Therocople ppt
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Thermocouple instruments work by generating a voltage at the junction of two different metals when heated. This voltage is proportional to temperature and is used to measure current. There are different types - contact uses a separate heater wire touching the thermocouple junction, non-contact separates them with insulation, and vacuum encloses them in glass to reduce cooling. Bridge type connects multiple thermocouples to form a bridge circuit for greater sensitivity. Advantages include accurate RMS measurement, immunity to stray fields, and wide frequency range. Disadvantages include high temperatures needed that could damage the instrument.
![ Principle of operation
Loss through heater coil I2R
Assuming temp. of cold junction maintained at room temp.
Then,
(Ta-Tb) ∝ I2R
Or, (Ta-Tb)= kI2R
So the Deflection angle of PMMC ,
x=KE=K[a(Ta-Tb)]
hence we can write k.K.a.I2R = k1I2, where k1 is some constant.
Instrument shows the square law of response](https://image.slidesharecdn.com/b482bbb4-407a-4ee7-9ac2-1ac1e8dc9594-150409205857-conversion-gate01/85/Therocople-ppt-5-320.jpg)
Therocople ppt
- 1. THERMOCOUPLE INSTRUMENTS PRINCIPLE OF OPERATION CONSTRUCTION THERMOELEMENTS ADVANTAGES DISADVANTAGES
- 2. What is Thermocouple? Thermocouple consists of two different metals which are placed in contact each other & a voltage is generated at the junction which is nearly proportional to the temperature .This junction is called a Thermocouple.
- 3. Purpose Of Operation: It is used to convert heat energy to electrical energy as shown in above fig. Due to the heater element temp. will increase at the junction & this causes production of an EMF (DC voltage) which is directly proportional to the root mean square value of electric current. A PMMC instrument is connected to the second part to read the current passing through heater
- 4. Principle of operation Generated EMF, E = a (Ta – Tb) + b (Ta – Tb)2 Where , Ta= temp. of heater element Tb= temp. of cold metal a & b are const. depends upon the type of metal we are using The above eqn. shows Parabolic fn. Value of b can be neglected if their air gap field of PMMC is uniform, E=a(Ta-Tb) ,b=0
- 5. Principle of operation Loss through heater coil I2R Assuming temp. of cold junction maintained at room temp. Then, (Ta-Tb) ∝ I2R Or, (Ta-Tb)= kI2R So the Deflection angle of PMMC , x=KE=K[a(Ta-Tb)] hence we can write k.K.a.I2R = k1I2, where k1 is some constant. Instrument shows the square law of response
- 6. Precaution • Measurement of current and voltage of high frequencies. So the instruments should be free from skin effect. • In order to reduce skin effect a very fine wire of non-magnetic material having a high resistivity Reason Behind using PMMC: Greater accuracy & sensitivity towards the measurement of DC value.
- 7. Contact Type • Have a separate heater wire and thermocouple has one junction(hot) in contact with heater wire.
- 8. Non Contact type • There is no electric contact between the heating element & the thermocouple. • This two are separated by electric insulation. • This is make the instrument sluggish & less sensitive than contact type. • This type of instrument basically use for measurement of currents in circuit operated at above the ground potential
- 9. Vacuum type • In order to attain a temperature rise (80c to 100c)needed to develop the required output in the thermo-couple instrument ,we must increase the efficiency & sensitivity of the thermo-element. • For that cause we enclose both the thermo-couple & heater in glass tube. • Efficiency in increased cause there is no cooling of heater due of convection of air current. • For this case heater element & thermo-couple element also remain isolated from each other by a bead of insulating material. • This is done to minimize the effect of capacitance between the thermo-couple & the rest of the circuit at high frequencies.
- 10. Vacuum type
- 11. Bridge type • For this case no heater element is used. • The thermo-couples connected to form a bridge • The current to be measured passes directly through the thermo-couple raises there temperature proportional to I2R • This thermo-couple gives greater output than is given by single thermo-couple used in vacuum thereby giving greater sensitivity. • It is much robust in withstanding mechanical shocks & overloads.
- 12. Bridge Type
- 13. ADVANTAGES 1. THE THERMOCOUPLE INSTRUMENTS CORRECTLY INDICATES THE RMS VALUE OF VOLTAGE OR CURRENT IRRESPECTIVE OF THE WAVE FORM. 2. THESE INSTRUMENTS ARE NOT AFFECTED BY STRAY MAGNETIC FIELDS. 3. THESE INSTRUMENTS ARE FREE FROM FREQUENCY ERRORS AND HENCE THEY CAN BE USED OVER A VERY WIDE FREQUENCY RANGE. 4. THESE INSTRUMENTS HAVE HIGH SENSITIVITY. 5. THEY ARE VERY CAREFUL WHEN USED AS TRANSFER INSTRUMENT.
- 14. DISADVANTAGES 1. AT NORMAL RATED CURRENT THE HEATER ATTAINS THE TEMPERATURE OF 300C. 2. AT NORMAL CURRENT THE HEATER WOULD GIVE A TEMPERATURE OF NEARLY 4 TIMES THE NORMAL TEMPERATURE,1200C. 3. IT IS OBVIOUS THAT SQUARE LAW RATE WILL BRING THE HEATER TO NEARLY ITS BURN OUT TEMPERATURE. 4. THE FUSES DO NOT PROVIDE ANY PROTECTION AS DUE TO OVERLOAD THE HEATER WIRE MAY BURN OUT BEFORE FUSE BLOWS OUT.
- 15. Prepared by- • Suman Mahato • Koustav Seth • Sayak Mondal • Amitava Ghosh • Sayak Mukherjee • Srinjoy Kar Students of 2nd year Electrical Engineering Dept UNIVERSITY OF ENGINEERING AND MANAGEMENT,JAIPUR
- 16. Thank You