Hysteresis Eddy Current Iron or Core Losses and Copper Loss in Transformer

As the electrical transformer is a static device, mechanical loss in
transformer normally does not come into picture. We generally consider
only electrical losses in transformer. Loss in any machine is broadly
defined as difference between input power and output power.

When input power is supplied to the primary of transformer, some
portion of that power is used to compensate core losses in transformer i.e.Hysteresis loss in transformer and Eddy Current loss in transformer core and some portion of the input power is lost as I²R loss and dissipated as heat in the primary and secondary winding, as because these windings have some internal resistance in them. The first one is called core loss or iron loss in transformerand later is known as ohmic loss or copper loss in transformer. Another loss occurs in transformer, known as Stray Loss, due to Stray fluxes link with the mechanical structure and winding conductors.

Copper loss in transformer

Copper loss is I²R loss, in primary side it is I₁²R₁ and in secondary side it is I₂²R₂loss, where I₁ & I₂ are primary & secondary current of transformer and R₁ & R₂are resistances of primary & secondary winding. As the both primary & secondary currents depend upon load of transformer, so copper loss in transformer vary with load.

Core losses in transformer

Hysteresis loss and eddy current loss, both depend upon magnetic properties of the materials used to construct the core of transformer and its design. So theselosses in transformer are fixed and do not depend upon the load current. So core losses in transformer which is alternatively known as iron loss in transformerand can be considered as constant for all range of load.

Hysteresis loss in transformer is denoted as,

W_h = K_hfB_m^1.6     watts

Eddy Current loss in transformer is denoted as,

W_e = K_ef²K_f²B_m²     watts

Where, K_h = Hysteresis Constant.

K_e = Eddy Current Constant.

K_f = form Constant.

Copper loss can simply be denoted as,

I_L²R₂′ + Stray loss

Where, I_L = I₂ = load of transformer, and R₂′ is the resistance of transformer referred to secondary.

Now we will discuss Hysteresis loss and Eddy Current loss in little bit more details for better understanding the topic of losses in transformer

Hysteresis loss in transformer

Hysteresis loss in transformer can be explained in different ways. We will discuss two of them, one is physical explanation other is mathematical explanation.

Physical explanation of Hysteresis loss

The magnetic core of transformer is made of ′Cold Rolled Grain Oriented Silicon Steel′. Steel is very good ferromagnetic material. This kind of materials are very sensitive to be magnetized. That means whenever magnetic flux passes through,it will behave like magnet. Ferromagnetic substances have numbers of domains in their structure. Domain are very small region in the material structure, where all the dipoles are paralleled to same direction. In other words, the domains are like small small permanent magnet situated randomly in the structure of substance. These domains are arranged inside the material structure in such a random manner, that net resultant magnetic field of the said material is zero. Whenever external magnetic field or mmf is is applied to that substance, these randomly directed domains are arranged themselves in parallel to the axis of applied mmf. After removing this external mmf, maximum numbers of domains again come to random positions, but some few of them still remain in their changed position. Because of these unchanged domains the substance becomes slightly magnetized permanently. This magnetism is called " Spontaneous Magnetism". To neutralize this magnetism some opposite mmf is required to be applied. The magneto motive force or mmf applied in the transformer core is alternating. For every cycle, due to this domain reversal there will be extra work done. For this reason, there will be a consumption of electrical energy which is known as Hysteresis loss of transformer.

Transformer Cooling System and Methods

The main source of heat generation in transformer is its copper loss or I²R loss. Although there are other factors contribute heat in transformer such as hysteresis & eddy current losses but contribution of I²R loss dominate them. If this heat is not dissipated properly, the temperature of the transformer will rise continually which may cause damages in paper insulation and liquid insulation medium of transformer. So it is essential to control the temperature within permissible limit to ensure the long life of transformer by reducing thermal degradation of its insulation system. In Electrical Power transformer we use external transformer cooling system to accelerate the dissipation rate of heat of transformer.There are different transformer cooling methods available for trans former, we will now explain one by one.

Different Transformer Cooling Methods

For accelerating cooling different transformer cooling methods are used depending upon their size and ratings. We will discuss these one by one below,

ONAN Cooling of Transformer

This is the simplesttransformer cooling system. The full form of ONAN is "Oil Natural Air Natural". Here natural convectional flow of hot oil is utilized for cooling. In convectional circulation of oil, the hot oil flows to the upper portion of the transformer tank and the vacant place is occupied by cold oil. This hot oil which comes to upper side, will dissipate heat in the atmosphere by natural conduction, convection & radiation in air and will become cold. In this way the oil in the transformer tank continually circulate when the transformer put into load. As the rate of dissipation of heat in air depends upon dissipating surface of the oil tank, it is essential to increase the effective surface area of the tank. So additional dissipating surface in the form of tubes or radiators connected to the transformer tank. This is known as radiator of transformer or radiator bank of transformer. We have shown below a simplest form on Natural Cooling or ONAN Cooling arrangement of an earthing transformer below.

ONAF Cooling of Transformer

Heat dissipation can obviously be increased, if dissipating surface is increased but it can be make further faster by applying forced air flow on that dissipating surface. Fans blowing air on cooling surface is employed. Forced air takes away the heat from the surface of radiator and provides better cooling than natural air. The full form of ONAF is "Oil Natural Air Forced". As the heat dissipation rate is faster and more in ONAF transformer cooling method than ONAN cooling system, electrical power transformer can be put into more load without crossing the permissible temperature limits.

OFAF Cooling of Transformer

In Oil Forced Air Natural cooling system of transformer, the heat dissipation is accelerated by using forced air on the dissipating surface but circulation of the hot oil in transformer tank is natural convectional flow.

The heat dissipation rate can be still increased further if this oil circulation is accelerated by applying some force. In OFAF cooling system the oil is forced to circulate within the closed loop of transformer tank by means of oil pumps. OFAF means "Oil Forced Air Forced" cooling methods of transformer. The main advantage of this system is that it is compact system and for same cooling capacity OFAF occupies much less space than farmer two systems of transformer cooling. Actually in Oil Natural cooling system, the heat comes out from conducting part of the transformer is displaced from its position, in slower rate due to convectional flow of oil but in forced oil cooling system the heat is displaced from its origin as soon as it comes out in the oil, hence rate of cooling becomes faster.

OFWF Cooling of Transformer

We know that ambient temperature of water is much less than the atmospheric air in same weather condition. So water may be used as better heat exchanger media than air. In OFWF cooling system of transformer, the hot oil is sent to a oil to water heat exchanger by means of oil pump and there the oil is cooled by applying sowers of cold water on the heat exchanger’s oil pipes. OFWF means "Oil Forced Water Forced" cooling in transformer.

ODAF Cooling of Transformer

ODAF or Oil Directed Air Forced Cooling of Transformer can be considered as the improved version of OFAF. Here forced circulation of oil directed to flow through predetermined paths in transformer winding. The cool oil entering the transformer tank from cooler or radiator is passed through the winding where gaps for oil flow or pre-decided oil flowing paths between insulated conductor are provided for ensuring faster rate of heat transfer. ODAF or Oil Directed Air Forced Cooling of Transformer is generally used in very high rating transformer.

ODWF Cooling of Transformer

ODAF or Oil Directed Water Forced Cooling of Transformer is just like ODAF only difference is that here the hot oil is cooled in cooler by means of forced water instead of air. Both of these transformer cooling methods are called Forced Directed Oil Cooling of transformer

Comparison between single Three Phase transformer and bank of three Single Phase transformers for three phase system

It is found that generation, transmission and distribution of electrical power are more economical in three phase system than single phase system. For three phase system three single phase transformers are required. Three phase transformation can be done in two ways, by using singlethree phase transformer or by using a bank of three single phase transformers. Both are having some advantages over other. Single 3 phase transformer costs around 15% less than bank of three single phase transformers. Again former occupies less space than later. For very big transformer, it is impossible to transport large three phase transformer to the site and it is easier to transportthree single phase transformers which is erected separately to form a three phase unit. Another advantage of using bank of three single phase transformersis that, if one unit of the bank becomes out of order, then the bank can be run as open delta.

Connection of Three Phase Transformer

A Verity of connection of three phase transformer are possible on each side of both a single 3 phase transformer or a bank of three single phase transformers.

Marking or labeling the different terminals of transformer.

Terminals of each phase of HV side should be labeled as capital letters, A, B, C, and those of LV side should be labeled as small letters, a, b, c. Terminal polarities are indicated by suffixes 1 & 2. Suffix 1’s indicate similar polarity ends and so do 2’s.

Star Star Transformer

Star Star Transformer is formed in a 3 phase transformer by connecting one terminal of each phase of individual side, together. The common terminal is indicated by suffix 1 in the figure below. If terminal with suffix 1 in both primary and secondary are used as common terminal, voltages of primary and secondary are in same phase. That is why this connection is called zero degree connection or 0^o – connection.

If the terminals with suffix 1 is connected together in HV side as common point and the terminals with suffix 2 in LV side are connected together as common point, the voltages in primary and secondary will be in opposite phase. Hence, Star Star Transformer connection is called 180^o – Connection, of three phase transformer.

Delta Detla Transformer

In delta delta transformer, 1 suffixed terminals of each phase primary winding will be connected with 2 suffixed terminal of next phase primary winding.

If primary is HV side, then A₁ will be connected to B₂, B₁ will be connected to C₂ and C₁ will be connected to A₂. Similarly in LV side 1 suffixed terminals of each phase winding will be connected with 2 suffixed terminals of next phase winding. That means, a₁ will be connected to b₂, b₁will be connected to c₂ and c₁ will be connected to a₂. If transformer leads are taken out from primary and secondary 2 suffixed terminals of the winding, then there will be no phase difference between similar line voltages in primary and secondary. This delta delta transformerconnection is zero degree connection or 0^o – Connection.

But in LV side of transformer, if, a₂ is connected to b₁, b₂ is connected to c₁ and c₂ is connected to a₁. The secondary leads of transformer are taken out from 2 suffixed terminals of LV windings, and then similar line voltages in primary and secondary will be in phase opposition. This connection is called 180^o – Connection, of three phase transformer.

Star Delta Transformer

Here in star delta transformer, star connection in HV side is formed by connecting all the 1 suffixed terminals together as common point and transformer primary leads are taken out from 2 suffixed terminals of primary windings.

The delta connection in LV side is formed by connecting 1 suffixed terminals of each phase LV winding with 2 suffixed terminal of next phase LV winding. More clearly, a₁ is connected to b₂, b₁ is connected to c₂ and c₁is connected to a₂. The secondary (here it considered as LV) leads are taken out from 2 suffixed ends of the secondary windings of transformer. The transformer connection diagram is shown in the figure beside. It is seen fron the figure that the sum of the voltages in delta side is zero. This is a must as otherwise closed delta would mean a short circuit. It is also observed from the phasor diagram that, phase to neutral voltage (equivalent star basis) on the delta side lags by − 30^o to the phase to neutral voltage on the star side; this is also the phase relationship between the respective line to line voltages. This star delta transformerconnection is therefore known as − 30^o – Connection.

Star – Delta + 30^o connection is also possible by connecting secondary terminals in following sequence. a₂ is connected to b₁, b₂ is connected to c₁ and c₂ is connected to a₁. The secondary leads of transformer are taken out from 2 suffixed terminals of LV windings,

Delta Star Transformer

Delta star transformerconnection of three phase transformer is similar to star – delta connection. If any one interchanges HV side and LV side of star – delta transformer in diagram, it simply becomes delta – star connected 3 phase transformer. That means all small letters of star delta connection should be replaced by capital letters and all small letters by capital in delta star transformer connection.

Material for Transformer Core

The main problem with transformer core is, its hysteresis loss and eddy current loss in transformer. Hysteresis loss in transformer mainly depends upon its core materials. It is found that a small quantity of silicon alloyed with low carbon content steel produces, material for transformer core which has low hysteresis loss and high permeability. As the increasing demand of power ratings, it is required to further reduce the core losses and for that aother technique is employed on steel, which is known as cold rolling. This technique arrange the orientation of grain in ferromagnetic steel in the direction of rolling.

The core steel which has under gone through the both silicon alloying and cold rolling treatments, is commonly known as CRGOS or Cold Rolled Grain Oriented Silicon Steel. This material is now universally used for manufacturing for transformer core.

Although this material has low specific iron loss but still it has some disadvantages,
Like it is susceptible to increase loss due to flux flow in direction other than grain orientation and it also susceptible to impaired performance due to impact of bending, blanking the cutting CRGOS sheet. Both surfaces of the sheets are provided with an insulating of oxide coating.

Optimum Design of Cross – Section of Transformer Core

The maximum flux density of CRGO steel is about 1.9 Tesla. Means the steel becomes saturated at the flux density 1.9 Tesla. One important criteria for design of transformer core, is that, it must not be saturated during transformer’s normal operation mode. Voltages of transformer depend upon its total magnetizing flux. Total magnetizing flux through core is nothing but product of flux density and cross – sectional area of the core. Hence, flux density of a core can be controlled by adjusting the cross sectional area of the core during its design.

The idea shape of cross – section of a transformer core is circular. For making perfect circular cross section, each and every successive lamination steel sheet should be cut in different dimension and size. This is absolutely uneconomical for practical manufacturing. In reality, manufacturers use different groups or packets of predefined number of same dimension lamination sheets. The group or packet is a block of laminated sheets with a predefined optimum height (thickness). The core is assembly of these blocks in such a successive manner as per their size from core central line that it gives a optimum circular shape of the cross – section. Such typical cross – section is shown in the figure below.

Oil ducts are needed for cooling the core. Cooling ducts are necessary because hot – spot temperature may rise dangerously high and their number depends on the core diameter, materials used for core. In addition to that clamp plates made of steel are needed on either sides of the core to clamping the lamination. The sheet steel lamination blocks, oil ducts, and clamping plates all should lie within the peripheral of optimum core circle.

The net sectional area is calculated from the dimensions of various packets and allowance is made for the space lost between lamination (known as stacking factor ) which for sheet steel of 0.28 mm thickness with insulation coating is approximately 0.96. Area is also deducted for oil ducts. The ratio of net cross sectional area of core to the gross cross – sectional area inside the imaginary peripheral circle is known as Utilization Factor of transformer core. By increasing numbers of steps of improves the Utilization Factor but at the same time it increases manufacturing cost. Optimum numbers of steps are between 6 (for smaller diameter) to 15 (larger diameter).

Manufacturing of transformer core

During core manufacturing in factory some factors are taken into consideration,

a) Higher reliability

b) Reduction in iron loss in transformer and magnetizing current

c) Lowering material cost and labor cost

d) abatement of noise levels

Quality checking is necessary at very step of manufacturing to ensure quality and reliability. The sheet steel must be tested for ensuring the specific core loss or iron loss values. The lamination should be properly checked and inspected visually, rusty and bend lamination to be rejected. For reducing the transformer noises the lamination should be tightly clamped together and punch holes should be avoided as far as possible to minimize cross flux iron losses. The air gap a the joint of limbs and yokes should be reduced as much as possible for allowing maximum smooth conducting paths for magnetizing current.

Corner Jointing of Limbs with Yokes

Core losses in transformer mainly due to,
1) magnetic flux flow along the direction of the grain orientation,
2) magnetic flux flow perpendicular to the direction of the grain orientation, this is also known as cross grain iron losses. The cross grain loss mainly occurs in the zones of Corner Jointing of Limbs with Yokes and it can be controlled to some extent by applying special corner jointing techniques. There are normally two types of joints used in transformer core

1) Interleaved Joints
2) Mited Joints

Interleaved Joints in transformer core

Interleaved Joint in transformer core is the simplest form of joints. This joint is is shown in the figure. The flux leaves and enters at the joint in perpendicular to grain orientation. Hence Cross Grain losses is high in this type of joints. But considering the low manufacturing cost it is preferable to use in small rating transformer.

Mitred Joints in transformer core

Here the lamination’s are cut at 45^o. The limbs and yoke lamination edges are placed face to face at the Mitred Joints in transformer core. Here the flux enters and leaves the lamination gets smooth path in the direction of its flow. Hence cross grain loss is minimum here. However it involves extra manufacturing cost but it is preferable to use in electrical power transformer where loss minimization is of the main criteria of design of transformer corial for Transformer Core

Core of Transformer and Design of Transformer Core

               In a electrical power transformerthere are primary, secondary and may be tertiary windings. The performance of a transformer mainly depends upon the flux linkages between these windings. For efficient flux linking between these winding one low reluctance magnetic path common to all windings, should be provided in the transformer. This low reluctance magnetic path in transformer is known as core of transformer.

                               

Influence of Diameter of Transformer Core

Let us consider, the diameter of transformer core be ′D′

Then, cross-sectional area of the core,

Now, voltage per turn

E = 4.44.φ_m.f

= 4.44.A.B_m.f

Where B_m is the maximum flux density of the core.

E is proportional to D²

Therefore voltage per turn is increased with increase in diameter of transformer core
Again if voltage across the winding of transformer is V

Then V = eN, where N is the number of turns in winding

If V is constant e is inversely proportional to N. And hence D² is inversely proportional to N. So diameter of the core is increased, the number of turns in the transformer winding reduced. Reduction of number of turns, reduction in height of the core legs. In-spite of reduction of core legs height, increased in core diameter, results, increased in overall diameter of magnetic core of transformer. This increased steel weight ultimately leads to increased core losses in transformer. Increased diameter of the core leads to increase the mean diameter on the winding. In – spite of increased diameter of the winding turns, reduced the number of turns in the winding, leads to less copper loss in transformer.

So we go on increasing diameter of the transformer core, losses in the transformer core will be increased but at the same time load loss or copper loss in transformer is reduced. On the other hand if diameter of the core is decreased, the weight of the steel in the core is reduced which leads to less core loss of transformer, but in the same time this leads to increase in number of turns in the winding, means increase in copper weight, which leads to extra copper loss in transformer. So diameter of the core must be optimized during design of transformer core, considering the both aspect.  

difference between mp&mc

When you start learning about  Microcontrollers(mc) (usually  8051) , the doubt will arise in your mind “hey… what’s the difference  between microprocessor and micro controller” ? In this article this is going to be  explaining i.e., the basic differences and similarities between a microprocessor and micro controller. In fact you can call this article a simple comparison of both micro computing devices. This comparison will be same (at the basic level) for any micro processor and controller.  So lets start.

At the basic level, a microprocessor and micro controller exist for performing some operations – they are – fetching instructions from the memory and executing these instruction (arithmetic or logic operations) and the result of these executions are used to serve to output devices. Are you clear? Both devices are capable of continuously fetching instructions from memory and keep on executing these instructions as long as the power is not turned off. Instructions are  electronic instructions represented by a group of bits. These instructions are always fetched from their storage area, which is named as memory.  Now lets take a  look at block diagrams of a microprocessor based system and a micro controller based system.

Microprocessor based system

Take a closer look at the block diagram and you will see a micro processor has many support devices like Read only memory, Read-Write memory, Serial interface, Timer, Input/Output ports etc. All these support devices are interfaced to microprocessor via a system bus. So one point is clear now, all support devices in a microprocessor based system are external.  The system bus is composed of an address bus, data bus and control bus.

Okay, now lets take a look at the microcontroller.

Micro controller system

The above block diagram shows a micro controller system in general. What’s the primary difference you see? All the support devices like Read only memory, Read – Write memory, Timer, Serial interface, I/O ports are internal. There is no need of interfacing these support devices and this saves a lot of time for the individual who creates the system. You got the basic understanding ? A micro controller is nothing but a microprocessor system with all support devices integrated inside a single chip. There is no need of any external interfacing in a micro controller unless you desire to create something beyond the limit, like interfacing an external memory or DAC/ADC unit etc. To make this microcontroller function, you need to give a DC power supply, a reset circuit and a quartz crystal (system clock) from external source.

Okay, so we have an idea about the basic difference between a microprocessor and microcontroller. Now lets compare some features of both systems.

Comparison

As you already know, support devices are external in a microprocessor based system where as support devices are internal for a micro controller. Micro controllers offer software protection where as micro processor base system fails to offer a protection system. This is made possible in microcontrollers by locking the on-chip program memory which makes it impossible to read using an external circuit. Okay! So that are basic differences, now you can come up with some more. As we need to interface support devices externally in a microprocessor based system, time required to build the circuit will be more, the size will be more and power consumption will be more in a microprocessor based system compared to microcontroller.