How and Why an increase in secondary or load current causes an increase in primary current?

http://avstop.com/ac/apgeneral/imageqju.jpg

If we connect a load to the secondary of the transformer as shown in fig-1, the induced emf in the secondary, causes the current to flow in the secondary winding. At the instant

shown in the diagram, the source of power causes the upper terminal of the primary to be positive, and the instantaneous direction of current is given by . In accordance with Lenz’s law, the secondary induced emf and the current it causes must be in such a direction as to oppose the setting up of the flux . Application of the right hand rule will show that this secondary current direction is given by. We now have a magnetomotive force in the secondary. , opposing that of the primary . It must be remembered, however, that the primary induced voltage is always directly proportional to the flux , and is also equal to the impressed voltage , with all values now taken as the effective, or rms ones. Since the impressed voltage does not change, the flux in the core must also be constant. Thus any increase in the secondary current will be balanced by an increase in the primary current. In this way load current causes an increase in primary current.