Electrical Machines

LSEGG216A9080V

Transformer

Basic of Transformer•A basic transformer consists of two sets of

coils or windings. •Each set of windings is simply an

inductor. AC voltage is applied to one of the windings, called the primary winding.

•The other winding, called the secondary winding, is positioned in close proximity to the primary winding,

•but is electrically isolated from it.

TransformerTransformerLosses & EfficiencyLosses & Efficiency

ObjectiveObjectivess1. Describe the power losses which occur in a transformer

2. Describe the tests which allow the power losses of a transformer to be calculated

3. Calculate transformer losses and efficiency using test results

4. Define the all day efficiency of a transformer5. Calculate the all day efficiency of a transformer

6. Describe the relationship between transformer cooling and rating

5. Calculate the all day efficiency of a transformer

6. Describe the relationship between transformer cooling and rating

7. Describe the methods of cooling

8. List the properties of transformer oil

9. Describe the tests conducted on transformer oil

Objectives

Transformer RatingsTransformer RatingsTransformers are rated to supply a given output in

Volt Ampsor

VAat a specified frequency and terminal voltage.

Transformer RatingsTransformer RatingsThey are NOT rated in Watts

The load power factor is unknown

IVS PFSPower

PFPowerS

Transformer RatingsTransformer RatingsThey are NOT rated in Watts

The load power factor is unknown

Student Exercise 1

EfficiencyPowerInputPowerOutputη

LossesOutputInput

LossesPowerOutputPowerOutputη

Ratio between Input power and Output Power

PowerInputLossesPowerInputη

Efficiency

100PowerInputPowerOutputη%

Efficiency is normally expressed as a percentage

Transformer Efficiency

PowerIn

PowerOut

OvercomeIron

Losses

Overcome Copper Losses

Some Poweris used to:

Transformer Losses

Copper Losses (Cu)

•Varies with load current

•Produces HEAT •Created by resistance of windings•Short circuit test supplies copper losses

Short Circuit Test

1000.5lossCopper 2

Copper Losses (Cu)•Finds Cooper losses at full load

•Copper losses vary with the square of the load Full load Cu loss = 100

WTransformer loaded at 50%

PCu = 25 W1000.25lossCopper

0102030405060708090

100110120130140150

0 10 20 30 40 50 60 70 80 90 100 110

Copper Losses (Cu)

% Load

Cu L

osse

s (W

)

Transformer LossesIron Losses

(Fe)•Fixed•Always present•Related to transformers construction

Eddy CurrentsReduced by laminationsProduces HEAT

HysteresisReduced by using special steels in laminations

Open Circuit TestFinds Iron Losses

(Fe)

Full Supply Voltage

SecondaryOpen Circuit

Wattmeter indicates Iron Losses (Fe)

100Losses OutputPowerOutputη%

Sout = 30 kVA Fe = 220 WCu FL= 840 W

Calculate η%at 75%Load

1000.220.75 22.522.5η% 2

84.0

η% = 97%

1000.220.4725 22.522.5η%

5.223075.0 outS 5.47284075.0 2 75%Cu

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 10 20 30 40 50 60 70 80 90 100 11096.00

97.00

% Load

Loss

es (

W)

η%

Cu Losses

Fe Losses

η%

Fe = Cu =Max η

Transformer Cooling• Transformer ratings can be increased if their windings are cooled

by some external means

• The most common cooling mediums are in direct with transformer windings;

and/orAir Oil• The most common methods of circulation are

Forced

and/or Natural

Transformer Classification• Transformers are allocated symbols which indicate the type of

cooling used• Can consist of up to 4 letters indicating the cooling system

1st Letter 2nd Letter 3rd Letter 4th Letter

The cooling medium in contact with the windings

The cooling medium in contact with the external cooling

system

Kind of Medium

Circulation type

Kind of Medium

Circulation type

Transformer ClassificationType AN

Dry Transformer withNatural Air Flow

Air Natural

Transformer ClassificationType AF

Dry Transformer withForced Air Flow

Air Forced

Transformer ClassificationType ONAF

Oil Tank Cooling Natural Oil Flow - Forced Air Flow

Oil Natural Air Forced

Transformer ClassificationType OFAF

Oil Tank Cooling Forced Oil Flow – Forced Air Flow

Oil Forced Air Forced

Transformer Oil

• Low Viscosity• High Flash point• Chemically inert• Good insulator

Acts as Coolant & Insulator

THE END