This document discusses measuring instruments used in electrical systems. It describes the classification of analog and digital instruments and the essential features of measuring instruments, which include the deflecting, controlling, and damping systems. It also provides details on various types of instruments like permanent magnet moving coil (PMMC) instruments, and how instrument ranges can be extended using ammeter shunts or voltmeter multipliers. PMMC instruments are described as having advantages like high sensitivity, uniformly divided scales, and low power consumption, but disadvantages like inability to measure AC and higher cost. Methods to use a given PMMC instrument as an ammeter with a 0-100A range or voltmeter with a 0-750V range are also presented
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Measuring Instruments
To monitor the operating variables that define the condition
of the supply system.
To govern the Quality of electric energy supplied.
For the purpose of billing the customers.
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Classification of Instruments
Analog instruments:
Mechanical representation is used as the analogue of an electrical
quantity.
Relatively cheaper
Accuracy is less
Digital instruments:
Used where accuracy is required.
Analog input is converted to BCD.
Used in Electronic and communication
circuits
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Essential features of measuring Instruments
Deflecting system:
The deflection produced by the operating torque is proportional to the
magnitude of the electrical quantity such as current, voltage, etc. being
measured.
The deflecting torque causes the moving mechanism to move from its initial
zero position.
Controlling system:
Torque produced by the controlling system is in opposition to the
deflecting torque.
Pointer comes to rest when deflecting and controlling torques are equal.
Damping system:
To minimize the oscillations in the deflecting system.
Air friction damping, Eddy current damping
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Principle: A coil which carries the operating current is attached to the
moving system. With the movement of the coil, the pointer moves over the
scale to indicate the electrical quantity to be measured.
Permanent magnet Moving coil (PMMC) Instruments
Construction:
•Rectangular coils of several turns
-aluminum former
•Coil is pivoted
•Controlling system: two phosphor
bronze hair springs
•Eddy current damping
Counter
weight
Scale
Pointer
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Working of PMMC
Mechanical force experienced by the coil
F = NBIL newton
Deflecting Torque Td α I
Controlling torque Tc α θ
At equilibrium Td = Tc
Therefore, θ α I
Hence, Scale is Uniform (Linear)
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•High sensitivity
•Scale is uniformly divided.
•The power consumption is very low.
•Not affected by stray magnetic fields.
Permanent magnet Moving coil (PMMC) Instruments
•Cannot be used for ac.
•Cost is higher.
Disadvantages:
Advantages:
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Ammeter shunts
Shunt is a very low resistance connected across the basic meter.
Rm = internal resistance of the basic meter.
Rsh = Resistance of the shunt
Im = full scale deflection of basic meter.
I = Current to be measured.
Extension of meter range:
I
ImIsh
Basic metershunt
Rm
Rsh
m
mm
sh
II
RI
R
−
=
mmshm RIRII =− )(
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Voltmeter multipliers
Multiplier is a very high resistance in series with the basic meter.
Rm = internal resistance of the basic meter.
Rs = Resistance of the multiplier.
Im = full scale deflection of basic meter.
v= Voltage across the meter for current Im
V = Full range voltage of instrument.
V
v
Rs
Rm
Im
m
mm
s
I
RIV
R
−
=
Extension of meter range:
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A permanent magnet moving coil instrument gives full scale
reading of 25mA when potential difference across the terminals
is 75mV. Show how can it be used as
(i)an ammeter for a range of 0-100A
(ii)a voltmeter for a range of 0-750V.
Ans: 0.00075Ω, 29997Ω
Illustration:
