What is an electrical transformer?
An electrical transformer is a static
device which is used for the transformation of AC electrical signal in
one circuit to the electrical signal of the same frequency in another
circuit with a little loss of power. The voltage in a circuit can be
increased or decreased, but with a proportional increase or decrease in
the current ratings.
Different Types of Transformers
Different types of transformers can be classified based on different criteria like function, core, etc.
Classification according to function:
Step-Up Transformer
A step up transformer is the one in
which the primary voltage of the coil is lesser than secondary voltage. A
Step-up transformer can be used for increasing voltage in the circuit.
It is used in flexible ac transmission systems or FACTS by SVC.
Step-Down Transformer
A step-down transformer is used for reducing the voltage. The type
of transformer in which the primary
voltage of the coil is greater than the secondary voltage is termed as
step down transformer. Most power supplies use a step-down transformer
to reduce the dangerously high voltage to a safer low voltage.
The ratio of the number of turns on each coil, called the turn’s ratio determines the ratio of the voltages. A step-down transformer
has a large number of turns on its primary (input) coil which is
connected to the high voltage mains supply, and a small number of turns
on its secondary (output) coil to give a low output voltage.
TURNS RATIO = (Vp / Vs) = (Np / Ns)
Where, Vp = primary (input) voltage Vs = secondary (output) voltage Np =
number of turns on primary coil Ns = number of turns on secondary coil
Ip = primary (input) current Is = secondary (output) current.
1. Core type 2. Shell type
Core Type Transformer
In this type of transformer, the
windings are given to the considerable part of the circuit in the core
type of the transformer. The coils used are of form-wound and
cylindrical type on the core type. It has a single magnetic circuit.
In core type transformer, the coils are
wounded in helical layers with different layers insulated from each
other by materials like mica. The core is having two rectangular limbs
and the coils are placed on both the limbs in the core type.
Shell Type Transformer
Shell type transformers are the most popular and efficient type of transformers. The shell type transformer
has a double magnetic circuit. The core has three limbs and both the
winding are placed on the central limbs. The core encircles most parts
of the winding. Generally multi-layer disc and sandwich coils are used
in shell type.
Each high voltage coil is in between two
low voltage coils and low voltage coils are nearest to top and bottom
of the yokes. The shell type construction is mostly preferred for
operating at very high voltage of transformer.
Natural cooling does not exist in the
shell type transformer as the winding in the shell type is surrounded by
the core itself. A large number of winding are needed to be removed
for better maintenance.
Other Types of Transformers
The types of transformers differ in the
manner in which the primary and secondary coils are provided around the
laminated steel core of the transformer:
• Based on winding, the transformer can be of three types
1. Two winding transformer (ordinary type) 2. Single winding (auto type) 3. Three winding (power transformer)
• Based on the arrangement of the coils the transformers are classified as:
1. Cylindrical type 2. Disc type
• According to use
1. Power transformer 2. Distribution transformer 3. Instrument transformer
Instrument transformer can subdivided into two types:
a) Current transformer b) Potential transformer
• According to the type of cooling the transformer can be of two types
1. Natural cooling 2. Oil immersed natural cooled 3. Oil immersed natural cooled with forced oil circulation
Working of Transformer
Let us now shift our attention to our basic requirement: How do transformers work? The operation of transformer
mainly works on the principle of mutual inductance between two circuits
linked by a common magnetic flux. A transformer is basically used for
transformation of electrical energy.
Transformers consist of types of conducting coils as primary winding and secondary windings.
The input coil is called the primary winding and the output coil is called the secondary winding of the transformer.
There is no electrical connection
between the two coils; instead they are linked by an alternating
magnetic field created in the soft-iron core of the transformer. The two
lines in the middle of the circuit symbol represent the core.
Transformers waste very little power so the power out is almost equal to
the power in.
The primary coil and the secondary coil
posses high mutual inductances. If one of the coils is connected to the
source of alternating voltage, then an alternating flux will set up in
the laminated core.
This flux gets linked up with the other
coil and an electromagnetic force is induced, as per Faraday’s law of
electromagnetic inductance.
e = M di/dt Where e is induced EMF M is mutual inductance
If the second coil is closed then the
current in the coil is transferred from primary coil of the transformer
to the secondary coil.
Ideal power equation of transformer
While we focus on our query of how do
transformers work, the basic we need to know is about the ideal power
equation of transformer.
If the secondary coil is attached to a
load that allows current to flow in the circuit, electrical power is
transmitted from the primary circuit to the secondary circuit.
Ideally, the transformer is perfectly
efficient; all the incoming energy is transformed from the primary
circuit to the magnetic field and into the secondary circuit. If this
condition is met, the incoming electric power must equal the outgoing
power:
Giving the ideal transformer equation
Transformers normally have high efficiency, so this formula is a reasonable approximation.
If the voltage is increased, then the
current is decreased by the same factor. The impedance in one circuit is
transformed by the square of the turn’s ratio.
For example, if impedance Zs is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns) 2 Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np) 2Zp.
We hope this article has been brief yet
precisely informative about how do transformers work. Here is a simple
yet important question for the readers- How is a transformer selected
for designing a power supply.
Photo Credits:
An electric transformer by wikimediaStep up transformer by imimg
Step down transformer by mpja
Core Type Transformer by electrical-info
Shell Type Transformer by electrical-info
Working of Transformer by encrypted
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