measuring instrument – APSEEE

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Induction Type Wattmeter

Induction Type Wattmeter

Induction type wattmeter can be used to measure as power only. In last article we have studied about dynamometer type wattmeter which can be used to measure dc as well as ac power.

Working Principle 

Induction type wattmeter works on induction principle.

Construction of induction type wattmeter

Induction type wattmeter consists of two laminated electromagnets. One is called series magnet and other is called shunt magnet. Series magnet is connected in series with the supply and carries the load current. Series magnet is made highly non-inductive so that angle of lag or lead is wholly determined by the load. Shunt magnet carries the current which is proportional to load voltage. This magnet is made highly inductive. So that the current in it lags behind the supply voltage by 90O.

A thin aluminum disc mounted on the spindle of the induction type wattmeter. It is mounted in such a way that it cuts the fluxes of both magnets. Hence, two eddy currents are produced in the disc. The controlling torque is provided by spiral springs. In this wattmeter, electro magnetic damping torque is provided. Shading rings are provided on the central limb of the shunt magnet.

induction type wattmeter


When induction type wattmeter is connected in the circuit to measure ac power, the current start flowing through both magnets. The shunt magnet carries the current proportional to the voltage across load while series magnet carries the load current. The fluxes produced by the series and shunt magnets induced eddy currents in the aluminum disc. The deflection torque is produced due to interaction of these eddy current and inducing fluxes. The deflecting torque on the disc causing the pointer connected to the moving system to move over the scale. The pointer comes to rest at a position where deflection torque becomes equal to controlling torque provided by spiral spring.

Deflecting Torque

Td α VI cos Φ


This is used to measure ac power.


Induction type wattmeter free from the effects of story field and have good damping.

Induction type wattmeter has fairly long scales.


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Permanent Magnet Moving Coil Instruments

Permanent Magnet Moving coil  Instruments (PMMC)

Permanent Magnet Moving coil  Instruments  are  moving coil instruments. These instruments are very sensitive and accurate. DC voltage or current can be measured by these instruments.

Principle of Operation

The operation of PMMC instrument is based on the principle when a current carrying conductor placed in a magnetic field, a mechanical force experienced by the conductor.

Construction of Permanent Magnet Moving coil Instruments

It consists of a permanent shoe magnet. Light rectangular coil of having many turns of fine wire wound on a light aluminum former. The coil acts as a moving element. This coil is mounted on a spindle. Two phosphor bronze spiral hair springs are attached to the spindle. The springs provide the controlling torque as well as they act as incoming and outgoing leads for the current. Eddy current damping is used and it is provided by the aluminum former.

permanent magnet moving coil instruments (PMMC)


When the instrument is connected in the circuit. The current flows through coil which is mounted on the spindle. Magnetic flux is set up in the coil. This magnetic field or flux interact with the field produced by permanent magnets, a force is exerted on the current carrying conductors of the coil which produces deflecting torque. This results pointer moves over the calibrated scale.

If current in the coil is reversed, the direction of deflecting torque will be reversed because field produced by the permanent magnets does not change.


  • These instruments have uniform scale.
  • These instruments require low power for their operation.
  • High torque/weight ratio.
  • No hysteresis loss as the former is of aluminum
  • Reliable damping torque.
  • These instruments are very accurate and reliable.


  1. The permanent magnet moving coil instruments cannot be used for ac.
  2. These instruments are costlier.

Errors in PMMC instruments

  1. Change of resistance of the moving coil with temperature.
  2. Due to ageing effects, wakening the stiffness of springs.

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Electrical Instrument

Electrical Instruments

Electrical instruments are used to measure electrical quantities such as current, voltage, power, energy, frequency, power factor, resistance etc. To measure these quantities several instruments such as ammeter, voltmeter, wattmeter, energy meter are used. In simple words we can say that the instruments which are used to measure electrical quantities are known as electrical instruments.

Classifications of Electrical Instruments

These instruments are divided into two types

Absolute Instruments

Secondary Instruments

  1. Absolute Instruments

Absolute instruments are the instruments which give the value of the quantity to be measured in terms of the constants. Such instruments do not require any previous calibration. Tangent galvanometer is the example of absolute instruments.

  1. Secondary Instruments

Secondary instruments are the instruments which determine the electrical quantities to be measured directly in terms defection.

Types of Secondary Instruments

The secondary instruments are further divided into three types.

Indicating Instruments

Integrating Instruments

Recording Instruments

  1. Indicating Instruments

These are the instruments that indicate the magnitude of electrical quantity being measured instantaneously. In such instruments, a pointer moves over the calibrated scale. Ammeter, voltmeter, wattmeter etc are the example of indicating instruments.

  1. Integrating Instruments

The instruments that add up the electrical quantity. Energy meter are the example of such instruments because these instruments measure the total energy (in KWH) in a given period.

  1. Recording Instruments

These are the instruments that give a continuous record of the variations of the electrical quantity being measure. ECG is the example of such instruments.

Essentials of Indicating Instruments

Indicating instruments are those instruments in which pointer moves over the calibrated scale to indicate the magnitude of electrical quantity which to be measured. The torques required for operation of indicating instruments are

Deflecting Torque

Controlling torque

Damping torque

  1. Deflecting Torque

It is a torque due to which pointer of instruments moves from its zero position in an indicating instrument is called deflecting torque.

  1. Controlling Torque

It is a torque which allow the pointer to deflect in accordance to the magnitude of electrical quantity or which brings the pointer to zero position when the instrument is disconnected from the circuit.

The controlling torque is provided by following methods.

  1. Spring Control
  2. Gravity Control

3. Damping Torque

It is the torque which suppresses the under oscillations of the pointers and brings the pointer to its final position quickly. The damping torque is provided by following methods.

  1. Air friction damping
  2. Fluid friction damping
  3. Eddy current damping




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Dynamo Type Wattmeter

Dynamo Type Wattmeter

A Dynamo type wattmeter variably used for measurement of ac power as well as dc power.

Working Principle

It works on the dynamo meter principle. According to this principle,  a mechanical force exists between two current carrying conductors when current passed through the conductors.


There are two coils are used in dynamo type wattmeter. One is called current coil and other is called potential coil. Current coil is a fixed coil which is connected in series with the load and carries load current. While potential coil is a moving coil connected across the load through a series resistance R and carries a current which is proportional the voltage across the load. The current coil or fixed coil is splitted into two parts. The controlling torque is provided by two spiral springs which also lead the current into and out of moving coil. Damping is provided by light aluminium vanes moving in an air dash pot. A pointer is attached to the movable coil that moves over a calibrated scale.

construction of dynamotype wattmeter

Working of dynamo type wattmeter

When the wattmeter is connected in the circuit to measure power. Current coil carries the load current I1 and produce a magnetic filed. Potential coil carries current (I2) proportional to the load voltage and produces another magnetic field. The magnetic fields of the current and potential coils react on one another causing the movable coil moves the pointer over the scale. The pointer comes to rest at a position where deflecting torque is equal to the controlling torque.

working of dynamo type wattmeter

The change of direction of current reverse current reverse current in both the current coils and potential or movable coil. So that, the direction of deflecting torque remains unchanged. Hence, these instrument suitable for the measurement of dc as well as ac power.

Deflecting Torque

Deflecting torque Td α I1 I2

Deflecting torque, Td α VI1 α load power


  1. High degree of accuracy
  2. Uniform scale


  1. Errors due to stray field acting on the potential coil.
  2. Error due to inductance of potential coil.
  3. It produce errors due to eddy currents.

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Wattmeter is an instrument which is used to measure electric power in the circuit. It measure electrical power in watt. Electric power is the product of current and voltage but in case of ac circuits power factor (cos Φ) is multiplied by an electric power. In this article, we will discuss about basic construction and working of wattmeter.

Construction of Wattmeter

A wattmeter basically consists of two coils, one coil is called the current coil and the other one is called pressure coil. As from definition of wattmeter, it measures electric power. So current coil (cc) measures current and pressure coil (PC) measures voltage. The current coil (cc) has less number of turns, which carries the current in the load and has very low impedance. The pressure coil (PC) has more number of turns having high impedance which is connected across the load. The load voltage is immersed across the pressure coil.  Circuit diagram of wattmeter is given below.

Construction of wattmeter

From above figure, M and L represents current coil. Common and V represents pressure coil. The terminal M denotes the main side, L denotes the load side, common denotes the common point between current coil(cc) and pressure coil (pc) and V denotes the second terminal of pressure coil. A wattmeter is a four terminal device.

Working Principle of wattmter

When current flows through the current coil (cc) and pressure coil (pc), they set up magnetic fields in space. The electromagnetic torque is produced by the interaction of magnetic fields set up in current coil and pressure coil. In both coils, one coil is fixed coil and an other coil is moveable. Under the influence of the torque, the moveable coil moves on a scale. The produce torque in coil is directly proportional to the current flowing through the coil. Large current flows in a coil produce large torque and vice-versa. The torque which moves the pointer is called deflecting torque. This torque is controlled by controlling torque. Controlling torque is provided by spiral spring. The deflection is proportional to the average power (VI Cos Φ) delivered to the circuit.

Construction of wattmeter

Sometimes, meter gives backward deflection or downscale reading. This is due to improper connection of the current coil and pressure coil of wattmeter.


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Electrical Transducer

Electrical Transducer

There are number of physical or non-electrical quantities which can be measured with the help of electrical or electronic instruments. Transducer plays very important role in industries.  Basically, an instrumentation system consists of three main parts, namely input device, a signal conditioning device and an output device. The output device may be a simple indicating meter, display etc. An Electrical Transducer used in industries.

electrical transducer

The Input device is usually  known as transducer. In this article we will discuss about transducer, advantages of transducer classification of transducer.

What is Electrical Transducer?

An electrical transducer is a device that converts non-electrical or physical quantity into electrical signal or quantity.

For most of the instrumentation systems, the input quantities are non-electrical, but these quantities are required to convert into electrical. Because electrical signal or quantity can be easily measured and processed.


  • Electrical signals can be easily processed as per need.
  • The electrical signals can be converted any desired form.
  • The power required for electrical or electronic system is very small.
  • These signals can easily transmitted and processed for the purpose of measurement.

Classification of Transducers

The transducer are classified on the basis of power utilized them for operations, there are two basic types of transducers (i) Active transducer (ii) Passive transducer.

Active Transducer

Active transducers are those transducers that does not require external power for their operation. Active transducers, also called self generating transducer. In these transducers, a voltage or current produced proportional to the quantity under measurement. The example of active transducer is piezoelectric crystal. When external force is applied to piezoelectric crystal it converts mechanical energy into desired output.

Passive Transducer

Electrical transducers are those transducers that require external power for their operation. In these transducers, some parameters is varied by the measurand which in turn varies the electrical quantity such as voltage or current.

The example of such type of transducers are resistive, inductive and capacitive. These are the basic types of transducer and further classified into many types.

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Conversion of Galvanometer into Ammeter and Voltmeter

Conversion of Galvanometer into Ammeter and Voltmeter

A Galvanometer is an electrical instrument. Which is used to detect electric current in a given circuit? Galvanometer is a very sensitive instrument. It can be converted into an ammeter as well as voltmeter by connecting a resistance with a suitable manner.

Conversion of Galvanometer into Ammeter

An ammeter is an electrical instrument which is used to measure electric in a given circuit. It is always connected in a series with the circuit whose current is to be measure.

In order to convert galvanometer in an ammeter a shunt is connected across the terminals of galvanometer.

What is Shunt?

A shunt is a wire having low resistance connected in parallel with the galvanometer to avoid flows of excessive current through the ammeter.

conversion of galvanometer into ammeter

The reason for connecting a wire having resistance (called shunt) in parallel with the galvanometer to make it an ammeter is very simple. We know that ammeter is connected in series with the circuit and therefore it necessary to ensure that its resistance should be very small so that circuit current is not affected. To achieve this low value of resistance is connected in parallel with the galvanometer. The resistance of galvanometer and shunt is in parallel and the equivalent resistance of the meter becomes very low or negligible.

In other ways we can say that an ammeter is a combination of galvanometer and shunt (low resistance) put in parallel with it.

galvanometer into ammeter circuit current

I = Circuit current.

IG = Galvanometer current.

S = Resistance of the shunt.

G = Resistance of the galvanometer.

Value of shunt resistance

Conversion of Galvanometer into Voltmeter

A voltmeter is an electrical instrument which is used to measure potential difference (Pd) or voltage between two terminals. Voltmeter always connected across with the circuit whose voltage is to be measured.

In order to convert galvanometer into voltmeter, a resistance having high value connected in series with the galvanometer.

The reason for connecting a high value resistance in series with galvanometer to make it a voltmeter is very simple. The voltmeter connected in parallel with the circuit the value of resistance which connected in series with the galvanometer should have very high value therefore it is desired that the whole current flows through the load in order to measure correct value of voltage. Ideally the value of series resistance should be infinity but practically it not possible.

conversion of galvanometer into voltmeter
Minimum value of current should be allowed to flow though the voltmeter. Thus a voltmeter is simply a combination of galvanometer in series with a high value of resistance.

Value of Resistance


R = Resistance to be connected in series.

V = Voltage.

IG = Current through galvanometer.

G = Resistance of the galvanometer.

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