ppt 7. heating & welding - large fonts - copy

8/10/2019 Ppt 7. Heating & Welding - Large Fonts - Copy

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EE 2802 Applied Electricity


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Domestic applications

Room heaters,

Water heating Kitchen appliances (hot plate, electrickettle, electric oven, toaster, etc.)

Industrial applications

Melting of metals

Heat treatment of metals (annealing,

tempering, soldering, brazing, etc..) Moulding of glass


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1. Cleanliness - no dust2. No pollution

3. Economical - cheap, low maintenance

4. Easy temperature control5. Special heating requirements can be

catered

Uniform heating Heating only a particular portion


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6. Higher efficiency

Most of the heat produced is

utilized for heating

7. Better working conditions

No irritating noise

Low radiation losses Low ambient temperature

8. Safety

Responds quickly to the controlledsignals


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1. Resistance heating / Joule heating

2. Arc heating

3. Induction heating

4. Eddy current heating5. Dielectric heating

6. Infrared heating


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When current passes, I2R losses

produce heat

Two methods

Direct resistance heating

Indirect resistance heating


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Direct Resistance Heating

Current pass through the material to be

heated High efficiency

Indirect Resistance Heating

Current pass through a resistanceelement which is surrounded by thecharge

Heat is delivered to the charge byradiation / convection


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A high voltage is applied across an air-gap

Air becomes ionized

Starts conducting in a form of continuousspark (arc) -> produce heat

Two methods:

Direct Arc Heating

Indirect Arc Heating


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Arc is formedbetween theelectrodes

and thecharge


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Arc is formed between electrodes

Produced heat is passed to the charge byradiation


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Similar to transformer working Primary winding is supplied from an AC

source

Primary is magnetically coupled to thecharge

It induces voltage in the charge (secondary)

Secondary current heats up the charge

Two methods:

Core type induction heating

Coreless type induction heating


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Primary is connected to high frequencyAC supply

Eddy currents induced in the charge and

heats it up


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Material is placed inside a high frequencycarrying coil

Material gets heated up by eddy currents


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Used for heating insulators like wood,plastic, ceramic etc


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Dielectric is placed between twoconducting plates

- to form a parallel plate capacitor

Power loss in the capacitor is convertedinto heat

Supply frequency between 10 50 MHz

Applied voltage up to 20 kV

Efficiency is around 50% Very high cost


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When tungsten filament lamps areoperated at about 23000C

they produce heat radiations

(infrared radiations)

Suitable reflectors can send these IR

radiations to the surface to be heatedApplications

Plaint drying

Low temperature heating of plastics


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Welding

Process of joining metallic and non-

metallic materials

By applying heat, pressure, or acombination


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Advantages

Does not waste excess fuel

Heat does not move far beyond the weldpoint - ideal for insulated wires

Methods

1. Arc welding2. Resistance welding


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Short arc length is necessary forgetting good welds


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Welding is never done directly from supplymains

Welding machines are essential To convert AC to DC when DC welding is

desired

To reduce voltage to a safer and suitable

value To provide required high current,

without drawing from the supply mains


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Consists of A reverse series winding DC generator DC / AC motor


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Operation

On open circuit, Only shunt filed is operative

Provide maximum voltage for striking thearc

After the arc has been established,

Current pass through the both windings

Fluxes oppose each other-> Decrease the voltage


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Portable operation

Suitable for all ferrous and non-ferrousmaterials

Suitable for all positions of welding

High initial cost and maintenance cost

Noisy in operation


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Consists of

A step-down transformer with tapped

secondary Adjustable reactor - for drooping VI

characteristics


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Heat generated due to I2R losses

Process has two stages:

Resistance heating Application of forging pressure


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Advantages

Welding action is rapid

No filler material is needed

Requires comparatively lesser skill

Suitable for large quantity production

Both similar and dissimilar metals can be

welded Parent metal is not harmed

Different shapes can be welded

Disadvantage

High initial cost

High maintenance cost


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Spot Welding


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Seam Welding


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Projection Welding


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Butt Welding


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A TEXTBOOK OF

ELECTRICAL TECHNOLOGY

VOLUME III

TRANSMISSION, DISTRIBUTION & UTILIZATION

B.L. THERAJA

A.K. THERAJA

ppt 7. heating & welding - large fonts - copy

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