Download EET-321 (2)-SWITCHGEAR AND PROTECTION

Transducer
Device when activated from one form energy
converted to another quantity
or
Device which converts one physical quantity
or condition to another
Physical quantity – heat , intensity of light,
flow rate, liquid level, humidity etc
Sensor : sense physical quantity
Classification of transducers
1.
2.
3.
4.
Based on principle of transduction
Active & passive
Analog & digital
Inverse transducer
Based on principle used
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Thermo electric
Magneto resistive
Electro kinetic
Optical
Passive transducer
Device which derive power reqd. for
transduction from auxiliary power
source
- externally powered
Eg : resistive, inductive, capacitive
Without power they will not work
Active transducer
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No extra power reqd. to produce I/p
Self generating
Draw power from input applied
Eg. Piezo electric x’tal used for accelartion
measurement
Analog transducer
• convert I/p quantity into an analog o/p
• Analog o/p- a continuous fn. Of time
• Eg. Strain gauge, L VDT, thermocouple
Digital transducer
• Converts I/p into an electrical o/p in the
form of pulses
Inverse transducer
• Which converts electrical signal to physical
• quantity
Transducer for pressure
measurement
• What is pressure?
force/ unit area
Unit – (N/sq.m) Pa
Pressure measured can be absolute , gauge or
differential depending on type of reference
Types of pressure transducers
• Gravitational
eg : manometer
• Elastic – force converted to strain
eg. Diaphragm, capsule, bellows,
bourdon tubes
Diaphragm
Common pressure sensing elastic element
Thin circular plate stretched & fastened at its
periphery
Made of elastic alloys of bronze,phosphor
bronze, stainless steel or alloys like Monel,
Nickel span -C
Structure – flat or corrugated
Top view of flat type
Top view of corrugated type
• Flat type – high natural frequency
• Corrugated – for large deflections
Capsule
Capsule
• Two corrugated metal diaphragms sealed
together at periphery
• Forms a shell like structure
• One diaphragm has a port at centre to admit
pressure to be measured
• Other diaphragm linked to a moveable
mechanical part.
Capsule
• Displacement proportional to difference b/w
outer & inner pressure.
Bourdon tubes
• curved or twisted metallic tubes with
elliptical cross section
• Sealed at one end
• Tends to straighten when pressure applied.
• Angular sensitivity proportional to pressure
applied
Spirial bourdon tubes
Bourdon tubes –C type
Helical type
• Tube in the form of helix
Bourdon tube
• More sensitive to shock & vibrations
• Good repeatability
Bellows
Bellows
• Thin walled cylindrical sheets with deep
convolutions sealed at one end
• Sealed end moves axially when pressure is
applied
• No. of convolution s – vary from- 2 to 50 –
depends on range, operating temp
• Used for low pressure measurement
Bellows
displacement y = 2.n. A q P Rx 2 /( Et 2 .)
where
n – no. of convolutions
A q- effective area
Et - young’s modulus of elasticity
Rx – radius of diaphragm
P – pressure
ie, Y  P
Resistive type
Eg. Strain gauge
Capacitive
Inductive type
• Eg. LVDT
Measurement of velocity
Velocity – linear or angular
Linear velocity
• Electromagnetic transducers
• O/p voltage E =  /t
•
= N I/ R2 . dR/dt
where N I/ R2 is a constant
R- reluctance
so E proportional to reluctance
R proportional to air gap & air gap proportional to
velocity
• Types
• Moving coil
• Moving magnetic
Angular velocity
• Tachometer - types
Mechanical
Electrical
MHD sensor – magneto hydrodynamic sensor
highly sensitive
High precision
Transducer for vibration
• Vibrations give early warning of impending
conditions which may develop &vlead to
complete failure & destruction of equipment
• Used in power plants, turbines
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Most vibrations are sinusodial in nature
Got amp & freq.
Amp gives displacement
By measuring displacement, velocity or
acceleration - vibrations measured
Vibration transducer
Accelometer – measures shock or vibration
Pot type or LVDT type
Pot type accelrometer
LVDT accelorometer
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