Why the centre tap of a centre tapped transformer grounded?

A transformer is a device by which the amplitude of an alternating emf can be increased or decreased. A transformer does not affect the frequency of the alternating voltage.

A centre tap in a center tapped transformer is actually a point of contact midway along the secondary winding of the transformer (i.e. a lead connection to the centre of the secondary winding). This wire connected in the halfway is adjusted in such a way that it is at zero volts or grounded , forming the neutral point for the winding.This allows the transformer to provide two separate output voltages which are equal in magnitude, but opposite in polarity to each other.We can say that the tapping point provides a common connection for two equal but opposite secondary voltages. The voltage from the centre tap to each of the outer winding terminal is equal to half the secondary voltage.Example: If we have a 20 V centre tapped transformer, the voltage from centre tap to each of the outer winding terminal will be 10 V. Centre tapped transformer plays a major role in the operation of full wave rectifier.

 

 

An ungrounded center tapped transformer produce unbalanced voltages in the two secondary windings due to the currents flowing in the common third connection.