Analog Meters Riandito Prakoso

Emir Kevin F

Lecturer :

Ir. Chairul Hudaya, ST, M.Eng., Ph.D., IPM

Electrical MeasurementsInternational Program

Department of Electrical Engineering

UNIVERSITAS INDONESIA

Outline

- Introduction

- Classification of Analog Instruments

- Principal Operation

- Operating Torque

- Construction Types of Analog Instruments

Introduction

An analog device is one in which the output or display is a continuous function of

time and bears a constant relation to its input.

Three general principles of operation are available:

(i) electromagnetic, (utilises the magnetic effects of electric currents)

(ii) electrostatic, (utilises the forces between electrically charged conductors)

(iii) electro-thermal, (utilises the heating effect)

Classification of Analog Instruments

Generally, analog instruments can be classified in two ways :

1. Absolute Instruments

1. Secondary Instruments

Principle of Operation

Analog instruments may be classified according to the principle of operation they

utilise. The effects are:

1. Magnetic Effect (moving iron, moving coil, electrodynamic)

2. Heating Effect (thermocouple, hotwire)

3. Electrostatic Effect (electrostatic voltmeter)

4. Electromagnetic Effect (induction wattmeter, induction energy meter)

5. Hall Effect

Operating Torque

There are three types of torques that needed for satisfactory operation of any

indicating instrument. Those are:

1. Deflecting torque

2. Controlling torque

3. Damping torque

Deflecting Torque

Any instrument’s deflection is found by the total effect of the deflecting torque,

control torque and damping torque.

The deflecting torque’s value is dependent upon the electrical signal to be

measured.

This torque helps in rotating the instrument movement from its zero position, it’s

called the deflecting system

Controlling Torque

Controlling torque works in the opposite of deflecting torque.

When the deflecting and controlling torques are equal in magnitude, then the

movement will be in definite position (equilibrium).

Spiral springs or gravity is usually given to produce the controlling torque. The

system which produces the controlling torque is called the controlling system.

Damping Torque

The moving system of the instrument will tend to move under the action of the deflecting

torque.

But the control torque, it will try to occupy a position of rest when the two torques are equal

and opposite.

Due to inertia of the moving system, the pointer will not come to rest immediately but

oscillate about its final deflected position and takes appreciable time to come to steady state.

To overcome this difficulty a damping torque is to be developed by using a damping device

attached to the moving system.

Construction Types of Analog Instruments

There are some construction types of analog measurement device based on the

principal operation, such as:

1. Permanent magnet moving coil

2. Moving iron

3. Electrodynamometer

4. Electrostatic

5. Electrothermal

6. Rectifier

7. True rms voltmeter

Permanent Magnet Moving Coil

The permanent magnet moving coil instrument or PMMC type instrument uses two

permanent magnets in order to create stationary magnetic field. These types of

instruments are only used for measuring the DC quantities as if we apply AC

current to these type of instruments the direction of current will be reversed during

negative half cycle and hence the direction of torque will also be reversed which

gives average value of torque zero. The pointer will not deflect due to high

frequency from its mean position showing zero reading. However it can measure

the direct current very accurate.

Let us move towards the constructions of permanent magnet moving coil instruments. We

will see the construction of these types of instruments in five parts and they are described

below:

Stationary Part or Magnet System: In the present time we use magnets of high field

intensities, high coercive force instead of using U shaped permanent magnet having soft

iron pole pieces. The magnets which we are using nowadays are made up of materials

like alcomax and alnico which provide high field strength.

Moving Coil: The moving coil can freely moves between the two permanent magnets as

shown in the figure given below. The coil is wound with many turns of copper wire and is

placed on rectangular aluminium which is pivoted on jeweled bearings.

Control System: The spring generally acts as control system for PMMC instruments.

The spring also serves another important function by providing the path to lead current

in and out of the coil.

Damping System: The damping force hence torque is provided by movement of

aluminium former in the magnetic field created by the permanent magnets.

Meter: Meter of these instruments consists of light weight pointer to have free

movement and scale which is linear or uniform and varies with angle.

Principal Operation

https://www.youtube.com/watch?v=ZtBKC6WSjD0

Moving iron instrument

This instrument is one of the most primitive forms of measuring and relay instrument. Moving iron type

instruments are of mainly two types. Attraction type and repulsion type instrument. Whenever a piece of

iron is placed nearer to a magnet it would be attracted by the magnet. The force of this attraction depends

upon the strength said magnetic field. If the magnet is electromagnet then the magnetic field strength can

easily be increased or decreased by increasing or decreasing current through its coil.

Accordingly the attraction force acting on the piece of iron would also be increased and decreased.

Depending upon this simple phenomenon attraction type moving iron instrument was developed.

Whenever two pieces of iron are kept side by side and a magnet is brought nearer to them the iron pieces

will repulse each other. This repulsion force is due to same magnetic poles induced in same sides the iron

pieces due external magnetic field.

Principal Operation

https://www.youtube.com/watch?v=L9wHaLyv94Q

Electrodynamometer type instrument

The electrodynamometer type instrument is a transfer instrument. A transfer

instrument is one which is calibrated with a d.c. source and used without any

modifications for a.c. measurements. Such a transfer instrument has same

accuracy for a.c. and d.c. measurements. The electrodynamometer type

instruments are often used in accurate a.c. voltmeters and ammeters, not only at

the power line frequency but also in the lower audiofrequency range. With some

little modifications, it can be used as a wattmeter for the power measurements.

Principal Operation

https://www.youtube.com/watch?v=z4zo7LHUtOc

Electrostatic instrument

As the name suggests the electrostatic type instrument use static electrical field to

produce the deflecting torque. These types of instrument are generally used for

the measurement of high voltages but in some cases they can be used in

measuring the lower voltages and powers of a given circuit

Force and torque equation

Advantage & Disadvantage

Advantages of Electrostatic Type Instruments

Now let us look at some advantages of electrostatic type instruments.

1. The first and the most important advantage is that we can measure both AC and DC voltage and the reason is very

obvious the deflecting torque is directly proportional to the square of the voltage.

2. Power consumption is quite low in these types of instruments as the current drawn by these instruments is quite low.

3. We can measure high value of voltage.

Disadvantages of Electrostatic Type Instruments

Instead of various advantages, electrostatic instruments posses few disadvantages and these are written below.

1. These are quite costly as compared to other instruments and also these have large size.

2. The scale is not uniform.

3. The various operating forces involved are small in magnitude.

Induction type instrument

Induction type instrument are only use for ac measurement and can be use for

either ammeter, voltmeter, and watt meter. However, the induction principle find its

widest application as watt hour or energy meter. In such instrument, the deflecting

torque is produced due to the reaction between the flux of an ac magnet and the

eddy current induced by another flux.

Principle of operation

https://www.youtube.com/watch?v=ldfJUL3JcaA

Electrothermal Instrument

Hot-wire instruments

An electrical measuring instrument (basically an ammeter) in which the current to

be measured is passed through a thin wire and causes its temperature to rise. The

temperature rise, which is proportional to the square of the current, is measured

by the expansion of the wire. Such instruments can be used for either direct

current or alternating current.

Advantage & Disadvantage of hot-wire

instrument+ Can be used for ac and dc system

+ Transfer type instrument meaning that the calibration is the sam in dc an ac

+ Free form stray magnetic field

+ Cheap in cost and simple construction

+ With suitable adjustment error due to temperature variation can be negligible

- High power consumption

- Nonuniform scale

- Takes time in action because of heating the wire

Thermo couple type instrument

A thermocouple is a device made by two different wires joined at one end, called junction end or measuring end. The two

wires are called thermoelements or legs of the thermocouple: the two thermoelements are distnguished as positive and

negative ones. The other end of the thermocouple is called tail end or reference end (Figure1). The junction end is

immersed in the enviroment whose temperature T2 has to be measured, which can be for instance the temperature of a

furnace at about 500°C, while the tail end is held at a different temperature T1, e.g. at ambient temperature.

Because of the temperature difference between junction end and tail end a voltage difference can be measured between

the two thermoelements at the tail end: so the thermocouple is a temperature-voltage transducer.

Advantage & Disadvantage

+ Not affected by stray magnetic field

+ Very high sensitivity

+ Can be used for measuring currents upto 50 MHz

+ Transfer instrument for calibration of dc instrument by potentiometer and a

standard cell

- Power loss due to poor efficiency of thermal conversion

- Low accuracy measurement and sensitivity oveloads

- Requires careful handling

Rectifier type instruments

Rectifier type instrument measures the alternating voltage and current with the help of rectifying elements and permanent

magnet moving coil type of instruments. However the primary function of rectifier type of instruments work as voltmeter.

Now one question must arises in our mind why we use rectifier type of instruments widely in the industrial world though we

have various other AC voltmeter like electrodynamometer type instruments, thermocouple type instruments etc?

1.Cost of electrodynamometer type of instruments is quite high than rectifier type of instruments. However rectifier type of

instruments as much accurate as electrodynamometer type of instruments. So rectifier type of instruments are preferred

over electrodynamometer type instruments.

2.The thermocouple instruments are more delicate than the rectifier type of instruments. However thermocouple type of

instruments is more widely used at very high frequencies

Sensitivity of rectifier type instrument

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The average value of a half-wave rectifier,

The sensitivity of half-wave rectifer instruments with ac is 0.45 times its sensitivity

with dc and the deflection is 0.45 times that produces with dc of equal magnitude

of V.

Average value of voltage/current for full wave rectifier,

The deflection is 0.9 times in full wave rectifier instrument with an ac than that

produced with dc of equal magnitude V.

Sensitivity of full wave rectifier instrument with an ac is 0.9 times its sensitivity with

dc.

Extention of range of rectifier instrument as voltmeter

Limitation

1.Rectifier instrument are only accurate on the waveform on which they are

calibrated, since calibration assumes pure sine waves the presences of harmonic

gives a pleasureable readings.

2.It is temperature sensitive, therefore it affect the instruments reading.

Application

1. Rectifier instrument is very suitable for measuring voltage of 50-250V

2. Micrometer or lo miliammeter(up to 10-15mA), not suitable for measuring

large current because in large current rectifier become too bulky and

providing shunt is impracticable due to rectifier characteristic

3. Rectifier instrument find their principal application in measurement in high

impedance circuit at low and audio frequencies. They are commonly use in

communication circuit because of their high sensitivy and low power

consumption.

True rms Voltmeter

As compared to the average and peak responding voltmeters the rmsresponding voltmeters present

special circuit design problems. RMS implies that the input quantity (say voltage in voltmeter) has to be

squared and then the square root of the average of the squared quantity is taken.

These meters are used to provide accurate rms readings of complex waveformsi.e., non-sinusoidal

waveforms having a crest factor of 10:1. Some of the applications are:

– Measurements of electrical or acoustical noise

– Low duty cycle pulse trains

– Voltages of undetermined waveforms

Disadvantages of a true rms-reading voltmeter

The accuracy of this technique has been difficult to control because of the

nonlinear behavior of the thermocouple which complicates the meter calibration,

Thermal variations & Sluggish response of the thermocouple which are also

susceptible to burnout also aggravate the problem.

Thermal variations are reduced by installing the heater and the thermo couple in

an evacuated glass bulb and by using fine wires of low thermal conductivity. Use

of null balance techniques reduces the effect of non linear behavior. Generally the

Nonlinear behavior of the measuring and feedback (balancing) thermocouples

cancel each other.

Comparison of different types of instruments