnet(s) are misaligned, and the rotorwill rotate until it is almost aligned with the stator's field magnets. As the rotor reaches

alignment, the brushes move to the next commutator contacts, and energize the next winding.

Given our example two-pole motor, the rotation reverses the direction of current through the

rotor winding, leading to a "flip" of the rotor's magnetic field, driving it to continue rotating.

Advantages of a brushed DC motor include low initial cost, high reliability, and simple controlof motor speed. Disadvantages are high maintenance and low life-span for high intensity uses.

Maintenance involves regularly replacing the brushes and springs which carry the electric

current, as well as cleaning or replacing the commutator. These components are necessary for

transferring electrical power from outside the motor to the spinning wire windings of the rotorinside the motor. Many of the limitations of the classic commutatorDC motor are due to the

need for brushes to press against the commutator. This creates friction. At higher speeds, brushes

have increasing difficulty in maintaining contact. Brushes may bounce off the irregularities in thecommutator surface, creating sparks. (Sparks are also created inevitably by the brushes making

and breaking circuits through the rotor coils as the brushes cross the insulating gaps between

commutator sections. Depending on the commutator design, this may include the brushes

shorting together adjacent sectionsand hence coil endsmomentarily while crossing the gaps.Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its

circuit is opened, increasing the sparking of the brushes.) This sparking limits the maximum

speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator.The current density per unit area of the brushes, in combination with their resistivity, limits the

output of the motor. The making and breaking of electric contact also causes electrical noise, and

the sparks additionally cause RFI (Radio Frequency Interference). Brushes eventually wear outand require replacement, and the commutator itself is subject to wear and maintenance (on larger

motors) or replacement (on small motors). The commutator assembly on a large machine is a

costly element, requiring precision assembly of many parts. On small motors, the commutator is

usually permanently integrated into the rotor, so replacing it usually requires replacing the wholerotor.

There are five types of brushed DC motor:

A. DC shunt wound motor: The field winding is connected in parallel with the armature
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B. DC series wound motor: The field winding is connected in series with the armature

C. DC compound motor (two configurations): It has both series & shunt field winding.

Cumulative compound: Series field aids shunt field

Differentially compounded : Series field opposes shunt field

D. Permanent Magnet DC Motor : No field winding

E. Separately-excited (sepex) : Field winding excited from separate DC source than that for

armature supply.

Brushless DC motors

Some of the problems of the brushed DC motor are eliminated in the brushless design. In this

motor, the mechanical "rotating switch" or commutator/brushgear assembly is replaced by an

external electronic switch synchronised to the rotor's position. Brushless DC motors use arotating permanent magnet in the rotor, and stationary electrical magnets on the motor housing.

A motor controller converts DC to AC. This design is simpler than that of brushed motors

because it eliminates the complication of transferring power from outside the motor to thespinning rotor. Advantages of brushless motors include long life span, little or no maintenance,

and high efficiency. Brushless motors are typically 85-90% efficient or more whereas DC motors

with brush gear are typically 75-80% efficient. Disadvantages include high initial cost, and morecomplicated motor speed controllers.

Universal motors:

A series-wound motor is referred to as a universal motor when it has been designed to operateon either AC or DC power. The ability to operate on AC is because the current in both the fieldand the armature (and hence the resultant magnetic fields) will alternate (reverse polarity) in

synchronism, and hence the resulting mechanical force will occur in a constant direction. An

advantage of the universal motor is that AC supplies may be used on motors which have some

characteristics more common in DC motors, specifically high starting torque and very compactdesign if high running speeds are used. The negative aspect is the maintenance and short life

problems caused by the commutator. As a result, such motors are usually used in AC devices

such as food mixers and power tools which are used only intermittently and often have highstarting-torque demands.

Parts

Every DC motor has six basic parts - axle, rotor (armature), stator, commutator, field magnet(s),and brushes.

Speed control
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The speed of a DC motor is proportional to Terminal Voltage applied to armature Armature

Voltage Drop and inversely proportional to Field flux per pole. Hence speed control can be

achieved by varying applied voltage (armature terminal voltage), varying field flux, Varyingresistance in armature circuit or by an electronically controlled switching device made of

thyristors,transistors, or, formerly, mercury arc rectifiers.. The direction of a DC motor can be

changed by reversing either the field or armature connections but not both. This is commonlydone with a special set ofcontactors (direction contactors).

In a circuit known as a chopper, the average voltage applied to the motor is varied by switchingthe supply voltage very rapidly. As the "on" to "off" ratio is varied to alter the average applied

voltage, the speed of the motor varies. The percentage "on" time multiplied by the supply voltage

gives the average voltage applied to the motor. Therefore, with a 100 V supply and a 25% "on"

time, the average voltage at the motor will be 25 V. During the "off" time, the armature'sinductance causes the current to continue through a diode called a "flyback diode", in parallel

with the motor. At this point in the cycle, the supply current will be zero, and therefore the

average motor current will always be higher than the supply current unless the percentage "on"

time is 100%. At 100% "on" time, the supply and motor current are equal. The rapid switchingwastes less energy than series resistors. This method is also called pulse-width modulation

(PWM) and is often controlled by a microprocessor. An output filteris sometimes installed tosmooth the average voltage applied to the motor and reduce motor noise.

Since the series-wound DC motor develops its highest torque at low speed, it is often used in

traction applications such as electric locomotives, and trams. Series motors must never be used inapplications where the drive can fail (such as belt drives). As the motor accelerates, the armature

(and hence field) current reduces. The reduction in field causes the motor to speed up until it

destroys itself.

DC motor starters

The counter-emf aids the armature resistance to limit the current through the armature. When

power is first applied to a motor, the armature does not rotate. At that instant the counter-emfis

zero and the only factor limiting the armature current is the armature resistance. Usually thearmature resistance of a motor is less than 1 ; therefore the current through the armature would

be very large when the power is applied. This current can make an excessive voltage drop

affecting other equipment in the circuit and even trip overload protective devices.

Therefore the need arises for an additional resistance in series with the armature to limit the

current until the motor rotation can build up the counter-emf. As the motor rotation builds up, theresistance is gradually cut out.

CIRCUIT BREAKER

What is a Circuit Breaker:

A circuit breaker is an automatically-operated electrical switch designed to protect an electrical

circuit from damage caused by overload orshort circuit. This can isolate a circuit in load & in off
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load situation (unlike an isolator, which can isolate the circuit in off load only). Its basic function

is to isolate a fault condition and by interrupting continuity, to immediately discontinue electrical

flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can bereset (either manually or automatically) to resume normal operation. Circuit breakers are made in

varying sizes, from small devices that protect an individual household appliance up to large

switchgeardesigned to protect high voltage circuits.

Principle of Operation:

All circuit breakers have common features in their operation, although details vary substantially

depending on the voltage class, current rating and type of the circuit breaker.

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually

done within the breaker enclosure. Circuit breakers for large currents or high voltages are usuallyarranged withpilot devices like over current & earth fault relay, to sense a fault current and to

operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized

by a separate DC source, i.e. battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit;some mechanically-stored energy (using something such as springs or compressed air) contained

within the breaker is used to separate the contacts. Small circuit breakers may be manually

operated; larger units have solenoids to trip the mechanism, and electric motors/compressed air

to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also

withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper

or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by theerosion due to interrupting the arc. Miniature and molded case circuit breakers are usually

discarded when the contacts are worn, but power circuit breakers and high-voltage circuitbreakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled, andextinguished in a controlled way, so that the gap between the contacts can again withstand the

voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the

medium in which the arc forms. Different techniques are used to extinguish the arc including:

Lengthening of the arc Intensive cooling (in jet chambers) Division into partial arcs Zero point quenching Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore

power to the interrupted circuit.
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Arc interruption

Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings willhave metal plates or non-metallic arc chutes to divide and cool the arc. Magnetic blowout coils

deflect the arc into the arc chute.

In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil

through the arc.

Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic

field, and then rely upon the dielectric strength of the sulfur hexafluoride (SF6) to quench thestretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact

material), so the arc quenches when it is stretched a very small amount (


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Front panel of a 1250 A air circuit breaker manufactured by ABB. This low voltage power

circuit breaker can be withdrawn from its housing for servicing.

Types of circuit breaker

Many different classifications of circuit breakers can be made, based on their features such as

voltage class, construction type, interrupting type, and structural features.

Low voltage circuit breakers

Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial

application, include:

MCB (Miniature Circuit Breaker)rated current not more than 100 A. Tripcharacteristics normally not adjustable. Thermal or thermal-magnetic operation.

MCCB (Molded Case Circuit Breaker)rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.

The LV circuit breakers are often installed in draw-out enclosures that allow removal and

interchange without dismantling the switchgear.

Large low-voltage molded case and power circuit breakers may have electrical motor operators,

allowing them to be tripped (opened) and closed under remote control. These may form part of

an automatic transfer switch system for standby power.
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Photo of inside of a circuit breaker

The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the mostcommon style in modern domestic consumer units and commercial electrical distribution boards.

The design includes the following components:

1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates thestatus of the circuit breaker (On or Off/tripped). Most breakers are designed so they can

still trip even if the lever is held or locked in the "on" position. This is sometimes referred

to as "free trip" or "positive trip" operation.2. Actuator mechanism - forces the contacts together or apart.3. Contacts - Allow current when touching and break the current when moved apart.4. Terminals5. Bimetallic strip6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the

device after assembly.7. Solenoid8. Arc divider/extinguisher

Magnetic circuit breaker

Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force increases with the

current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. Thecircuit breaker contacts are held closed by a latch. As the current in the solenoid increases

beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows

the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic

time delay feature using a viscous fluid. The core is restrained by a spring until the currentexceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by

the fluid. The delay permits brief current surges beyond normal running current for motor

starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force torelease the latch regardless of core position thus bypassing the delay feature. Ambient

temperature affects the time delay but does not affect the current rating of a magnetic breaker.

Thermal magnetic circuit breaker
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Thermal magnetic circuit breakers, which are the type found in most distribution boards,

incorporate both techniques with the electromagnet responding instantaneously to large surges in

current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions.

Common trip breakers

When supplying a branch circuit with more than one live conductor, each live conductor must be

protected by a breaker pole. To ensure that all live conductors are interrupted when any poletrips, a "common trip" breaker must be used. These may either contain two or three tripping

mechanisms within one case, or for small breakers, may externally tie the poles together via their

operating handles. Two pole common trip breakers are common on 120/240 volt systems where240 volt loads (including major appliances or further distribution boards) span the two live wires.

Three-pole common trip breakers are typically used to supply three-phase electric powerto large

motors or further distribution boards.

Two and four pole breakers are used when there is a need to disconnect the neutral wire, to besure that no current can flow back through the neutral wire from other loads connected to the

same network when people need to touch the wires for maintenance. Separate circuit breakersmust never be used for disconnecting live and neutral, because if the neutral gets disconnected

while the live conductor stays connected, a dangerous condition arises: the circuit will appear de-

energized (appliances will not work), but wires will stay live. This is why only common tripbreakers must be used when switching of the neutral wire is needed.

Medium-voltage circuit breakers

Medium-voltage circuit breakers rated between 1 and 36 kV may be assembled into metal-

enclosed switchgear line ups for indoor use, or may be individual components installed outdoorsin a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are

now themselves being replaced by vacuum circuit breakers (up to about 36 kV). Like the high

voltage circuit breakers described below, these are also operated by current sensing protectiverelays operated through current transformers. Medium-voltage circuit breakers can be classified

by the medium used to extinguish the arc:

Vacuum circuit breakerWith rated current up to 3200 A, these breakers interrupt thecurrent by creating and extinguishing the arc in a vacuum container. These are generally

applied for voltages up to about 36,000 V, which corresponds roughly to the medium-

voltage range of power systems. Vacuum circuit breakers tend to have longer lifeexpectancies between overhaul than do air circuit breakers.

Air circuit breakerRated current up to 10,000 A. Trip characteristics are often fullyadjustable including configurable trip thresholds and delays. Usually electronicallycontrolled, though some models are microprocessorcontrolled via an integral electronic

trip unit. Often used for main power distribution in large industrial plant, where the

breakers are arranged in draw-out enclosures for ease of maintenance.

SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.
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High-voltage circuit breakers

Electrical power transmission networks are protected and controlled by high-voltage breakers.The definition of high voltage varies but in power transmission work is usually thought to be

higher than normal system voltage of 33KV. High-voltage breakers are nearly always solenoid-

operated, with current sensing protective relays operated through current transformers. Insubstations the protection relay scheme can be complex, protecting equipment and busses from

various types of overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

Minimum oil SF6

Some of the manufacturers are ABB, GE (General Electric) , AREVA, Mitsubishi Electric,Pennsylvania Breaker, Siemens, Toshiba, Konar HVS, BHEL and others.

Due to environmental and cost concerns over insulating oil spills, most new breakers use SF 6 gasto quench the arc.

High-voltage AC circuit breakers are routinely available with ratings up to 765 kV.

High-voltage circuit breakers used on transmission systems may be arranged to allow a singlepole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this

improves the system stability and availability.
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SIEMENS INDOOR VCB

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