How Does a Transformer Work

Now that you know the main parts of a transformer, let's look at how a basic transformer works.
Induced Voltage .When an input voltage is applied to the primary winding, alternating current starts to flow in the primary winding. As the current flows, a changing magnetic field is set up in the transformer core. As this magnetic field cuts across the secondary winding, alternating voltage is produced in the secondary winding. In short, a voltage is being induced on the secondary winding.

Eddy Currents

As a magnetic field expands and collapses about the windings of the iron-core transformer, its flux lines cut across both of the Turns of the winding and the core. As a result, voltages are induced in the core itself. These voltages in the core create Eddy Currents. These currents move through the core in circular paths. Because eddy currents create heat in the core and do not aid the induction process, they are a waste of energy, referred to as Eddy-Current Losses. Core designs have been created in an attempt to minimize these losses. For example, basic transformers use a laminated core made up of insulated layers, rather than a solid core.

Because the sheets are insulated from one another, the resistance across the core is high. Eddy currents are thereby reduced.

Transformers are often subjected to abnormally high voltage stresses caused by abnormal operating conditions, such as lightning and switching. This is especially true of high-voltage transformers, like the one pictured here, in use at a power relay station.

The end turns are the ones subjected to abnormal voltages, often high enough to break down the insulation. If this were to happen, current could arc between the turns. As a protective measure, the turns on the top and bottom of this coil are more widely spaced. They are also more heavily insulated.

Turns Ratio
The ratio between the number of actual turns of wire in each coil is the key in determining the type of transformer and what the output voltage will be. The ratio between output voltage and input voltage is the same as the ratio of the number of turns between the two windings.

The relationship between the number of turns in the secondary and the number of turns in the primary is commonly called the Turns Ratio or Voltage Ratio. It is common practice to write the turns ratio with the primary (input) number first, followed by the secondary (output) number. The two numbers are often separated by a colon.

Consider this example:


Transformer Primary Voltage: 480 volts
Transformer Secondary Voltage: 120 volts


This transformer has four primary turns for every one secondary turn. Turns ratio is written as 4 to 1, or 4:1.

A transformer's output voltage is greater than the input voltage if the secondary winding has more turns of wire than the primary winding. The output voltage is stepped up, and we have a Step-Up Transformer. If the secondary winding has fewer turns than the primary winding, the output voltage is lower. This is a Step-Down Transformer.

Step-Up vs. Step-Down
Step-Up Transformer: The primary winding of a step-up transformer has fewer turns than the secondary winding, with the resultant secondary voltage being higher than the primary.

Step-Down Transformer: The primary winding of a step-down transformer has more turns than the secondary winding, so the secondary voltage is lower than the primary.

Is There Something for Nothing?
In Figure 10, the step-up transformer has a 1 to 2 ratio. As a result, the output voltage is doubled. At first, this might seem like we are gaining or multiplying voltage without sacrificing anything. Of course, this is not the case. Ignoring small losses, the amount of power transferred in the transformer is equal on both the primary and secondary sides.

Power is equal to Voltage multiplied by Current. This is expressed by the formula:
P = V x I

Power is also always equal on both sides of the transformer, meaning both sides of the equation must have the same value. This means we cannot change the voltage without changing the current also.

In Figure we can see that when voltage is stepped down from 240 V to 120 V in a 2 to 1 ratio, the current is increased from 1 to 2 amps, keeping the power equal on each side of the transformer. In contrast, in Figure 11, when the voltage is stepped up from 120 V to 240 V in a 1 to 2 ratio, the current is reduced from 2 to 1 amp to maintain the power balance. In other words, voltage and current may be changed for particular reasons, but power is merely transferred from one point to another.

One big advantage of increasing the voltage and reducing the current is that power can be transmitted through smaller gauge wire. Think about how much wire is used by a utility company to get electricity to where it is used. For this reason, the generated voltages are stepped up very high for distribution across large distances, then stepped back down to meet consumer needs.

Voltage Taps

As you know, the turns ratio determines the voltage transformation that takes place. There are times when the actual incoming voltage is different than the expected normal incoming voltage. When this happens, it could be advantageous to be able to change the turns ratio in order to get the desired (rated) output voltage. You might view this as fine tuning the input voltage to get the desired output. Voltage Taps, designed into the transformer's primary winding, deliver this desired flexibility.

Suppose a transformer has a 4 to 1 turns ratio. Remember, that means the primary has 4 times as many turns as the secondary, which tells you the transformer is a step-down transformer. If the input voltage is 480 volts, the output would be 120 volts.

What if the input delivered to the transformer primary is less than the expected normal of 480 volts, say 456 volts for this example? This could be significant if getting 120 volts from the secondary is critical. Tapping the primary in a number of different spots helps to eliminate the problem by providing a means to adjust the turns ratio, and fine-tune the secondary output voltage.

This transformer has taps at 2 1/2% and 5% below the normal voltage of 480 volts. In the industry, this would be referred to as having two 2 1/2% Full Capacity Below Normal Taps (FCBN). These two taps provide a 5% voltage range below the normal 480 volts. When taps are provided above the normal as illustrated, they are called Full Capacity Above Normal Taps (FCAN).

For standardization purposes, taps are in 2 1/2% or 5% steps. The tap arrangement used on many transformers is two@2 1/2% FCAN and four@2 1/2% FCBN, which provides a 15% total range of tap voltage adjustments.