ch-3measuring instruments eepw

7/25/2019 Ch-3Measuring Instruments EEPW

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Chapter-3

Electrical Measuring Instruments &

MeasurementsIntroduction

Tests and measurements are important in designing evaluating, maintaining

and servicing electrical circuits and equipment. In order to detect electrical quantities

such as current, voltage, resistance or power, it is necessary to transform an electrical

quantity or condition into a visible indication. This is done with the aid of instrumentsthat indicate the magnitude of quantities either by the position of a pointer moving

over a graduated scale (analogue instrument) or in the form of a decimal number

(digital instrument).

Analogue Instruments

The main devices in an analogue instrument are deflecting device, controlling deviceand damping device.

(a)

Deflecting or operating device: A mechanical force produced by current andvoltage causes the pointer to deflect from its zero position.

(b)Controlling device: The controlling force acts in opposition to the deflecting

force. It ensures that the deflection shown on the meter is same for a given

measured quantity. Also prevents pointer going to maximum deflection.

Types: Spring control and gravity control.

Damping device:The damping force ensures that the pointer comes to rest in

its final position quickly and without oscillation. There are three main typesof damping used.

Types: Eddy current, Air friction and Fluid friction damping

Moving Iron Instrument:

Attraction Type Fig.(a)When current flows in the solenoid, a pivoted soft iron disc is attracted

towards the solenoid and the movement causes a pointer to move across a scale.

The force moving the pointer I2

Repulsion Type Fig.(b)

As in fig. 2 pieces of iron are placed inside the solenoid. One is fixed and the

other is attached to the pointer. When current passes through the solenoid, the 2

pieces of iron are magnetized in the same direction and therefore repel each other.The pointer thus moves across the scale.


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*

Fig. Moving Iron Instrument

Moving Coil Instrument

A moving coil instrument operates on the motor principle. When a conductor

carrying current is placed in a magnetic field, a force F is exerted on a conductor,given by F=BIL where B-flux density, L-conductor length, I-current flowing in the

conductor.

**

*http://www.electrical-engineering-assignment.com/repulsion-type## http://yourelectrichome.blogspot.com/2014/05/moving-iron-instruments.html

** http://electrical4u.com/electrical-measuring-instruments-types-accuracy-

precision-resolution-speed/


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Comparison of moving coil and moving iron instruments


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Ammeters and voltmeters

Voltmeters and ammeters are devices used to measure the electrical

characteristics of parts of a circuit.

Ammeters measure current.

Voltmeters measure voltage (potential difference).

Ammeters

Ammeters are always wired in series to the circuit.

*Since the current in a series circuit is the same all the way, the current

measure with the ammeter is also the current flowing through the resistor. If the ammeter was wired in parallel, part of the current would branch

off into the ammeter, part into the resistor, and we can not have anaccurate reading of the current in the circuit.

Since the ammeter is in series in the circuit it must has the lowestresistance possible.

If it has very much resistance at all, it would increase the overall

resistance of the circuit and change the current what we are

measuring.

Voltmeters

Voltmeters are always wired in parallel to the circuit.

*

* www.studyphysics.ca/30/voltamm.pdf


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Anything that is connected in parallel has the same voltage acrosseverything, so the voltage drop measured across the resistor is

accurate.

If there is no source of electricity or resistance between theconnections of the voltmeter it will read zero, since no potential

difference exists between the two points.

If the voltmeter was wired in series, it would have a voltage drop

of its own, but would not be able to measure the potentialdifference between two points in the circuit.

Since the voltmeter is in parallel, voltmeters must always havevery high resistances.

Shunts and Multipliers

Shunts and Multipliers are the resistances connected in parallel orseries with the voltmeters or ammeters to enhance their measuring capacity.

Shunts

A resistance placed in parallel with an instrument (or Galvanometer)to control the current passing through it, when placed in a circuit carrying afairly large current is called a shunt. The resistance of the shunt should not

vary with time.For shunt, Iara= IsRs

**

Multipliers

A high resistance in series with a galvanometer is connected, to limit thecurrent flowing through the meter so that it does not exceed the value for full

scale deflection and thus prevents the instrument from being damaged. Sucha resistance is called multiplier.

For Multiplier

,

**

**A Text book on Electrical-and-Electronic-Principles-and-Technology-Third-

Edition by john bird.


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Practical Multirange Voltmeter

The range of the basic d.c. voltmeter can be extended by using number ofmultipliers and a selector switch. Such a meter is called multi range voltmeter

#

#http://yourelectrichome.blogspot.com/2014/07/multirange-voltmeter.html

Ohmmeters

The purpose of an ohmmeter, of course, is to measure the resistanceplaced between its leads. This resistance reading is indicated through a

mechanical meter movement which operates on electric current. Theohmmeter must then have an internal source of voltage to create the

necessary current to operate the movement, and also have appropriateranging resistors to allow just the right amount of current through themovement at any given resistance.

*

*http://electriciantraining.tpub.com/14188/css/14188_94.htm


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The megger

Megger is a portable instrument which is used to measure insulation

resistance of the electrical machinery or system. It can be battery operated ormechanically operated (hand crank dc generator) and gives direct reading in

ohms. An ohmmeter designed for measuring high values of resistance (i.e.

megohms) is called an insulation resistance tester (e.g. Megger).

CIRCUIT DIAGRAM

**

(1) Deflecting & Control coil: Connected parallel to the generator,

mounted at right angle to each other and maintain polarities in such a way toproduced torque in opposite direction.

2) Permanent Magnets: Producemagnetic field to deflect pointer with

North-South pole magnet.

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 themegger from range zero to

infinity, enable us to read the value.

5) D.C generator orBattery connection: Testingvoltage is produced by

hand operatedD.C generator for manual operated Megger.Battery /

electronicvoltage charger is provided for automatic type Megger for same

purpose.

** http://avstop.com/ac/apgeneral/ohmmeters.html
http://www.electrical4u.com/what-is-magnetic-field/http://www.electrical4u.com/megger-working-principle-types-history-uses-of-megger/http://www.electrical4u.com/battery-history-and-working-principle-of-batteries/http://www.electrical4u.com/voltage-or-electric-potential-difference/http://www.electrical4u.com/principle-of-dc-generator/http://www.electrical4u.com/battery-history-and-working-principle-of-batteries/http://www.electrical4u.com/voltage-or-electric-potential-difference/http://avstop.com/ac/apgeneral/ohmmeters.htmlhttp://avstop.com/ac/apgeneral/ohmmeters.htmlhttp://www.electrical4u.com/voltage-or-electric-potential-difference/http://www.electrical4u.com/battery-history-and-working-principle-of-batteries/http://www.electrical4u.com/principle-of-dc-generator/http://www.electrical4u.com/voltage-or-electric-potential-difference/http://www.electrical4u.com/battery-history-and-working-principle-of-batteries/http://www.electrical4u.com/megger-working-principle-types-history-uses-of-megger/http://www.electrical4u.com/what-is-magnetic-field/


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A hand driven dc generator, G, which supplies the necessary current for

making the measurement

6) Pressure coil resistance and Current coil resistance: Protect

instrument from any damage because of low externalelectrical

resistanceunder test.

Working

Coil A is connected in series with R3 and the unknown resistance, Rx, to

be measured. Coil B is connected in series with R2 .

If the terminals are open circuited, no current flows in coil A, and the

current in coil B alone controls the movement of the moving element.

When a resistance is connected between the terminals, current flows

in coil A, tending to move the pointer clockwise. At the same time,coil B tends to move the pointer counterclockwise.

Therefore, the moving element comes to rest at a position at which

the two forces are balanced.

This position depends upon the value of the external resistance.

ANALOGUE AND DIGITAL MULTIMETER

Multimeters are An instrument designed to measure electrical

quantities. A typical multimeter can measure alternating- and direct-current

potential differences (voltages), current, and resistance, with several full-scale ranges provided for each quantity.

*

*https://rodzah.files.wordpress.com/2011/07/topic_2-1f37518.pdf
http://www.electrical4u.com/electrical-resistance-and-laws-of-resistance/http://www.electrical4u.com/electrical-resistance-and-laws-of-resistance/http://www.electrical4u.com/electrical-resistance-and-laws-of-resistance/http://www.electrical4u.com/electrical-resistance-and-laws-of-resistance/http://www.electrical4u.com/electrical-resistance-and-laws-of-resistance/http://www.electrical4u.com/electrical-resistance-and-laws-of-resistance/


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Wattmeter

The wattmeter is an indicating type instruments, generally used for

power measurement of the electrical circuit . A wattmeter consists of(1)

a low resistance current coil which is inserted in series with the line

carrying the current and(2) a high resistance pressure coil which is connected across the twopoints whose potential difference is to be measured.

**

Energy meter

An energy meter is a device that measures the amount of electricalenergy consumed by a residence, business, or an electrically powered

device.The most common unit of measurement on the electricity meter is the

kilowatt hour,which is equal to the amount of energy used by a load of one

kilowatt over a period of onehour,or 3,600,000joules.

Energy is the product of power and time for which power is used (i.e)

Energy=powertime

Principle of operation

The meters fall into two basic categories, electromechanical andelectronic.

The electromechanical induction meter operates by counting the

revolutions of an aluminium disc which is made to rotate at a speedproportional to the power. The number of revolutions is thus proportional to

the energy usage. The metallic disc is acted upon by two coils. One coil isconnected in such a way that it produces a magnetic flux in proportion to the

voltage and the other produces a magnetic flux in proportion to the current.The type of meter described above is used on a single-phase AC supply.
http://en.wikipedia.org/wiki/Kilowatt_hourhttp://en.wikipedia.org/wiki/Kilowatthttp://en.wikipedia.org/wiki/Hourhttp://en.wikipedia.org/wiki/Joulehttp://en.wikipedia.org/wiki/Joulehttp://en.wikipedia.org/wiki/Hourhttp://en.wikipedia.org/wiki/Kilowatthttp://en.wikipedia.org/wiki/Kilowatt_hour


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**A Text book on Electrical-and-Electronic-Principles-and-Technology-Third-

Edition by john bird.

**

Creep: In an induction type meter, creep is a phenomenon that can adverselyaffect accuracy that occurs when the meter disc rotates continuously with

potential applied and the load terminals open circuited. A test for error dueto creep is called a creep test.

DIGITAL INSTRUMENTS:

The digital measuring instruments indicate the values of the quantity indigital format that is in numbers, which can be read easily.

Examples of digital instruments:

Digital voltmeter

Digital millimeter

Digital frequency meter Digital measurement of time etc.

Digital voltmeter (DVM):

Any digital instrument requires analog to digital converter at its input.

Hence first block in a general DVM is ADC.

Types of DVM

1.

Ramp type DVM

2. Dual slope DVM3.

Successive approximation type DVM

**A TEXT BOOK OF ELECTRICAL TECHNOLOGY BY RAJPUT


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Successive approximation type DVM

A D/A converter is used to provide the estimates. The "equal to orgreater than" or "less than" decision is made by a comparator. The D/A

converter provides the estimate and is compared to the input signal. A

special shift register called a successive-approximation register (SAR) is

used to control the D/A converter and consequentially the estimates. At thebeginning of the conversion all the outputs from the SAR are at logic zero.

If the estimate is greater than the input, the comparator output ishigh and the first SAR output reverses state and the second

output changes to a logic "one."

If the comparator output is low, indicating

that the estimate is lower than the input signal, the first outputremains in the logic one state and the second output assumes

the logic state one. This continues to all the states until theconversion is complete.

**http://www.allsyllabus.com/aj/note/EEE/Electric_and_Electronic_Measurement_and_Instrum

entation/unit5/Successive-approximation%20DVM.php#.VT8tedKqqko

Problems


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1. Calculate the power dissipated by the voltmeter and by resistor R when (a)R=300 and (b) R=3M. The voltmeter sensitivity (figure of merit) is 12k/V,

f.s.d=200V.

2. For the figure below, determine the (a) Power dissipated in the load (b) Wattmeter

reading

3. A 0-1A ammeter having a resistance of 50 is used to measure the current flowingin a 1k resistor when the supply voltage is 250V. Calculate: (a) the approximate

value of current (neglecting the ammeter resistance), (b) the actual current in the

circuit, (c) the power dissipated in the meter, (d) the power dissipated in 1k resistor.

4. Electrical equipment in an office takes a current of 13A from a 240V supply.

Estimate the cost per week of electricity if the equipment is used for 30 hours each

week and 1 kWh of energy costs 12.5p.

5. An electric heater consumes 3.6 MJ when connected to a 250V supply for 40 minutes.Find the power rating of the heater and the current taken from the supply.

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