Step down transformers are designed to reduce electrical voltage. When it comes to step down transformers, their primary voltage is always greater than their secondary voltage. A step down transformer "steps down" the voltage fed to it. For example, a step down transformer is needed to output 110v in a country with a 220v supply.
Step down transformers are designed to convert voltage from a higher level or phase configuration, in most cases down to a lower voltage level. Step down transformers can include features for electrical isolation, power distribution, and control and instrumentation applications. Step down transformers typically rely on the principle of magnetic induction between coils to convert voltage and/or current levels.
Step down transformers are made from two or more coils of insulated wire wound around a core made of iron. When voltage is applied to one coil (frequently called the primary or input) it magnetizes the iron core, which induces a voltage in the other coil, (frequently called the secondary or output). The turns ratio of the two sets of windings determines the amount of voltage transformation.
An example of this would be: 100 turns on the primary and 50 turns on the secondary, a ratio of 2 to 1.
Step down transformers can be considered nothing more than a voltage ratio device.
When it comes to step down transformers, the voltage ratio measured between primary and secondary will always mirror the "turns ratio" (except for single phase smaller than 1 kva which have compensated secondaries). A practical application of this would be: a 2 to 1 turns ratio would be a 480 to 240 voltage step down. Note that if the voltage input were 440 volts, then the votlage output would be measured at 220 volts. The ratio between input and output voltage will stay constant. Step-down transformers should not be operated at voltages higher than the nameplated rating, but it may be possible to operate stepdown transformers at lower voltages than rated. Because of this, it is possible to do some non-standard step down voltage applications using standard transformers.
Single phase step down transformers 1 kva and larger may also be reverse connected to step-down or step-up voltages. (Note: when it comes to single phase step up or sing phase step down transformers, sized less than 1 KVA, they should not be reverse connected because the secondary transformer windings have additional turns to overcome a voltage drop when the load is applied. If reverse connected, the output voltage will be less than desired.)
For more information consult any manufacturer listed in our Step Down Transformer Buyer's Guide.
See also: Step Up Transformer 110v 220v