Three Phase Transformers Theory and Calculations 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 1
DELTA Transformer 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 2
WYE Transformer 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 3
Voltage and Current in a Delta Transformer 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 4
Voltage and Current in a WYE Transformer 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 5
D V W - I D = Delta configuration V = Volts W = Wye configuration I = Current 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 6
Delta Volts are Equal Delta Configuration Transformer Voltage is EQUAL Phase and Line The Phase Voltage = Line Voltage Current is NOT EQUAL Phase Current = Line Current / 1.732 Line Current = Phase Current X 1.732 D V W I, Phase Divide, Line Multiply, by 1.732 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 7
Wye Current is Equal Wye Configuration Transformer Current is EQUAL Phase and Line The Phase Voltage = Line Voltage Voltage is NOT EQUAL Phase Voltage = Line Voltage / 1.732 Line Voltage = Phase Voltage X 1.732 D V W I, Phase Divide, Line Multiply, by 1.732 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 8
DELTA Configuration WYE ( Y ) Configuration 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 9
Ratio of Transformation 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 10
(1) 1. Primary Line 2. Primary Phase 3. Secondary Phase (2) (3) (4) 4. Secondary Line Core 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 11
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 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 12
1920 V 15.6 amps 125 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 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 13
26 a 7200v X 480v 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 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 14
1440 V 120 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 1. Ratio Primary volts / Secondary volts = 1440/120 = 12 2. Secondary Line to Secondary Phase 140 = 140a (Wye =) 3. Primary Phase current using ratio = 140 / 12 = 11.66 amps 4. Delta Line Current = 11.66a X 1.732 = 20.20 amps Pri Line Delta 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 15
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 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 16
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 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 17
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 Whatever your Impedance simply Divide it into 100 I - 3 phase = VA / E X 1.732 Convert K (Kilo) by dividing by 1000 265,000 / 208 X 1.732 265,000 / 360.256 735.58802 Multiplier = 100 / 2.5 = 40 Do this calc first 736 amps X 40 = 29,440 amps fault current 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 18
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 Grounding Electrode Conductor Feeder Circuit Electrical Equipment The HIGH LEG IS ALWAYS ORANGE 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 19
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 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 20
Open Delta System 1. Requires 3 transformers 2. Advantage: 3 phase power 3. Disadvantage: Reduced capacity Approx: 58% of total 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 21
You can derive two (2) Different voltages from this Transformer 120 volts and 208 volts 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 22
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 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 23
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Transformer Connection Identification 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 26
Transformer Connection Identification 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 27
Transformer Connection Identification 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 28
Three Phase Transformer Exam 12 Questions 30 Minutes 8/29/2016 Copyright 2016 Dan Dudley & Associates Page 29