For Free PPTs, Prjoects, Materials
Mail Us :

Differences Between Instrument and Power Transformers

In this article differences between power transformer and instrument transformer are discussed.Current transformer and Potential transformers come under instrument transformers.

Differences Between Instrument and Power Transformers

Power Transformers Instrument Transformers
1. Mainly used to change voltage levels in
a power system.
1. Mainly used to extend the ranges of the
instruments while measuring parameters like
voltage,current,power etc.
2. They are required to transform huge amount
of power to the load.
2. They are required to transform very small power
as their loads are generally delicate moving elements
of the instruments.
3. They can be used to step up or step down the
3. They are basically step down transformers and
used along with devices such as protective
relays,indicators etc.
4. The exciting current is a small fraction of the
secondary winding of load current.
4. As the load it self is small, the exciting current is
of the order of the secondary winding.
5. The cost is main consideration in the design
while efficiency and regulations are secondary
5. Accuracy is the main consideration while designing
to keep ratio and phase angle errors minimum.
Cost is the second consideration.
6. As they handle large power, the heat
dissipation is the major consideration and
cooling method is necessary.
6. The power output is very small as loads are light.
Hence heating is not severe.
7. The limitation on the load is due to temperature
7. Accuracy is the main load limitation factor and not
the temperature rise.
8. Example : Distribution transformer used in
8. Example : Current transformer and Potential

difference between power transformer and current transformer
difference between power and potential transformer
instrument transformer pdf
how current transformers work
difference between current transformer and potential transformer
difference between power transformer and distribution transformer
difference between current and voltage transformer pdf
what is power transformer 
Read More

Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments

Let us have a comparison between two major type of electrical measuring instruments they are moving coil (MC) and moving iron (MI) instruments.In this article all major differences between moving iron and  moving coil   instruments are discussed.When you go viva or interview most often question on electrical measurements is what is the difference between mi and mc type instruments?.In this article you will get answer to all differences between MI & MC typ instruments.
Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments

Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments

M.C Instruments M.I Instruments
1. MC type instruments are more accurate. 1. MI type are less accurate than MC type.
2. Manufacturing cost is high. 2. Cheap in cost.
3. Reading scale is uniformly distributed. 3. Non-uniform scale
(scale cramped at beginning and finishing)
4. Very sensitive in construction & for input. 4. Robust in construction.
5. Low power consumption 5. Slightly high power consumption.
6. Eddy current damping is used. 6. Air friction damping is used.
7. Can be used only for D.C measurements. 7. Can be used for A.C as well as for D.C
8. Controlling torqure is provided by spring. 8. Controlling torque is provided by
gravity or spring
9. Deflection proportional to current.( θ α I ). 9. Deflection proportional to square of current.
( θ α I² ).
10. Errors are set due to aging of control
springs,permanent magnet (i.e. No Hysteresis loss)
10. Errors are set due to hysteresis and stray fields.
(i.e. hysteresis loss takes place).

In this comparison between MC and MI Instruments we shared top points.

difference between moving coil and moving iron instruments
difference between moving coil and moving iron instruments pdf
Read More

Torque Equation Of Moving Iron Instruments

In previous tutorial on Moving Iron Instrument Operation construction & working principle was discussed. In this post moving iron instrument torque equation will be derived.

Torque Equation of Moving Iron Instruments

READ HERE Moving Iron Instrument Working Operation CLICK HERE 

Consider a small increment in current supplied to the coil of the instrument. due to this current let dθ be the deflection under the deflecting torque Td. Due to such deflection, some mechanical work will be done.
Mechanical Work = Td .dθ
There will be a change in the energy stored i the magnetic field due to the change in inductance. This is because the vane tries to occupy the position of minimum reluctance. The inductance is inversely proportional to the reluctance of  the magnetic circuit of coil.

Let   I = initial current
L = instrument inductance
θ = deflection
dI = increase in current
dθ = change in deflection
dL = change in inductance

In order to effect an increment dL in the current, there must be an increase in the applied voltage given by,

e = d(L*I)/dt
   = I * dL/dt + T * dI/dt     as both I and L are changing.

The electrical energy supplied is given by 

eIdt = { I * dL/dt + T * dI/dt }Idt
       =I²dL + ILdI
The stored energy increases from 1/2*(LI²) to 1/2*[(L+dL)(I+dI)²]
Hence the change in stored energy is given by,
1/2*[(L+dL)(I+dI)²] - 1/2*(LI²)
Neglecting higher order terms,this becomes ILdI +  1/2 * I² dL
The energy supplied in nothing but increase in stored energy plus the energy required for mechanical work done.

I²dL + ILdI = ILdI + 1/2*(I²)dL +Td.
Td.dθ =  1/2( I².dL )

Td = 1/2  I²dL/dθ

While the controlling torque is given by,
Tc = Kθ
where K = spring constant 
Kθ = 1/2  I²dL/dθ
θ = 1/2  I²dL/dθ * 1/K      under equilibrium 

Thus the deflection is proportional to the square of the current through the coil. And the instrument gives square law response.

Read More

Moving Iron Instrument Working Operation

Moving Iron Instruments or MI Instruments

In our previous article we have discussed PMMC Instrument Working Opeartion. In this tutorial on Moving Iron Instrument Working Operation we go through the construction & basic principle of MI type instrument.  

The moving iron instruments are classified as:
i) Moving iron attraction type instruments
ii) Moving iron repulsion type instruments

Moving Iron Attraction Type Instrument Working Operation

Moving Iron Instrument Working Principle : The basic working principle of these instruments is very simple that a soft iron piece if brought near magnet gets attracted by the magnet.

The construction of the attraction type moving iron instrument is shown in the below figure.

lt consists of a fixed coil C and moving iron piece D. The oil is flat and has narrow slot like opening. The moving iron is a flat disc which is eccentrically mounted on the spindle. The spindle is supported between the jewel bearings. The spindle carries a pointer which moves over a graduated scale.The number of turns of the fixed coil are dependent on the range of the instrument. For passing large current through the coil only few turns are required.

The controlling torque is provided by the springs but gravity control may also be used for vertically mounted panel type instruments.

The damping torque is provided by the air friction. A light aluminium piston is attached to the moving system. it moves in a fixed chamber. The chamber is closed at one end. it can also be provided with the help of vane attached to the moving system.

The operating magnetic field in moving iron instruments is very weak. Hence eddy current damping is not used since it requires a permanent magnet which would affect or distort the operating field.This is the reason Why why eddy current damping is not used in moving iron instrument.

Moving Iron Repulsion Type Instrument

Moving iron repulsion Type instruments have two vanes inside the coil. the one is fixed and other is movable. When the current flows in the coil, both the vans are magnetized with like polarities induced on the same side. Hence due to the repulsion of like polarities, there is a force of repulsion between the two vanes causing the movement of the moving vane. The repulsion type instruments are the most commonly used instruments.

The two different designs of repulsion type instruments are:
i) Radial vane type and
ii) Co-axial vane type

Radial Vane Emulsion Typo Instrument

Below shows the radial vane repulsion type instrument. Out of the other moving iron mechanisms, this is the moat sensitive and has most linear scale.

The two vanes are radial strips of iron. The fixed vane is attached to the coil. The movable vane is attached to the spindle and suspended in the induction field of the coil. The needle of the instrument is attached to this vane.

Even-though the current through the coil is alternating, there is always repulsion between the like poles of the fixed and the movable vane. Hence the deflection of the pointer is always in the same direction. The deflection is effectively proportional to the actual current and hence the scale is calibrated directly to read amperes or volts. The The calibration is accurate only for the frequency for which it is designed because the impedance is different for different frequencies.

Concentric Vane Repulsion Type Instrument

Figure shows the concentric vane repulsion type instrument. The instrument has two concentric vanes. One is attached to the coil frame rigidly while the other can rotate co-axially inside the stationary vane.

Both the vanes are magnetized to the same polarity due to the current in the coil. Thus the movable vane rotates under the repulsive force. As the movable vane is attached to the pivoted shaft, the repulsion results in a rotation of the shaft. The pointer deflection is proportional to the current in the coil. The concentric vane type instrument is moderately sensitive and the deflection is proportional to the square of the current through coil. Thus the instrument said to have square low response. Thus the scale of the instrument is non-uniform in nature. Thus whatever may be the direction of the current in the coil, the deflection in the moving iron instruments is in the same direction. Hence moving iron instruments can be used for both a.c. and d.c. measurements. Due to square low response, the scale of the moving iron instrument is non-uniform.

moving iron instrument pdf
working principle moving iron instruments
moving coil instrument
classification of moving iron instruments
moving iron instrument can be used for measuring
attraction type moving iron instrument
moving iron instrument symbol
moving iron instrument working animation
Read More

Cooling Methods of A Transformer

 Power Transformer Cooling Methods 

The transformers get heated due to iron and copper looses occurring in them. It is necessary to dissipate this heat so that the temperature of the windings is kept below the value at which the insulation begins to deteriorate. The cooling of transformer is more difficult than that of rotating machines because the rotating machines create a turbulent airflow which assists in removing the heat generated due to losses. Luckily the losses in transformers are comparatively small. Nevertheless the elaborate cooling arrangements have been devised to deal with the whole range of sizes.

Types of transformer cooling methods:

As far as cooling methods are concerned. the transformers are of following two types

1. Dry type. 2. Oil immersed type.

Dry Type Transformers. Small transformers up to 25 KVA size are of the dry type and have the following cooling arrangements:

(i) Air natural. In this method the natural circulation of surrounding air is utilized to carry away the heat generated by losses. A sheet metal enclosure protects the winding from mechanical injury.

(ii) Air blast. Here the transformer is cooled by a continuous blast of cool air forced through the core and windings . The blast is produced by a fan. The air supply must be filtered to prevent accumulation of dust in ventilating ducts.

Oil Immersed Transformers. In general most transformers are of oil immersed types. The oil provides better insulation than air and it is a better conductor of heat than air. Mineral oil is used for this purpose.Oil immersed transformers are classified as follows:

i) Oil immersed self-cooled transformer. The transformer is immersed in oil and heat generated in cores and windings is passed to oil by conduction. Oil in contact with heated parts rises and its place is taken by cool oil from the bottom. The natural oil transfer: its heat to the tank walls from where heat is taken away by the ambient air. The oil gets cooler and falls to the bottom from where it is dissipated into the surroundings. The tank mince is the best dissipate of heat but a plain tank will have to be excessively large, if used without any auxiliary means for high rating transformers. As both space and oil are costly, these auxiliary means should not increase the cubic capacity of the tank. 
The heat dissipating capacity can be increased by providing (i) corrugations. (ii) fins (iii) tubes and (iv) radiator tanks.

The advantages of ‘oil natural' cooling is that it does not clog the ducts and the windings are fire from effect of moisture.

(ii) Oil immersed forced air-cooled transformers. In this type of cooling, air is directed over the outer surfaces of the tank of the transformer immersed in oil.

(iii) Oil immersed water-cooled transformers. Heat is extracted from the oil by means of a stream of water pumped through a metallic coil immersed in the oil just below the top of the tank.The heated water is in turn cooled in a spray pond or a cooling tower.

(iv) Oil immersed forced oil cooled transformers. In such transformers heat is extracted from the oil by pumping the oil itself upward through the winding and then back by way of external radiators which may themselves be cooled by fans. The extra cost of oil pumping equipment must of course economically justified but it has incidentally the advantage of reducing the temperature difference between top and bottom of enclosing tank. 

  • transformer cooling methods pdf
  • transformer cooling methods ppt
  • cooling methods of transformer wikipedia
  • types of transformer cooling methods
  • transformer cooling class
  • transformer cooling oil
  • the function of conservator in a transformer is
  • air blast cooling of transformer
Read More

Principles of Power Systems By V.K Mehta PDF Free Download

Principles of Power Systems By V.K Mehta PDF Free Download

Now you can read & learn electrical power systems in offline.You can download Principles of Power Systems By V.K Mehta. This is a very nice book with attractive colors & amazing images.All concepts of power systems are explained very clearly.You can get idea behind every power concept in single read.You can use principle of power system by vk mehta free download in doc format as text in image can be copied.

We are sharing only the link of this book which is already on internet.We respect copy right policy of principle of power system by vk mehta book publishers.

What You Get In Principles of Power Systems By V.K Mehta 

Generating stations 
Variable load on power stations 
Economics power generation 
Power factor improvement 
Supply systems 
Mechanical design of overhead lines 
Electrical desing of overhead lines 
Performance of transmission lines 
Underground cables 
Distribution system general 
D.C Distribution 
A.C Distribution 
Voltage control 
Introduction to switchgear 
Symmetrical fault calculations 
Unsymmetrical fault calculations 
Circuit breakers 
Protective relays 
Protection of Alternators and transformers 
Protection of busbars and lines 
Protection againts overvoltages 
Neutral Grounding 

Download Here

  • principles of power system by vk mehta pdf solution manual
  • principle of power system by vk mehta free download in doc format
  • principles of electronics vk mehta pdf free download
  • principles of power systems by vk mehta ebook
  • principles of power system by v.k.mehta (4th edition) colored book free download
  • principles of electronics by v k mehta rohit mehta.pdf free download
  • principle of power system by vk mehta pdf download
  • principle of power system by vk mehta solution pdf
Read More

Power Factor | It's Calculation, Power Factor Improvement Methods

Power Factor | Calculation and Power Factor Improvement Methods

What Is Electrical Power Factor ?

The cosine of angle between voltage and current in an a.c. circuit is known as power factor. In an a.c. circuit, there is generally a phase difference φ between voltage and current. The term cos φ is called the power factor of the circuit. If the circuit is inductive, the current lags behind the voltage and the power factor is referred to as lagging. However, in a capacitive circuit, current leads the voltage and power factor is said to be leading.

Consider an inductive circuit taking a lagging current I from supply voltage V; the angle of lag being φ. The phasor diagram of the circuit is shown in fig. The circuit current I can be resolved into two perpendicular components, namely ;

(a) I cos φ in phase with V
(b) I sin φ 90°  out of phase with V

The component I cos φ is known as active or wattful component, whereas component I sin φ is called the reactive or wattless component. The reactive component is a measure of the power factor. If the reactive component is small, the phase angle φ is small and hence power factor cos φ will be high. Therefore, a circuit having small reactive current (i.e., I sin φ) will have high power factor and vice-versa. It may be noted that value of power factor can never be more than unity.

(i) It is a usual practice to attach the word ‘lagging’ or ‘leading’ with the numerical value of power factor to signify whether the current lags or leads the voltage. Thus if the circuit has a p.f. of 0·5 and the current lags the voltage, we generally write p.f. as 0·5 lagging.

(ii) Sometimes power factor is expressed as a percentage. Thus 0·8 lagging power factor may be expressed as 80% lagging.

Power Triangle 

The analysis of power factor can also be made in terms of power drawn by the a.c. circuit. If each side of the current triangle oab of below figure is multiplied by voltage V, then we get the power triangle OAB shown in figure where
OA = VI cos φ and represents the active power in watts or kW
AB = VI sin φ and represents the reactive power in VAR or kVAR
OB = VI and represents the apparent power in VA or kVA
The following points may be noted form the power triangle :

(i) The apparent power in an a.c. circuit has two components viz., active and reactive power at right angles to each other.
OB² = OA² + AB²
or (apparent power)² = (active power)² + (reactive power)²
or (kVA)²= (kW)² + (kVAR)²

(ii) Power factor, cos φ = OA/OB = active power/apparent power = kW/kVA
Thus the power factor of a circuit may also be defined as the ratio of active power to the apparent power. This is a perfectly general definition and can be applied to all cases, whatever be the waveform.

(iii) The lagging reactive power is responsible for the low power factor. It is clear from the power triangle that smaller the reactive power component, the higher is the power factor of the circuit.
kVAR = kVA sin φ = (kW * sin φ)/cos φ
∴ kVAR = kW tan φ

(iv) For leading currents, the power triangle becomes reversed. This fact provides a key to the power factor improvement. If a device taking leading reactive power (e.g. capacitor) is connected in parallel with the load, then the lagging reactive power of the load will be partly neutralized, thus improving the power factor of the load.

(v) The power factor of a circuit can be defined in one of the following three ways :

(a) Power factor = cos φ = cosine of angle between V and I

(b) Power factor = R/Z= Resistance/Impedance

(c) Power factor = VI/(VI * cosφ) = Active power/Apparent Power

(vi) The reactive power is neither consumed in the circuit nor it does any useful work. It merely flows back and forth in both directions in the circuit. A wattmeter does not measure reactive power.

Disadvantages of Low Power Factor

The power factor plays an importance role in a.c. circuits since power consumed depends upon this

P = VL IL cos φ

It is clear from above that for fixed power and voltage, the load current is inversely proportional to the power factor. Lower the power factor, higher is the load current and vice-versa. A power factor less than unity results in the following disadvantages :

(i) Large kVA rating of equipment. The electrical machinery (e.g., alternators, transformers, switchgear) is always rated in kVA.
Now, kVA = kW/cos φ
It is clear that kVA rating of the equipment is inversely proportional to power factor. The smaller the power factor, the larger is the kVA rating. Therefore, at low power factor, the kVA rating of the equipment has to be made more, making the equipment larger and expensive.

(ii) Greater conductor size. To transmit or distribute a fixed amount of power at constant voltage, the conductor will have to carry more current at low power factor. This necessitates large conductor size. 

(iii) Large copper losses. The large current at low power factor causes more I²R losses in all the elements of the supply system. This results in poor efficiency.

(iv) Poor voltage regulation. The large current at low lagging power factor causes greater voltage drops in alternators, transformers, transmission lines and distributors. This results in the decreased voltage available at the supply end, thus impairing the performance of utilization devices. In order to keep the receiving end voltage within permissible limits, extra equipment (i.e., voltage regulators) is required.

(v) Reduced handling capacity of system. The lagging power factor reduces the handling capacity of all the elements of the system. It is because the reactive component of current prevents the full utilisation of installed capacity. The above discussion leads to the conclusion that low power factor is an objectionable feature in the supply system.

Causes of Low Power Factor

Low power factor is undesirable from economic point of view. Normally, the power factor of the whole load on the supply system in lower than 0·8. The following are the causes of low power factor:

(i) Most of the a.c. motors are of induction type (1φ and 3φ induction motors) which have low lagging power factor. These motors work at a power factor which is extremely small on light load (0·2 to 0·3) and rises to 0·8 or 0·9 at full load.

(ii) Arc lamps, electric discharge lamps and industrial heating furnaces operate at low lagging power factor.

(iii) The load on the power system is varying ; being high during morning and evening and low at other times. During low load period, supply voltage is increased which increases the magnetization current. This results in the decreased power factor.

Power Factor Improvement Methods 

The low power factor is mainly due to the fact that most of the power loads are inductive and, therefore, take lagging currents. In order to improve the power factor, some device taking leading power should be connected in parallel with the load. One of such devices can be a capacitor. The capacitor draws a leading current and partly or completely neutralists the lagging reactive component of load current. This raises the power factor of the load.
Read More

Instantaneous Relay Operation

Instantaneous Relay Working Operation 

What Is Instantaneous Relay ?

An instantaneous relay is one in which there is no time delay provided intentionally. More specifically ideally there is no time required to operate the relay. Although there is some time delay which can not be avoided.

Instantaneous relay. An instantaneous relay is one in which no intentional time delay is provided. In this case, the relay contacts are closed immediately after current in the relay coil exceeds the minimum calibrated value. Figure shows an instantaneous solenoid type of relay. Although there Will be a short time interval between the instant of pickup and the closing of relay contacts, no intentional time delay has been added.

Instantaneous Relay Working Operation

The instantaneous relays have operating time less than 0-1 second. The instantaneous relay is effective only Where the impedance between the relay and source is small compared to the protected section impedance. The operating time of instantaneous relay is sometimes expressed in cycles based on the power-system frequency
e.g. one-cycle would be If 50 second in a 50-cycle system.


  • instantaneous relay wikipedia
  • instantaneous relay definition
  • instantaneous relay operating time
  • difference between idmt and instantaneous relay

Read More