A step-up transformer is used when the secondary voltage is greater than the primary voltage. It has more wire turns in the secondary. The common usage of it is a starter of an electric motor. It takes a lot of voltage to initially start the motor’s turning. A step up transformer is also needed to use a 220v product in a country with 110v supply. In other words, the voltage has been “stepped up”. As you can see in the diagram, input is 110V and output is 220V. That is the main purpose of a Step-up Transformer.
Step- down transformers are designed to reduce electrical voltage. It has less wire turns in the secondary. Thus, decreases voltage. For instance, here in the Philippines, we have 220V electric supply. But since we have relatives or friends abroad who send us appliances that requires 110V supply, there is a need to use a Step down Transformer. Using a step-down transformer actually means that the voltage has been “stepped down”.