L41 instruments

Dept of E&E ELE101/102 1

Measuring Instruments


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.


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


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


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


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)


•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:


Ammeter shunts

Shunt is a very low resistance connected across the basic meter.

Rm = internal resistance of the basic meter.Rsh = Resistance of the shuntIm = full scale deflection of basic meter.I = Current to be measured.

Extension of meter range:

I

ImIsh

Basic metershunt

RmRsh

m

mmsh II

RIR

mmshm RIRII )(


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.

Vv

Rs

Rm

Im

m

mms I

RIVR

Extension of meter range:


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:

L41 instruments

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Transcript of L41 instruments