5/4/2009
Three Phase Transformers
Three Phase
Transformers
Theory and
Calculations
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
DELTA Transformer
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
WYE Transformer
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Voltage and Current in a Delta
Transformer
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Voltage and Current in a WYE
Transformer
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
D–V
W-I
• D = Delta configuration
• V = Volts
• W = Wye configuration
• I = Current
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Delta – Volts are Equal
9 Delta Configuration Transformer
– Voltage is EQUAL Phase and Line
¾The Phase Voltage = Line Voltage
– Current is NOT EQUAL
¾Phase Current = Line Current / 1.732
¾Line Current = Phase Current X 1.732
– D V W I , Phase Divide, Line Multiply, by 1.732
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Wye – Current is Equal
9 Wye Configuration Transformer
• Current is EQUAL Phase and Line
¾ The
•
Phase Voltage = Line Voltage
Voltage is NOT EQUAL
¾ Phase
Voltage = Line Voltage / 1.732
¾ Line Voltage = Phase Voltage X 1.732
„
D V W I , Phase Divide, Line Multiply, by 1.732
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
DELTA
Configuration
WYE (“Y”)
Configuration
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Ratio of
Transformation
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
(4)
(1)
(2)
(3)
1. Primary Line
2. Primary Phase
3. Secondary Phase
Core
4. Secondary Line
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
230 V
6.25
amps
6.25
amps
X
25
amps
What is the Primary Line voltage of a Delta-Wye connected
transformer with a secondary line voltage of 230 volts,
secondary line current of 25 amps, and primary line current of
6.25 amps
Answer
•1. Ratio Primary amps / Secondary amps = 25/6.25 = 4
•2. Secondary Line to Secondary Phase 230 / 1.732 = 132.8v
•3. Primary Phase volts using ratio = 132.8 X 4 = 531.2 volts
•4. Delta Phase Volts = Line Volts = 531.2 volts Primary Line
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
1920 V
125
amps
15.6
amps
What is the Secondary Line voltage of a Delta-Wye connected
transformer with a Primary line voltage of 1920 volts, secondary
line current of 125 amps, and primary line current of 15.6 amps
Answer
1. Ratio Primary amps / Secondary amps = 125/15.6 = 8
2. Primary Line to Primary Phase 1920v = 1920v Primary Phase
3. Secondary phase using ratio = 1920 / 8 = 240 volts
4. Wye Line Volts = Phase Volts X 1.732 = 415.68 volts Sec Line
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
7200v
480v
26 a
X
What is the Secondary Line Current of a Delta-Wye connected
transformer with a Primary line voltage of 7200 volts, secondary
line voltage of 480, and primary line current of 26 amps
Answer
1. Ratio Primary volts / Secondary volts = 7200/480 = 15
2. Pri Line amps to Pri Phase amps = 26 / 1.732 = 15.01 amps
3. Sec phase amps using ratio = 15 X 15.01 = 225.17 amps
4. Wye Line amps = Phase amps = 225 amps Sec Line Current
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
120 V
1440 V
140 amps
What is the Primary Line current of a Delta-Wye connected
transformer with a secondary line voltage of 120 volts,
secondary line current of 140 amps, and primary line voltage
of 1440 volts.
Answer
ƒ1. Ratio Primary volts / Secondary volts = 1440/120 = 12
ƒ2. Secondary Line to Secondary Phase 140 = 140a (Wye =)
ƒ3. Primary Phase current using ratio = 140 / 12 = 11.66 amps
ƒ4. Delta Line Current = 11.66a X 1.732 = 20.20 amps Pri Line Delta
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Making a Delta out of a Wye
Open these
to make a
WYE a
Delta
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Three Phase example #1
Secondary Phase amps for a WYE
CONNECTED TRANSFORMER, 480 V, 45
AMP, are ?
Phase Amps = Line Amps (Wye)
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Three Phase example #2
Phase volts for a 3 phase transformer are
208 v, what is the line voltage connected
Wye ?
Line Volts = Phase Volts X 1.732
208 X 1.732
360.256 Volts
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Three Phase example #3
Phase Current for a Delta connected
transformer, 480 v, 45 kva, with 20 amps
on the line are ?
Phase Amps = Line amps / 1.732
20 / 1.732
11.547 amps
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Three Phase Fault Current
Calculation
Transformer: 3phase, 265 kva, 208/120 volt with 2.5%
impedance
I = W / E (no Power Factor W = VA
3 Phase use 1.732
I - 3 phase = VA / E X 1.732
Convert K (Kilo) by dividing by 1000
Whatever your
Impedance simply
Divide it into
100
Do this
calc first
265,000 / 208 X 1.732
265,000 / 360.256
735.58802
Multiplier = 100 / 2.5 = 40
736 amps X 40 = 29,440 amps fault current
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Service Entrance
Raceway & Conductors
NESC
HIGH LEG PHASE “C”
Meter Base
NEC 230.56 & 215.8
HIGH LEG – PHASE “B”
Service Equipment
Feeder CB
HIGH LEG
Phase “B”
408.3(E)
215.8
Feeder
Circuit
Grounding
Electrode
Conductor
The HIGH LEG IS ALWAYS
Electrical
Equipment
ORANGE
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
NEC – Phase Arrangement
• 408.3(E)
–A – B – C
• Front to Back
• Top to Bottom
• Left to Right
–Higher Voltage to Ground
• B Phase – Center Phase
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Open Delta System
1. Requires 3
transformers
2. Advantage:
3 phase power
3. Disadvantage:
Reduced capacity
Approx: 58% of
total
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
You can derive two (2)
Different voltages from this
Transformer
120 volts and 208 volts
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
You can derive three (3)
Different voltages from this
Transformer
120, 208 and 240 volts
This transformer configuration
Is also called a
“Center Tapped Delta”
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Delta - Wye Transformer Connection
L1
L2
L3
N
L1
L2
L3
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Delta - Delta Transformer Connection
L1
L2
L3
N
L1
L2
L3
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Wye - Wye Transformer Connection
N
L1
L2
L3
N
L1
L2
L3
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Wye - Delta Transformer Connection
N
L1
L2
L3
N
L1
L2
L3
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Three Phase
Transformer Exam
12 Questions
30 Minutes
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Three Phase Answers
1.
B
Equal to
Line Amps = Phase Amps
2.
A
51.96 Amps
Ratio = 480 / 120 = 4
120 / 4 = 30 Amps
30 Amps X 1.732 = 51.96 Amps
3.
C
120.09 V
Line Volts / 1.732 = Phase Volts
208 / 1.732 = 120.09 V
Copyright 1999-2008 Dan Dudley
Three Phase Transformers
Three Phase Answers
4.
D
Are Equal To
5.
A
3.23 Amps
Phase Amps = Line Amps / 1.732
Phase Amps = 5.6 amps / 1.732
Phase Amps = 3.23 Amps
6.
A
Wye three phase, 4 wire
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley
5/4/2009
Three Phase Transformers
Three Phase Answers
7. A
108.25
I – 3phase = VA / E X 1.732
45000 / 240 X 1.732 = 108.25
8. B
5,412 Amps
100 / 2% impedance = 50
108.25 X 50 = 5,412.5 amps
9.
A A,B,C left to right
408.3(a)
10. C
35000
450.23(A)(1)(a)
11. D
3 hours
450.43(A)
12. D
450.45(E)
Copyright 1999-2008 Dan Dudley
Copyright 2000-2007 Dan Dudley