7-heating & welding

8/10/2019 7-Heating & Welding

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EE 2850 Applied Electricity


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Domestic applications: Room heaters,

Water heating, hot plate, electric kettle, electricoven, toaster,.

Industrial applications: Melting of metals

Heat treatment of metals (annealing, tempering,

soldering, brazing, etc..) Moulding of glass


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Cleanliness - no dust

No pollution

Economical - cheap, low maintenance

Easy temperature control

Special heating requirements can be

catered Uniform heating Heating only a particular portion


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Higher efficiency

Most of the heat produced is utilized forheating

Better working conditions

No irritating noise

Low radiation losses

Low ambient temperature

Safety

Responds quickly to the controlled signals


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1. Resistance heating / Joule heating

2. Arc heating

3. Induction heating

4. Eddy current heating

5. Dielectric heating

6. Infrared heating


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Based on effect

When current passes, losses produce heat

Two methods

Direct resistance heating

Indirect resistance heating


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Direct Resistance Heating

Current pass through the material (or charge) tobe heated

High efficiency as heat is produced in the

charge it self

Indirect Resistance Heating

Current pass through a resistance element which

is surrounded by the charge Heat is delivered to the charge by radiation /

convection


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A high voltage is applied across an air-gap

Air becomes ionized

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

Two methods:

Direct Arc Heating

Indirect Arc Heating

For carbon/graphite electrodes -> 30000C 35000C


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Arc is formedbetween theelectrodes andthe charge


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Arc is formed between electrodes Produced heat is passed to the charge by radiation


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Based on the principle of transformer working

Primary winding is supplied from an AC source

Primary is magnetically coupled to the charge

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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Direct Core Type

Indirect Core Type

A suitable element is heated by induction

Heat is transferred to the charge by radiation


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No heavy iron core Primary is connected to high frequency AC supply

Eddy currents induced in the charge and heats it up

Eddy current loss

Needs a high frequency supply


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Material to be heated is placed inside a highfrequency carrying coil

Magnetic filed produced by the coil sets up eddycurrents in the material

Material gets heated up


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Advantages:

Can take place in vacuum or special environments

-> where other heating types cannot be used

Can made to penetrate any depth, by selectingproper frequency


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Used for heating insulators like wood, plastic,ceramic etc

Supply frequency between 10 50 MHz

Applied voltage up to 20 kV

Efficiency is around 50%


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Dielectric is placed between two conducting platesto form a parallel plate capacitor

When connected to the AC source,capacitor draws a current

which leads the voltage by an angle which islittle less than 900

Power loss in the capacitor is converted into heat


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Advantages:

Heat is uniform as it is generated within thedielectric material

Only method for heating dielectrics

Heating can be stopped immediately as andwhen desired

Disadvantage:

Cost is very high


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When tungsten filament lamps are operated atabout 23000C

they produce heat radiations

(infrared radiations)

Using suitable reflectors, these IR radiations arefocused on the surface to be heated

Applications: Paint drying

Low temperature heating of plastics


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Welding

Process of joining metallic and non-metallicmaterials

By applying heat, pressure, or a combination

Electric welding

Process of welding two pieces of metal togetherusing a powerful electric current


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Does not waste excess fuel

Heat does not move far beyond the weld point

- ideal for insulated wires

Can be used for both large and small jobs


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Welding can be done by using Electric energy

Chemical energy

Mechanical energy

2 common methods (by electric energy)

1. Arc welding

2. Resistance welding


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Arc welding:

Electricity is conducted in the form of an arc

Established between the two metallic surfaces

Resistance welding:

Current is passed though the inherent resistanceof the joint to be welded

Heat generated by losses


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When current passes between two metallicelectrodes which are separated by a short distance

- An electric arc is formed

The arc is started by touching the (+) ve metal tothe (-) ve metal

Then separating them about 3 to 6 mm

When touched, large short circuit current flows When separated, continue to flow in form of spark

(arc)


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DC power - heat generated at cathode is much lesscompared to the anode

AC power heat developed equally at the anode

and the cathode Short arc length is necessary for getting good

welds


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Welding is never done directly from supply mains

Welding machines provide current of variouscharacteristics

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 drawingfrom the supply mains


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Two types of welding machines:

1. DC welding machines

Motor generator set

AC transformer with rectifiers

2. AC welding machines


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


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On open circuit, Only shunt filed is operative Provide maximum voltage for striking the arc

After the arc has been established, Current pass through the both windings Fluxes oppose each other -> Decrease the voltage

Advantage:

Portable operation Suitable for all ferrous and non-ferrous materials Suitable for all positions of welding

Disadvantages:

High initial cost and maintenance cost Noisy in operation


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Consists of

A transformer (single-phase or three-phase)

A rectifier unit


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Advantages:

Long life as it has no moving parts

Efficient

Quite in operation

Applications:

Pipes

Non ferrous metals

Mild steel in thin gauges


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Consists of

A step-down transformer with tapped secondary

Adjustable reactor - for drooping VI characteristics


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Advantages:

Low initial cost

Low maintenance cost

Low wear

Disadvantages:

Not suitable for cast iron and non-ferrous metals


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No extra metal is added to the joint

Heat generated as per the equation

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 OFELECTRICAL TECHNOLOGYVOLUME III

TRANSMISSION, DISTRIBUTION & UTILIZATION


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Saving energy in HVAC applications with VariableSpeed Drives VSDs)

Should include: Introduction - HVAC systems, use of electric

machines, need for VSDs How VSDs can save energy Example / Case study - to give a quantitative

idea Any other which is relevant

No. of words: 750 - 1000

7-heating & welding

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