Electrical Machines

1
Electrical Machines

• LSEGG216A
• 9080V

2
TransformerLosses Efficiency
Week 3
3
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

4
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

5
Transformer Ratings

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

6
Transformer Ratings

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

7
Transformer Ratings

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

8
Problem
S 2 kVA
V2 230 V
V1 6,351 V
Power output at unity PF ?
Power S x PF
P 2 kVA x 1
P 2 kW
9
Problem
S 2 kVA
V2 230 V
V1 6,351 V
Full load secondary current at 0.8 PF ?
I 10.87 A
10
Student Exercise 1
11
S 20 kVA
V2 200 V
V1 1270 V
(a) Power output at unity power factor
P 20 kW
12
S 20 kVA
V2 200 V
V1 1270 V
(b) Power output at 0.8 power factor
P 16 kW
13
S 20 kVA
V2 200 V
V1 1270 V
(c) Full-load secondary current at unity power
factor
I 100 A
14
S 20 kVA
V2 200 V
V1 1270 V
(d) Secondary current when transformer supplies
10 kW at 0.8 power factor
I 62.5 A
15
Efficiency
Ratio between Input power and Output Power
16
Efficiency
Efficiency is normally expressed as a percentage
17
Transformer Efficiency
PowerIn
PowerOut
? 100
? 95
? 90
Overcome Copper Losses
OvercomeIron Losses
Some Poweris used to
18
Student Exercise 2
19
S 20 kVA
V1 230 V
V2 32 V
? 90
PF 0.85
(a) Power output of transformer
P 85 W
20
S 20 kVA
V1 230 V
V2 32 V
? 90
PF 0.85
(b) Power input
P 94.4 W
21
S 20 kVA
V1 230 V
V2 32 V
? 90
PF 0.85
(c) Losses
P 9.4W
22
Transformer Losses
Copper Losses (Cu)

• Varies with load current
• Produces HEAT
• Created by resistance of windings
• Short circuit test supplies copper losses

23
Short Circuit Test
Copper Losses (Cu)
Limited Supply Voltage 5-10
Secondary Short Circuited
Wattmeter indicates Copper Losses (Cu)
24
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
25
Copper Losses (Cu)
Cu Losses (W)
Load
26
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
27
Open Circuit Test
Finds Iron Losses (Fe)
Full Supply Voltage
Secondary Open Circuit
Wattmeter indicates Iron Losses (Fe)
28
Transformer Efficiency

• Student Exercise 3

29
Fe 220 W
Cu FL 840 W
Sout 30 kVA
Calculate ?at Full Load
? 96.6
30
Fe 220 W
Cu FL 840 W
Sout 30 kVA
Calculate ?at 75Load
? 97
31
Fe 220 W
Cu FL 840 W
Sout 30 kVA
Calculate ?at 50Load
? 97.21
32
Fe 220 W
Cu FL 840 W
Sout 30 kVA
Calculate ?at 25Load
? 96.49
33

• 100 ? 96.6
• 75 ? 97
• 50 ? 97.21
• 25 ? 96.49

34
?
Cu Losses
Fe Losses
Losses (W)
?
Load
Fe Cu Max ?
35
Maximum Efficiency
Fe 220
Cu 840

• Fe Cu Max ?

Load 51.18
? 97.21
36
(No Transcript)
37
All Day Efficiency

• Most Transformers are connected permanently
• The time that the transformer has to be
  calculated when determining efficiency
• Able to determine the best transformer for the
  application by its efficiency

38
All Day Efficiency

• Transformer A

Sout 300 kVA
Fe 1.25 kVA
Cu 3.75 kVA
39
All Day Efficiency

• Transformer B

Sout 300 kVA
Fe 2.5 kVA
Cu 2.5 kVA
40
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
41
(No Transcript)
42
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 windings The cooling medium in contact with the external cooling system The cooling medium in contact with the external cooling system
Kind of Medium Circulation type Kind of Medium Circulation type
43
Transformer Classification
Type AN
Air Natural
Dry Transformer withNatural Air Flow
44
Transformer Classification
Type AF
Air Forced
Dry Transformer withForced Air Flow
45
Transformer Classification
Type ONAN
Oil Natural Air Natural
Oil Tank Cooling Natural Oil Flow - Natural Air
Flow
46
Transformer Classification
Type ONAF
Oil Natural Air Forced
Oil Tank Cooling Natural Oil Flow - Forced Air
Flow
47
Transformer Classification
Type OFAF
Oil Forced Air Forced
Oil Tank Cooling Forced Oil Flow Forced Air Flow
48
Transformer Oil
Acts as Coolant Insulator

• Low Viscosity
• High Flash point
• Chemically inert
• Good insulator

49
Transformer Oil Tests

• Dielectric Strength
• Acidity
• Power factor
• Interfacial tension
• Dissolved Gas

50
THE END