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Department of Technical Education

Andhra PradeshName : V. Narsimha Reddy

Designation : Lecturer in EEE

Branch : D.E.C.E

Institute : GMR. Polytechnic Gajwel

Semester : VI Semester

Subject : Electrical Utilization & Automation

Code : EE605 A

Topic : Electric Heating

Duration : 100 Minutes

Sub topic : Methods of Temperature control

Teaching Aids : PPT, Diagrams, Animation

Revised By : K. Chandra Sekhar, L/EEE, GPT, HYD

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Recap

In the previous class you have learnt about

• Resistance heating

• Classification of resistance heating

• Applications of resistance heating

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ObjectivesObjectives

On completion of this topic ,you would be able to know

• Different methods of temperature control of resistance heating

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Temperature Control of Resistance Heating

• In resistance heating supply voltage and resistance of heating elements are independent parameters

• Current is a dependent parameter• The temperature of resistance furnace is

controlled by • Supply voltage• By varying the number of heating elements• Switching ON and OFF the supply

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Temperature Control Methods

Temperature control of resistance heating

Periodically switching on & off of the power

supply

By varying supply voltage

By varying resistance

By variable supply

By Auto-T/F or

Induction regulator

By series impedance

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Temperature Control Methods

Temperature control of resistance heating

Periodically switching on & off of the power

supply

By varying supply voltage

By varying resistance

By varying the number of

heating elements

By changing the

arrangement & heating

elements

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By Variable Supply Voltage

• If the furnace has separate generating set for electric

power supply, then variable voltage can be generated and

supplied to the furnace depending upon the temperature

• Voltage can be variable

• This is usually employed for large capacity furnace

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By Auto-transformer or Induction Regulator

V1=Primary side supply voltage.V2=Secondary output voltage.I1=Primary currentI2=Secondary current.

Fig.1

• FURNACE

• 4

• 3

• 2

• 1

• Supp

• l• y

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By Auto-transformer or Induction Regulator

• Fig 1 shows an auto - transformer

• By changing the position of the tap the voltage supplied to heating element will change and accordingly the temperature

• Supp

• l• y

Fig.1

• FURNACE

• 4

• 3

• 2

• 1

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1 – Phase Induction Regulator

• Fig 2 shows a single

phase induction regulator

• By changing the position

of rotor (primary) the

output voltage v2 will

change

• The change controls the

furnace temperature

Fig 2

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By Series Impedance

• A variable impedance will be connected in series with the furnace

• A part of voltage drop across the impedance included and the remaining voltage appears across the furnace

• Due to inclusion of impedance of the coil,the power factor reduces

• A capacitor is connected across the supply to improve the power factor i.e., Dotted Lines

Fig.3

• Supp

• l• y

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By Varying The Number of Heating Elements

• In this method the number of heating elements are changed by operating the switches shown in fig 4

• if the supply is reduced to all/few elements, the temperature also reduces and vice-versa.

Fig.4

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By Varying The Number of Heating Elements

• The number of heating elements are changed by

operating the switches shown in fig 4

• Each element is provided with one switch in fig 4 (a)

• One switch controls three elements in fig 4(b)

• If the supply voltage is reduced to few / all elements the

temperature reduces and vice – versa

• Non uniform heating taken place

• If uniform heating is required the even or odd number

elements should be switched off in fig 4 (b)

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V

V/2 V/2

V/4 V/4 V/4 V/4

V

V

Four parallel paths Two parallel paths

one parallel path

By Changing The Arrangement of Heating Elements

Fig.5

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By Changing The Arrangement of Heating Elements

• Fig 5 (a) shows the

arrangement of four

heating elements for a

single phase supply

• All the elements are in

parallel, the heat

produced will be equal to

ratingFig 5 (a)

V

Four parallel paths

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By Changing The Arrangement of Heating Elements

• If four elements are

arranged in two paths

shown in fig 5 (b)

• The heat produced will be

one – fourth

• Because the voltage

across each element is

v/2

Fig 5 (b)

V

V/2 V/2

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By Changing The Arrangement of Heating Elements

• If all four elements are

arranged in series shown

in fig 5 (c)

• The heat produced will be

one – sixteenth

• Because the voltage in

each element is v/4 Fig 5 (c)

V/4 V/4 V/4

V

V/4

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By Changing The Arrangement of Heating Elements

• Fig 6 shows circuit configuration for 3 phase system

• When the same elements are changed from delta to star, the power consumption reduced to one - third

Fig.6Connected in star Connected in delta

Three-Phase System

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Periodically / Intermittent On / Off

Final temperature α Time interval the switch remains ON Total time interval of the

ON/OFF cycle.

• The ON and OFF arrangement can be had by a thermostat switch.

• Which switches ON and OFF the supply at predetermined temperatures.

• Final temperature attained is proportional to ratio.

• The method is more efficient than series impedance method.

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Summary

In this class we have discussed about

• Methods of temperature control of resistance heating

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Quiz

1. When four elements connected in parallel produce heat

energy equal to the furnace. If the same four elements

are arranged in two paths then the heat produced will

be__________ that of the rated value.

a. Half

b. One-fourth

c. One-sixteenth

d. Equal

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Frequently Asked Questions

1) List the methods of controlling the temperature in resistance heating.

2) Explain the Auto T/F method of controlling the temperature.

3) Explain the method of controlling the temperature by varying the number of elements