Why transformer frequency does not change?

 atransfoemer works on voltage rise or fall in the primary and induced in the secondary .if frequecy is changing then harmoi=nics are produced in secondary which may demage the system .so transformer works on fix frequency.

Transformer is a static electrical device, which transfers electrical power from one voltage level to another voltage level without change in frequency. 

Here transformer main job is to transfer the power from one voltage level to another voltage level by mutual induction. Hence whatever be the emf frequency in primary winding, same frequency emf will be induced in secondary winding (emf amplitude depends upon the turns ratio).