Transcript File

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
1.
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.
5.
Define the all day efficiency of a transformer
Calculate the all day efficiency of a transformer
Objectives
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 Ratings
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
S V I
Power  S  PF
Power
S
PF
Transformer Ratings
They are NOT rated in Watts
The load power factor is unknown
Student Exercise 1
Efficiency
Ratio between Input power and Output Power
Output Power
η
Input Power
Input  Output  Losses
Output Power
η
Output Power  Losses
Input Power  Losses
η
Input Power
Efficiency
Efficiency is normally expressed as a percentage
Output Power
η% 
 100
Input Power
Transformer Efficiency
Power
In
Some Power
is used to:
Power
Out
Overcome
Copper
Losses
Overcome
Iron
Losses
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
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%
Copper loss  0.5  100
2
Copper loss  0.25  100
PCu = 25 W
Cu Losses (W)
Copper Losses (Cu)
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
% Load
70
80
90
100
110
Transformer Losses
•Fixed
Iron Losses (Fe)
•Always present
•Related to transformers construction
Eddy Currents
Reduced by laminations
Produces HEAT
Hysteresis
Reduced by using special
steels in laminations
Open Circuit Test
Finds Iron Losses (Fe)
Full Supply
Voltage
Secondary
Open Circuit
Wattmeter indicates Iron Losses (Fe)
Cu FL= 840 W
Sout = 30 kVA
Fe = 220 W
Calculate η%at 75%Load
Output Power
η% 
 100
Output  Losses
22.5
η% 
 100
2
22.5  0.75  0.84  0.22

η% 

22.5
 100
22.5  0.4725  0.22
η% = 97%
S out  0.75  30  22 .5
Cu75%  0.75  840  472.5
2
1.4
Losses (W)
1.0
0.8
η%
Cu Losses
1.2
Fe Losses
97.00
0.6
η%
0.4
0.2
0.0
96.00
0
10
20
30
40
50
60
% Load
Fe = Cu =Max η
70
80
90
100
110
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
and/or
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
The cooling medium in contact
with the windings
3rd Letter
4th Letter
The cooling medium in contact
with the external cooling system
Kind of Medium Circulation type 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
ONAF
Oil Natural Air Forced
Type
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
Transformer Oil
Acts as Coolant & Insulator
•
Low Viscosity
•
High Flash point
•
Chemically inert
•
Good insulator
THE END