MM0704, Lesson 4
The voltage induced per turn in the secondary will be:
24 : 300 = .08V.
.08V X 3,000 = 240V
The above transformer is said to be a step-down transformer, since from the 240 V applied you are getting only 24 V
from the secondary. A step-up transformer then, would be one in which electric energy would come from the
secondary at a higher potential than the primary voltage.
Current Ratio. The preceding explanation of transformer operation was considered with an unloaded secondary. If a
load is placed on the secondary, there will be current flowing in the secondary. For example, if an AC motor is
connected across the secondary terminals of a transformer, the motor draws current. The amount of current depends
upon the power required by the motor to perform its work.
Within the transformer, this secondary current tends to magnetize the core in a direction opposite to that of the
magnetizing action of the primary current. This magnetizing action of the secondary tends to lower the induced EMF
in both the primary and secondary windings. The counter EMF is lowered in the primary circuit, therefore permitting a
greater current to flow in the primary.
The load on the secondary is consequently a determining factor of the primary current, considering the proportion of
voltage turns previously discussed. Being an ideal transformer, there are no losses and the primary power is equal to
the secondary power. This can be expressed as Pp = Ps, where Pp is the primary power and Ps is the secondary power.
Now,
Using the two equations above:
This equation shows the current and turns ratio of an ideal transformer.
Figure 4-4 shows a transformer having a primary winding of 5,500 turns and a secondary of 250 turns, rated at 10 V
amps. The primary is connected to a 110-VAC power source. The secondary voltage is found by using the following
formula:
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