Transformer

Introduction to Electrical Transformer

What is a Transformer?

In Very Simple words .

Transformer is a device which :

1. Transfer Electrical power from one electrical circuit to another Electrical circuit.
2. It’s working without changing the frequency.
3. Work through on electric induction.
4. When, both circuits take effect of mutual induction.
5. Can’t step up or step down the level of DC voltage or DC Current.
6. Can step up or step down the level of AC voltage or AC Current.

                  1500 kVA Transformer by Siemens | Electrical Technology

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
  

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

• Types of Transformers

There are two basic Types of Transformers

1. Single Phase Transformer
2. Three Phase Transformer

Below are the more types of transformer derived via different functions and operation etc.

• Types of Transformers w.r.t Cores

• Core Type Transformer
• Shell Type Transformer
• Berry Type Transformer

• Types of Transformer w.r.t uses

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

• Types of Transformer w.r.t 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
• Auto Transformer

• Operating & 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.

                                 Click image to enlarge | Working of a Transformer

 

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.

• 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.

• Electromagnetism

The interaction between magnetic field and electric current is termed electromagnetism. Current carrying conductors produces magnetic field when current passes through it. Movement of electrons in a conductor will result to electric current (drifted electrons) which occurs as a result of the EMF set up across the conductor.

The EMF set up across the conductor can be in form of that stored in chemical energy or magnetic field. Current carrying conductor placed in a magnetic fields will experience mechanical force while a conductor placed in a magnetic field will have its electrons drifted which will results to electric current.

• Field Flux

Two magnets of unlike poles will attract each other while magnets of like poles will repel each other (so it is with electric charges). Every magnet is surrounded by a force field and is represented by imaginary lines emanating from the north pole of a magnet going into the south pole of the same magnet.

Read the important terms related to Field Flux and Magnetic Filed with formulas Here

“The lines linking the north and south pole of a magnet which represents force field which links coils in a transformer is termed as magnetic flux”.

• Electromagnetic Induction

Electromagnetic induction is a phenomenon that explains how EMF and current is or can be induced in a coil when a coil and a magnetic field interact. This phenomenon”electromagnetic induction”is explained by Faraday’s laws of electromagnetic induction. The direction of induced EMF in a coil is explained by Lenz’s law and Fleming’s right hand rule.

• Faraday’s Laws Of Electromagnetic Induction

After Ampere and others investigated the magnetic effect of current, Michael Faraday tried the opposite. In the course of his work he discovered that when there was change in a magnetic field in which a coil was placed, EMF was induced in the coil.

This happened only whenever he moved either the coil or the magnet he used in the experiment. EMF was induced in the coil only when there was change in the field flux (if the coil is fixed, moving the magnet towards or away from the coil causes EMF to be induced). Thus Faraday’s laws of electromagnetic induction states as follows;

• Faraday’s First Law

Faraday’s first law of electromagnetic induction states that “EMF is induced in a coil when there is a change in the flux linking the coil”.

• Faraday’s Second Law
  

Faraday’s second law of electromagnetic induction states that “the magnitude of induced EMF in a coil is directly proportional to the rate of change of flux linking the coil”.

e = N dϕ/dt

Where

e = Induced EMF

N = the number of turns

dϕ = Change in flux

dt = Change in time

• Lenz’s Law

Lenz’s law entails how the direction of an induced EMF in a coil can be determined. “It thus states that the direction of induced EMF is such that it opposes the change causing it.

In other words, When an E.M.F is induced in a circuit, the current setup always opposes the motion, or change in current, which produces it. OR

An induced EMF will cause a current to flow in a close circuit in such a direction what its magnetic effect will oppose the change that produced it.

According to this law (which introduced by Lens in 1835), the direction of current can be found. when the current through a coil changes magnetic field, the voltage is created as a result of changing magnetic field, the direction of the induced voltage is such that it always opposes the change in current.

in very simple words, lenz’s law stating that the induced effect is always such as to oppose the cause that produced it.

• 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
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 / Xl)   …. 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?

• Uses and Application of Transformer

Uses and applications of transformer is discussed already in this previous post.

Advantages of 3-Phase Transformer over 1-Phase Transformer

Read the advantages and disadvantages of Single Phase & three phase transformer here.

You may also read :

• Transformer Efficiency, All day Efficiency & Condition for Maximum Efficiency
• How to Calculate/Find the Rating of Transformer in kVA (Single Phase and Three Phase)?
• A (50/60 Hz) Transformer. Which one will give more Output? (When operates on 50 or 60 Hz frequency)
• Can we operate a 60HZ Transformer on 50Hz Supply Source and Vice Versa?
• Is it possible to Operate a 50Hz Transformer on 500Hz frequency Supply?
• EMF Equation of Transformer
• Losses in Transformer