Basic of Electrical12 Engg

7/30/2019 Basic of Electrical12 Engg

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BASIC OF ELECTRICAL ENGINEERING

Date. 15.12.10

ROLLING STREAM


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Topics for Discussion

Introduction AC & DC Current

Various measures of producing Voltage

Imp terms & Formulae

Power & Control Circuit

Transformer

Motor

LT switchgear Drives

PLC


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Introduction


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Alternating Current (A.C.)

Alternating Current (A.C.):The current or voltage which alternates its directionand magnitude every time. Nowadays 95% of the total energy is produced,transmitted and distributed in A.C. supply.

Advantages of A.C. over D.C.:

More voltage can be generated than D.C.(up to 33KV when DC, 650V only)

AC voltage can be stepped up/down with the help of transformer.

AC transmission and distribution is more economical, as line material(say copper) can besaved by transmitting power at higher voltages.

AC motors for the same h.p. as of DC motors are cheaper, lighter in weight, require lesserspace and require lesser attention in operation and maintenance.

AC can be converted in DC easily, when and where required but DC can not be converted toAC so easily and it will not be economical.


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Direct Current (D.C.)

Direct Current (D.C.):The current or voltage does not change its direction andmagnitude and is steady at all times. The cells, accumulators and dynamos or

generators give pure DC.

Even we produce 95% of electrical energy as AC, but we can not forget the DC for itsfollowing merits: DC series motor are more suitable for traction purposes in tramway, railways, cranes andlifts.For electroplating, electrolytic and electrochemical processes(battery charging etc.,) DC is

required.

Arc lamps for search lights and cinema projects work on DC.

DC Arc welding is better than AC.

Relays and operating time switches,etc., and circuit-breakers, DC work more efficiently.

In rolling mills, paper mills where fine speed and torque control with high h.p. is required,DC motors are required


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Methods of Producing Voltage

Electrochemistry - Combining chemicals with certain metals causes a chemicalreaction that transfers electrons.Example: A battery can maintain a potential difference between its positive and

Negative terminals by chemical action

Static electricity - When an object with a normally neutral charge loses electrons, due to friction,and comes in contact with another object having a normal charge, an electric charge is exerted betweenthe two objects.

Magnetic induction - Rotating coils of wire through a stationary magnetic field

or by rotating a magnetic field through a stationary coil of wire produces a potential.This is one of the most useful and widely employed applications of producing vastquantities of electric power.

Piezoelectric effect - Bending or twisting certain materials will cause electrons toDrive out of orbit in the direction of the force. Example: Load Cells.


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Conductors

Conductors are materials with electrons that are loosely bound to their atoms, or materialsthat permit free motion of a large number of electrons. Atoms with only one valence electron, such as copper, silver, and gold, are examples of good

conductors.

Most metals are good conductors.

Insulators

Insulators, or nonconductors, are materials with electrons that are tightly bound to theiratoms and require large amounts of energy to free them from the influence of the nucleus.

The atoms of good insulators have their valence shells filled with eight electrons, whichmeans they are more than half filled. Any energy applied to such an atom will be distributedamong a relatively large number of electrons.

Examples of insulators are rubber, plastics, glass, and dry wood.

Resistors

Resistors are made of materials that conduct electricity, but offer opposition to current flow. These types of materials are also called semiconductors because they are neither good

conductors nor good insulators. Semiconductors have more than one or two electrons intheir valence shells, but less than seven or eight.

Examples of semiconductors are carbon, silicon, germanium, tin, and lead. Each has fourvalence electrons.

Conductors, insulators, and electron flow


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Imp. Definitions & Formulae


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Kirchoffs Law


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Kirchoffs Law


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AC Power System


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AC Power System


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Electrical Circuits


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Capex Report for the month of OCT_10Electrical Symbols


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Control circuits


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Power & Control circuit


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Power & Control circuit


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Transformer


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Transformer


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Motors


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MOTOR PROTECTIONMotor protection safeguards the motor, the supply system and personnel from various operating conditions of the

driven load, the supply system or the motor itself.

Over current ProtectionOver current protection interrupts the electrical circuit to the motor upon excessive current demand on the supply

system from either short circuits or ground faults.

Overcurrent protection is required to protect personnel, the motor branch circuit conductors, control equipment,and motor from these high currents.

Overcurrent protection is usually provided in the form of fuses or circuit breakers. These devices operate when a

short circuit, ground fault or an extremely heavy overload occurs.

Most over current sources produce extremely large currents very quickly.

Motors


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Overload ProtectionOverload protection is installed in the motor circuit and/or motor to protect the motor from damagefrom mechanical overload conditions when it is operating/running.

LT Switch Gear

The effect of an overload is an excessive rise in temperature in the motor windings due to current higher than fullload current.Properly sized overload protection disconnects the motor from the power supply when the heat generated in themotor circuit or windings approaches a damaging level for any reason.The larger the overload, the more quickly the temperature will increase to a point that is damaging to theinsulation and lubrication of the motor.

Unlike common instantaneous type fuses and breakers, overload devices are designed to allow high currents toflow briefly in the motor to allow for: Typical motor starting currents of 6 to 8 times normal running current whenstarting.Overload protection trips when an overload exists for more than a short time. The time it takes for an overload totrip depends on the type of overload device, length of time the overload exists, and the ambient temperature inwhich the overloads are located.


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LT Switch Gear

Other Motor Protection Devices

Low Voltage ProtectionLow Voltage Disconnects - Protection device operates to disconnect the motor when the supply voltagedrops below a preset value. The motor must be manually restarted upon resumption of normal supplyvoltage.Low Voltage Release - Protection device interrupts the circuit when the supply voltage drops below apreset value and re-establishes the circuit when the supply voltage returns to normal.

Phase Failure Protection

Interrupts the power in all phases of a three-phase circuit upon failure of any one phase.Normal fusing and overload protection may not adequately protect a polyphase motor from damaging

single phase operation. Without this protection, the motor will continue to operate if one phase is lost.Large currents can be developed in the remaining stator circuits which eventually burn out.. Phase failureprotection is the only effective way to protect a motor properly from single phasing.

Ground Fault ProtectionOperates when one phase of a motor shorts to ground preventing high currents from

damaging the stator windings and the iron core.

Other Motor Protection DevicesBearing Temperature Monitors & ProtectionWinding Temperature Monitors & Protection DevicesCurrent Differential Relays (Phase Unbalance)Vibration Monitors & Protection


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Fuse :

A fuse is a short piece of metal, inserted in the circuit, which melts when excessive current flowsthrough it and thus breaks the circuits.

Circuit Breakers:

A circuit breaker is a piece of equipment which can

(i) Make or break a circuit either manually or by remote control under normal conditions.

(ii) Break a circuit automatically under fault conditions .

(iii) Make a circuit either manually or by remote control under fault conditions.

Protective Relays:

A protective relay is a device that detects the fault and initiates the operation of the circuitbreaker to isolate the defective element from the rest of the system.

LT Switch Gear


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LT Switch Gear

Time-Delay (Dual Element) Fuses

These are generally dual element fuses with both thermal and instantaneous trip features that allow themotor starting current to flow for a short time without blowing the fuse. Time delay fuses can also be used to provide some degree of overload protection which standard fuses cannot. The time delay fuses to be sized up to a maximum of 175% of a motors FLA for overcurrent protection.

Time-delay fuses will hold 500% of their amp rating for 10 seconds which will allow most motors to startwithout opening the circuit.

Under normal conditions, a 100-amp time-delay fuse will start any motor with a locked-rotor currentrating of 500 amps or less.


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Inverse Time Circuit BreakersInverse time circuit breakers have both thermal and instantaneous trip features and are preset to trip at

standardized levels. This is the most common type of circuit breaker used in the building trades for residential,commercial, and heavy construction.The thermal action of this circuit breaker responds to heat.If a motors ventilation inlets and outlets are not adequate to dissipate heat from the windings of themotor, the heat will be detected by the thermal action of the circuit breaker. If a short should occur, the magnetic action of the circuit breaker will detect the instantaneous values ofcurrent and trip the circuit breaker.The National Electrical Code requires inverse time circuit breakers to be sized to a maximum of 250% of

the motor FLA.

LT Switch Gear


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LT Switch Gear

Motor Overload Protection

Motors larger than 1 horsepower must be provided separate motor overload protection devices.The most common devices typically used include:1) magnetic or thermal overload devices2) electronic overload relays3) Fuses

Magnetic & Thermal OverloadsOverload devices are usually located in the motors starter and connected in series with the motorselectrical supply circuit and can be operated by either magnetic or thermal action.

The same amount of current passes through the overload relay and the motor.If the current or heat through the overload device is higher than the devices rating, it trips and shutsdown the electric power to the motor.

Thermal Overload Relays

A thermal overload relay is an electro-mechanical relay that is operated by heat developed in the relay.

When the level of current in a circuit reaches a preset value, the increased temperature opens a set ofcontacts. The increased temperature opens the contacts through a bimetallic strip or by melting an alloy thatactivates a mechanism that opens the contacts.Two types include melting alloy and the bi-metallic strip.


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LT Switch Gear

Melting-Alloy Thermal Overload Relays:These are probably the most popular type of overload protection.

The motor current passes through a small heater winding and under overload conditions, the heat causesa special solder to melt allowing a ratchet wheel to spin thus opening the control circuit contacts.Must be reset by hand operation.Heater coil and solder pot in one unit non-tamperable


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Bimetallic Thermal Overload Relays:

This design uses a bimetal strip associated with a current-carrying heater coil.

When an overload occurs, the heat causes the bimetal to deflect and actuate a tripping mechanism whichopens a set of contacts in the control circuit interrupting power to the coil and opening the power contacts.Most relays are adjustable over a range from 85% to 115% of their value.They are available with ambient compensation. An ambient compensated devices trip point is notaffected by ambient temperature and performs consistently at the same value of current.

LT Switch Gear


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Electronic Overloads

Electronic overloads sense the load current and the heating effect on the motor iscomputed. If an overload condition exists, the sensing circuit interrupts the power circuit.

The tripping current can be adjusted to suit the particular application.

Electronic overloads often perform additional protective functions such as groundfault and phase loss protection.

Fuses

Fuses have limited application as the primary means of overload protection for motorsbut can be effectively used to provide back up overload protection.

Their basic function is to protect against short circuits and ground faults.

If sized to provide overload protection, they would blow when the motor starts due tohigh motor inrush current.

Dual-element fuses can provide motor overload protection, but they have to bereplaced

when they blow which can be a disadvantage.

There is a risk of single-phasing damage to the motor when only one fuse blowsunless single-phase protection is provided.


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Drive Basic

TORQUE REF

TORQUE REF

SELECTOR

SPEED REF.

LOCAL/REM

RAMP GEN. SPEED CONTROL

CURRENT LIMITER

CURRENT CONTROL

CURRENT ACTUAL

FORMING

FILRING

UNIT

EMF REF

EMF F/B

POWER

FIELD

EXITER

SPEED ACTUAL

SCALING

SPEED REF. VIACTRL PANEL

A

D

ENCODER

T.G.


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AC Drives


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DC Drives

DC Motors are used extensively in adjustable-speed drives and position control applications.

There are two Control in the DC Drives

Armature-Voltage control :

Speed below base speed can be controlled by armature voltage control

Field flux control :

Speed above base speed are obtained by field-flux control.

As speed control methods for dc motors are simpler and less expensive than those for ac motors,dc motorsare preferred where wide speed control range is required.

Depending upon the type of ac Source or the Method of voltage control,dc Drives are classified as under :

1. Single phase dc drives

2. Three phase dc drives

3. Chopper drives.


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AC Drives

PLC


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PLC


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How Tube Lights are Made

The process

begins with glass tubes that are cleaned so that no foreign particles remain. The tubes are shape

with a flame, called a folder's shaper.

They are heated for thirty seconds and then shaped mechanically.

A thin coat of phosphorous is applied to the inner part of the tube.

The cathode mount is inserted into the tube end, and the tube is sealed at high temperature.

The tubing is then emptied of air, and mercury is added to the tube. A tube cap is then sealed onto the tube.

Light:

Light is produced when electricity is passed through the phosphorous, which creates an ionization of the mercuryinside the tube.

The filament in the cathode mount fires when electricity is put through it, shooting electrons through the tubing.

The tubing creates an ionization avalanche, which reaches the tiny drop of mercury in the tube.

The mercury reacts to the electrons and begins to glow.


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Fluorescent light

A fluorescent light does not have the usual glowing filament of an incandescent bulb, but instead containsa mercury vapor that gives off ultraviolet light when ionized.

The ultraviolet light makes particles that coat the inside of the tube, and these particles glow or fluoresce

Fluorescent starters are used in several types of fluorescent lights. The starter is there to help the lamp light.When voltage is applied to the fluorescent lamp, here's what happens:

1.The starter (which is simply a timed switch) allows current to flow through the filaments at the ends of the tube.

2.The current causes the starter's contacts to heat up and open, thus interrupting the flow of current. The tubelights.

3.Since the lighted fluorescent tube has a low resistance, the ballast now serves as a current limiter.

Working :

When you turn on a fluorescent tube, the starter is a closed switch. The filaments at the ends of the tubeare heated by electricity, and they create a cloud of electrons inside the tube. The fluorescent starter is a timedelay switch that opens after a second or two. When it opens, the voltage across the tube allows a stream ofelectrons to flow across the tube and ionize the mercury vapor.

Without the starter, a steady stream of electrons is never created between the two filaments, and the lamp flickers.Without the ballast, the arc is a short circuit between the filaments, and this short circuit contains a lot

of current. The current either vaporizes the filaments or causes the bulb to explode.


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WCM Communication


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Thank you

Basic of Electrical12 Engg

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