Three-Phase Transformers - EdRenzi.com

1 Three-Phase TransformersWhen more power is needed - three Transformers can be tied together. Thisis called Three-Phase . Here s a simple way of comparing single -phase to Three-Phase power. single -PhaseThree-PhaseThink of single -phase as one guy driving a stake into the ground; three -phaseas three guys, working together, driving the same stake into the three -PhaseThree-phase power is delivered from a generator with three separatewindings. These windings are equally spaced around a rotating cylinder (rotor)with each winding occupying one-third of the rotor circumference. three -phasegenerator windings are spaced 120 apart for a total of 360.

2 120 120 120 Because the three source voltages timed at three different intervals, they canbe transmitted over just three conductors (rather than the six conductorsneeded to transmit three separate single -phase loads).120 120 120 A Three-Phase system can produce 173% higher effective voltage than asingle-phase system. This is very useful for higher power loads such as largemotors. 26We calculate the total voltage (of all three lines of a Three-Phase trans-former) by multiplying the voltage of each single winding (phase) by the squareroot of 3 (!3). The reason for this is the angles of each phase are always 120 120 By using this trigonomic formula (don t get scared) we can see where wederive the square root of 120 --------- = = !

3 3sin 30 ConnectionOne way of tying three Transformers together is in parallel (+ to +) and(- to -). This is called a Wyeconnection. Here s how a Wye connection is wired:NeutralL1L2L3 LinePhase27We call each of the single -phase transformer windings (hooked together) of the three single -phase Transformers (L1-L2-L3) is called the Line. We really only draw Three-Phase from the Line side. Also,Line Amps on a Wye connected transformer equals Phase Amps. It would seemthat current would not remain the same in a parallel circuit. But, threetransformers wired in parallel are not like three resistors in produce, instead of consume voltages, so the opposite ruleswould would apply in this more simplified representation of a Wye looks like this:NL1L2L3 ABCIf we tie together three single -phase, 120 volt, Transformers in a Wyeconfiguration our output voltage would look like this:NL1L2120 v120 v120 v208 v3 L3 Why do we get 208 volts, Three-Phase on the Line side (L1-L2-L3) ?

4 Justmultiply 120 volts times the square root of 3 (!3). Remember phase angles ofthree 120 volt sources. 120 volts x (!3) = 208 volts28We can also get 208 volts, single -phase on the Line side of this transformerby connecting (L1-L2);NL1L2L3"A"120 v208 v1 "B"120 v"C"120 vAs you see, we are connecting only two of the three single -phasetransformers available. We still only get 208 volts (120 x !3) from thisconnection, and it is just single -phase. But, we have the capability of using twotransformers ( A & B ) to help share the load. Also, we can use two otherpossible connections (L2-L3) and (L1-L3) to feed other 208 volt, what !

5 Yes, it s even possible to get 120 volts, single -phase out of thesame "A"120 v120 v1 "B"120 v"C"120 vL3 Remember, we have the benefit of a neutral conductor and, by connecting(L2-N) we can feed 120 volt, single -phase loads. And, seeing that we are onlyusing one single -phase transformer ( B ), that leaves two other possibilities (L1-N) and (L3-N) for serving other 120 volt circuits. What s also very useful is thatwe can balance three 120 volt circuits on just one other words, A208/120 volt (Wye configured) transformer could supply120 volt, single -phase equipment (like fluorescent lights), 208 volt, single -phase29equipment (like range or oven), and 208 volt, Three-Phase equipment (like heavymachinery).

6 This is what makes Three-Phase so great !Delta ConnectionsAnother way of tying three Transformers together is in series (+ to -). This iscalled a Deltaconnection. Here s how a Delta connection is wired:L1L2L3 PhaseLineA more simplified representation of a Delta configuration looks like this:L1L2L3 ACB30By connecting three (240 volt) single -phase Transformers (Delta) we get 240volts, Three-Phase . We don t multiply by !3 because Line volts equals Phasevolts on Delta Transformers . It would seem that voltage, would not remain thesame in a series type circuit. But, three Transformers wired in series are not likethree resistors in series.

7 Transformers produce, (instead of consume) voltages,so the opposite rules would would apply in this v240 v240 vL3240 v3 Connect (L1-L2) and you ll get 240 volts, single -phase. Connecting (L2-L3)or (L1-L3) will also give us 240 volts, single -phase. Notice how three different240 volt (1 ) loads utilize three different v240 v240 vL3240 v1 We can still get 120 volts, ( single -phase) out of a 240 volt Delta system, butit s going to take some additional work. We are going to have to center-tap oneof the 240 volt, single -phase Transformers . It s customary to center-taptransformer C ..L3L1L2"C"240 v120 v, 1 N120 v, 1 31As you can see, we can get 120 volts, ( single -phase) from a Deltaconfiguration (L2-N) (L3-N).

8 Unfortunately, all 120 volt loads must be carriedon transformer C . Types Of Three-Phase TransformersWith the possibility of wiring both the primary and secondary of Three-Phase Transformers , there are four possible configurations:(1) Wye/Wyeis commonly used for interior wiring systems.(2) Wye/Deltais used to step-down utilities high line voltages.(3) Delta/Deltais often used for industrial applications.(4) Delta/Wyeis popular for stepping down transmission lines to four-wire services when neutrals are most commonly used Three-Phase transformer is a Delta/Wyeconfiguration. It s used primarily for power distribution from utilities toresidential and commercial v480 v480 vL3NL1L2120 v120 v120 v208 v3 208 v1 120 v1 480 v3 PrimarySecondaryThe given voltage designations for a Delta/Wye Three-Phase transformer is: Primary-Line/Secondary-Line/Secondary-Ph ase Or in this case:Delta/Wye (480/208/120)32 Phase-To-PhaseLet s not forget that Three-Phase is a product of three individual single -phase Transformers .

9 When calculating the Phase-To-Phaserelationship the samerules apply as any single -phase transformer. In other words, Our Phase-To-Phase voltage below is 480 to 120 which is a ratio of 4 "A"480 v"B"480 v"C"480 v"A"120 v"A"120 v"A"120 vWe can also use the the same ladder chart we used for a typical x 480120 Don t forget that this is just one of three single -phase Transformers . Theremay be efficiency or power factor losses between the primary and secondaryphases. But, we ll assume that this transformer is 100% efficient with no powerfactor loss so the Primary vA will equal the Secondary the Primary vA, Primary Amps and Secondary Watts assumingthat our Secondary Amps = x 480120100 Did you get 12,000 Primary vA ?

10 Multiplying 12,000 by 3 (threetransformers) will give us 36 kVa (36,000 vA). We ll call this our Line kVa, whichis the sum total of all three - phases . In other words, our transformer rating is s important to understand the difference between Phase and Line. Phaseis the single -phase transformer relationship. Line is the result of the combinationof all three Transformers hooked are a few of rules we must remember. (1) Line Watts = Phase Watts x 3 (or) Phase Watts = Line Watts 3(2) Delta Phase Volts = Delta Line Volts(3) Wye Phase Amps = Wye Line AmpsYes, we can use the ladder Line-To-Line on Three-Phase Transformers Checkthis !