Step-up Transformers are used to increase electrical voltage when required for electrical devices, equipment or machinery. Transformers work by way of electromagnetic induction.
Electromagnetic induction is the result of a changing magnetic field which induces an electromotive force (emf) or voltage in a conductor (wire). In the case of a step-up transformer, AC electric current flowing through a primary coil of wire creates a changing magnetic field. A secondary coiled conductor (with a larger number of turns) that is closely exposed to the changing magnetic field of the primary coil will be charged with a electric potential (voltage) which is higher than the voltage applied to the primary coil.
The voltage increase in the secondary coil (in a perfect world) is directly proportional to the number of turns of the primary coil compared to the number of turns in the secondary coil.
e.g. 100 turns on the primary and 1000 turns on the secondary would produce 10 times more voltage in the secondary coil compared to the voltage that has been applied to the primary coil.