Transfomer Power:

        The mutual induction between the primary and secondary of a
transformer makes it possible to transfer electric energy from one
circuit to another. In doing so, it is possible to change the value
of the induced voltage. However, a transformer cannot increase total
power. If the transformer steps up voltage, the secondary 
current is correspondingly less than the primary current so that
the power (voltage times current) is the same in both windings.

        With an ac voltage applied across the primary, the induced
counter emf holds the current to the small amount necessary to 
replace the energy lost in the resistance of the wire. A voltage
is induced in each turn of the primary and secondary windings, 
but with no external circuit, there is no secondary current. 
However, when an external circuit path is provided, the induced
secondary voltage causes current in that circuit.
        Remember that the current direction in the oposite to that
in the primary. Therefore, the magnetic field set up by the secondary
current opposes and weakens the field caused by the primary current.
This weakened field induces a lower counter emf and, therefore, the
primary current increases.
        The higher the secondary current, the greater the weakening
of the overall field and the greater the primary current. These actions
are so balanced that any power delivered by the secondary causes a
corresponding increase of power taken from the source by the primary.
        The power in a transformer is measured in Watts and
is equal to the voltage times the current. Because of losses
in the core and those due to resistance of the wire (copper losses)
in the primary winding, the power available to the secondary is always
slightly less than that in the primary. For most circuits, these 
losses are a very small part of the total power handled. Therefore,
for all practical purposes, the power in each winding is so nearly
equal that it can be written in abbreviated form as:

                        EpIp  = EsIs
                        
                        Ep = primary voltage
                        
                        Ip = primary current
                        
                        Es = secondary voltage
                        
                        Is = secondary current

This means Ep times Ip is the same number of watts as Es times Is. 
These terms can be rearranged to calculate any one value when the
other three are known. For Example:
                        
                        
                                  Ep
                        Is = Ip  ---
                                  Es

        A transformer developed 5 volts in it's secondary when
        a 100 volt source was connected across it's primary.
        Assuming a primary current of 2 amperes, the secondary 
        current is:
                             2 x 100      200
                        Is = --------- = -----  = 40 amperes
                                5          5

        For the primary, the power is:

                100 volts x 2 amperes = 200 watts

        For the secondary, the power is:

                5 volts x 40 amperes = 200 watts

Thus, with no increase in available power, a transformer secondary can
provide higher voltage with lower current or lower voltage with higher
current than that existing in the primary.



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