Show Mobile Navigation

Featured post

Working of Light Emitting Diode (LED)

Youtube Channel

Latest Stories

Current Transformer (CT) - Construction and Working Principle

The large alternating currents which can not be sensed or passed through normal ammeter, and current coils of wattmeters, energy meters can easily be measured by use of current transformers along with normal low range instruments.

A transformer is a device which consists of two windings called primary and secondary. It transfers energy from one side to another with suitable change in the level of current or voltage.
Related: Working Principle of Transformer
A current transformer (CT) is a type of transformer that is used to measure AC current. It produces an alternating current (AC) in its secondary which is proportional to the AC current in its primary. Current transformers, along with voltage or potential transformers are Instrument transformer.

Current Transformer Symbol / Circuit Diagram 

A current transformer (CT) basically has a primary coil of one or more turns of heavy cross-sectional area. In some, the bar carrying high current may act as a primary. This is connected in series with the line carrying high current.
The secondary of the current transformer is made up of a large number of turns of fine wire having small cross-sectional area. This is usually rated for 5A. This is connected to the coil of normal range ammeter.


Related: Why Current Transformer (CT) Seconday Should not be Open ?

Working Principle of CT

These transformers are basically step up transformers i.e. stepping up a voltage from primary to secondary. Thus the current reduces from primary to secondary. 

So from current point of view, these are step down transformer, stepping down the currant value considerably from primary to secondary. 

Let,
N1 = Number of Primary Turns
N2 = Number of Secondary Turns
I1 = Primary Current
I2 = Secondary Current

For a transformer,
I1∕I = N2/N1

As N2 is very high compared to N1, the ratio I1 to I2 is also very high for current transformers. Such a current ratio is indicated for representing the range of current transformer. 

For example, consider a 500:5 range then it indicates that C.T. steps down the current from primary to secondary by a ratio 500 to 5. 

I1∕I = 500/5

Knowing this current ratio and the meter reading on the secondary, the actual high line current flowing through the primary can be obtained. 

Construction of Current Transformer

There are three types of constructions used for the current transformers which are,
  1. Wound Type CT
  2. Torroidal (Window) Type CT
  3. Bar Type CT

Wound Type Current Transformer – The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turn’s ratio of the transformer. 

Torroidal (Window) Type Current Transformer – These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the torroidal transformer. Some current transformers have a “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached. 

Bar-type Current Transformer – This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device. 

1. Wound Type Current Transformer

In wound type construction the primary is wound for more than one full turn on the core.
The construction is shown below.
In a low voltage wound type current transformer, the secondary winding is wound on a bakelite former. The heavy primary winding is directly wound on the top of the secondary winding with a suitable insulation in between the two. Otherwise the primary is wound completely separately and then taped with suitable insulating material and assembled with the secondary on the core.

The current transformers can be ring type or window type. Some commonly used shapes for the stampings of window type current transfonncrs are shown in the figure below.

The core material for wound type is nickel-iron alloy or an oriented electrical steel. Before installing the secondary winding on core it is insulated with the help of end collars and circumferential wraps of pressboards. Such pressboards provide additional insulation and protection to the winding from damage due to the sharp corners.

2. Bar Type Current Transformer

In this type of current transformer, the primary winding is nothing but a bar of suitable size. The construction is shown in the Figure.

The insulation on the bar type primary is bakelized paper tube or a resin directly moulded on the bar. Such bar type primary is the integral part of the current transformer. The core and the secondary winding are same in bar type transformer.

The stampings used for the laminations in current transformers must have high cross-sectional area than the ordinary transformers. Due to this, the reluctance of the interleaved comers remains as low as possible. Hence the corresponding magnetizing current is also small. The windings are placed very close to each other so as to reduce the leakage reactance. To avoid the corona effect, in bar type transformer, the external diameter of the tube is kept large.

The windings are so designed that without damage, they can withstand short circuit forces which may be caused due to short circuit in the circuit in which the current transfomter is inserted.

For small line voltages, the tape and varnish are used for insulation. For line voltages above 7 kV the oil immersed or compound filled current transformers are used.

Uses / Advantages of Current Transformer

Current transformers are used extensively for measuring current and monitoring the operation of the power grid.

Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building with three-phase service and single-phase services greater than 200 amperes. 

High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground. 

Current transformers can be mounted on the low voltage or high voltage leads of a power transformer.

Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits, and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices.

Classification or Types of Protective Relays


The IEEE defines protective relays as: “relays whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action ”. 

Relays detect and locate faults by measuring electrical quantities in the power system which are different during normal and intolerable conditions. The most important role of protective relays is to first protect individuals, and second to protect equipment

All the relays consist of one or more elements which get energized and actuated by the electrical quantities of the circuit. Most of the relays used are electromechanical type which work on the principles of electromagnetic attraction and electromagnetic induction.

There are different types of protective relays available now. They can be broadly classified in to the following types. Each of them will be explained in detail in coming articles.

  1. Electromagnetic attraction type Relays
    1. Solenoid Type
    2. Attracted Armature
    3. Balanced Beam Type
  2. Induction Type Relays
    1. Induction Disc Type
    2. Induction Cup Type
  3. Direction Type Relays
    1. Reverse Current Type
    2. Reverse Power Type
  4. Relays based on Timing
    1. Instantaneous Type
    2. Definite Time Lag Type
    3. Inverse Time Lag type
  5. Distance type Relays
    1. Impedance Type
    2. Reactance Type
    3. Admittance Type
  6. Differential Type Relays
    1. Current Differential Type
    2. Voltage Differential Type
  7. Other Types of Relays
    1. Under voltage, current, power relay
    2. Over voltage, current, power relay
    3. Thermal Relay
    4. Rectifier Relay
    5. Permanent Magnet Moving Coil Relay
    6. Static Relay
    7. Gas Operated Relay
Protection relays can also be classified in accordance with their construction, actuating
signal, application and function.

Classification based on  Construction
Depending upon the principle of construction, the following four broad categories are found.

  • Electromechanical Relay
  • Solid State Relay
  • Microprocessor Relay
  • Numerical Relay
Classification based on Actuating Signals

The actuating signal may be any of the following signals including a numbers of different combinations of these signals depending upon whether the designed relay require a single or multiple inputs for its realization.
 Current
• Voltage
• Power
• Frequency
• Temperature
• Pressure
• Speed
• Others

Classification based on Function
The functions for which the protection system is designed classify the relays in the following few categories.
• Directional Over current Relay
• Distance Relay
• Over voltage Relay
• Differential Relay
• Reverse Power Relay
• Others

It is important to notice that the same set of input actuating signals may be utilized to design to relays having different function or application. 
For example, the voltage and current input relays can be designed both as a Distance and/ or a Reverse Power relay.

Electromagnetic Attraction Type Relays

The electromagnetic attraction type relays operate on the principle of attraction of an armature by the magnetic force produced by undesirable current or movement of plunger in a solenoid. These relays can be actuated by AC or DC quantities. The various types of these relays are. 

  1. Solenoid Type
  2. Attracted Armature Type
  3. Balanced Beam Type


1. Solenoid Type 

In this relay, the plunger or iron core moves into a solenoid and the operation of the relay depends on the movement of the plunger. 

2. Attracted Armature Type : 

This relay operate on the current setting. When currant in the circuit exceeds beyond the limit, the armature gets attracted by the magnetic force produced by the undesirable current. The current rating of the circuit in which relay is connected plays an important role in the operation of the relay. 

3. Balanced Beam Type : 

In this relay, the armature is fastened to a balanced beam. For normal current, the beam remains horizontal but when current exceeds, the armature gets attracted and beam gets tilted causing the required operation 

Induction Type Relays

These relays work on the principle of an electromagnetic induction. The use of these relays is limited to AC quantities. The various types of these relays are, 

  1. Induction Disc Type
  2. Induction Cup Type


1. Induction Disc Type

In this relay. a metal disc is allowed to rotate between the two electromagnets. The electromagnets are energised by alternating currents. The two types of constructions used for this type are shaded pole type and watthour meter type. 

2. Induction Cup Type : 

In this relay, electromagnets act as a stator and energised by relay coils. The rotor is metallic cylindrical cup type. 

Directional Type Relays

These relays work on the direction of current or power flow in the circuit. The various types of these relays are 

  1. Reverse Current Type
  2. Reverse Power Type


1. Reverse Current Type : 

The relay is actuated when the direction of the current is reversed ur the phase of the current becomes more than the predetermined calve 

2. Reverse Power Type : 

The relay is actuated when the phase displacement between applied voltage and current attains a specified value. 

Relays Based on Timing

In relays the time between instant of relay operation and instant at which tripping of contacts takes place can be controlled. The time is called operation time. Based om this, the time relays are classified as, 
  1. Instantaneous Type
  2. Definite Time Lag Type
  3. Inverse Time Lag type


1. Instantaneous Type : 

In this type no time is lust between operation of relay and tripping of contacts. No intentional time delay is provided. 

2 Definite Time Lag Type : 

In this type intentionally a definite time lag is provided between operation of relay and tripping of contact. 

3 Inverse Time Lag type : 

In this type, the operating time Is approximately Inversely proportional to the magnitude of the actuating quantity. 

Distance Type Relays 

These relays work on the principle of measurement of voltage to current ratio. In this type, there are two coils. One coil is energized by current while other by voltage. The torque produced is  proportional to the ratio of the two quantities. When the ratio reduces below a set value, the relay operates. 

The various types of these relays are, 
  1. Impedance Type
  2. Reactance Type
  3. Admittance Type


1. Impedance Type : 

In this type, the ratio of voltage to current is nothing but an impedance which is proportional to the distance ol the relay form the fault point. 

2. Reactance Type : 

The operating time is proportional to the reactance which is proportional to the distance of the relay from the fault point. 

3. Admittance Type : 

This is also called mho type. In this type, the operating time is proportional to the admittance. 

Differential Type Relays 

A differential relay operates when the vector difference of two or more electrical quantities in the circuit in which relay is connected. exceeds a set value. These are classified as, 
  1. Current Differential Type
  2. Voltage Differential Type


1. Current Differential Type : 

In this type, the relay compares the current entering a section of the system and the current leaving the section. Under fault condition, these currents are different.

2. Voltage Differential Type :

In this type, two transformers are used. The secondaries of the transformers are connected in series with the relay in such a way that the induced e.m.fs are in opposition under normal conditions. Under fault condition, primaries carry different currents due to which induced e.m.f.s no longer remain in opposition and the relay operates. 

Other Types of Relays 

Various other types of relays which are used in practice are,

1. Under voltage, current, power relay : 
This relay operates when the voltage, current or power in a circuit falls below a set value

2. Over voltage, current, power relay : 
This relay actuates when the voltage, current or power In a circuit rises above a set value.

3. Thermal Relay : 
This relay actuates due to the heat produced by the current in the relay coil.

4. Rectifier Relay : 
In this relay, the quantities to be sensed are rectified and then given to the moving coil unit of the relay.

5. Permanent Magnet Moving Coil Relay : 
In this relay, the coil carrying current is free to rotate in the magnetic field of a permanent magnet. This is used for DC only.

6. Static Relay : 
This relay uses some electronic method for sensing the actuating quantity. It uses a stationary circuit.

7. Gas Operated Relay : 
The gas pressure is adjusted according to the variations in the actuating quantity. This gas pressure is used to actuate the relay. Buchholz relay is an example of such type of relay. 

Voltage Surge in Power System


What is Voltage Surge or Transient Voltage?

The sudden rise in voltage for a very short duration on power system is known as voltage surge or transient voltage.

Surges or transients can damage, degrade, or destroy electronic equipment within any home, commercial building, industrial, or manufacturing facility. Transients can reach amplitudes of tens of thousands of volts.  Surges are generally measured in microseconds (μs).

How does Surges Occur?

Transients or surges are of temporary nature and exist for a very short duration (a few hundred microseconds) but they cause overvoltages on the power system. 

They originate from switching and from other causes but by far the most important transients are those caused by lightning striking a transmission line. When lightning strikes a line, the surge rushes along the line, just as a flood of water rushes along a narrow valley when the retaining wall of a reservoir at its head suddenly gives way. 

In most of the cases, such surges may cause the line insulators (near the point where lightning has struck) to flash over and may also damage the nearby transformers, generators or other equipment connected to the line if the equipment is not suitably protected.

Lightning Surge



Voltage Surge Waveform
Voltage Surge Waveform
Waveform of a typical lightning Surge

The figure above shows the waveform of a typical lightning surge. The voltage build-up is taken along the y-axis and the time along the x-axis. 

It may be seen that lightning introduces a steep-fronted wave. The steeper the wavefront, the more rapid is the build-up of voltage at any point in the network. In most of the cases, this build-up is comparatively rapid, being of the order of l-5 μs. 

Voltage surges are generally specified in terms of the rise time t1 and the time t2 to decay to half of the peak value. For  example, a 1/50μs surge is one which reaches its maximum value in 1μs and decays to half of its peak value is 50μs.

Causes of Surges

60-80% of surges are created within a facility. A common source for surges generated inside a building are devices that switch power on and off. This can be anything from a simple thermostat switch operating a heating element to a switch-mode power supply found on many devices.  Surges that originate from outside the facility include those due to lightning and utility grid switching. 

Why Voltage Control is Important?



In a modern power system, electrical energy  from the generating station is delivered to the ultimate consumers through a network of transmission and distribution. 

For satisfactory operation of motors, lamps and other loads, it is desirable that consumers are supplied with substantially constant voltage. 

Too wide variations of voltage may cause erratic operation or even malfunctioning of consumer's appliances. 

To safeguard the interest of the consumers, the government in each country has enacted a law in this regard. The statutory limit of voltage variation is 6% of declared voltage at consumer's terminals.

Causes of Voltage Variation

The principal cause of voltage variation at consumer's premises is the change in load on the supply system. 

When the load on the system increases, the voltage at the consumer's terminals falls due to the increased voltage drop in 

(i) alternator synchronous impedance 
(ii) transmission line 
(iii) transformer impedance 
(iv) feeders and
(v) distributors. 

The reverse would happen should the load on the system decrease. These voltage variations are undesirable and must be kept within the prescribed limits (i.e. i 6% of the declared voltage). 

This is achieved by installing voltage regulating equipment at suitable places in the power system.

Importance of Voltage Control

When the load on the supply system changes, the voltage at the consumer's terminals also changes.

The variations of voltage at the consumer’s terminals are undesirable and must be kept within prescribed limits for the following reasons:

  • In case of lighting load, the lamp characteristics are very sensitive to changes of voltage. For instance, if the supply voltage to an incandescent lamp decreases by 6% of rated value, then illuminating power may decrease by 20%. On the other hand, if the supply voltage is 6% above the rated value, the life of the lamp may be reduced by 50% due to rapid deterioration of the filament.

  • In case of power load consisting of induction motors, the voltage variations may cause erratic operation. If the supply voltage is above the normal, the motor may operate with a saturated magnetic circuit, with consequent large magnetizing current, heating and low power factor. On the other hand, if the voltage is too lo\v, it will reduce the starting torque of the motor considerably.

  • Too wide variations of voltage cause excessive heating of distribution transformers. This may reduce their ratings to a considerable extent.

It is clear from the above discussion that voltage variations in a power system must be kept to minimum level in order to deliver good service to the consumers. 

With the trend towards larger and larger interconnected system, it has become necessary to employ appropriate methods of voltage control.

Practical Transformer on No Load

Please read Ideal Transformer and Practical Transformer before continuing.

Consider a practical transformer on no load i.e., secondary on open-circuit as shown in figure below.
The primary will draw a small current I0 to supply 
(i) the iron losses and 
(ii) a very small amount of copper loss in the primary. 

Hence the primary no load current I0 is not 90° behind the applied voltage V1 but lags it by an angle Φ0 < 90° as shown in the phasor diagram.

No load input power, W0 = VIcosΦ0    

As seen from the phasor diagram, the no-load primary current Ican be resolved into two rectangular components viz.


  1. Iw 
  2. Im 

(i) The component Iw in phase with the applied voltage V1. This is known as active or working or iron loss component and supplies the iron loss and a very small primary copper loss.

Iw = I0 cosΦ0   

(ii) The component Im lagging behind V1 by 90° and is known as magnetizing component. It is this component which produces the mutual flux Φ in the core.

Im = I0 sinΦ0   

Clearly, I0 is phasor sum of Im and Iw.

I0 = √ (Im2 +Iw2)

No load power factor, cosΦ0 = Iw∕I0  

It is emphasized here that no load primary copper loss (i.e. I02R1 ) is very small and may be neglected. Therefore, the no load primary input power is practically equal to the iron loss in the transformer i.e.,

No load input power, W0 = Iron loss

Note
At no load, there is no current in the secondary so that V2 = E2
On the primary side, the drops in R1 and X1, due to I0 are also very small because of the smallness of I0
Hence, we can say that at no load, V1 = E1.

Constant Current Driver Design based on LM317

Using a resistor to limit the current can be used for small LEDs. For such low power LEDs, where the supply current is not critical it works okay. 

But for high-power LEDs, a better way is to use the Constant Current Driver.

As the name suggests, the constant current driver will supply the same current whatever voltage it is supplied with and whatever the forward voltage of the LED. You just set the current and that is how much current will flow through the high-power LED.

A very useful IC that is often used for this purpose is the LM317. This IC is primarily intended as an adjustable voltage regulator, but can easily be adapted for use in regulating current.

Design 

Figure below shows the schematic diagram for regulating the current to a high-power LED.

The LM317 is very easy to use in a constant current mode. It will always strive to keep its output voltage at exactly 1.25V above whatever voltage the Adj (adjust) pin is at.

The LED we are going to use is a 1W white light LED. It has an If (forward current) of 300mA and a Vf (forward voltage) of 3.4V.

The formula for calculating the right value for R for use with the LM317 is:

R = 1.25V / I 
so in this case, R = 1.25 / 0.3 = 4.2Ω

If we used a standard resistor value of 4.7Ω, then this would reduce the current to:

I = 1.25V / 4.7Ω = 266 mA

Checking the power rating for the resistor, the LM317 will always have 1.25V between Out and Adj. So:
P = V × I = 1.25V × 266mA = 0.33W

A half-watt resistor will therefore be fine.

The LM317 also needs its input to be about 3V higher than its output to guarantee 1.25V between Adj and the output.

This means that a 6V battery would not be quite high enough because the forward voltage is 3.4V. However, we could drive the circuit using a 9V battery or even a 12V power supply without modification, since whatever the input voltage, the current will always be limited to about 260mA.

A quick calculation of the power consumed by the LM317 will reassure us that we are not going to come near exceeding its maximum power rating.

For a 9V battery, the voltage between In and Out will be 
9 – (1.25 + 3.4) = 4.35V. 

The current is 260mA, so the power is: 
4.35 × 0.26 = 1.13W.

According to its data sheet, the maximum power handling capability of the LM317 is 20W, and it can cope with a current of up to 2.2A for a supply voltage of less than 15V. So we are fine.


What Jobs Electrical Engineers Do? | Roles and Responsibilities

Have you ever thought about what jobs an electrical engineer do after their graduation. Then this article is for you. Keep reading. 



This article explains what are the roles and responsibilities or duties on different positions handled electrical engineers in the Electrical / Technical Industry. After going through this article you will get a clear idea about the various roles handled by electrical engineers in the industry.

Any electrical or technical organization is made up of a group of individuals with various duties, all working together for their own good, the good of their employer and their customers. 

Here is the list of some of those positions.

1. Design Engineer
2. Estimator / Cost Engineer
3. Contracts Manager
4. Project manager
5. Service manager
6. Technician
7. Supervisor / Foreman
8. Operative or skilled operative
9. Mechanic / Fitter
10. Maintenance Manager / Engineer 

There is often no clear distinction between the duties of the individual employees, each do some of the others work activities. Responsibilities vary, even by people holding the same job title and some individuals hold more than one job title. However, let us look at some of the roles and responsibilities of those working in the electrotechnical industry.

1. Design Engineer : Roles and Responsibilities

  • The design engineer will normally meet with clients and other trade professionals to interpret the customer's requirements.
  • He or she will produce the design specification which enables the cost of the project to be estimated.

 2. Estimator / Cost Engineer 

  • This person measures the quantities of labor and material necessary to complete the electrical project using the plans and specifications for the project.
  • From these calculations and the company’s fixed costs, a project cost can be agreed.

 3. Contracts Manager 

  • Contracts manager may oversee a number of electrical contracts on different sites
  • A contract manager will monitor progress in consultation with the project manager on behalf of the electrical companies.
  • They cost out variations to the initial contract.
  • Contract managers may have health and safety responsibilities because he or she has an overview of all company employees and contracts in progress

 4. Project manager

  • Project manager is responsible for the day-to-day management of one specific contract.
  • He/She will have overall responsibility on that site for the whole electrical installation.
  • Project manager attends site meetings with other trades as the representative of the electrical contractor.

 5. Service manager 

  • Service manager monitors the quality of the service delivered under the terms of the contractor
  • He/She checks that the contract targets are being met.
  • All Service managers checks that the customer is satisfied with all aspects of the project.
  • The Service Manager’s focus is customer specific while the Project Manager’s focus is job specific.

 6. Technician 

  • A technician will be more office based than site based
  • He/She will carry out surveys of electrical systems
  • update electrical drawings
  • obtain quotations from suppliers
  • He maintain records such as ISO 9000 quality systems.
  • carry out testing inspections and commissioning of electrical installation
  • troubleshoot

7. Supervisor / Foreman 

  • This person will probably be a mature electrician
  • have responsibility for small contracts
  • have responsibility for a small part of a large contract
  • be the leader of a small team (e.g. electrician and trainee) installing electrical systems.

8. Operative or skilled operative 

  • This person will carry out the electrical work under the direction and guidance of a supervisor
  • will demonstrate a high degree of skill and competence in electrical work
  • will have, or be working towards, a recognized electrical qualification and status as an electrician, approved electrician or electrical technician. 

 9. Mechanic / Fitter 

  • An operative who usually has a ‘ core skill ’ or ‘ basic skill ’ and qualification in mechanical rather than electrical engineering
  • in production or process work, he or she would have responsibility for the engineering and fitting aspects of the contract, while the electrician and instrumentation technician would take care of the electrical and instrumentation aspects
  • all three operatives must work closely in production and process work
  • ‘additional skilling’ or ‘multi-skilling’ training produces a more flexible operative for production and process plant operations 

 10. Maintenance Manager / Engineer 

  • Maintenance Engineer is responsible for keeping the installed electrical plant and equipment working efficiently.
  • He takes over from the builders and contractors the responsibility of maintaining all plant equipment and systems under his or her control
  • This engineer might be responsible for a hospital or a commercial building, a university or college complex.
  • Maintenance Engineers will set up routine and preventative maintenance programmes to reduce possible future breakdowns
  • When faults or breakdowns do occur he or she will be responsible for the repair using the company’s maintenance staff.


Why Current Transformer (CT) Seconday Should not be Open ?


It is very important that the secondary of Current Transformer should not be kept open. 

Either it should be shorted or must be connected in series with a low resistance coil such as current coils of watt meter, coil of ammeter etc.
Related: Why Transformers are Rated in kVA, Not in kW?
If it is left open, then current through secondary becomes zero hence the ampere turns produced by secondary which generally oppose primary ampere turns becomes zero. 

As there is no counter m.m.f., unopposed primary m.m.f. (ampere turns) produce high flux in the core. 

  • This produce excessive core losses, heating the core beyond limits. 
  • Similarly heavy e.m.f.s will be induced on the primary and secondary side. This may damage the insulation of the winding. 

This is danger from the operator point of view as well. It is usual to ground the C.T. on the secondary side to avoid a danger of shock to the operator. 

Hence never open the secondary winding circuit of a current transformer while its primary winding is energized. 

Thus most of the current transformers have a short circuit link or a switch at secondary terminals. When the primary is to be energized, the short circuit link must be closed so that there is no danger of open circuit secondary. 

Ideal Transformer and Practical Transformer

Ideal Transformer

An ideal transformer is one that has
  1. no winding resistance
  2. no leakage flux i.e., the same flux links both the windings
  3. no iron losses (i.e., eddy current and hysteresis losses) in the core

Although ideal transformer cannot be physically realized, yet its study provides a very powerful tool in the analysis of a practical transformer. In fact, practical transformers have properties that approach very close to an ideal transformer.



Consider an ideal transformer on no load i.e., secondary is open-circuited as shown in figure. Under such conditions, the primary is simply a coil of pure inductance. 

When an alternating voltage V₁ is applied to the primary, it draws a small magnetizing current Iₘ which lags behind the applied voltage by 90°. This alternating current Iₘ produces an alternating flux ϕ which is proportional to and in phase with it. 

The alternating flux ϕ links both the windings and induces e.m.f. E₁ in the primary and e.m.f. E₂ in the secondary. The primary e.m.f. E₁ is, at every instant, equal to and in opposition to V₁  (Lenz’s law). Both e.m.f.s E₁ and E₂ lag behind flux ϕ by 90°. However, their magnitudes depend upon the number of primary and secondary turns.

Phasor Diagram of Ideal Transformer

The phasor diagram of an ideal transformer on no load is also shown above.

Since flux ϕ is common to both the windings, it has been taken as the reference phasor. 

The primary e.m.f. E₁ and secondary e.m.f. E₂ lag behind the flux ϕ by 90°. 

Note that E₁ and E₂ are in phase. But E₁ is equal to V₁ and 180° out of phase with it.

Practical Transformer

A practical transformer differs from the ideal transformer in many respects. The practical transformer has 
  1. iron losses 
  2. winding resistances and 
  3. magnetic leakage, giving rise to leakage reactances.

1. Iron Losses 

Since the iron core is subjected to alternating flux, there occurs eddy current and hysteresis loss in it. These two losses together are known as iron losses or core losses. 

The iron losses depend upon the supply frequency, maximum flux density in the core, volume of the core etc. 

It may be noted that magnitude of iron losses is quite small in a practical transformer.

2. Winding resistances

Since the windings consist of copper conductors, it immediately follows that both primary and secondary will have winding resistance. The primary resistance R₁ and secondary resistance R₂ act in series with the respective windings as shown in figure. 

When current flows through the windings, there will be power loss as well as a loss in voltage due to IR drop. This will affect the power factor and E₁ will be less than V₁ while V₂ will be less than E₂. 



3. Leakage reactances

Both primary and secondary currents produce flux. The flux ϕ which links both the windings is the useful flux and is called mutual flux. 

However, primary current would produce some flux ϕ which would not link the secondary winding. Similarly, secondary current would produce some flux ϕ that would not link the primary winding. 

The flux such as ϕ₁ or ϕ₂ which links only one winding is called leakage flux. The leakage flux paths are mainly through the air. The effect of these leakage fluxes would be the same as though inductive reactance were connected in series with each winding of transformer that had no leakage flux as shown in figure. 

In other words, the effect of primary leakage flux ϕ₁ is to introduce an inductive reactance X₁ in series with the primary winding as shown. Similarly, the secondary leakage flux ϕ₂ introduces an inductive reactance X in series with the secondary  winding. 

There will be no power loss due to leakage reactance. However, the presence of leakage reactance in the windings changes the power factor as well as there is voltage loss due to IX drop.




Note
Although leakage flux in a transformer is quite small (about 5% of ϕ)  compared to the mutual flux ϕ, yet it cannot be ignored. It is because leakage flux paths are through air of high reluctance and hence require considerable e.m.f. It may be noted that energy is conveyed from the primary winding to the secondary winding by mutual flux f which links both the windings.


Previous
Editor's Choice