The number of turns in primary coil are never made equal to number of turns in secondary coil in a transformer, why?

The transformer is a device either to increase or to decrease the ac output voltage. If the number of turns in primary coil and number of turns in secondary coil are made equal then transformer neither increases nor decreases the ac output voltage. This means transformer has no work. Hence, the number of turns primary and secondary coils is made different. 

A transformer is defined as a passive electrical device that transfers electrical energy from one circuit to another through the process of electromagnetic induction. It is most commonly used to increase ('step up') or decrease ('step down') voltage levels between circuits.A transformer is not designed to convert AC to DC or DC to AC. The transformer can step-up or step -down current. A transformer that increases the voltage from the primary to secondary is called a step-up transformer.

Transformers generally have one of two types of cores: Core Type and Shell Type. These two types are distinguished from each other by the manner in which the primary and secondary coils are place around the steel core.Transformers do not pass direct current (DC), and can be used to take the DC voltage (the constant voltage) out of a signal while keeping the part that changes (the AC voltage). In the electrical grid transformers are key to changing the voltages to reduce how much energy is lost in electrical transmission.Three basic types of transformer oil used are mineral oil (mostly naphthenic), silicone, and bio-based. Mineral oil based transformer oils dominate the consumption as it has good electrical and cooling properties, and provides cost-effective solution.

A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple circuits.