ELECTRICAL ENGINEERING DEPARTMENT�Semester: 4th �Subject: Energy conversion – I�Topic: TRANSFORMER �CHAPTER-3�AY: 2021 – 22
PREPARED By
ER SUBHALAXMI ROUT
TRANSFORMER
Types of transformers
Transformers Based on Voltage Levels
Step-Up Transformer
Step-Down Transformer
Transformer Based on the Core Medium Used
Air Core Transformer
Iron Core Transformer
Types of transformers
Transformers Based on Winding Arrangement
AutoTransformer
Transformers Based on Usage
Power Transformer
Distribution Transformer
Types of transformers
Measurement Transformer
Current Transformer
Potential Transformer
Protection Transformers
Constructional details
In all types of transformer construction, the central iron core is constructed from of a highly permeable material made from thin silicon steel laminations.
These steel transformer laminations vary in thickness’s from between 0.25mm to 0.5mm
Minimization of hysteresis loss
Every Ferromagnetic material used in transformer cores exhibits hysteresis phenomena.Hysteresis loss is caused by the magnetization and demagnetization of the core as current flows in the forward and reverse directions.
Pb = η * Bmaxn * f * V
choose a core material that has low Kh and high permeability. usually silicon steel and Cold Rolled Grain Oriented steel are used.
Minimization of hysteresis loss
Remedies to reduce hysteresis loss
Air core transformer eliminates loss due to hysteresis in the core material but has more leakage flux. Air core provides very low inductance in most situations. Hence it is not a plausible solution.
Another remedy is to use soft magnetic materials with low hysteresis, such as silicon steel, steel alloys, Mn-Zn ferrite,. Soft magnetic materials are optimal to be used in transformer core because of following advantages
High saturation magnetization, hence the core saturation happens at higher magnetic fields
They are characterized by Low coercivity and remanent magnetic flux density, which means low hysteresis losses.
High resistivity
High magnetic permeability’s e.t.c.
Minimization of eddy current loss
In order to reduce the eddy current loss, the resistance of the core should be increased. In other words, low reluctance should be retained.
In devices like transformers, the core is made up of laminations of iron. ie,the core is made up of thin sheets of steel, each lamination being insulated from others.
As the laminations are thin, they will have relatively high resistance.
Each lamination sheet will have an eddy current circulates within it.
The sum of individual eddy current of the laminations are very less compared to that of using single solid iron core.
The eddy current loss is proportional to f2. So at higher frequencies, the eddy current loss is very high.
Under such conditions, the use of lamination sheets are not enough.
E. M. F. Equation
As shown in the above figure that the flux changes from + ϕm to – ϕm in half a cycle of 1/2f seconds.
By Faraday’s Law
Let E1 is the emf induced in the primary winding
Where Ψ = N1ϕ�Since ϕ is due to AC supply ϕ = ϕm Sinwt
So the induced emf lags flux by 90 degrees.
Maximum valve of emf
E. M. F. Equation
But w = 2πf
Root mean square RMS value is
Putting the value of E1max in equation (6) we get
Putting the value of π = 3.14 in the equation (7) we will get the value of E1 as
Similarly
Operation on no load
consider one electrical transformer with only core losses, which means, it has only core losses but no copper loss and no leakage reactance of transformer.
Total current supplied from the source has two components, one is magnetizing current which is merely utilized for magnetizing the core, and another component of the source current is consumed for compensating the core losses in transformers.
source current is not exactly at 90° lags of the supply voltage, but it lags behind an angle θ is less than 90o.
If total current supplied from source is Io, it will have one component in phase with supply voltage V1 and this component of the current Iw is core loss component.
This component is taken in phase with the source voltage because it is associated with active or working losses in transformers. Another component of the source current is denoted as Iμ. This component produces the alternating magnetic flux in the core, so it is watt-less; means it is a reactive part of the transformer source current. Hence Iμ will be in quadrature with V1 and in phase with alternating flux Φ.
Phasor diagrams
Operation on load
As I2 is flowing through the secondary, a self mmf in secondary winding will be produced. Here it is N2I2, where, N2 is the number of turns of the secondary winding of transformer.
Phasor diagrams
Equivalent circuit
Where,�R1 = Primary Winding Resistance.�R2= Secondary winding Resistance.�I0= No-load current.�Iµ = Magnetizing Component,�Iw = Working Component,
This Iµ & Iw are connected in parallel across the primary circuit. The value of E1 ( Primary e.m.f ) is obtained by subtracting vectorially I1 Z1 from V1 . The value of X0 = E1 / I0 and R0 = E1 /Iw. We know that the relation of E1 and E2 is E2 /E1 = N2 /N1 = K , ( transformation Ratio )
Equivalent circuit
The secondary circuit is shown in fig-1. and its equivalent primary value is shown in fig- 2,
The total equivalent circuit of the transformer is obtained by adding in the primary impedance as shown in – Fig-3 .
Equivalent circuit
And It can be simplified the terminals shown in fig – 4 & further simplify the equivalent circuit is shown in fig.- 5 ,
At last, the circuit is simplified by omitting I0 altogether as shown in fig- 5 .
Losses & Efficiency
Losses of transformer are divided mainly into two types:
1. Iron Loss
2. Copper Losses
Iron Loss:
This is the power loss that occurs in the iron part. This loss is due to the alternating frequency of the emf. Iron loss in further classified into two other losses.
a) EDDY CURRENT LOSS: This power loss is due to the alternating flux linking the core, which will induced an emf in the core called the eddy emf, due to which a current called the eddy current is being circulated in the core. As there is some resistance in the core with this eddy current circulation converts into heat called the eddy current power loss.
Eddy current loss is proportional to the square of the supply frequency.
b) HYSTERISIS LOSS: This is the loss in the iron core, due to the magnetic reversal of the flux in the core, which results in the form of heat in the core. This loss is directly proportional to the supply frequency.
Eddy current loss can be minimized by using the core made of thin sheets of silicon steel material, and each lamination is coated with varnish insulation to suppress the path of the eddy currents.
Hysterisis loss can be minimized by using the core material having high Permeability
Copper Loss:
This is the power loss that occurs in the primary and secondary coils when the transformer is on load. This power is wasted in the form of heat due to the resistance of the coils. This loss is proportional to the sequence of the load hence it is called the Variable loss where as the Iron loss is called as the Constant loss as the supply voltageand frequency are constants
Efficiency
The efficiency of a transformer at a particular land and power factor is defined as the ratio of power output to power input .
���Condition for maximum efficiency
Condition for maximum efficiency:
Iron losses, Pi = hysteresis loss + eddy current loss
=Ph + Pe
Copper losses, Pc = I12R01 or I22R02
Considering primary side:
Input to primary = V1I1 cos ɸ1
Efficiency,
Variation of efficiency with power factor
Variation of efficiency with power factor. We know that transformers efficiency,
The variations of efficiency with power factor at different loading on a typical transformer are shown in Fig. 41.
Regulation
Voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component.
Voltage regulation at Lagging Power factor
Angle between OC and OD may be very small, so it can be neglected and OD is considered nearly equal to OC i.e.
Voltage regulation of transformer at lagging power factor,
Regulation
Voltage Regulation for Leading Power Factor
Angle between OC and OD may be very small, so it can be neglected and OD is considered nearly equal to OC i.e.
Voltage regulation of transformer at leading power factor,
All day efficiency
It is defined as the ratio of output power to the input power in kWh or wh of the transformer over 24 hours.
Some transformer efficiency cannot be judged by simple commercial efficiency as the load on certain transformer fluctuate throughout the day.
Thus, the iron or core loss occurs for the whole day in the distribution transformer
The copper loss occurs only when the transformers are in the loaded condition.
Hence, the performance of such transformers cannot be judged by the commercial or ordinary efficiency, but the efficiency is calculated or judged by All Day Efficiency also known as operational efficiency or energy efficiency which is computed by energy consumed during 24 hours.
Effect of variations of frequency & supply voltage on iron losses
As long as the flux variations are sinusoidal with respect to line, hysteresis loss (P), and eddy current loss (P) varies according to the following relations
Where x lies between 1.5 and 2.5 depending on the grade of iron used in transformer core
and
If the transformer is operated with the frequency and Voltage changed in the same proportion, the flux density will remain unchanged as obvious from Eq. (10.2) and apparently the no-load current will also remain unaffected.
Effect of variations of frequency & supply voltage on iron losses
The transformer can be operated safely at frequency less than rated one with correspondingly reduced voltage. In this case iron losses will be reduced. But if the transformer is operated with increased Voltage and frequency in the same proportion, the core losses may increase to an intolerable level. Increase in frequency with constant supply voltage will cause reduction in hysteresis loss and leave the eddy current losses unaffected. Some increase in voltage could, therefore, be tolerated at higher frequencies, but exactly how much depends on the relative magnitude of the hysteresis and eddy current losses and the grade of iron used in the transformer core.
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