Unit 3
Contents
The response of a material when subjected to an external magnetic field is the root of magnetism. The spinning electrons in the material behave like tiny magnets. These tiny magnets are aligned in the direction of applied magnetic field and thereby the material is magnetized.
An applied magnetic field H incites a response from a 2 material called magnetic induction B. The relationship between B and H can be defined by
B = μo H + μo I ------------------------(1)
Where I is the magnetic moment per unit volume. μo is a constant called the permeability of free space. So, the ratio of I to H is called the susceptibility and is indicative of the ease with which a material can be magnetized by an applied magnetic field .
----------------------------------------(2)
Equation(2) is called the suspectability.
The ratio of B to H is called the permeability and it is the property of the material which allows magnetic lines of force (field) to penetrate the material.
μ=B/H -----------------------------(3)defines magnetic permeability.
- When tested experimentally, a ferromagnetic material such as iron will produce a curve similar to that shown above.
- First there is an upper/lower limit to the magnetic flux density which may be achieved, which occurs at positive or negative saturation, respectively. This is related to the crystalline structure of the iron, where each crystal has its own – initially random – magnetic orientation.
- Increasing the magnetic field strength in either direction causes more and more magnetic 'domains' to align with the external magnetic field, but once almost all of the domains have aligned themselves, then little further increase in magnetic flux density is possible. The ferromagnetic material is said to be saturated.
- The curve demonstrates magnetic hysteresis or 'lag' as the sample is alternatively magnetized in the positive and negative directions. When initially magnetized, the curve follows points a–b on the graph, but on reducing H to zero, some residual magnetism remains (point c - also known as the remanent flux density).
- In order to fully demagnetize the specimen, it is necessary to apply a negative magnetic field strength (point d - called the coercive force). Making H increasingly negative leads to negative saturation (point e). If H is reduced back to zero, point f is reached (negative residual magnetism). As H becomes positive, the flux density reduces to zero (point g) and then becomes positive.
- Finally returning back to point b (positive saturation), after which the cycle b–g repeats. The area enclosed by the B-H curve (shaded light blue above) is proportional to the energy loss as the ferromagnetic material is magnetized with varying polarity by connection to an alternating (AC) power supply.
- This energy loss is undesirable and causes unwanted heating of the material. In general, harder ferromagnetic materials have higher hysteresis losses, since more energy is required to realign the magnetic domains. Steel, to which a small proportion of silicon has been added, is commonly used for applications such as transformer cores and motor rotors, due to its lower hysteresis loss.
- For smaller applications, such as inductor cores, passive filters, miniature transformers and antennas, then ferrite (a ceramic-like combination of metallic oxides including ferric oxide) is a popular choice.
- Types of transformers
Acc to input supply : I phase and 
phase
Acc to construction : core and shell type
Acc to 0/P : step up and step down
Construction of transformers (study only for MCQs)
- Laminated steel core
Material used for core is (silicon steel) it is used for its (high permeability) and (low magnetic reluctance) 
magnetic field produced is very strong
The core is formed of (stacks of laminated thin steel sheets) which are electrically isolated from each other. They are typically (0.35 to 0.5 mm thick)
We can used 2 ‘L’ shaped sheets or 2 
shaped sheets for laminations
4.7 Construction and types
- There a 2 types of winding
- Concentric or cylindrical
- Sandwiched type
- Cylindrical
L.V. = low voltage H.V. = high voltage are mounted on same limb to minimum leakage .
L.V. Winding placed inside and H.V. Winding placed outside with (proper insulation between the winding as it is easy to insulated L.V. Winding) than H.V. Winding.
2. Sandwiched
The H.V. And L.V. Winding are divided into no. Of small coils and there small windings are interleaved.
(the top and bottom winding are L.V. Coils because they are close to core)
- Transformer tank : wholes assembly of winding and core placed inside the Transformer tank (sheet metal tank) which is filled with Transformer oil or insulating oil which acts as an (insulator or coolant ) MCQ
- Transformer oil : (The function of oil is to remove efficiently the heat generated in core and in winding)
- Moisture should not be allowed which creeps the insulation which achieved by closed Transformer tank.
(To increase cooling surface are tubes or fins are provided)
- Conservator tank : above tank T/F tank there is one small tank in which same empty space is always provided above the oil level. (this space is required for oil to expand or contract due to temperature change) MCQ
However during contraction outside air can have moisture which will deteriorate the insulating properly of oil.
- Breather : the air goes in or out through the breather (To reduce the moisture content of their air . Same drying agents such as (silica gel or) calcium chloride) is used in the breather (The dust particles present in air are also removed by breather)
- Buccholz Relay : (for incipient (slowly increasing) faults
There is pipe connecting rain tank and conservator. On the pipe a protective device called Buccholz Relay is mounted.
When the Transformer is about to be faulty and draw range current the oil becomes very hot and decompose.
During this process different types of gases are liberated.(The Bucchoz Relay get operated by these gases) and gives an alarm to the operator. ɡȴ the fault continues to persist then there lay will trip off main circuit breaker to protect the Transformer.
- Explosion Vent :
An explosion Vent or relief value is the bent up pipe filled on the main tank.
(The explosion vent consist of aluminium of oil ) when the T/F becomes faulty the cooling oil get decomposed and various types of gases are liberated
(ɡȴ the gas pressure exodus certain level then the aluminium of oil (diagram) in explosion vent will burst) to release pressure. The will save main tank from getting damaged.
- Single phase transformer - for MCQs
(Symbol and principal of operation)
It is a static device which can transfer electrical energy from one ac circuit to another ac circuit without change in frequency
It can increase or decrease the voltage but with corresponding decrease or increase in current
It works on Principle 
“Mutual Induction”
A major application of transformer is to increase voltage before transmitting electrical energy over a long distance through conductors and to again reduce voltage at place where it is to be used.
Symbol
- Primary winding : (ac supply side) For MCQs
The winding which is connected to supply is called primary side
2. Secondary winding : (load side) for MCQs
The winding which is connected to load is called as secondary winding
- Working : For MCQs
It works on principle of mutual Induction ie “when 2 coil are inductively coupled and if current in one coil is changed uniformly then an emf get induced in another coil”
- When alternating voltage V1 is applied to primary winding am alternating current I1 flows in it producing alternating flux in the core.
- As per Faradays Law of Electromagnetic Induction 1 an emf E1 is induced in the primary winding E = N1
- The emf induced in the primary winding is nearly equal and opporite to applied voltage V1
- Assuming leakage flux to be negligible almost whole flux produced in primary winding links with secondary winding thence emf induced in the secondary winding E2 = N2
- Core and shall type transformers
Core Diagram
Shell Diagram:
- It consist of magnetic frame with 1. 3 Limbs
2 limbs
2. single magnetic circuit 2. Double magnetic ckt.
3. Winding encircles the core 3. Core encircles the winding
4. One window 4. Two window
5. Cylindrical type winding is used 5. Sandwich type is used
6. Easy to repair 6. Not easy repair
7. Preferred for Low voltage application 7. Preferred for high voltage application
8. Provides better cooling since winding 8. Does not provide effective cooling as
Are uniformly distributed on 2 limbs the winding are surrounding core.
The B-H curve of transformer core is never linear rather it becomes flat as the value of magnetizing current increases.
This means, transformer exciting current will not be in phase with the core flux. In fact, exciting current leads the core flux by some angle “α”. This angle is known as Hysteretic Angle.
This angle is dependent on the hysteresis loop of the core material.
In an ideal transformer, magnetization curve is assumed to be linear. This means, magnetizing current and core flux are in phase.
In practical/real transformer, the exciting current (current in primary winding under no load condition) drawn from the source is composed of magnetizing current (Im) and core loss component (Ic). This is because, core loss is supplied by the primary source only. However, in an Ideal Transformer, as there is no core loss, the exciting current is equal to magnetizing current.
An auto Transformer is a special types of transformer such that a part of the winding is common to both primary as well as secondary
It has only One winding wended on a laminated magnetic core
With the help of auto Transformer the voltage can be stepped up and stepped down at any desired value
Fig A shows auto T/F as step down T/F variable terminal B C is connected to load and it acts as secondary wdg.
The position of point C is called as topping point can be selected as per requirement
Fig. B show auto T/F as step up T/F variable terminal B C is connected to supply side ie ac side and it acts as secondary winding
The operating principle of auto Transformer is same as that of 2 winding Transformer
Advantages
- Weight of copper required in an auto Transformer is always loss than that of the conventional 2 winding Transformer and hence it is chaper
- Com[act in size and loss costly.
- Losses taking place in Transformer is reduced hence efficiency is higher than conventional Transformer.
- Due to reduced resistance, voltage, regulation is better than conventional T/F.
Disadvantage :
As low voltage and high voltage sides are not separate then there is always risk of electric shocks when use for high vtg. Applications
Applications
- Starting squirrel cage induction motor and synchronous motor.
- Auto transformer as dimmer stat
- Used as vari ac to vary a. c Voltage
References:
Getting Started in Electronics Book by Forrest Mims
Practical Electronics for Inventors, Fourth Edition Book by Paul Scherz and Simon Monk
Electronics for dummies Book by Cathleen Shamieh and Gordon McComb
