What do street lamps, large motors, data centers, and stadiums have in common? They all rely on ready access to electricity—and lots of it.
But getting electricity for a specific purpose isn’t as simple as hooking up directly to the power lines. The high voltage electricity in power lines is only suitable for transmitting power over long distances. To be usable in everyday applications, the electricity must pass through a transformer which converts the power to a suitable voltage.
Many people know what a transformer looks like. Understanding how they work, though, is a different story. Whether you’re budgeting for a transformer or installing one, knowing what transformers do and how they work provides greater clarity on what you need. In this article, you’ll find an introduction to transformers, why we need them, how they work, and a run-through of their most important parts.
In the simplest of terms, a transformer is an electrical device that takes a given input voltage and changes it to a different output voltage. This change can either be an increase or a decrease in voltage.
Electrical energy consists of two key elements: current and voltage.
Think of electricity as water flowing through a pipe.
To move water from the city reservoir to homes, businesses, and factories, you need a big pipe and a lot of water pressure. City water lines are built to handle huge volumes of water and that water moves quickly because of powerful water pressure.
Now imagine hooking up a high-volume, high-pressure city water pipe directly to your kitchen sink. The faucet would burst as soon as you turn it on and you’d have a river gushing into your house. To be usable, the water pressure from the main water line must be reduced using pressure regulators.
Once the water pressure has been reduced, it can finally be used for showers, cleaning dishes, watering your garden, and any other household and business chores.
Transformers do the same thing to electricity. The electricity running through power lines can exceed 300,000 volts—a massive amount of “electrical pressure”. Transformers make electricity usable by lowering the voltage at the point of use. These types of transformers are called step-down transformers.
These range from massive substation transformers found in utility substation yards, to those big green padmount transformers sitting outside your business, to small polemount transformers found atop power poles.
Commercial and industrial operations use large transformers, which provide three-phase voltages like 480 or 208 volts. Homes and small businesses use smaller single-phase transformers to provide 120/240v single-phase. Here, we will focus on three-phase distribution transformers.
A transformer functions under the law of energy conservation, which states that energy can neither be created nor destroyed, only transformed. Therefore, a transformer does not make electricity, it merely changes the voltage to suit the needs of the user.
Transformers accomplish this change in voltage through the process of electromagnetic induction.
When you run an alternating electric current through a wire (conductor), an invisible, moving magnetic field is created around the electrified conductor. When you place a second conductor within this changing magnetic field, the moving flux lines in the field induce a current in the second conductor.
You can use electromagnetic induction to increase or decrease voltage between the two conductors by wrapping the two conductors into coils with one being longer (having more loops in the coil), and the other shorter (having fewer loops in the coil). If you then electrify the coil having more loops, a current will be induced in the coil with fewer loops at a lower voltage than is present in the first coil.
The first coiled conductor where electricity enters the transformer is called the primary coil, and the other coil where current is induced is called the secondary coil. Both the primary and secondary coils (also called windings), made of aluminum or copper, are wrapped around an iron core which strengthens, and directs the changing magnetic field for better induction.
Each loop in the coil around the iron core is called a “turn”.
How do we get the exact voltage that we need? First, we have to understand one simple rule: the ratio of turns between the primary and secondary coils determines the ratio of voltage between the coils.
If the ratio of turns between the coils is 25:1, then the voltage will be transformed at a ratio of 25:1. To get the precise voltage you need, you would build a transformer with the exact desired ratio of turns in each coil. A transformer with a turns ratio of 25:1 would be used to transform 12,000 volts to 480 volts.
There are many configurations and types of transformers, like unit substation, polemount, medium voltage dry-type, and general purpose low voltage dry-type transformers. Despite different design goals, they all use similar components. Below we will explore the parts found in oil-filled padmount transformers.