Transformer

Sobuj Kumar RayEEN 257, Machine-I

Transformer

DEFINATION

Transformer is a static piece of apparatus by means of which power in one circuit is transformed into electric power of the same frequency in another circuit.

Working PrincipleIt consists of two inductive coils which are

electrically separated but magnetically linked. The two coils possess high mutual inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set up in the laminated core, most of which is linked with the other coil in which it produces mutually induced e.m.f according to Faraday’s laws. If the second coil ckt is closed a current flows in it and electric energy is transferred from the first coil to second coil. The first coil, in which electric energy is fed from the ac supply mains, is called the primary winding and the other from which energy is drawn out, is called secondary winding.

Transformer

Transformer

In brief, a transformer is a device thata) TX transfers electric power from one ckt to another.b) It does so without a change of frequency.c) It accomplishes this by electromagnetic induction.d) Where the two electric ckt are in mutual inductive influence of each

other.

Components of a Transformer

• Winding• Iron core• Transformer tank• Yoke• Conservator• Transformer Oil• Bushing• Oil level indicator

• Breather• Radiator tubes for cooling• Tap changer• Buchholz relay

7

Transformer Construction

The simple element of a transformer consists of two coils having mutual inductance and a

laminated steel core.Necessary part are:

1. Some suitable container for assemble core and winding

2. A suitable medium for insulating the core and its winding from container

3. Suitable bushing for insulating and bringing out the terminals of winding from the tank.

Transformer Construction

Transformer Construction

The eddy current loss is minimize by laminating the core, the lamination being insulated from each other by alight coat of core- plate vanish

or by an oxide layer on the surface.The thickness 0.35mm for 50Hz frequency

0.50 for 25Hz frequency

Transformer Construction

Transformer Construction

Transformer ConstructionConstructionally, the TX are of two general types. The two type are known

as : a) Core-type Transformerb) Shell-type Transformer

• Core-type Transformer-Winding surrounds a considerable part of Core.• Shell-type Transformer-Core Surrounds a considerable part of Winding.

Core-type &Shell-type Transformer Construction

Shape of core

Joint of core

Bushing

• The bushing provides insulation of the terminals of winding. These bushings are made of well china clay. These are housed on the upper side of tank through gasket. Service line and terminals of the winding is connected through bushing.

• High side bushings are large and low side bushings are small  

17Low voltage bushing High voltage bushing

Conservator

• It is transformer oil storage tank and if the main tank oil reduces the conservator will feed the oil to main tank.

• And if the oil get expansion that oil will go to conservator

18

Fig: Conservator

Breather

• A transformer breather is an accessory of an oil filled type transformer which is attached into the oil conservator tank.

• It protect moisture entering the conservator tank.

• In the refrigeration breather system, an air dryer is fitted to the conservator vessel.

Fig: Breather

19

Assembling of Transformer

Elementary Theory of an Ideal Transformer

An ideal Transformer is one which has no magnetic leakage and hence which has no cupper and core losses.

Elementary Theory of an Ideal Transformer

Elementary Theory of an Ideal Transformer

E.M.F Equation of a Transformer

Let• N1= No of turns in

primary• N2= No of turns

secondary• Φm=maximum flux in

core

Voltage transformer ratio

Transformer on No-load

At no-load condition the primary current has to supply

1. Iron loss in core i.e hysteresis loss and eddy current loss

2. Small amount of copper loss in primary

Transformer on load

When the secondary is loaded the magnitude & phase of I2 with respect to

v2 is determined by the characteristic of the load.

1. Current I2 is phase with v2 if load is non inductive

2. It lags if load is inductive3. It leads if load is

capacitive

Vector diagram for a loaded transformer

The total primary current is the vector sum of I0 and

Transformer with winding resistance but mo magnetic leakage

𝑉2 = 𝐸2 − 𝐼2𝑅2 𝐸1 = 𝑉1 − 𝐼1𝑅1

Equivalent circuit

Magnetic leakage

The flux linked with primary does not link the secondary but part of it complete its magnetic circuit by passing through air rather the core. This flux is known as leakage flux. This flux is directly proportional to the e primary ampere turn.

Leakage flux equivalent circuit

A transformer with magnetic leakage is equivalent to an ideal transformer with inductive coils connected in both primary and secondary circuits.

Equivalent circuit of transformer with resistance and leakage reactance

𝑍1 = ට𝑅12 + 𝑋12

𝑋2 = ට𝑅22 + 𝑋22

Vector diagram with resistance and leakage reactance

𝑉1 = 𝐸1 + 𝐼1𝑅1 + 𝑗𝐼1𝑋1= 𝐸1 + 𝐼1 𝑍1 𝐸2 = 𝑉2 + 𝐼2𝑅2 + 𝑗𝐼2𝑋2= 𝑉2 + 𝐼2𝑍2

Equivalent circuit with resistance and leakage reactance

𝑍01 = ට𝑅012 + 𝑋012

𝑍02 = ට𝑅022 + 𝑋022

Equivalent circuit

𝑋0 = 𝐸1𝐼0

𝑅0 = 𝐸1𝐼𝑤

Equivalent circuit

Transformer testThe purpose of this is to

determine core loss.High voltage winding left open

and other is connected to its supply of normal voltage and frequency.

No load current is small(2 to 10% of rated current) Cu loss is negligible small in primary and nil in secondary.

If W is the wattmeter reading

𝑊= 𝑉1𝐼0 cos𝛷0

cos𝛷0 = 𝑊𝑉1𝐼0

𝐼µ = 𝐼0 sin𝛷0 𝐼𝑤 = 𝐼0 cos𝛷0 𝑋0 = 𝑉1𝐼µ and 𝑅0 = 𝑉1𝐼𝑤

Short-circuit or impedance test

Purpose of this test is to determine copper loss at full load

A low voltage (usually 5 to 10% of primary voltage) is applied to the primary and is cautiously till full-load current are flowing both in primary and secondary.

Short circuit test

If Vsc is the voltage required to circulate rated load current,

then 𝑍01 = 𝑉𝑠𝑐𝐼1 𝑊= 𝐼12𝑅01 𝑅01 = 𝑤𝐼0

𝑋01 = ට𝑍012 + 𝑅012

Why transformer rating KVA?

1. Cu loss of transformer depends on current

2. Iron or core loss depend on voltageTotal loss depend on volt-ampere(VA) and

not on phase angle between V & I i.e it is independent of load power factor. That is Why transformer rating KVA not kw.

Losses in transformer

In static transformer , there are no friction or windage losses.

1. Core or iron loss2. Cupper lossCore loss two typesHysteresis & Eddy current

loss

Efficiency & condition of maximum efficiency

𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦= 𝑂𝑢𝑡𝑝𝑢𝑡𝐼𝑛𝑝𝑢𝑡 Condition for maximum efficiency 𝑊𝑖 = 𝐼12𝑅02

Cu loss = Iron loss

Auto-transformer

It is a transformer with one winding only, part of this being common to both primary and secondary. The primary and secondary are not electrically isolated from each other.

Uses of transformer

1. To give small boost to a distribution cable to correct the voltage drop.

2. As auto- starter transformer to give upto 50 to 60% of full voltage to an induction during starting.

Parallel operation of single-phase transformer

For supplying a load in excess of the rating , we have to connect more transformer in parallel.

Condition of parallel operation1. Primary winding of the transformer should be suitable for

the supply system voltage & frequency2. The transformer should be properly connected with regard

polarity.3. The voltage rating of both primaries and secondary's

should be identical.4. The percentage impedance should be equal in magnitude

and have same X/R ratio in order to avoid circulating currents and operation at different power factor.

5. With transformer having different kVA ratings, the equivalent impedance should be inversely proportional to the individual kVA rating.