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There are many transformer configurations available today, each applicable to a certain situation or design requirement. Let's discuss one of the more basic configurations and how it can be used.
Multiple winding transformers
In many instances, the solution to a multiple line voltage problem is a multiple winding transformer. One example is the transformer shown in Fig. 1. Here, we have two primary windings, each rated at 240V. The "S" and "F" on the diagram denote the start and finish of the windings. Let's assume that each is capable of handling 5kVA. Then, this transformer would be rated at 10kVA and would be capable of handling 10kVA of load, if both primary windings are fully utilized. Depending on how these primary windings are connected, this transformer could be used with either a 240V or 480V primary. Let's see how.
Series connection. In Fig. 2, we see that the Fig. 1 transformer's two 240V windings are now connected in series. If we connect the "finish" of one primary to the "start" of the other primary, it's fairly obvious that we end up with one continuous winding. And, since each winding has enough turns for 240V, then the combination of both windings, in series, have enough turns for 480V. The current, then, will be the same in both windings, since they're in series; thus, both windings contribute their full kVA capability, and the total primary can handle 10kVA.
The voltage ratio for each primary winding, as shown in Fig. 1, is 240V primary:120V secondary, or 2:1. When we connect both primary windings together in series, we double the number of turns (and the voltage). Thus, the voltage ratio of the Fig. 2 configuration is 480V primary:120V secondary, or 4:1. While the primary is now 480V, the secondary remains 120V, and the transformer is still capable of handling its rated 10kVA load.
Parallel or multiple connections. If we want to make our Fig. 1 transformer suitable for a 240V line while still maintaining a 120V output for our 10kVA load rating, we can connect the primary windings in parallel, as shown in Fig. 3. Note that the two primary "starts" are connected together, as are the two "finishes." This has the same effect as having wound only one 240V winding, but with a double wire (or a wire having twice the circular mil area). By having the two windings in parallel, each winding can operate at its rated voltage and can carry its rated current. Thus, the primary winding can handle the 10kVA for which the transformer is rated.
These series/parallel connections allow our transformer to operate at fully rated kVA, at either input voltage, with the same secondary voltage. Taps, as an additional feature, can be added to the primary to increase the transformer's application versatility. However, both primary and secondary windings must be tapped in exactly the same manner.
The tap percentages change with series and parallel connections. For example, a 5% tap at 240V is 12V in a parallel connection. In a series connection, this same 12V represents only 2 1/2% of 480V. Thus, a transformer with a 480V series connection may have two 2 1/2% taps above normal and four 2 1/2% taps below normal. However, this same transformer, now with a 240V parallel connection, will only have one 5% tap above normal and two 5% taps below normal.
Choosing the best configuration
Because there are a variety of transformer configurations available, your choice should be guided by economics as well as suitability. Evaluating the number of leads to be connected and the kVA capacity of individual windings should point you in the right direction and guide you to the best choice.