Transformers - Transfer (as if) by Magic – Voltage Regulation
Regulation of voltage across
the secondary terminals of a transformer under varying conditions of load or
slight changes in the input voltage is crucial or at least desirable for
the equipments to operate smoothly. Just
learn how transformers could be used for this purpose
Introduction
We studied about Faraday’s law
in our previous article. In this article we will study how a steady voltage
could be gained by using a transformer as a voltage regulator when either the
load varies or the input voltage varies within reasonable limits
Voltage Regulation
As we know a transformer steps
up or steps down the input voltage depending on the ratio of turns between the
primary and the secondary windings. It also leads to an increase or decrease of
the associated current. Now the input power and voltage (and hence the current)
is constant and so should be the output voltage (and current). But there is
another factor in the secondary circuit which comes into play and that factor
is the load. It is not always possible to have a fixed or constant load but it
may vary considerably within the given limits over a period of time depending
on where the transformer is actually used.
Let us take an example of a
step down transformer supplying electricity to a home which has a lot of rooms
and electrical gadgets such as heaters, geysers, lightings and so forth.
Obviously the load cannot be steady and will depend on usage of the household
people. The change in load will cause change in current and hence the voltage.
If the output voltage varies too much with current it is certainly not good for
the equipments. Though sometimes there are other
situations where perfect voltage regulation may not be desirable. For example
let us think about a step down transformer which is used in arc-welding. The
basic function of that transformer is to supply low voltage and high current
for the welding process to take place. Nevertheless the voltage required when
the welder is striking an arc is much greater than the voltage required for
keeping the arc sustained.
The Regulation Effectiveness
of a transformer is measured as a percentage between no load and full load
conditions and is given by the formula
Regulation Percentage = [No
load voltage – Full load voltage/Full load voltage] * 100
The typical value for this
percentage should be less than say 3% for the transformer to be considered to
have good voltage regulations characteristics
How to Regulate Voltage
We looked at the need for
voltage regulation but how do we actually go about achieving it? Several types
of voltage regulators are based on different principles but we will limit ourself to those using transformers as the main
ingredient for the voltage regulation recipe.
In the example of the
household arrangement, there are transformers which have a servo mechanism for
varying the output voltage based on load feedback.
In case of variable regulation
requirements such as the case of the welding transformer discussed above, most
of the welding transformers by having a mechanical system for moving the iron
core in and out of the magnetic couplings so as the deliberately reduce the
regulation effect.
Voltage regulation is not only
required from the load point of view but also the primary voltage may change
and the secondary voltage needs to be maintained constant for reasonable
variations of the primary voltage. A method which is effective in such a case
is the use of a ferroresonant transformer for
automatic voltage control. These transformers operate under conditions of
persistent core saturation which means that any variations in the voltage
within reasonable limits does not affect the output voltage significantly thus
achieving the voltage regulation. These types of transformers usually have two
secondary windings. One is used to supply the output voltage while the other
acts as a harmonics filter to filter out harmonics created by core saturation.
The figure below illustrates this concept clearly.
Hence we have seen that
transformers can also be used for partial voltage regulation apart from
stepping up or stepping down of the voltage.