Step Down Transformer: How Does it Work? (Formula & Working Principle)

What is a Step Down Transformer?

A step-down transformer is a type of transformer that converts the high voltage (HV) and low current from the primary side of the transformer to the low voltage (LV) and high current value on the secondary side of the transformer. The reverse of this is known as a step up transformer.

A transformer is defined as a static electrical device that converts electrical energy from the primary side windings to magnetic energy in the core, and then back to electrical energy on the secondary side. Step-down transformers are widely used in various electrical systems and transmission lines.

Step-down transformers serve two main voltage ranges: low voltage (LV) applications up to 1 kV and high voltage (HV) applications above 1 kV.

A transformer that takes a higher primary voltage to a lower secondary voltage steps the voltage down. Connected the other way round, it raises a lower primary voltage to a higher secondary voltage; those are step-up transformers.

The transformer turns ratio (n) for a step down transformer is approximately proportional to the voltage ratio:

Where V_P,S are voltages, and N_P,S are the turns numbers on the primary (LV) and secondary (HV) sides respectively. The primary side of a step-down transformer (HV side) has a larger number of turns than the secondary side (LV side).

That means energy flows from the HV to the LV side. The voltage is stepped down from the primary voltage (input voltage) to the secondary voltage (output voltage).

Rearranging this equation gives the output (secondary) voltage, sometimes called the step down transformer formula:

A transformer calculator can help you easily calculate the transformer turns ratio and whether the device is a step down or step up transformer.

The first low voltage (LV) application sits in electronic devices, which need a low supply such as 5 V to run efficiently.

A step-down transformer provides that low voltage. It converts the household supply (230 V or 120 V) on the primary to a low secondary voltage that powers the electronics.

Electronic devices with higher power needs use transformers running at kilohertz frequencies, because a high-power 50/60 Hz unit would be too large and heavy. Step-down transformers also appear in everyday battery chargers.

Step Down Transformer Applications

Step-down transformers carry out a central function in a power system: they lower the voltage level and adapt it to energy consumers. This happens in several steps:

• A long-distance transmission system should run at the highest practical voltage. High voltage with low current cuts the transmission power loss sharply. Because a power grid connects to transmission systems at several voltage levels, step-down transformers interconnect those levels. They reduce the voltage from a higher to a lower value (e.g. 765/220 kV, 400/220 kV, 220/110 kV). These transformers are huge, with very high nominal power (up to 1000 MVA). When the turns ratio is not large, autotransformers are usually installed.
• The next step adapts the transmission voltage to the distribution level. The characteristic ratios here are 220/20 kV and 110/20 kV, with secondary LV levels of 35 kV and 10 kV also in service. These transformers reach 60 MVA in nominal power (usually about 20 MVA). An on-load tap changer is almost always installed; its main function is voltage regulation. In the USA the tap changer usually sits on the LV side, and in the rest of the world mostly on the HV side.
• The final step adapts the voltage to the home level . These small distribution transformers have nominal power up to 5 MVA (mostly below 1 MVA) and nominal voltages of 35, 20 or 10 kV on the HV side and 400/200 V on the LV side. They also have a

  high turns ratio. They usually carry a de-energized tap changer with 5 tap positions (+/- 2) and no on-load tap changer.