Transformers

Prepared by

V.BHUVANESWARI

TRANSFORMERS

• Transformer works on the principle of electromagnetic induction.

• Transformer Is an electrical device having no moving parts, it transfers

electric energy by mutual induction from one circuit to another at same

frequency with changed values of V and I.

• It consists of two winding insulated from each other and wound on a

common core made up of magnetic material.

• Alternating voltage is connected across one of the winding called primary

winding.

• In both the windings emf is induced by electromagnetic induction.

• And the second winding is the secondary winding of transformer.

WORKING PRINCIPLE OF TRANSFORMER

• When primary winding is connected to an ac source an exciting current flows through

the winding. As the current is alternating it will produce an alternating flux in the core

which will be linked by both the primary and secondary windings.

E1=V1

• Where ,E1 is induced emf in primary

V1 is applied voltage in primary

• Power transferred from the primary to the secondary circuit by electromagnetic

induction.

• Flux in the core will alternate at same frequency of the supply voltage.

• Frequency of induced emf in secondary is the same as that of the supply voltage.

• Magnitude of emf induced in secondary winding will depend upon its number of turns.

In a transformer if the number of turns in secondary winding is less than that in the

primary winding it is called step-down transformer.

In a transformer if the number of turns in secondary winding is higher than that in the

primary winding it is called step-up transformer.

CLASSIFICATION OF TRANSFORMERS

Transformers are classified on basis of

1. Duty they perform

2. Construction

3. Voltage output

4. Application

5. Cooling

6. Input supply

• Duty they perform

I. Power transformer – from transmission and distribution

II. Current transformer- instrument transformers

III. Potential transformer- instrument transformers

• Construction

I. Core type transformer

II. Shell type transformer

III. Berry type transformer

• Voltage output

I. Step down transformer(Higher to Lower)

II. Step up transformer(Lower to Higher)

III. Auto transformer(Variable from ‘0’ to rated value)

• Application

I. Welding transformer

II. Furnace transformer

• Cooling

I. Duct type transformer

II. Oil immersed transformer

self cooled

Forced air cooled

Water cooled

Forced oil cooled

• Input supply

I. Single phase transformer

II. Three phase transformer

star- star Delta-Star

Star-delta Open- Delta

Delta-delta Scott connection

Constructional details

Transformer is a static device and has no moving parts.

Main components of a transformer are:

• Magnetic core

• Primary and secondary windings

• Insulation of windings

• Expansion tank or conservator

• Tank , oil , cooling arrangement , temperature gauge, oil gauge

• Buchholz relay

• Silica gel breather

(1) MAGNETIC CORE

• Magnetic core consists of an iron core. The core is laminated and made of silicon steel.

• Thickness varies from 0.35mm to 0.5mm.

• Laminations are insulated from each other by coating then with a thin coat of varnish.

• Various types of stampings and laminations employed in the construction of transformers.

There are two types of transformer cores ,they are

(a) Shell type (b)Core type

Shell type – Two windings are carried by central limb. Core is made up of E and I stampings

and has three limbs. Has two parallel paths for magnetic flux.

Core type- Has two limbs for two windings and is made up of two L-type stampings. Has only

one magnetic path.

(2) Winding

• There are two windings in a transformer.

• They are primary and secondary windings.

• Made up of copper.

(3) Insulation

• Paper is still used as basic conductor insulation.

• For power transformers enamelled copper with paper insulation is also used.

(4) Insulating oil

• The coil used in transformer protects the paper from dirt and moisture and removes the heat

produced in the core and coils,

• It also acts as insulating medium.

Oil must possess following properties:

• High dielectric strength

• Free from inorganic acid , alkali and corrosive sulphur.

• Low viscosity to provide good heat transfer.

• Good resistance to emulsion so that the oil may throw down any moisture entering the tank

instead of holding it.

(5)EXPANSION TANK or Conservator

• A small auxiliary oil tank mounted above the

transformer and connected to main tank by a pipe.

• Its function is to keep transformer tank full of oil.

(6)TEMPERATURE GAUGE

• Is to indicate hot oil or hottest spot temperature.

• It is self contained weather proof unit made of alarm contacts.

(7) OIL GAUGE

• Every transformer is fitted with an oil gauge to indicate the oil level

present inside the tank.

• It is provided with an alarm contact which gives an alarm when the

oil level drops beyond permissible height due to oil leak or due to any

other reason.

(8) BUCHHOLZ RELAY

• First warning that fault is present is given by presence of bubbles in oil.

• It gives an alarm in case of minor fault and to disconnect transformer

From the supply mains in case of severe faults.

(9) Breather

• It is a chamber which prevents entry of moisture

inside the transformer tank.

• It is filled with drying agent such as calcium chloride

or silica gel.

This absorbs moisture and allows dry air to enter

transformer tank. It is replaced regularly.

EMF EQUATION OF TRANSFORMER

Wkt, T=1/f

Average emf (e)= dΦ/dt

dΦ = Φm

dt = 1/4f

Average rate of change of flux= Φm / (1/4f) = 4f Φm volts

Average emf induced per turn = Average rate of change of flux= 4f Φm volts

Form factor = RMS value/ Average value = 1.11

RMS value= Form factor x Average value

=1.11 x Average value

RMS value of emf induced/turn = 1.11 x 4f Φm = 4.44 f Φm volts

Primary and Secondary winding having N1 and N2 turns.

RMS value of emf induced Primary winding, E1 = 4.44 f Φm N1 volts

RMS value of emf induced Secondary winding, E1 = 4.44 f Φm N2 volts

TRANSFORMER RATIO

For an ideal transformer, E1 = V1 and E2 = V2

There is no voltage drop in the windings.

V1 I1 = V2 I2

V2 / V1 = E2 / E1 = I1 / I2 = N2 / N1 = K

V2 / V1 = K -> Voltage ratio

E2 / E1 = K -> Transformation ratio

N2 / N1 = K -> Turns ratio

I1 / I2 = K -> Current ratio

(i) If K > 1 , then the transformer is called step-up transformer.

(ii) If K < 1 , then the transformer is called step-down transformer.

RATING OF TRANSFORMER

• The rating of transformer is expressed by Volt- Ampere (VA)

• Cu loss depends on Current (A)

• Iron loss depends on Voltage (V)

• Total loss depends upon Volt- Ampere (VA)

• It is independent of Load power factor cos Φ.

Ideal transformer

Ideal transformer has following properties :

• No winding resistance i.e., purely inductive

• No magnetic leakage flux

• No cu loss

• No core loss

Ideal transformer secondary is open. Ac supply is connected to primary winding.

Current flows through primary winding. This current is called MAGNETISING CURRENT

(Iμ).

Value of Magnetising current is small. The Magnetising current produces an alternating

flux (Φ).

Iμ and Φ are in-phase. This changing flux links primary with secondary winding .

Due to alternating flux a self-induced emf (E1) is produced in primary winding which is

equal to and in opposition with V1. It is known as counter emf or back emf of primary

winding.

Induced emf E2 is produced in secondary winding because of alternating flux linking

with secondary winding. This emf is known as mutually induced emf.

PRACTICAL TRANSFORMER ON NO-LOAD

If the primary winding is connected to alternating voltage and secondary winding is left open then transformer is said to be on NO-LOAD. Since secondary is open this current is called no-load primary current (Io).

No load input power, P0= V1 I0 cos Φ0

Active or working or iron loss or wattful component (Iw) which is in-phase with ‘V1’ and

supplies iron loss and small amount of primary cu loss.

Iw = I0 cos Φ0

Where, cos Φ0 is no load power factor.

Reactive or magnetizing or wattles component Iμ

which is in quadrature with V1 and its function is

to sustain flux in core.

Iμ= I0 Sin Φ0

I0 = 𝐈𝐰𝟐 + 𝐈𝛍

𝟐

TRANSFORMER ON LOAD

When the secondary winding is connected to load then the transformer is said to be

on load.

Phase angle between V2 and I2 depends on type of load.

Resistive = I2 in-phase with V2

Load Inductive = I2 will lag V2

capacitive= I2 will lead V2

When transformer is loaded,

Flux is constant at no-load as well as at loaded condition, therefore transformer is

called as constant flux apparatus.

Total primary current will be vector sum of I0 and I2’

Transformer winding resistance

In practical transformer the winding have some resistances. Primary winding has

primary resistance(R2).

Transformer winding leakage reactance

Primary leakage flux(ΦL1) – All the flux generated by the primary winding does not

link with secondary winding. Some part of flux passes through air rather than

around the core. This flux is in-phase with I1.

Secondary leakage flux(ΦL2) – Leakage flux is set up in secondary winding. This

flux induces eL2 in secondary winding. This flux ΦL2 does not link with primary is

also in-phase with I2.

VOLTAGE REGULATION

Regulation of transformer is defined as reduction in magnitude of terminal

voltage due to load wrt no-load terminal voltage.

% Regulation = |V2 on no-load| - |V2 when loaded|

|V2 on no-load|

LAGGING POWER FACTOR

% Regulation = [I1 R01 cos Φ+ I1X01sin Φ ] X 100

V1

LEADING POWER FACTOR

% Regulation = [I1 R01 cos Φ - I1X01sin Φ ]

V1

UNITY POWER FACTOR

% Regulation = I1 R01

V1

EQUIVALENT CIRCUIT OF TRANSFORMER

Where V1 = Voltage applied to the primary winding. I0 = No load primary current. Im = Magnetizing component of no load primary current Ic = Core loss component of no load primary current I1 = Primary current on load I2 = Secondary load current V2 = Voltage across the secondary load terminals

R0 =Resistance representing the core loss

R1=Resistance of the primary winding

X1=Reactance of the primary winding

R2 =Resistance of the secondary winding

X2=Reactance of the secondary winding

E1 = E.M.F. induced in the primary winding

E2=E.M.F. induced in the secondary winding

• The equivalent circuit consists of two circuits, one representing the primary winding and

another is the secondary winding.

• The transfer of power from one circuit to other takes place due to mutual induction.

APPROXIMATE EQUIVALENT CIRCUIT OF TRANSFORMER

The equivalent circuit can be simplified by transferring the exciting branch to left position of

circuit. This circuit is known as approximate equivalent circuit of transformer.

EQUIVALENT CIRCUIT OF TRANSFORMER REFFERED TO PRIMARY

If all the secondary parameters are transferred to primary side we get equivalent circuit of

transformer referred to primary.

Here, Resistance and Reactance > Divided by K2

Voltages > Divided by K

Currents > Multiplied by K

Where, K is transformation ratio.

EQUIVALENT CIRCUIT OF TRANSFORMER REFFERED TO SECONDARY

Why is transformer rating in KVA?

• Cu loss depends on Current and Iron loss depends upon voltage.

• Hence total loss in transformer depends upon Volt- Ampere(VA) only and

not on the phase angle between voltage and current ie., it is independent

of load power factor.

TESTING OF TRANSFORMER

The performance of transformer can be calculated by following equivalent

parameters:

R01 – Equivalent Resistance referred to primary side

R02 – Equivalent Resistance referred to secondary side

X01 – Equivalent Leakage reactance referred to primary side

X02 - Equivalent Leakage reactance referred to secondary side

Rm or R0 – core loss resistance

Xm or X0 – Magnetising reactance

TESTING IN TRANSFORMER

TESTING

INDIRECT

OPEN CIRCUIT

TEST

SHORT CIRCUIT

TEST

DIRECT

LOAD TEST

OPEN CIRCUIT / NO-LOAD TEST ON TRANSFORMER

PURPOSE OF THIS TEST IS TO DETERMINE

• Core loss or Iron loss Or Magnetic loss (Pi)

• No load current (I0)

• Shunt branch parameters R0 and X0

One of the winding is kept open.

Rated voltage at rated frequency is applied to other(LV) winding.

A voltmeter, wattmeter, and an ammeter are connected in LV side of the

transformer.

Ammeter > Reads No-Load Current, I0

Voltmeter > Reads Applied Voltage, V0

Wattmeter> Reads No-Load Input Power, W0 or P0

DETERMINATION OF EQUIVALENT CIRCUIT CONSTANTS THROUGH NO- LOAD TEST

No load power factor, CosΦ0 = W0 / V0 I0

Core loss component, Iw = I0 CosΦ0

Magnetising component, Im = I0 SinΦ0

Core Loss, Pi = No load power (W0)

Core loss resistance, R0 = V0 / Iw = V0 / I0 CosΦ0

Magnetising reactance, X0= V0 / Im = V0 / I0 SinΦ0

SHORT CIRCUIT / IMPEDANCE TEST ON TRANSFORMER

PURPOSE OF THIS TEST IS TO DETERMINE

• Z01 or Z02 – Total impedance referred to either primary or secondary side

• R01 or R02- Total resistance referred to either primary or secondary side

• X01 or X02- Total reactance referred to either primary or secondary side

• Full load cu loss I22 R02

In this test one of the winding is short circuited by thick conductor.

Current rating of HV side is low compared with LV side.

Power input gives total cu loss at rated load.

Unity power factor wattmeter is used for measuring power in SC test.

DETERMINATION OF EQUIVALENT CIRCUIT CONSTANTS THROUGH LOAD TEST

SC power factor, CosΦsc = Wsc / Vsc Isc

Resistance of transformer referred to primary side , R01 = Wsc / (Isc)2

Reactance of transformer referred to primary side , X01= Z01 SinΦsc =

𝐙𝟎𝟏𝟐 − 𝐑𝟎𝟏

𝟐

Impedance of transformer referred to primary side, Z01 = Z01 Cos

Φsc= Vsc / Isc