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When to use I²R, V²/R or VI while calculating Electric Power?

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I came across a question that was asked to me quite a couple of time: When to use I²R, V²/R or VI while calculating power? At first, the question looks simple and as if it needs no formal explanation but that's not the case. Use of any of them will give the correct result provided that you apply it correctly. Correct application is possible only when you understand the type of circuit you are dealing with. Another important point is to use the one which is going to take less time and help you calculate easily. All of the three equations tell the same thing but their usage is subject to the type of problem you're trying to solve. Let us understand when, how an what to use while calculating power in an electrical circuit.

Open-Delta or V-V connection of a Three Phase Transformer

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We know, there are various types of three-phase connected transformers like Star-Star, Star-Delta, Delta-star and Delta-Delta. Open-Delta connected transformer is a special case of three-phase Delta connected Transformer where one phase is removed due to some fault linked to it and the transformer we are left with is called an Open-Delta Transformer or V-V Transformer. It might look like the transformer will not be working but that's not true. The transformer is able to supply the loads but at reduced capacity. Let's see, how it works and what is the reduced capacity. I am also going to discuss about the over-loading capacity of the open-delta transformer. The transformer is called V-V connected transformer because the transformer looks like the English letter 'V' after one of the phases has been disconnected.       Download PDF