Precise lead or terminal designation is very important. There are specific parameters affected by terminal markings and coil relationships, and how we actually terminate winding leads.
Additive and subtractive voltage
In Fig. 1, we see a graphic representation of a single-phase transformer with primary and secondary windings. Note that the primary terminations are designated "H1" and "H2," and the secondaries are designated "X1" and "X2." These designations are common in the industry and well recognized. The numbers "1" and "2" indicate voltage polarization.
Looking closely, we notice that "H1" and "X1" mark the starts (noted by the letter "S") of the primary and secondary windings, respectively, and that "H2" and "X2" mark their finishes (noted by the letter "F"), respectively.
Additive voltage. If Termination X1 is connected to Termination H2, the primary and secondary winding voltages add together. Thus, the total voltage between X1 and H2 is 600V (480V plus 120V). As we can see, this connection produces the same voltage as if there were only one winding, but with the same number of turns as the primary winding plus the secondary winding. All we did was connect the start of one winding to the finish of the other.
Notice that in Fig. 1, both coils are wound in the same direction. This arrangement of coils and terminations is called additive voltage.
Subtractive voltage. Suppose the coils of our transformer are wound in opposite directions or one set of markers are reversed, and we make the same connection (X1 to H2). What would happen then? Looking at Fig. 2, we see just that scenario: the secondary coil is wound in the reverse direction as that shown in Fig. 1. Now, the voltage between H1 and X2 is 360V (480V minus 120V). The Fig. 2 transformer has a subtractive voltage.
Coil terminations
Transformer coil terminations are usually done with the coil end anchored on the surface of the coil itself to form a terminal, connected to a piece of insulated cable called a lead, or attached to terminals on a terminal board.
On smaller AWG-sized leads, color coding is used. On larger-sized leads, wire markers are used. With leads connected to terminal boards, the terminals themselves may be stamped for identification, or some form of ID may be placed adjacent to the terminal.
Multiple coil transformers
In Fig. 3, we see a single-phase transformer having multiple primary and secondary windings. In fact, both the primary and secondary have two coils, in what is called a series multiple arrangement. If we connect H2 to H3, H4 to X1, and X2 to X3, we end up with one winding having a voltage equal to the total of each separate winding voltage.
Suppose we connect X1 to X4; what would happen? Well, the voltage between X2 and X3 would be zero, since the coils are connected in opposition to one another. This is called bucking.
Let's do a sample problem to see how all of the above applies.
Sample problem
Suppose our Fig. 3 transformer has a 240/480V primary and a 120/240V secondary. If we connect the primary windings in series, the secondary windings in parallel, and H4 to X4, what is the maximum voltage that can be applied between H1 and X1: a) 240V, b) 360V, c) 480V, or d)600V?
Because the primary windings are connected in series, the primary voltage is 480V (twice 240v), and because the secondary windings are connected in parallel, the secondary voltage is 120V. With the above in mind, our answer really depends on whether the 120V section aids or bucks the 480V. Remember how we connected H4 and X4 together? Because these two leads are the winding finishes, the resultant voltage is the same as if 120V worth of turns were subtracted from the 480V winding. Thus, the correct answer is 360V.
Actually, the connecting configuration stated in our problem is the hard way to get a 360V input transformer since, in effect, you're wasting the 120V secondary as well as 120V of the primary. A better way would be to connect the primary in parallel for 240V. Then, if the H2, H4 primary leads are connected to the X1, X3 secondary leads, the voltages will add to 360V (240V plus 120V). The 360V input lines will be connected to H1, H3 and X2, X4. Such a connection can be used as an autotransformer to transform 360V to 120V or 240V.
