Upload
aman-ansari
View
239
Download
1
Embed Size (px)
Citation preview
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