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Electrical Machines

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.

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Electrical Machines

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  1. Electrical Machines LSEGG216A 9080V

  2. Transformer

  3. 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.

  4. TransformerLosses & Efficiency

  5. 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

  6. 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

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

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

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

  10. Student Exercise 1

  11. Efficiency Ratio between Input power and Output Power

  12. Efficiency Efficiency is normally expressed as a percentage

  13. Transformer Efficiency PowerIn PowerOut Overcome Copper Losses OvercomeIron Losses Some Poweris used to:

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

  15. 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

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

  17. 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

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

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

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

  21. 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; Air Oil and/or • The most common methods of circulation are Forced Natural and/or

  22. Transformer Classification • Transformers are allocated symbols which indicate the type of cooling used • Can consist of up to 4 letters indicating the cooling system

  23. Transformer Classification Type AN Air Natural Dry Transformer withNatural Air Flow

  24. Transformer Classification Type AF Air Forced Dry Transformer withForced Air Flow

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

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

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

  28. THE END

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