What is an Electrical Transformer? Construction, Working, Types and Applications

Electrical Technology

What is a Transformer? Its Parts, Operation, Types, Limitation & Application

What is a Transformer?

As the name suggests, an Electrical Transformer transfers electrical power from one electrical circuit to another electrical circuit. It does not change the value of power.
A Transformer only steps-up or steps-down the level of voltage or current.
A Transformer doesn’t change the circuit frequency during operation.
A Transformer works on the principle of electric i.e. mutual induction.
A Transformer operates when both circuits take effect by mutual induction.
A Transformer can’t step-up or step-down the level of DC voltage or DC Current.
A Transformer only step-up or step-down the level of AC voltage or AC Current.
A Transformer doesn’t change the value of flux.
A Transformer won’t operate on DC Voltage.

Without transformers the electrical energy generated at generating stations won’t probably be sufficient enough to power up a city. Just imagine that there are no transformers. How many power plants do you think have to be set up in order to power up a city? It’s not easy to set up a power plant. It is expensive.

Numerous power plant have to be set up in order to have sufficient power. Transformers help by amplifying the Transformer output (stepping up or down the level of voltage or current).

When the number of turns of the secondary coil is greater than that of primary coil, such a transformer is known as step up transformer.

Likewise when the number of turns of coil of primary coil is greater than that of secondary transformer, such a transformer is known as step down transformer.

Construction of a Transformer (Parts of a Transformer)

1	Oil filter valve	17	Oil drain valve
2	Conservator	18	Jacking boss
3	Buchholz relay	19	Stopper
4	Oil filter valve	20	Foundation bolt
5	Pressure-relief vent	21	Grounding terminal
6	High-voltage bushing	22	Skid base
7	Low-voltage bushing	23	Coil
8	Suspension lug	24	Coil pressure plate
9	B C T Terminal	25	Core
10	Tank	26	Terminal box for protective devices
11	De-energized tap changer	27	Rating plate
12	Tap changer handle	28	Dial thermometer
13	Fastener for core and coil	29	Radiator
14	Lifting hook for core and coil	30	Manhole
15	End frame	31	Lifting hook
16	Coil pressure bolt	32	Dial type oil level gauge

Related Post: Open Delta Connections of Transformers

Working Principle of a Transformer

Transformer is a static device (and doesn’t contain on rotating parts, hence no friction losses), which convert electrical power from one circuit to another without changing its frequency. it Step up (or Step down) the level of AC Voltage and Current.

Transformer works on the principle of mutual induction of two coils or Faraday Law’s Of Electromagnetic induction. When current in the primary coil is changed the flux linked to the secondary coil also changes. Consequently an EMF is induced in the secondary coil due to Faraday law’s of electromagnetic induction.

The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

A simple transformer has a soft iron or silicon steel core and windings placed on it(iron core). Both the core and the windings are insulated from each other. The winding connected to the main supply is called the primary and the winding connected to the load circuit is called the secondary.

Winding (coil) connected to higher voltage is known as high voltage winding while the winding connected to low voltage is known as low voltage winding. In case of a step up transformer, the primary coil (winding) is the low voltage winding, the number of turns of the windings of the secondary is more than that of the primary. Vice versa for step down transformer.

Good to Know:

Transformer Always rated in kVA instead of kW.

As explained earlier, EMF is induced only by variation of the magnitude of the flux.

When the primary winding is connected to ac mains supply, a current flows through it. Since the winding links with the core, current flowing through the winding will produce an alternating flux in the core. EMF is induced in the secondary coil since the alternating flux links the two windings. The frequency of the induced EMF is the same as that of the flux or the supplied voltage.

By so doing (variation of flux) energy is transferred from the primary coil to the secondary coil by means of electromagnetic induction without the change in the frequency of the voltage supplied to the transformer. During the process, a self induced EMF is produced in the primary coil which opposes the applied voltage. The self induced EMF is known as back EMF.

Ideal Transformer verses Practical Transformer

In our previous post, we have discussed the ideal transformer with phasor and circuit diagrams and detailed comparison with practical transformers. Keep in mind that an ideal transformer has no losses at all e.g. the input power to the transformer is equal to the output power. In addition, it should be noted that ideal transformer is an imaginary (theoretical concept) which does not exist in real life.

Equivalent Circuit of a Transformer

In our explanatory article about equivalent circuit of an electrical Transformer which is a graphical representation of a transformer circuit in which the resistance and leakage reactance are imagined to be external to the winding. The exact equivalent circuit of a transformer can be referred to as the primary side or secondary side.

EMF Equation of a Transformer

Magnitude of the induced EMF (or Voltage) in a transformer can be found by EMF equation of the transformer. When a source of alternating current (AC) is applied to the primary winding of the transformer which is known as magnetizing current, it produces alternating flux in the core of a transformer.

Losses in a Transformer

Unlike an ideal transformer, A real and practical transformer has some losses like ohmic losses, magnetic flux losses, copper and core losses and energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those of distribution transformers usually perform better than 98%.

Transformer’s Efficiency

On specific power factor and load, the transformer efficiency and all day efficiency can be found by dividing its output on Input (similar to other electrical machines i.e. motors, generators etc). But the values of both Input and Output should be same in unites (i.e. in Watts, kilowatts, megawatts etc).

Types of Transformers

There are different types of transformer based on their usage, design, construction as follow.

Types of Transformers based on its Phases

Single Phase Transformer
Three Phase Transformer

Types of Transformers based on its Core Design

Core Type Transformer
Shell Type Transformer
Berry Type Transformer

Types of Transformers based on its Core

Air core Transformer
Ferromagnetic/Iron Core Transformer

Types of Transformer based on its usage

Large Power Transformer
Distribution Transformer
Small Power Transformer
Sign Lighting Transformer
Control & Signaling Transformer
Gaseous Discharge Lamp Transformer
Bell Ringing Transformer
Instrument Transformer
Constant Current Transformer
Series Transformer for Street Lighting

Types of Transformer based on Insulation & Cooling

Self Air Cooled or Dry Type Transformer
Air Blast-Cooled Dry Type
Oil Immersed, Self Cooled (OISC) or ONAN (Oil natural, Air natural)
Oil Immersed, Combination of Self Cooled and Air blast (ONAN)
Oil Immersed, Water Cooled (OW)
Oil Immersed, Forced Oil Cooled
Oil Immersed, Combination of Self Cooled and Water Cooled (ONAN+OW)
Oil Forced, Air forced Cooled (OFAC)
Forced Oil, Water Cooled (FOWC)
Forced Oil, Self Cooled (OFAN)

Types of Instrument Transformer

Current Transformer
Potential Transformer
Constant Current Transformer
Rotating Core Transformer or Induction regulator
Autotransformer

Limitation of the Transformer

To understand the main points, we have to discuss some basic terms related to transformer operation. So lets back to basic for a while.

A transformer is an AC machine that steps up or steps down an alternating voltage or current. A transformer being an AC machine however cannot step up or down a DC voltage or DC current. It sounds a bit weird though. You might be thinking “so are there not DC transformers?”

To answer the two questions whether there are or there are not DC transformers and know “why transformer cannot step up or step down a DC voltage” it’s necessary we know how electric current and magnetic field interact with each other in transformer operation.

Fleming’s Right Hand Rule

It states that “if the thumb, the forefinger and the middle finger are held in such a way that they are mutually perpendicular to each other (makes 90° of Angles), then the forefinger points the direction of the field, the thumb points the direction of motion of the conductor and the middle finger points the direction of the induced Current (from EMF).

Why Transformers Can’t step Up Or Step Down A DC Voltage or Current?

A transformer cannot step up or step down a DC voltage. It is not recommendable to connect a DC supply to a transformer because if a DC rated voltage is applied to the coil (primary) of a transformer, the flux produced in the transformer will not change in its magnitude but rather remain the same and as a result EMF will not be induced in the secondary coil except at the moment of switching on, So the transformer may start to smock and burn because;

In case of DC supply, Frequency is zero. When you apply voltage across a pure inductive circuit, then according to

X_L= 2 π f L

Where:

X_L = Inductive Reactance
L = Inductance
f = Frequency

if we put frequency = 0, then the overall X_L (inductive reactance) would be zero as well.

Now come to the current, I = V / R (and in case of inductive circuit, I = V / X_L) …. basic Ohm’s Law

If we put Inductive reactance as 0, then the current would be infinite (Short circuit)…

So, If we apply DC voltage to a pure inductive circuit, The circuit may start to smoke and burn.

Thus transformers are not capable of stepping up or stepping down a DC voltage. Also there will be no self induced EMF in such cases in the primary coil which is only possible with a varying flux linkage to oppose the applied voltage. The resistance of the primary coil is low and as such a heavy current flowing through it will result to the primary coil burning out due to excessive heat produced by the current.

Also read: Under what condition is D.C supply applied safely to the primary of a transformer?