EE1008- ELECTRICAL AND ELECTRONICS MEASUREMENTS AND INSTRUMENTATION UNIT -1 Measurement of R, L, C 10/21/2015www.noteshit.com1

Wheatstone bridge 10/21/2015www.noteshit.com2

Balance Condition 10/21/2015www.noteshit.com3

Bridge balanced equation 10/21/2015www.noteshit.com4

Sensitivity Current Sensitivity:- Voltage Sensitivity:- Bridge Sensitivity:- 10/21/2015www.noteshit.com5

Under Small Unbalance 10/21/2015www.noteshit.com6

Under Small Unbalance 10/21/2015www.noteshit.com7

Under Small Unbalance 10/21/2015www.noteshit.com8

Thevenin’s voltage 10/21/2015www.noteshit.com9

Sensitivity under unbalance 10/21/2015www.noteshit.com10

Kelvin’s Bridge 10/21/2015www.noteshit.com11

One of the major drawback of the Wheatstone bridge is that, it can measure the resistance from few ohm to several mega ohm but to measure low resistance it gives significant error.Wheatstone bridgeresistance So, we need some modification in Wheatstone bridge itself, and the modified bridge so obtained is Kelvin bridge, which is not only suitable for measuring low value of resistance but has wide range of applications in the industrial world.Wheatstone bridgeresistance The kelvin’s bridge may be regarded as a modified of the wheatstone bridge to secure increased accuracy in the measurement of low resistance. It is used to measure resistance from few micro ohm to 1.0 ohm. 10/21/2015www.noteshit.com12

Bridge Balance Equation /21/2015www.noteshit.com13

From Equ Sub. Equ. 5 in 4 From 5 Sub 7 in Sub. equ. 7 & 10 in equ /21/2015www.noteshit.com14

Kelvin’s Double Bridge 10/21/2015www.noteshit.com15

The kelvin’s double bridge incorporates the idea of a second set of ratio arms - hence the name double bridge- and the use of four terminal resistors for the low resistance arms. 1 st R 1 /R 2 2 nd a/b 10/21/2015www.noteshit.com16

Balance condition Consider the path back to 5 through the battery Sub equ. 4 in equ. 2 10/21/2015www.noteshit.com17

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Again we reaches to the same result i.e. t has no effect. It indicates that the resistance of connecting lead has no effect on the measurement provided that the two sets of ratio arms have equal ratios. 10/21/2015www.noteshit.com20

Practical Kelvin’s double Bridge 10/21/2015www.noteshit.com21

AC Bridges When the four resistive arms of the basic Wheatstone bridge are replaced by impedances and the bridge is excited by an AC source, the result is an AC Bridge. To balance the bridge, two conditions must be satisfied, the resistive (R) and the reactive components (X C or X L ). Once balanced, the AC Bridge indicates a null. AC bridge circuits are also used for shifting phase, providing feedback paths for oscillators and amplifiers, filtering out undesired signals, and measuring the frequency of audio and radio frequency (RF) signals. 10/21/2015www.noteshit.com22

Bridge balance condition In admittance form Polar form of impedance Sub. The polar values in balance condition 10/21/2015www.noteshit.com23

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Maxwell's Bridge – Inductor L Maxwell’s Inductance Bridge Maxwell’s Inductance Capacitance Bridge 10/21/2015www.noteshit.com25

1 2 Maxwell’s Inductance Bridge 10/21/2015www.noteshit.com26

3 4 10/21/2015www.noteshit.com27

Maxwell’s Inductance Capacitance Bridge 10/21/2015www.noteshit.com28

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Quality Factor 10/21/2015www.noteshit.com30

Disadvantage: Maxwell bridgeMaxwell bridge is that, they are unsuitable of measuring the low and high quality factor coils. 10/21/2015www.noteshit.com31

Anderson's Bridge Need of Anderson's bridge though we have Maxwell bridge to measure quality factor of the circuit.Maxwell bridge The main disadvantage of using Maxwell bridge is that, they are unsuitable of measuring the low and high quality factor.Maxwell bridge However Maxwell bridge are suitable for measuring accurately medium quality factor respectively.Maxwell bridge So, there is need of bridge which can measure low quality factor and this bridge is modified Maxwell's bridge and known as Anderson's bridge.Maxwell's bridge 10/21/2015www.noteshit.com32

Convert star connected R2, R4, r into delta connected 10/21/2015www.noteshit.com33

Balance Equation star to delta connection equivalent 10/21/2015www.noteshit.com34

Sub. R 5 and R 6 values in the balance equation 10/21/2015www.noteshit.com35

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This bridge is used to measure to the capacitance of the capacitor, dissipation factor and measurement of relative permittivity. Schering Bridge 10/21/2015www.noteshit.com37

Balance equation 10/21/2015www.noteshit.com38

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Measurement of high Resistance - Megger Insulation resistance quality of an electrical system degrades with time, environment condition i.e. temperature, humidity, moisture & dust particles.resistance It also get impacted negatively due to the presence of electrical & mechanical stress, so it’s become very necessary to check the IR (Insulation resistance) of equipment at a constant regular interval to avoid any measure fatal or electrical shock.resistance 10/21/2015www.noteshit.com40

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1)Deflecting & Control coil : Connected parallel to the generator, mounted at right angle to each other and maintain polarities in such a way to produced torque in opposite direction. 2) Permanent Magnets: Produce magnetic field to deflect pointer with North-South pole magnet.magnetic field 3) Pointer : One end of the pointer connected with coil another end deflects on scale from infinity to zero. 4) Scale : A scale is provided in front-top of the megger from range ‘zero’ to ‘infinity’, enable us to read the value. 10/21/2015www.noteshit.com42

5) D.C generator or Battery connection : Testing voltage is produced by hand operated D.C generator for manual operated Megger. Battery / electronic voltage charger is provided for automatic type Megger for same purpose.D.C generatorBattery 6) Pressure coil resistance and Current coil resistance : Protect instrument from any damage because of low external electrical resistance under test.electrical resistance 10/21/2015www.noteshit.com43

Working Principle of Megger Voltage for testing produced by hand operated megger by rotation of crank in case of hand operated type, a battery is used for electronic tester. 500 Volt DC is sufficient for performing test on equipment range up to 440 Volts. 1000V to 5000V is used for testing for high voltage electrical systems. Deflecting coil or current coil connected in series and allows flowing the electric current taken by the circuit being tested.electric current The control coil also known as pressure coil is connected across the circuit. 10/21/2015www.noteshit.com44

Current limiting resistor (CCR & PCR ) connected in series with control & deflecting coil to protect damage in case of very low resistance in external circuit.resistance In hand operated megger electromagnetic induction effect is used to produce the test voltage i.e. armature arranges to move in permanent magnetic field or vice versa.voltage Where as in electronic type megger batteries are used to produce the testing voltage. As the voltage increases in external circuit the deflection of pointer increases and deflection of pointer decreases with a increases of current. Hence, resultant torque is directly proportional to voltage & inversely proportional to current. 10/21/2015www.noteshit.com45

When electrical circuit being tested is open, torque due to voltage coil will be maximum & pointer shows ‘infinity’ means no shorting throughout the circuit and has maximum resistance within the circuit under test.resistance If there is short circuit pointer shows ‘zero’, which means ‘NO’ resistance within circuit being tested.resistance The deflection torque is produced with megger tester due to the magnetic field produced by voltage & current, similarly like ‘Ohm's Law’Ohm's Law Torque of the megger varies in ration with V/I, (Ohm's Law :- V=IR or R=V/I). Electrical resistance to be measured is connected across the generator & in series with deflecting coil.Electrical resistance 10/21/2015www.noteshit.com46

Produced torque shall be in opposite direction if current supplied to the coil. 1. High resistance = No current: - No current shall flow through deflecting coil, if resistance is very high i.e. infinity position of pointerresistance 2. Small resistance = High current :- If circuit measures small resistance allows a high electric current to pass through deflecting coil, i.e. produced torque make the pointer to set at ‘ZERO’.resistance electric current 3. Intermediate resistance = varied current: - If measured resistance is intermediate, produced torque align or set the pointer between the range of ‘ZERO to INIFINITY’resistance 10/21/2015www.noteshit.com47

Loss of Charge Method 10/21/2015www.noteshit.com48

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