TYPES OF DC GENERATORS

Types of DC GENERATORS : The magnetic field in a DC generator is normally produced by electromagnets rather than permanent magnets.

Generators are generally classified according to their methods of field excitation.

On this basis, dc generators are divided into the following two classes:

(i) Separately excited dc generators

(ii) Self-excited dc generators

The behaviour of a dc generator on load depends upon the method of field excitation adopted.

TYPES OF DC GENERATORS (i)Separately Excited DC Generators

A dc generator whose field magnet winding is supplied from an independent external dc source (e.g., a battery etc.) is called a separately excited generator.

Fig shows the connections of a separately excited generator.

The voltage output depends upon the speed of rotation of armature and the field current (Eg =PfØ ZN/60 A).

The greater the speed and field current, greater is the generated e.m.f.

It may be noted that separately excited dc generators are rarely used in practice.

The dc generators are normally of self-excited type.

Armature current, Ia = IL

Terminal voltage, V = Eg – IaRa

Electric power developed = EgIa

Power delivered to load = EgIa  – Ia²Ra

TYPES OF DC GENERATORS (ii)Self-Excited DC Generators

A dc generator whose field magnet winding is supplied current from the output of the generator itself is called a self-excited generator.

There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely;

(a)Series generator;

(b) Shunt generator;

(c) Compound generator

(a) Series generator

In a series wound generator, the field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load.

Fig. shows the connections of a series wound generator.

Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance.

Series generators are rarely used except for special purposes e.g., as boosters.

Armature current, Ia = Ise = IL = I(say)

Terminal voltage, V = EG – I(Ra + Rse)

Power developed in armature = EgIa

Power delivered to load

 (b) Shunt generator

In a shunt generator, the field winding is connected in parallel with the armature winding so that terminal voltage of the generator is applied across it.

The shunt field winding has many turns of fine wire having high resistance.

Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load.

Fig. shows the connections of a shunt-wound generator.

Shunt field current, Ish = V/Rsh

Armature current, Ia = IL + Ish

Terminal voltage, V = Eg – IaRa

Power developed in armature = EgIa

Power delivered to load = VI_L

(c) Compound generator

In a compound-wound generator, there are two sets of field windings on each pole—one is in series and the other in parallel with the armature.

A compound wound generator may be:

Short Shunt in which only shunt field winding is in parallel with the armature winding.

Long Shunt in which shunt field winding is in parallel with both series field and armature winding.

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Single phase Energy meter

Construction of DC GENERATOR

PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

DC Generator EMF Equation

CHARACTERISTICS OF DC GENERATOR

Application of DC Generator

Working of DC motors

Principles of DC motor and Operation of DC motor

Classification and Types of DC Motor

Basic Equations of DC Motor and Applications of DC Motor

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

EMF Equation of Transformer

Transformer on No Load and Load

Equivalent Circuit of Transformer

Voltage Regulation of Transformer

Single phase induction Motor – Construction, Principle of Operation

Types of Single phase induction Motor

 

DC GENERATOR EMF EQUATION

DC GENERATOR EMF EQUATION

Let

Φ = flux/pole in weber

Z = total number of armture conductors = No.of slots x No.of conductors/slot

P = No.of generator poles

A = No.of parallel paths in armature

N = armature rotation in revolutions per minute (r.p.m) E = emf induced in any parallel path in armature

Generated emf Eg = emf generated in any one of the parallel paths i.e E. Average emf geneated /conductor = dΦ/dt volt (n=1)

Now, flux cut/conductor in one revolution dΦ = ΦP Wb No.of revolutions/second = N/60

Time for one revolution, dt = 60/N second

Hence, according to Faraday’s Laws of Electroagnetic Induction,

EMF generated/conductor is For a simplex wave-wound generator

No.of parallel paths = 2

No.of conductors (in series) in one path = Z/2

EMF generated/path is

For a simplex lap-wound generator

No.of parallel paths = P

No.of conductors (in series) in one path = Z/P

EMF generated/path

In general generated emf

where A = 2 for simplex wave-winding A = P for simplex lap-winding

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Single phase Energy meter

Construction of DC GENERATOR

PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

Types of DC Generator

CHARACTERISTICS OF DC GENERATOR

Application of DC Generator

Working of DC motors

Principles of DC motor and Operation of DC motor

Classification and Types of DC Motor

Basic Equations of DC Motor and Applications of DC Motor

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

EMF Equation of Transformer

Transformer on No Load and Load

Equivalent Circuit of Transformer

Voltage Regulation of Transformer

Single phase induction Motor – Construction, Principle of Operation

Types of Single phase induction Motor

 

PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

Principle of DC Generator:

The PRINCIPLE OF DC GENERATOR  is to converts mechanical energy into electrical energy.

when a conductor move in a magnetic field in such a way conductors cuts across a magnetic flux of lines and emf produces in a generator and it is defined by faradays law of electromagnetic induction emf causes current to flow if the conductor circuit is closed.

The pole pieces (marked N and S) provide the magnetic field.

The pole pieces are shaped and positioned as shown to concentrate the magnetic field as close as possible to the wire loop.

The loop of wire that rotates through the field is called the ARMATURE.

The ends of the armature loop are connected to rings called SLIP RINGS.

They rotate with the armature.

The brushes, usually made of carbon, with wires attached to them, ride against the rings.

The generated voltage appears across these brushes.

The elementary generator produces a voltage in the following manner (fig. 1-3).

The armature loop is rotated in a clockwise direction.

The initial or starting point is shown at position A.

This will be considered the zero-degree position.

At 0º_ the armature loop is perpendicular to the magnetic field.

The black and white conductors of the loop are moving parallel to the field.

The instant the conductors are moving parallel to the magnetic field, they do not cut any lines of flux.

Therefore, no emf is induced in the conductors, and the meter at position A indicates zero.

This position is called the NEUTRAL PLANE.

As the armature loop rotates from position A (0º) to position B (90º), the conductors cut through more and more lines of flux, at a continually increasing angle.

At 90º  they are cutting through a maximum number of lines of flux and at maximum angle.

The result is that between 0º and 90º , the induced emf in the conductors builds up from zero to a maximum value.

Observe that from 0º_ to 90º_, the black conductor cuts DOWN through the field.

At the same time the white conductor cuts UP through the field.

The induced emfs in the conductors are series-adding.

This means the resultant voltage across the brushes (the terminal voltage) is the sum of the two induced voltages.

The meter at position B reads maximum value.

As the armature loop continues rotating from 90º_ (position B) to 180º_ (position C), the conductors which were cutting through a maximum number of lines of flux at position B now cut through fewer lines.

They are again moving parallel to the magnetic field at position C. They no longer cut through any lines of flux.

As the armature rotates from 90º_ to 180º_, the induced voltage will decrease to zero in the same manner that it increased during the rotation from 0º_ to 90º_.

The meter again reads zero.

From 0º_ to 180º_ the conductors of the armature loop have been moving in the same direction through the magnetic field.

Therefore, the polarity of the induced voltage has remained the same.

This is shown by points A through C on the graph.

As the loop rotates beyond 180º_ (position C), through 270º_ (position D), and back to the initial or starting point (position A), the direction of the cutting action of the conductors through the magnetic field reverses.

Now the black conductor cuts UP through the field while the white conductor cuts DOWN through the field.

As a result, the polarity of the induced voltage reverses.

Following the sequence shown by graph points C, D, and back to A, the voltage will be in the direction opposite to that shown from points A, B, and C.

The terminal voltage will be the same as it was from A to C except that the polarity is reversed (as shown by the meter deflection at position D).

The voltage output waveform for the complete revolution of the loop is shown on the graph in figure.

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Single phase Energy meter

Construction of DC GENERATOR

DC Generator EMF Equation

Types of DC Generator

CHARACTERISTICS OF DC GENERATOR

Application of DC Generator

Working of DC motors

Principles of DC motor and Operation of DC motor

Classification and Types of DC Motor

Basic Equations of DC Motor and Applications of DC Motor

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

EMF Equation of Transformer

Transformer on No Load and Load

Equivalent Circuit of Transformer

Voltage Regulation of Transformer

Single phase induction Motor – Construction, Principle of Operation

Types of Single phase induction Motor

 

Construction of DC GENERATOR

The construction of DC generator are explained.

Construction of DC GENERATOR

An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction.

The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of mechanical energy.

The Dynamo was the first electrical generator capable of delivering power for industry.

The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current.

A dynamo machine consists of a stationary structure which generates a strong magnetic field, and a set of rotating winding’s which turn within that field.

On small machines the magnetic field may be provided by a permanent magnet; larger machines have the magnetic field created by electromagnets.

The energy conversion in generator is based on the principle of the production of dynamically induced e.m.f. whenever a conductor cuts magnetic flux, dynamically induced e.m.f is produced in it according to Faraday’s Laws of Electromagnetic induction.

This e.m.f causes a current to flow if the conductor circuit is closed.

CONSTRUCTION OF DC MACHINES

A D.C. machine consists mainly of two part the stationary part called stator and the rotating part called rotor.

The stator consists of main poles used to produce magnetic flux ,commutating poles or interpoles in between the main poles to avoid sparking at the commutator but in the case of small machines sometimes the inter poles are avoided and finally the frame or yoke which forms the supporting structure of the machine.

The rotor consist of an armature a cylindrical metallic body or core with slots in it to place armature winding’s or bars,a commutator and brush gears The magnetic flux path in a motor or generator is show below and it is called the magnetic structure of generator or motor.

The major parts in the Construction of DC GENERATOR can be identified as

5. Frame
6. Poles
7. Armature
8. Field winding
9. Commutator
10. Brush
11. Other mechanical parts

Frame

Frame is the stationary part of a machine on which the main poles and commutator poles are bolted and it forms the supporting structure by connecting the frame to the bed plate.

The ring shaped body portion of the frame which makes the magnetic path for the magnetic fluxes from the main poles and inter poles is called Yoke.

Why we use cast steel instead of cast iron for the construction of Yoke?

In early days Yoke was made up of cast iron but now it is replaced by cast steel.

This is because cast iron is saturated by a flux density of 0.8 Wb/sq.m where as saturation with cast iron steel is about 1.5 Wb/sq.m.

So for the same magnetic flux density the cross section area needed for cast steel is less than cast iron hence the weight of the machine too.

If we use cast iron there may be chances of blow holes in it while casting.

So now rolled steels are developed and these have consistent magnetic and mechanical properties.

poles:

Solid poles of fabricated steel with separate/integral pole shoes are fastened to the frame by means of bolts.

Pole shoes are generally laminated.

Sometimes pole body and pole shoe are formed from the same laminations.

The pole shoes are shaped so as to have a slightly increased air gap at the tips. Inter-poles are small additional poles located in between the main poles.

These can be solid, or laminated just as the main poles.

These are also fastened to the yoke by bolts.

Sometimes the yoke may be slotted to receive these poles.

The inter poles could be of tapered section or of uniform cross section. These are also called as commutating poles or com poles.

The width of the tip of the com pole can be about a rotor slot pitch.

Armature

The armature is where the moving conductors are located.

The armature is constructed by stacking laminated sheets of silicon steel.

Thickness of these lamination is kept low to reduce eddy current losses.

As the laminations carry alternating flux the choice of suitable material, insulation coating on the laminations, stacking it etc are to be done more carefully.

The core is divided into packets to facilitate ventilation.

The winding cannot be placed on the surface of the rotor due to the mechanical forces coming on the same.

Open parallel sided equally spaced slots are normally punched in the rotor laminations.

These slots house the armature winding.

Large sized machines employ a spider on which the laminations are stacked in segments.

End plates are suitably shaped so as to serve as ’Winding supporters’.

Armature construction process must ensure provision of sufficient axial and radial ducts to facilitate easy removal of heat from the armature winding.

Field windings:

In the case of wound field machines (as against permanent magnet excited machines) the field winding takes the form of a concentric coil wound around the main poles.

These carry the excitation current and produce the main field in the machine.

Thus the poles are created electromagnetically.

Two types of windings are generally employed.

In shunt winding large number of turns of small section copper conductor is used.

The resistance of such winding would be an order of magnitude larger than the armature winding resistance.

In the case of series winding a few turns of heavy cross section conductor is used.

The resistance of such windings is low and is comparable to armature resistance.

Some machines may have both the windings on the poles.

The total ampere turns required to establish the necessary flux under the poles is calculated from the magnetic circuit calculations.

The total mmf required is divided equally between north and south poles as the poles are produced in pairs.

The mmf required to be shared between shunt and series windings are apportioned as per the design requirements.

As these work on the same magnetic system they are in the form of concentric coils. Mmf ’per pole’ is normally used in these calculations.

Armature winding As mentioned earlier, if the armature coils are wound on the surface of the armature, such construction becomes mechanically weak.

The conductors may fly away when the armature starts rotating.

Hence the armature windings are in general pre-formed, taped and lowered into the open slots on the armature.

In the case of small machines, they can be hand wound.

The coils are prevented from flying out due to the centrifugal forces by means of bands of steel wire on the surface of the rotor in small groves cut into it.

In the case of large machines slot wedges are additionally used to restrain the coils from flying away.

The end portion of the windings are taped at the free end and bound to the winding carrier ring of the armature at the commutator end.

The armature must be dynamically balanced to reduce the centrifugal forces at the operating speeds.

Compensating winding One may find a bar winding housed in the slots on the pole shoes.

This is mostly found in d.c. machines of very large rating.

Such winding is called compensating winding.

In smaller machines, they may be absent.

Commutator:

Commutator is the key element which made the d.c. machine of the present day possible.

It consists of copper segments tightly fastened together with mica/micanite insulating separators on an insulated base.

The whole commutator forms a rigid and solid assembly of insulated copper strips and can rotate at high speeds.

Each commutator segment is provided with a ’riser’ where the ends of the armature coils get connected.

The surface of the commutator is machined and surface is made concentric with the shaft and the current collecting brushes rest on the same.

Under-cutting the mica insulators that are between these commutator segments has to be done periodically to avoid fouling of the surface of the commutator by mica when the commutator gets worn out.

Some details of the construction of the commutator are seen in Fig

Brush and brush holders:

Brushes rest on the surface of the commutator.

Normally electro-graphite is used as brush material.

The actual composition of the brush depends on the peripheral speed of the commutator and the working voltage.

The hardness of the graphite brush is selected to be lower than that of the commutator.

When the brush wears out the graphite works as a solid lubricant reducing frictional coefficient.

More number of relatively smaller width brushes are preferred in place of large broad brushes.

The brush holders provide slots for the brushes to be placed.

The connection Brush holder with a Brush and Positioning of the brush on the commutator from the brush is taken out by means of flexible pigtail.

The brushes are kept pressed on the commutator with the help of springs.

This is to ensure proper contact between the brushes and the commutator even under high speeds of operation.

Jumping of brushes must be avoided to ensure arc free current collection and to keep the brush contact drop low.

Other mechanical parts End covers, fan and shaft bearings form other important mechanical parts.

End covers are completely solid or have opening for ventilation.

They support the bearings which are on the shaft.

Proper machining is to be ensured for easy assembly.

Fans can be external or internal.

In most machines the fan is on the non-commutator end sucking the air from the commutator end and throwing the same out.

Adequate quantity of hot air removal has to be ensured.

Bearings Small machines employ ball bearings at both ends.

For larger machines roller bearings are used especially at the driving end.

The bearings are mounted press-fit on the shaft.

They are housed inside the end shield in such a manner that it is not necessary to remove the bearings from the shaft for dismantling.

End Shields or Bearings

If the armature diameter does not exceed 35 to 45 cm then in addition to poles end shields or frame head with bearing are attached to the frame.

If the armature diameter is greater than 1m pedestral type bearings are mounted on the machine bed plate outside the frame.

These bearings could be ball or roller type but generally plain pedestral bearings are employed.

If the diameter of the armature is large a brush holder yoke is generally fixed to the frame.

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Single phase Energy meter

PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

DC Generator EMF Equation

Types of DC Generator

CHARACTERISTICS OF DC GENERATOR

Application of DC Generator

Working of DC motors

Principles of DC motor and Operation of DC motor

Classification and Types of DC Motor

Basic Equations of DC Motor and Applications of DC Motor

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

EMF Equation of Transformer

Transformer on No Load and Load

Equivalent Circuit of Transformer

Voltage Regulation of Transformer

Single phase induction Motor – Construction, Principle of Operation

Types of Single phase induction Motor

 

Single phase Energy meter

Single phase Energy meter single phase induction type energy meter is also popularly known as watt-hour meter.

This name is given to it.

This article is only focused about its constructional features and its working.

Induction type energy meter essentially consists of following components:

1. Driving system
2. Moving system
3. Braking system and
4. Registering system

Driving system

It consists of two electromagnets, called “shunt” magnet and “series” magnet, of laminated construction.

A coil having large number of turns of fine wire is wound on the middle limb of the shunt magnet.

This coil is known as “pressure or voltage” coil and is connected across the supply mains.

This voltage coil has many turns and is arranged to be as highly inductive as possible.

In other words, the voltage coil produces a high ratio of inductance to resistance.

This causes the current and therefore the flux, to lag the supply voltage by nearly 90 degree

Adjustable copper shading rings are provided on the central limb of the shunt magnet to make the phase angle displacement between magnetic field set up by shunt magnet and supply voltage is approximately 90 degree.

The copper shading bands are also called the power factor compensator or compensating loop.

The series electromagnet is energized by a coil, known as “current” coil which is connected in series with the load so that it carry the load current.

The flux produced by this magnet is proportional to, and in phase with the load current.

Moving system

The moving system essentially consists of a light rotating aluminium disk mounted on a vertical spindle or shaft.

The shaft that supports the aluminium disk is connected by a gear arrangement to the clock mechanism on the front of the meter to provide information that consumed energy by the load.

The time varying (sinusoidal) fluxes produced by shunt and series magnet induce eddy currents in the aluminium disc

The interaction between these two magnetic fields and eddy currents set up a driving torque in the disc.

The number of rotations of the disk is therefore proportional to the energy consumed by the load in a certain time interval and is commonly measured in kilowatt-hours (Kwh).

Braking system

Damping of the disk is provided by a small permanent magnet, located diametrically opposite to the a.c magnets.

The disk passes between the magnet gaps.

The movement of rotating disc through the magnetic field crossing the air gap sets up eddy currents in the disc that reacts with the magnetic field and exerts a braking torque.

By changing the position of the brake magnet or diverting some of the flux there form, the speed of the rotating disc can be controlled.

Registering or counting system

The registering or counting system essentially consists of gear train, driven either by worm or pinion gear on the disc shaft, which turns pointers that indicate on dials the number of times the disc has turned.

The energy meter thus determines and adds together or integrates all the instantaneous power values so that total energy used over a period is thus known.

Therefore, this type of meter is also called an“integrating” meter.

Working of Single phase induction type Energy Meter

The basic working of Single phase induction type Energy Meter is only focused on two mechanisms:

2. Mechanism of rotation of an aluminum disc which is made to rotate at a speed proportional to the power.
3. The mechanism of counting and displaying the amount of energy transferred. Mechanism of rotation of an aluminum disc

The metallic disc is acted upon by two coils.

One coil is connected or arranged 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 field of the voltage coil is delayed by 90 degrees using a lag coil.

This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current and voltage.

A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc

– this acts as a brake which causes the disc to stop spinning when power stops being drawn rather than allowing it to spin faster and faster.

This causes the disc to rotate at a speed proportional to the power being used.

Mechanism of displaying the amount of energy transferred

The aluminum disc is supported by a spindle which has a worm gear which drives the register.

The register is a series of dials which record the amount of energy used.

The dials may be of the cyclometer type, an odometer-like display that is easy to read where for each dial a single digit is shown through a window in the face of the meter, or of the pointer type where a pointer indicates each digit.

It should be noted that with the dial pointer type, adjacent pointers generally rotate in opposite directions due to the gearing mechanism.

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DC Circuits and Ohm’s Law

AC Circuits and Kirchhoff’s law

Steady State Solution of DC Circuits and Problems based on ohm’s law

AC Instantaneous Value and RMS Value

RL Series Circuit And LR Series Circuit

Star Delta transformation

Electrical Instruments and Classification of instruments

Gravity control

Damping Torque

Permanent Magnet Moving Coil Instruments (PMMC)

Operating Principles of Moving Iron Instruments Ammeters and Voltmeters

Electrodynamometer Type Wattmeter

Electrodynamometer Type Wattmeter

Electrodynamometer Type Wattmeter in general, a watt meter is used to measure the electric power of a circuit, or sometime it also measures the rate of energy transferred from one circuit to another circuit.

When a moving coil (that is free to rotate) is kept under the influence of a current carrying conductor, then automatically a mechanical force will be applied to the moving coil, and this force will make a little deflection of the moving coil.

If a pointer is connected with the moving coil, which will move of a scale, then the deflection can be easily measured by connecting the moving coil with that pointer.

This is the principle of operation of all dynamo meter type instruments, and this principle is equally applicable for dynamo meter type watt meter also.

This type of watt meter consists of two types of coil, more specifically current coil and voltage coil.

There are two current coils which are kept at constant position and the measurable current will flow through those current coils.

A voltage coil is placed inside those two current coils, and this voltage coil is totally free to rotate.

The current coils are arranged such a way, that they are connected with the circuit in series.

And the voltage coil is connected in parallel with the circuit.

As simple as other voltmeter and ammeter connection.

In fact, a watt meter is a package of an ammeter and a voltmeter, because the product of voltage and current is the power, which is the measurable quantity of a watt meter

When current flows through the current coils, then automatically a magnetic field is developed around those coils.

Under the influence of the electromagnetic field, voltage coil also carries some amount of current as it is connected with the circuit in parallel.

In this way, the deflection of the pointer will proportional to both current and voltage of the circuit. In this way, Watt = Current × Voltage equation is satisfied and the deflection shows the value of power inside the circuit.

A dynamo meter type watt meter is used in various applications where the power or energy transfer has to be measured.

Construction and Working Principle of Electrodynamometer Type Wattmeter

Now let us look at constructional details of Electrodynamometer Type Wattmeter.

It consists of following parts There are two types of coils present in the electrodynamometer.

They are :

(a) Moving coil : 

Moving coil moves the pointer with the help of spring control instrument.

A limited amount of current flows through the moving coil so as to avoid heating.

So in order to limit the current we have connect the high value resistor in series with the moving coil.

The moving is air cored and is mounted on a pivoted spindle and can moves freely. In electrodynamometer type wattmeter, moving coil works as pressure coil.

Hence moving coil is connected across the voltage and thus the current flowing through this coil is always proportional to the voltage.

(b) Fixed coil: 

The fixed coil is divided into two equal parts and these are connected in series with the load, therefore the load current will flow through these coils.

Now the reason is very obvious of using two fixed coils instead of one, so that it can be constructed to carry considerable amount of electric current.

These coils are called the current coils of electrodynamometer type wattmeter.

Earlier these fixed coils are designed to carry the current of about 100 amperes but now the modern wattmeter are designed to carry current of about 20 amperes in order to save power.

(c) Control system: 

Out of two controlling systems i.e.

(1). Gravity control

(2) Spring control, only spring controlled systems are used in these types of wattmeter.

Gravity controlled system cannot be employed because they will appreciable amount of errors.

(d) Damping system: 

Air friction damping is used, as eddy current damping will distort the weak operating magnetic field and thus it may leads to error.

(e) Scale:

There is uniform scale is used in these types of instrument as moving coil moves linearly over a range of 40 degrees to 50 degrees on either sides.

Now let us derive the expressions for the controlling torque and deflecting torques. In order to derive these expressions let us consider the circuit diagram given below:

We know that instantaneous torque in electro dynamic type instruments is directly proportional to product of instantaneous values of currents flowing through both the coils and the rate of change of flux linked with the circuit.

Let I₁ and I₂ be the instantaneous values of currents in pressure and current coils respectively. So the expression for the torque can be written as:

T = I₁*I₂*(dM / dx)

Where x is the angle

Now let the applied value of voltage across the pressure coil be V=  – V sin ωt

Assuming the electrical resistance of the pressure coil be very high hence we can neglect reactance with respect to its resistance.

In this the impedance is equal to its electrical resistance therefore it is purely resistive

The expression for instantaneous current can be written as I₂ = v / Rp where Rp is the resistance of pressure coil.

I₂ =    V sin ωt / Rp

If there is phase difference between voltage and electric current, then expression for instantaneous current through current coil can be written as

I₁ = I(t) = – I sin (ωt – Φ)

As current through the pressure coil in very very small compare to current through current coil hence current through the current coil can be considered as equal to total load current.

Hence the instantaneous value of torque can be written as – V sin ωt / Rp * – I sin (ωt – Φ) * (dM / dx)

Average value of deflecting torque can be obtained by integrating the instantaneous torque from limit 0 to T where T is the time period of the cycle Td = deflecting torque = VI cosΦ /Rp *(dM / dx)

Controlling torque is given by Tc = Kx where K is spring constant and x is final steady state value of deflection.

Advantages of Electrodynamometer Type Wattmeter

Following are the advantages of electrodynamometer type wattmeters and they are written as follows:

(a). Scale is uniform up to certain limit

(b). They can be used for both to measure AC as well as DC quantities as scale is calibrated for both

Errors in Electrodynamometer Type Wattmeter

Following are the errors in the electrodynamometer type watt meters:

(a)     Errors in the pressure coil inductance.

(b)     Errors may be due to pressure coil capacitance.

(c)      Errors may be due to mutual inductance effects.

(d)     Errors may be due connections.

         (i.e. pressure coil is connected after current coil )

(e)      Error due to Eddy currents.

(f)      Errors caused by vibration of moving system.

(g)     Temperature error.

(h)     Errors due to stray magnetic field.

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AC Instantaneous Value and RMS Value

RL Series Circuit And LR Series Circuit

Star Delta transformation

Electrical Instruments and Classification of instruments

Gravity control

Damping Torque

Permanent Magnet Moving Coil Instruments (PMMC)

Operating Principles of Moving Iron Instruments Ammeters and Voltmeters

Single phase Energy meter

Moving Iron instruments

Moving iron instruments are generally used to measure alternating voltages and currents.

In moving-iron instruments the movable system consists of one or more pieces of specially-shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in coil.

There are two general types of moving iron instruments namely:

1. Repulsion (or double iron) type (figure 1)
2. Attraction (or single-iron) type (figure 2)

The brief description of different components of a moving-iron instrument is given below:

Moving element:

 A small piece of soft iron in the form of a vane or rod.

Coil:

To produce the magnetic field due to current flowing through it and also to magnetize the iron pieces.

Repulsion type

In repulsion type, a fixed vane or rod is also used and magnetized with the same polarity. Control torque is provided by spring or weight (gravity).

Damping torque is normally pneumatic, the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.

Deflecting torque produces a movement on an aluminum pointer over a graduated scale.

The deflecting torque in any moving-iron instrument is due to forces on a small piece of magnetically ‘soft’ iron that is magnetized by a coil carrying theoperating current.

In repulsion type moving–iron instrument consists of two cylindrical soft iron vanes mounted within a fixed current-carrying coil.

One iron vane is held fixed to the coil frame and other is free to rotate, carrying with it the pointer shaft.

Two irons lie in the magnetic field produced by the coil that consists of only few turns if the instrument is an ammeter or of many turns if the instrument is a voltmeter.

Current in the coil induces both vanes to become magnetized and repulsion between the similarly magnetized vanes produces a proportional rotation.

The deflecting torque is proportional to the square of the current in the coil, making the instrument reading is a true

‘RMS’ quantity Rotation is opposed by a hairspring that produces the restoring torque .

Only the fixed coil carries load current, and it is constructed so as to withstand high transient current.

Moving iron instruments having scales that are nonlinear and somewhat crowded in the lower range of calibration.

Measurement of Electric Voltage and Current

Moving iron instruments are used as Voltmeter and Ammeter only.

Both can work on AC as well as on DC.

Ammeter

Instrument used to measure current in the circuit.

Always connected in series with the circuit and carries the current to be measured. This current flowing through the coil produces the desired deflecting torque.

It should have low resistance as it is to be connected in series.

Voltmeter

Instrument used to measure voltage between two points in a circuit.

Always connected in parallel.

Current flowing through the operating coil of the meter produces deflecting torque.

It should have high resistance. Thus a high resistance of order of kilo ohms is connected in series with the coil of the instrument.

Ranges of Ammeter and Voltmeter

For a given moving-iron instrument the ampere-turns necessary to produce full-scale deflection are constant.

One can alter the range of ammeters by providing a shunt coil with the moving coil.

Voltmeter range may be altered connecting a resistance in series with the coil. Hence the same coil winding specification may be employed for a number of ranges.

Advantages

1. The instruments are suitable for use in AC and DC circuits.
2. The instruments are robust, owing to the simple construction of the moving parts.
3. The stationary parts of the instruments are also simple.
4. Instrument is low cost compared to moving coil instrument.
5. Torque/weight ratio is high, thus less frictional error.

Errors

(i). Error due to variation in temperature.

(ii). Error due to friction is quite small as torque-weight ratio is high in moving coil instruments.

(iii). Stray fields cause relatively low values of magnetizing force produced by the coil. Efficient magnetic screening is essential to reduce this effect.

(iv). Error due to variation of frequency causes change of reactance of the coil and also changes the eddy currents induced in neighbouring metal.

(v). Deflecting torque is not exactly proportional to the square of the current due to non -linear characteristics of iron material.

Attraction type

The basic construction of attraction type moving iron instrument is illustrated bellow A thin disc of soft iron is eccentrically pivoted in front of a coil.

This iron tends to move inward that is from weaker magnetic field to stronger magnetic field whencurrent flowing through the coil.

In attraction moving instrument gravity control was used previously but now gravity control method is replaced by spring control in relatively modern instrument.

By adjusting balance weight null deflection of the pointer is achieved.

The required damping force is provided in this instrument by air friction.

The figure shows a typical type of damping system provided in the instrument, where damping is achieved by a moving piston in an air syringe.

Theory of Attraction Type Moving Iron Instrument

Suppose when there is no current through the coil, the pointer is at zero, the angle made by the axis of the iron disc with the line perpendicular to the field is φ.

Now due current I and corresponding magnetic field strength, the iron piece is deflected to an angle θ.

Now component of H in the direction of defected iron disc axis is Hcos{90 – (θ + φ) or Hsin(θ + φ).

Now force F acting on the disc inward to the coil is thus proportional to H²sin(θ + φ) hence the force is also proportional to I²sin(θ + φ) for constant permeability.

If this force is acting on the disc at a distance l from the pivot, then deflection torque,

Td = Fl cos (θ+Φ)

Thus Td = I² sin (θ+Φ) cos (θ+Φ)

Td = kI² sin 2(θ+Φ)

Where k is constant.

Now, as the instrument is gravity controlled, controlling torque will be

Tc = k’ sin θ

Where k ‘is constant

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AC Circuits and Kirchhoff’s law

Steady State Solution of DC Circuits and Problems based on ohm’s law

AC Instantaneous Value and RMS Value

RL Series Circuit And LR Series Circuit

Star Delta transformation

Electrical Instruments and Classification of instruments

Gravity control

Damping Torque

Permanent Magnet Moving Coil Instruments (PMMC)

Electrodynamometer Type Wattmeter

Single phase Energy meter

Permanent Magnet Moving Coil Instruments

Permanent Magnet Moving Coil Instruments

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 accurately.

Construction of permanent magnet moving coil instruments:

We will see the construction of these types of instruments in four 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 aluminum 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

Deflecting torque Equation:

Let us derive a general expression for torque in permanent magnet moving coil instruments or PMMC instruments.

We know that in moving coil instruments the deflecting torque is given by the expression:

T_d = N B l dI

where N is number of turns,

B is magnetic flux density in air gap, l is the length of moving coil,

d is the width of the moving coil, And I is the electric current.

Now for a moving coil instruments deflecting torque should be proportional to current, mathematically we can write T_d = GI.

Thus on comparing we say G = NBIdl.

At steady state we have both the controlling and deflecting torques are equal.

T_c is controlling torque, on equating controlling torque with deflection torque we have GI = K.x where x is deflection thus current is given by

I = K / G x

Since the deflection is directly proportional to the current therefore we need a uniform scale on the meter for measurement of current.

Now we are going to discuss about the basic circuit diagram of the ammeter. Let us consider a circuit as shown below:

The current I is shown which breaks into two components at the point A.

The two components are Is and Im.

Before I comment on the magnitude values of these currents, let us know more about the construction of shunt resistance.

The basic properties of shuntresistance are written below,

The electrical resistance of these shunts should not differ at higher temperature, it they should posses very low value of temperature coefficient.

Also the resistance should be time independent. Last and the most important property they should posses is that they should be able to carry high value of current without much rise in temperature.

Usually manganin is used for making dc resistance.

Thus we can say that the value of Is much greater than the value of Im as resistance of shunt is low.

From the we have, Is .Rs = ImRm

Where Rs is resistance of shunt and Rm is the electrical resistance of the coil. Is = I – Im

M= I / Im = 1+ (Rm + Rs)

Where m is the magnifying power of the shunt.

Errors in Permanent Magnet Moving Coil Instruments

There are three main types of errors

(a) Errors due to permanent magnets:

Due to temperature effects and aging of the magnets the magnet may lose their magnetism to some extent.

The magnets are generally aged by the heat and vibration treatment.

(b) Error may appear in PMMC Instrument due to the aging of the spring:

However the error caused by the aging of the spring and the errors caused due to permanent magnet are opposite to each other, hence both the errors are compensated with each other.

(c) Change in the resistance of the moving coil with the temperature:

Generally the temperature coefficients of the value of coefficient of copper wire in moving coil is 0.04 per degree Celsius rise in temperature.

Due to lower value of temperature coefficient the temperature rises at faster rate and hence the resistance increases.

Due to this significant amount of error is caused.

Advantages of Permanent Magnet Moving Coil Instruments

(1)The scale is uniformly divided as the current is directly proportional to deflection of the pointer.

Hence it is very easy to measure quantities from these instruments.

(2)Power consumption is also very low in these types of instruments.

(3)Higher value of torque is to weight ratio.

(4)These are having multiple advantages, a single instrument can be used for measuring various quantities by using different values of shunts and multipliers.

Disadvantages of Permanent Magnet Moving Coil Instruments

(1)     These instruments cannot measure ac quantities.

(2)     Cost of these instruments is high as compared to moving iron instruments.

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AC Instantaneous Value and RMS Value

RL Series Circuit And LR Series Circuit

Star Delta transformation

Electrical Instruments and Classification of instruments

Gravity control

Damping Torque

Operating Principles of Moving Iron Instruments Ammeters and Voltmeters

Electrodynamometer Type Wattmeter

Single phase Energy meter

Electrical Measuring Instruments

Electrical Measuring Instruments

Classification of instruments

(i). Depending on the quality measured

(ii). Depending on the different principles used for their working

(iii). Depending on how the quantity is measured

Electrical Measuring Instruments classification Depending on the quality measured

• Voltmeter
• Ammeter
• Energy meter
• Ohm meter

Electrical Measuring Instruments classification Depending on the different principles used for their working

• Moving Iron type
• Moving coil type
• Dynamometer type
• Induction type

Electrical Measuring Instruments classification Depending on how the quantity is measured

• Deflecting type
• Integrating type
• Recording type

Deflecting Torque

The deflecting torque moves the moving system and the pointer from the zero position.

The deflecting torque can be obtained through magnetic, thermal, electromagnetic or electro dynamic effects

Controlling torque

The controlling torque acts in a direction opposite to that of deflecting torque.

When the controlling torque (TC) and the deflecting torque (TD) are numerically equal the pointer takes a definite position.

In the absence of TC the pointer would deflect to maximum position irrespective of the quantity to be measured.

Moreover TC also helps in bringing the moving system to zero position when the instrument is disconnected from the circuit.

The controlling torque is obtained through spring control and gravity control

Spring Control:

 The arrangement for spring control consists of two phosphor bronze spiral hair springs attached to a moving system.

The springs are made of materials which

(i). are not affected by fatigue.

(ii). Have low temp-coefficient of resistance

(iii). Have low specific resistance

(iv). Are non-magnetic

As the pointer deflects the springs get twisted in the opposite direction.

The combined twist produces the necessary controlling torque which is proportional to angle of deflection of moving system θ.

If we consider a permanent magnet moving coil meter with spring control system the deflecting torque will be proportional to the current passing through it and the controlling torque will be proportional to the angle of deflection

Thus T_D α I

T_C α θ

Since T_D = T_C

We have θ α I

Thus the spring controlled instruments having uniform scale

Eddy current damping

Eddy current damping is the most efficient form of damping.

The essential components in this type of damping are a permanent magnet; and a light conducting disc usually of alumninum.

When a sheet of conducting material moves in a magnetic field so as to cut through lines of force, eddy currents are set up in it and a force exists between these currents and the magnetic field, which is always in the direction opposing the motion.

This force is proportional to the magnitude of the current, and to the strength of field.

The former is proportional to the velocity of movement of the conductor, and thus, if the magnetic field is constant, the damping force is proportional to the velocity of the moving system and is zero when there is no movement of the system.

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AC Instantaneous Value and RMS Value

RL Series Circuit And LR Series Circuit

Star Delta transformation

Gravity control

Damping Torque

Permanent Magnet Moving Coil Instruments (PMMC)

Operating Principles of Moving Iron Instruments Ammeters and Voltmeters

Electrodynamometer Type Wattmeter

Single phase Energy meter

Damping Torque

Damping Torque : We have already seen that the moving system of the instrument will tend to move under the action of the deflecting torque.

But on account of the control torque, it will try to occupy a position of rest when the two torques are equal and opposite.

However, due to inertia of the moving system, the pointer will not come to rest immediately but oscillate about its final deflected position as shown in Fig 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.

The damping torque is proportional to the speed of rotation of the moving system, that is Tv = kv d dt θ

where kv = damping torque constant

d dt θ = speed of rotation of the moving system

Depending upon the degree of damping introduced in the moving system, the instrument may have any one of the following conditions as depicted in Fig.

3. Under damped condition: The response is oscillatory
4. Over damped condition: The response is sluggish and it rises very slowly from its zero position to final position.
5. Critically damped condition: When the response settles quickly without any oscillation, the system is said to be critically damped.

In practice, the best response is slightly obtained when the damping is below the critical value i.e., the instrument is slightly under damped.

The damping torque is produced by the following methods: Air Friction Damping & Fluid friction damping

Air Friction Damping

In this type of damping a light vane or vanes having considerable area is attached to the moving system to develop a frictional force opposing the motion by reason of the air they displace.

Two methods of damping by air friction are depicted

The arrangement of a light aluminum vane which moves in a quadrant (sector) shaped air chamber.

The chamber also carries a cover plate at the top.

The vane is mounted on the spindle of the moving system.

The aluminum vane should not touch the air-chamber walls otherwise a serious error in the deflection of the instrument will be introduced.

Now, with the motion, the vane displaces air and thereby a damping force is created on the vane that produces a torque (damping) on the spindle.

When the movement is quicker the damping force is greater; when the spindle is at rest, the damping force is zero.

The arrangement of consists of a light aluminum piston which is attached to the moving system.

This piston moves in a fixed chamber which is closed at one end. Either circular or rectangular chamber may be used.

The clearance (or gap) between the piston and chamber walls should be uniform throughout and as small as possible.

When the piston moves rapidly into the chamber the air in the closed space is compressed and the pressure of air thus developed opposes the motion of the piston and thereby the whole moving system.

If the piston is moving out of the chamber, rapidly, the pressure in the closed space falls and the pressure on the open side of the piston is greater than that on the opposite side.

Motion is thus again opposed.

With this damping system care must be taken to ensure that the arm carrying the piston should not touch the sides of the chamber during its movement.

The friction which otherwise would occur may introduce a serious error in the deflection.

The air friction damping is very simple and cheap.

But care must be taken to ensure that the piston is not bent or twisted.

This method is used in moving iron and hot wire instruments.

Fluid Friction Damping

This form is damping is similar to air friction damping.

The action is the same as in the air friction damping.

Mineral oil is used in place of air and as the viscosity of oil is greater, the damping force is also much greater.

The vane attached to the spindle is arranged to move in the damping oil.

It is rarely used in commercial type instruments.

The oil used must fulfill the following requirements.

It should not evaporate quickly.

It should not have any corrosive effect on metals.

Its viscosity should not change appreciably with temperature.

It should be good insulator.

Advantages of Fluid Friction Damping

4. The oil used for damping can also be used for insulation purpose in some forms of instruments which are submerged in oil.
5. The clearance between the vanes and oil chamber is not as critical as with the air friction clamping system.
6. This method is suitable for use with instruments such as electrostatic type where the movement is suspended rather than pivoted.
7. Due to the up thrust of oil, the loads on bearings or suspension system is reduced thereby the reducing the frictional errors.

Disadvantages of Fluid Friction Damping

1. The instruments with this type of damping must be kept always in a vertical position.
2. It is difficult to keep the instrument clean due to leakage of oil.
3. It is not suitable for portable instruments.
4. The fluid friction damping can be used for laboratory type electrostatic instruments.

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Steady State Solution of DC Circuits and Problems based on ohm’s law

AC Instantaneous Value and RMS Value

RL Series Circuit And LR Series Circuit

Star Delta transformation

Electrical Instruments and Classification of instruments

Gravity control

Permanent Magnet Moving Coil Instruments (PMMC)

Operating Principles of Moving Iron Instruments Ammeters and Voltmeters

Electrodynamometer Type Wattmeter

Single phase Energy meter