Transformer Losses & Efficiency, Slides of Electric Machines

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Electrical MachinesElectrical Machines

LSEGG216A 9080V

TransformerTransformer

Losses & Efficiency Losses & Efficiency

Week 3 Week 3

ObjectiveObjectivess

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

Transformer RatingsTransformer Ratings

Transformers are rated to supply a given output in

Volt Amps

or

VA

at a specified frequency and terminal voltage.

Transformer RatingsTransformer Ratings

They are NOT rated in Watts

The load power factor is unknown

ProblemProblem

V

1

V

V

2

= 230 V

S = 2 kVA Power output at unity PF? P = 2 kVA x 1 P = 2 kW Power = S x PF

Power SPF P  20 , 000  1. 0

V

2

V = 200 V

1

V

S = 20 kVA P = 20 kW (a) Power output at unity power factor

VxPF S I  200x1. 20, I 

V

2

V = 200 V

1

V

S = 20 kVA I = 100 A (c) Full-load secondary current at unity power factor

VxPF S I  200x0. 10, I 

V

2

V = 200 V

1

V

S = 20 kVA I = 62.5 A (d) Secondary current when transformer supplies 10 kW at 0.8 power factor

EfficiencyEfficiency

100 Input Power Output Power η%   Efficiency is normally expressed as a percentage

TransformerTransformer

Efficiency Efficiency

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

Power  SPF P  100  0.

V

1

= 230 V V

2

= 32 V

S = 20 kVA η = 90% PF = 0. (a) Power output of transformer P = 85 W

In Out η  η Out In 

V

1

= 230 V V

2

= 32 V

S = 20 kVA η = 90% PF = 0. (b) Power input P = 94.4 W

85 W In 