Connecting single-phase transformers to form a 3-phase bank must be done with extreme caution. It is essential that the windings be connected in a certain way only. Reversing a winding can damage the transformer. Figure 14-17 shows three individual single-phase trans-Primary A 480 v c A B A B formers connected to step down 480-V delta to 240-V delta. It is best to obtain a 3-phase transformer rather than to try to connect single-phase transformers. To illus-trate the complexity, standard dual-voltage, single-phase transformers are used to change a 480-V, 3-phase delta to a 120/208-V 3-phase wye in Figure 14-18. WINDING TAPS Transformers, except for small sizes, are often sup-plied with winding taps to compensate for abnormally low or high primary voltage. Assume, for example, that a transformer is rated 480 V primary and 240 V second-ary. This means that 240 V will be the output if the input is 480 V. But, what if the input is only 444 V? The turns ratio for this transformer is 2 to 1; therefore, using Equa-tion 14.3, the output will be 222 V. 444 v Secondary voltage = ---= 222 V 2 Secondary A B c c c Secondary Figure 14-17 Dual-voltage, single-phase transformers with 240/480-V primary windings, and 120/240-V secondary wind-ings are shown connected to form a 480-V delta to 240-V delta, 3-phase, step-down transformer bank. Unit 14 Transformers 2 8 7
Primary A Secondary A B c .-----120 v ----~Pt.,._ ___ 208 v ---Jo>i Secondary N A B c Figure 14-18 Dual-voltage, single-phase transformers with 240/480-V primary windings, and 120/240-V secondary wine ings are shown connected to form a 480-V delta to 120/208-V wye, 3-phase, step-down transformer bank. In order to get an output of 240 V with an input of only 444 V, the turns ratio will have to be changed to 1. 85 to 1. The purpose of the tap connections, usually on the primary, is to change easily the transformer turns ratio. A typical single-phase transformer nameplate with pri-mary taps is shown in Figure 14-19. Winding taps each make a 21/2% change in the volt-age. A transformer will often have two taps above nor-mal voltage and four taps below normal voltage. A trans-former usually comes preconnected for normal voltage. If an abnormal voltage is present, it is up to the installer to change the tap connections. Problem 14-4 A 25-k VA, single-phase transformer is used to sup-ply 120/240 V, 3 wires, to a 100-A panelboard. The pri-mary voltage is only 450 V instead of the normal480 V. Show how to connect the transformer of Figure 14-19 to compensate for the low input voltage. 282 Unit 14 Transformers Solution The primary windings must be connected in seric because the normal voltage should be 480 V. Therefon check the transformer nameplate for series tap conne< tions. Move down the voltage column until the actu: input voltage fits between two numbers. Usually, th higher voltage on the chart is used. If the lower numbt is used, the output will be greater than 240 V. Tr proper connection is shown in Figure 14-20 with jumper wire placed between taps 5 and 6.
You've reached the end of your free preview.
Want to read all 24 pages?
- Spring '16
- HEAVY CURRENT(ELECTRICAL)
- Transformer Description, NEC Section