emt 462 electrical system technology chapter 4 : dc meters by: en. muhammad mahyiddin ramli

EMT 462EMT 462ELECTRICAL ELECTRICAL

SYSTEM SYSTEM TECHNOLOGTECHNOLOG

YY

EMT 462EMT 462ELECTRICAL ELECTRICAL

SYSTEM SYSTEM TECHNOLOGTECHNOLOG

YYChapter 4 :Chapter 4 :DC MetersDC MetersChapter 4 :Chapter 4 :DC MetersDC Meters

By:En. Muhammad Mahyiddin Ramli

Chap 4: DC Meters 2

Today’s Lecture Motivation

To familiarize the d’Arsonval meter movement, how it is used in ammeters, voltmeters, and ohmeters, some of its limitations (effects), as well as some of its applications.

Chap 4: DC Meters 3

After completing today topic, students should be able to…….

Explain the principle of operation of the d’Arsonval meter movement

Describe the purpose of shunts across a

meter movement and multipliers in series with a meter movement

Define the term sensitivity

Chap 4: DC Meters 4

Introduction

Meter: Any device built to accurately detect & display an electrical quantity in a

readable form by a human being.

Readable form

• Visual

• Motion of pointer on a scale

• Series of light (digital)

Chap 4: DC Meters 5

The D’Arsonval Meter

Hans Oersted (1777-1851)

Jacques d’Arsonval (1851-1940)

Danish physicist who discovered the relationship between current and magnetism – from the deflection of a compass needle

French physiologist who discovered the moving-coil galvanometer – from muscle contractions in frogs using a telephone, which operates on an extremely feeble currents similar to animal electricity

Chap 4: DC Meters 6

The D’Arsonval Meters

In 1880s, two French inventors: Jacques d’Arsonval and Marcel Deprez patented the moving-coil galvanometer.

Jacques d’Arsonval(1851 – 1940)

Marcel Deprez(1843 – 1918)

Deprez-d'Arsonval Galvanometer

Chap 4: DC Meters 7

Types of Instrument

• Permanent Magnet Moving-Coil (PMMC) – most accurate type for DC measurement

• Moving Iron

• Electrodynamometer

• Hot wire

• Thermocouple

• Induction Type

• Electrostatic

• Rectifier

Chap 4: DC Meters 8

The D’Arsonval Meter Movement

Fig 1-1 The d’Arsonval meter movement

The basic moving coil system generally referred to as a d’Arsonval meter movement or Permanent Magnet Coil (PMMC) meter movement.

Current-sensitive device capable of directly measuring only very small currents.

Its usefulness as a measuring device is greatly increased with the proper external circuitry.

Chap 4: DC Meters 9

Current from a circuit in which measurements are being made with the meter passes through the windings of the moving coil. Current through the coil causes it to behave as an electromagnet with its own north and south poles. The poles of the electromagnet interact with the poles of the permanent magnet, causing the coil to rotate. The pointer deflects up scale whenever current flows in the proper direction in the coil. For this reason, all dc meter movements show polarity markings.

Chap 4: DC Meters 10

D’Arsonval Used in

DC Ammeter


Since the windings of the moving coil are very fine wire, the

basic d’Arsonval meter movement has only limited

usefulness without modification.

One desirable modification is to increase the range of

current that can be measured with the basic meter

movement.

This done by placing a low resistance called a shunt (Rsh),

and its function is to provide an alternate path for the total

metered current, I around the meter movement.

D’Ársonval Meter Movement Used In A DC Ammeter


Basic DC Ammeter Circuit

In most circuits, Ish >> Im

Fig. 1-2 D’Ársonval meter movement used in ammeter

circuit

Where

Rsh = resistance of the shunt

Rm = internal resistance of the meter movement (resistance of the moving coil)

Ish = current through the shunt

Im = full-scale deflection current of the meter movement

I = full-scale deflection current for the ammeter

Ammeter


Cont’

Knowing the voltage across, and the current through, the shunt allows us to determine the shunt resistance as:

mm

m

msh

m

sh

mm

sh

shsh RII

I

RI

I

I

RI

I

VR

Ohm


Example 3.1

Calculate the value of the shunt resistance required to convert a 1-mA meter movement, with a 100-ohm internal resistance, into a 0- to 10-mA ammeter.


Solution

VmARIV mmm 1.01001

VVV msh 1.0

mAmAmAIII msh 9110

11.119

1.0

mA

V

I

VR

sh

shsh


Ayrton Shunt or Universal Shunt

William Edward Ayrton studied under Lord

Kelvin at Glasgow. In 1873 he was

appointed to the first chair in natural

philosophy and telegraphy at Imperial

Engineering College, Tokyo. In 1879 he

was the first to advocate power

transmission at high voltage, and with

John Perry (1850-1920) he invented the

spiral-spring ammeter, the wattmeter, and

other electrical measuring instruments.

The ammeter (a contraction of ampere

meter) was one of the first to measure

current and voltage reliably. They also

worked on railway electrification,

produced a dynamometer and the first

electric tricycle. British Engineer

William Edward Ayrton (1847-1908)


The Ayrton Shunt

The purpose of designing the shunt circuit is to allow to measure current, I that is some number n times larger than Im.

I = nIm

= 1nRm


Advantages of the Ayrton

Fig 1-3 Ayrton shunt circuit

Eliminates the possibility of the meter movement being in the circuit without any shunt resistance.

May be used with a wide range of meter movements.


Con’t

The individual resistance values of the shunts are calculated by starting with the most sensitive range and working toward the least sensitive range.

The shunt resistance is:

On this range the shunt resistance is equal to Rsh and can be computed by Eqn

cbash RRRR

1

n

RR m

sh


Con’t

2

)(

I

RRIRR mshm

cb

3

)(

I

RRIR mshm

c

)( cbsha RRRR

ccbb RRRR )(


D’Arsonval Used in

DC Voltmeter


The basic d’Ársonval meter movement can be converted to a dc voltmeter by connecting a multiplier Rs in series with the meter movement

The purpose of the multiplier: is to extend the voltage range of

the meter to limit current through the

d’Arsonval meter movement to a maximum full-scale deflection current.

Fig 1.4 The basic d’Arsonval meter Movement Used In A DC Voltmeter

D’Ársonval Meter Movement Used In A DC Voltmeter


Con’t

To find the value of the multiplier resistor, first determine the sensitivity, S, of the meter movement.

/V)( 1

ySensitivit fsI

Resistance InternalRange SRs


Example 3.2

Calculate the value of the multiplier resistance on the 50V range of a dc voltmeter that used a 500A meter movement with an internal resistance of 1k.


Solution

Sensitivity, VkI

Sfs

2500

11

Multiplier, Rs = S X Range – internal Resistance

= (2k X 50) – 1k = 99k


Voltmeter And Ammeter

Effect


Voltmeter Loading Effect

When a voltmeter is used to measure the voltage across a circuit

component, the voltmeter circuit itself is in parallel with the circuit

component.

Since the parallel combination of two resistors is less than either

resistor alone, the resistance seen by the source is less with the

voltmeter connected than without.

Therefore, the voltage across the component is less whenever the

voltmeter is connected.

The decrease in voltage may be negligible or it may be appreciable,

depending on the sensitivity of the voltmeter being used.

This effect is called voltmeter loading. The resulting error is called a

loading error.


Example 3.3

Two different voltmeters are used to measure the voltage across resistor RB in the circuit of Figure 2-2. The meters are as follows.

Meter A : S = 1k/V, Rm = 0.2k, range = 10VMeter B : S = 20k/V, Rm = 1.5k, range=10V

Calculate:a) Voltage across RB without any meter

connected across it.b) Voltage across RB when meter A is used.

c) Voltage across RB when meter B is used

d) Error in voltmeter readings.


Solution

(a) The voltage across resistor RB without either meter connected is found Using the voltage divider equation:

V5

5k25k

kΩ5V30

BA

BRB RR

REV


Solution

(b) Starting with meter A, the total resistance it presents to the circuit is:

The parallel combination of RB and meter A is:

Therefore, the voltage reading obtained with meter A, determined by the voltage divider equation, is:

V

RR

REV

Ae

eRB

53.3kΩ25kΩ33.3

kΩ33.3V30

1

1

kΩ33.310kΩ5kΩ

10kΩ5kΩ

1

TAB

TABe RR

RRR

kΩ10V10k/V1Range SRTA


Solution

(c) The total resistance that meter B presents to the circuit is:

RTB = S x Range = 20k/V x 10 V = 200 k

The parallel combination of RB and meter B is:

Re2 = (RB x RTB)/(RB + RTB) = (5kx200k)/(5k+200k) = 4.88 k

Therefore, the voltage reading obtained with meter B, determined by use of the voltage divider equation, is:

VRB = E(Re2)/(Re2+RA) = 30 V x (4.88k)/(4.88k+25k) = 4.9 V


Solution(d)

Voltmeter A error = (5 V – 3.53 V)/5 V x (100%

= 29.4%Voltmeter B error = (5 V – 4.9 V)/5 V x

(100%) = 2 %

%100 valueExpected

value)Measured- value(ExpectederrorA Voltmeter


Ammeter Insertion Effects

Inserting an ammeter in a circuit always

increases the resistance of the circuit and

reduces the current in the circuit.

This error caused by the meter depends on the

relationship between the value of resistance in

the original circuit and the value of resistance in

the ammeter.


Con’t

** For high range ammeter, the internal

resistance in the ammeter is low.

** For low range ammeter, the internal resistance

in the ammeter is high.


1R

EI e

Expected current value in a series circuit

mm RR

EI

1

Series circuit with ammeter


Con’t

Hence;

me

m

RR

R

I

I

1

1

Therefore;

%1001

e

m

I

IInsertion error =


Example 3.4

A current meter that has an internal resistance of 78 ohms is used to measure the current through resistor Rc in below circuit.

Determine the percentage of error of the reading due to ammeter insertion.


Solution

The current meter will be connected into the circuit between points X and Y in the schematic as shown above.

When we look back into the circuit from terminals X and Y, we can express Thevenin’s equivalent resistance as:

ba

bacTH RR

RRRR

RTH = 1 k + 0.5 k =

1.5 k


Solution

Therefore, the ratio of meter current to expected current:

Im/Ie= 1.5 k/(1.5 k + 78) = 0.95

Solving for Im yields, Im = 0.95Ie

Insertion error = [1 – (Im/Ie)] x 100% = 5.0%

me

m

rR

R

I

I

1

1


The Ohmmeter (Series Ohmmeter)

The ohmmeter consists of battery, resistor and PMMC.

*function of Rz and Rm are to limit the current through the meter.

mZfs RR

EI

The full-scale deflection current,

Basic ohmmeter circuit


Con’t

To determine the value of unknown resistor, Rx, The Rx is connected to terminal X and Y.

Above figure shows the basic ohmmeter circuit with unknown resistor, Rx connected between probes.

Basic ohmmeter circuit with unknown resistor, Rx connected between probes.

Rz = variable resistor


Con’t

The circuit current,

xmZ RRR

EI

The ratio of the current, I to the full-scale deflection current, Ifs is

xmZ

mZ

mZ

xmZ

fs RRR

RR

RRE

RRRE

I

IP


Summary

Basic d’Arsonval meter movement – current sensitive device capable of directly measuring only very small currents.

Large currents can be measured by adding shunts.

Voltage can be measured by adding multipliers. Resistance – adding battery and a resistance

network. All ammeters & voltmeters introduce some error –

meter loads the circuit (common instrumentation problem).


It is possible to fail in many ways....while to succeed its only possible in one way.

- Aristotle

emt 462 electrical system technology chapter 4 : dc meters by: en. muhammad mahyiddin ramli

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