Electrical Machines LSEGG216A 9080V

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.

Transformer Losses & Efficiency

Objectives Describe the power losses which occur in a transformer Describe the tests which allow the power losses of a transformer to be calculated Calculate transformer losses and efficiency using test results Define the all day efficiency of a transformer Calculate the all day efficiency of a transformer Describe the relationship between transformer cooling and rating

Objectives Calculate the all day efficiency of a transformer Describe the relationship between transformer cooling and rating Describe the methods of cooling List the properties of transformer oil Describe the tests conducted on transformer oil

Transformer Ratings Volt Amps VA Transformers are rated to supply a given output in Volt Amps or VA at a specified frequency and terminal voltage.

Transformer Ratings They are NOT rated in Watts The load power factor is unknown

Transformer Ratings They are NOT rated in Watts The load power factor is unknown

Student Exercise 1

Ratio between Input power and Output Power Efficiency Ratio between Input power and Output Power

Efficiency is normally expressed as a percentage

Transformer Efficiency Power In Power Out Overcome Copper Losses Overcome Iron Losses Some Power is used to:

Transformer Losses Copper Losses (Cu) Varies with load current Produces HEAT Created by resistance of windings Short circuit test supplies copper losses

Transformer loaded at 50% Short Circuit Test Copper Losses (Cu) Finds Cooper losses at full load Copper losses vary with the square of the load Full load Cu loss = 100 W Transformer loaded at 50% PCu = 25 W

Copper Losses (Cu) Cu Losses (W) % Load

Transformer Losses Iron Losses (Fe) Fixed Always present Related to transformers construction Eddy Currents Hysteresis Reduced by laminations Produces HEAT Reduced by using special steels in laminations

Wattmeter indicates Iron Losses (Fe) Open Circuit Test Finds Iron Losses (Fe) Full Supply Voltage Secondary Open Circuit Wattmeter indicates Iron Losses (Fe)

Cu FL= 840 W Fe = 220 W Sout = 30 kVA Calculate η%at 75%Load η% = 97%

η% Cu Losses Fe Losses Losses (W) η% % Load Fe = Cu =Max η

Air Oil Forced Natural Transformer Cooling and/or and/or 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; Air Oil and/or The most common methods of circulation are Forced Natural and/or

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

Transformer Classification Type AN Air Natural Dry Transformer with Natural Air Flow

Transformer Classification Type AF Air Forced Dry Transformer with Forced Air Flow

Transformer Classification Type ONAF Oil Natural Air Forced Oil Tank Cooling Natural Oil Flow - Forced Air Flow

Transformer Classification Type OFAF Oil Forced Air Forced Oil Tank Cooling Forced Oil Flow – Forced Air Flow

Acts as Coolant & Insulator Transformer Oil Acts as Coolant & Insulator Low Viscosity High Flash point Chemically inert Good insulator

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