Top Questions to Consider When Choosing a Transformer - EMR

There are many types of transformers and choosing the right one for your worksite is very important. The cost of your transformer can easily vary 50% for the same size transformer based on the type of transformer, materials used, available accessories selected and required testing. Not only do you need to determine the size of the load and voltage to be transformed but also the type of load, location of the transformer and how it is to be applied within its applied facility.

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To make this process easier, we have put together an easy guide of questions to consider when selecting a three phase transformer. Follow these questions to make the best selection.

Is it a liquid filled or dry type?

If you are looking for liquid filled, ask the following.
1. What size kVA?

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2. What is the type? Compartmental Padmount or Substation
3. Determine primary voltage: Delta or Wye?
4. Determine secondary voltage: Delta or Wye?

Once you have chosen a type liquid filled transformer, there are several more steps to help finalize your decision. For a Padmount type, these include determining live front or dead front, loop feed or radial feed, which winding material and the temperature rise. Lastly, for a Substation type, decide which enclosure, temperature, if forced air cooling is necessary and lightning arresters are required.

If you are looking for a dry type, ask the following.
1. What size kVA?

2. What is the type? General Purpose, Drive Isolation, Substation or K-Rated
3. Determine primary voltage: Delta or Wye?
4. Determine secondary voltage: Delta or Wye?
5. What is preferred winding material? Aluminum or Copper
6. What temperature rise? 115 degrees Celsius rise or 115 degrees Celsius rise; etc.
7. What is the enclosure type? Nema 1, Nema 3R or Nema 12 (If no enclosure, unit will be core and coil.)

Answering these questions will help you pick the best transformer. And, your service provider can use the results to expedite the process.

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Volts per turn is a metric often used in the design of transformers (and motors).

It is simply a way of describing the rate change of flux carried by the transformer core, by the voltage that would be induced in a _single_ coil wrapped around that core.

Turns electrically in series add voltage, so if you have 192 turns with 1.25V induced per turn, then you get 240V. Of course, you might have something like a center tapped 192 turn coil, providing 120/240V, or some other design feature that we've not been told about.

-Jon Test Question

Test Question

This is a question that was presented on a Journeyman's exam this week. The question never mention the primary or the secondary, It simply said that a transformer has 1.25 volts per turn. There are 192 turns. So the first thing someone would do is multiply 1.25 times 192 and the answer is 240 volts. Well there it is right next to the letter C. So you circle it and move on. Well after finishing the test you start your review and this question pops up again. That answer is to straight forward for a test. A transformer has a ratio. Most are 2:1. So if the primary is 240 volts and the ratio is 2:1 the answer would be 120 volts. 120 volts happened to be answer B. The trouble is that with such limited information you will never be able to agree on the correct answer. There is no way to ask the state for the answer unless you fail the test and sit for a review. So I quess we will never know. Has anyone else had that same question from the ocean state.
Thanks,
Fran