TRANSFORMERS – What They Are, How They Work, How To Size Them

Let’s open up a transformer and dig through it to see how they work. Transformers are extremely useful devices used in electrical systems to raise and lower voltage. They’re also used for isolation, typically in cleaning up power. They only work with Alternating Current, however, so in DC circuits you won’t find transformers used.

As an apprentice, I was always fascinated by transformers. They take incoming power and through thin-air (magnetic coupling) change that power to something different. You can feed 480-volts into the PRIMARY side of the transformer and get 240-volts out of the SECONDARY side…and there are no moving parts. Maybe I’m weird but that just always seemed COOL AS HELL to me!

What is a transformer?

Transformers are essentially constant-power devices. This means that you can change the voltage and amperage values but the available power through the device will remain the same. Transformers are not power-loss devices, meaning they don’t function from resistance to dissipate or radiate heat. Instead, they can be more thought of like a an energy storage device due to the inductance inside of it. This is why transformers are rated in KVA, or thousands of Volt-Amperes. A 75kva transformer might be able to handle 156 amperes of current flowing through it at 480 volts. Or it could handle 313 amperes of current flowing at 240 volts. Either way, the available power is 75,000 volt-amps.

One thing you may also notice is that the relationship between the voltage and amperage is inverse through a transformer. If you connect a load at 480-volts that draws 100 amps from the secondary, the primary will show 200 amps of current flowing at 240 volts. If you do the math, both sides equal 48,000 VA.

30 kVA 3-phase Delta/Wye transformer

There are many different types of transformers out there, but most commonly you’ll come across the Dry-Type Delta/Wye transformer. This means that the PRIMARY (incoming) side is wound differently than the SECONDARY (outgoing) side. The PRIMARY windings are wound in series with one another (DELTA), whereas the SECONDARY windings are wound in parallel with one another (WYE). There are several more types that we don’t get into in this video, but examples are Wye/Delta, Wye/Wye, Delta/Delta, and the ZigZag. These all operate the same way, but the results vary because the windings all address a specific problem or need.

Grounding and bonding of transformers is a really important thing to get right. Transformers are considered “separately derived systems” for the purposes of the National Electrical Code, so knowing how to bond the grounding and grounded conductors, as well as any grounding electrode, the metal enclosure of the transformer, and building steel is important. We will cover grounding and bonding transformers in a later article.

Transformer Sizes

Comparison of single-phase and three-phase transformer sizes – Source:

How to calculate the size you need

First of all, there are a lot of factors to consider when picking a transformer for any specific application. Whether incoming power is single-phase, three-phase delta, or three-phase wye. Also are there a lot of harmonic loads the transformer will be driving? If so you’ll need to look into K-rated transformers that have reinforced neutrals, cores, and conductor insulation to handle excess heating. Other things like noise level, whether it’s an isolation transformer, buck-boost, and an assortment of other things.

Now assuming you’ve got all of that figured out, we need to know what size transformer we’ll need. There are two common transformer sizing formulas we use to calculate this, depending on if you’re in a single-phase or three-phase environment.

Example of a Three-Phase Transformer calculation

The formula we use is essentially a modified ohm’s law power calculation, so some may use E as Voltage, I as Amperes – while others may simply use V or A as I’ve done in the picture above.

Single Phase

V x A = VA / 1000

another way this can be organized:

kVA = (V x A) / 1000

It’s easiest to use the load for our calculations, so we’ll calculate based on the secondary rather than the primary numbers. Say we have a 208v load that will draw 200a and we want to know what size the single-phase transformer should be. We plug those numbers into the formula as follows:

kVA = (240 x 200) / 1000

kVA = 48kVA

Since there is no transformer rated at 48kVA, we’d go up to the next single-phase transformer size, which is a 50kVA transformer.

Three Phase

V x A x √ 3 = VA / 1000

another way this can be organized:

kVA = (V x A x √ 3) / 1000

Now let’s look at what happens when we turn the same size single-phase load into a three-phase load, and select a transformer for the application

kVA = (240 x 200) / 1000

kVA = 83kVA

The size of the transformer increases by a factor of 1.732 (or √ 3). You could just take the 48kVA from the single-phase calculation and multiply it by 1.732 to get the same 83kVA. Now again, we don’t have an 83kVA transformer so we need to go up to the next available three-phase size, which is a 112.5kVA transformer.

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