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The Basics of 3-Phase Transformers — Part 1

Jul 1, 1999 12:00 PM, By Paul Frank, Tel-A-Train

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When connecting transformers, you may have to connect them to single- and 3-phase power sources as well as single- and 3-phase loads. Making the wrong connections may damage equipment and cause injuries. This month, we'll see how to wire transformers to single- and 3-phase power sources, then connect them for single-phase loads. This forms the basis for understanding 3-phase transformers.

When you connect transformers, you'll face one of several supply combinations for single-phase power or 3-phase power. Let's look at what these are and how you connect them.

Single-phase power, single voltage. You can have one hot conductor and one grounded conductor, or two hot conductors. Connect between any two conductors for single voltage. Single-phase power, dual voltage (3-wire Edison system). This has two hot conductors and one grounded neutral conductor. Connect two hot conductors for full voltage. Connect one hot conductor and the grounded neutral for lower voltage.

Three-phase power, single voltage. Here, you have three hot conductors. Connect all three conductors for three-phase voltage. Connect any pair for single-phase voltage.

Three-phase power, dual voltage (4-wire, 3-phase system). This includes three hot conductors and a grounded neutral conductor. Connect all three hot conductors for 3-phase power. Connect two or three of the hot conductors and the neutral for lower, single-phase voltage.

Connecting single-phase transformers. Primary connections on single-phase transformers may have one or two high-voltage terminals. If you have only one high-voltage terminal, connect between a hot conductor and a neutral. With two high-voltage terminals, you may connect two hot conductors or a hot conductor and a neutral.

Secondary connections may have three or four terminals supplying full-voltage loads and half-voltage loads. If you have three terminals, the middle one is usually a grounded neutral. You can connect half-voltage loads between the middle terminal and either outer terminal. You can connect full-voltage loads between the outer terminals. With four terminals, you must short the transformer windings together, then connect for half-voltage loads, full-voltage loads, or both.

The secret to sizing. To size transformers, you must calculate voltage, current, and power. To do that, you need the transformer's "turns ratio," primary voltage, and total load. The secondary voltage is the primary voltage divided by the turns ratio. Secondary current is the primary current times the turns ratio. Power (kVA) entering the transformer will be slightly more than the power leaving the transformer. Beware, some texts show transformer calculations without defining the turns ratio: thus making it easy to use the wrong calculation method.

Next month, we'll see how to wire transformers to 3-phase power sources and 3-phase loads.

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