design and manufacture of transformers

INDUSTRIAL ENGINEERING ASSIGNMENT

                         ON

TRANSFORMER LAMINATIONS

DEFINITION:

A transformer is a device with two or more stationary electrical circuits that are conductively disjointed but magnetically coupled time varying magnetic field and is used for transferring power one circuit to another by means of electromagnetic induction at the same frequency.

Working principle:

The physical basis of a transformer is mutual inductance between two circuits linked by common magnetic flux through a path of low reluctance.

A transformer can rise or lower the voltage with a corresponding decrease or increase in current. In its simplest form, a transformer consists of two conducting coils having a mutual inductance. The primary is the winding, which receives electric power, and the secondary is the one which may deliver it. The coils are wound on a laminated core of magnetic material.

The two coils possess high mutual inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set up in the laminated core, most of which is linked up with the other coil in which it produces mutually induced emf (electromotive force) according to the Faraday’s laws electromagnetic induction,

 e = M * di/dt

                                                          Where, e = Induced emf

                                                                      M = Mutual inductance

               If the second circuit is closed, a current flow in it and so electric energy is transferred from the first coil to the second coil.

MANUFACTURING:

The physical basis of a transformer is the mutual induction between two circuits linked by a common magnetic field. Accordingly it needs a magnetic circuit and two or more electric circuits. Apart from active materials like cold rolled grain oriented silicon steel and copper, a number of ferrous, non-ferrous and insulating materials are employed for building up a transformer.

 The building of a transformer comprises of designing, manufacturing and testing. The designing is done based on specifications specified by the customers. After the design of the transformer according to the specifications, the detailed report of the design is sent to the manufacturing unit. The manufacturing unit manufactures the transformer according to the specifications of the design department. The manufacturing unit has various sections under it to complete the specified job.

Each of these sections is assigned a set of jobs. All these jobs are done in parallel so as to maximize production. The general flow chart of the manufacturing section is as follows:

FLOW CHART:

The manufacturing of a transformer can be classified into following sections:

• Core cutting
• Winding
• Core building assembly
• Dummy coil assembly
• Active part assembly
• Final assembly / tanking
• Testing
• Packing and dispatch.

PROPERTIES:

                    Transformer core for the power frequency application is made of highly permeable material. The high value of permeability helps to give a low reluctance for the path of the flux and the flux lines mostly confine themselves to the iron. Silicon steel in the form of thin laminations is used for the core material. Over the years progressively better magnetic properties are obtained by going in for Hot rolled non-oriented to Hot rolled grain oriented steel. The thickness of the laminations progressively got reduced from over 0.5mm to the present 0.25mm per lamination. These laminations are coated with a thin layer of insulating varnish, oxide or phosphate. The magnetic material is required to have a high permeability µ and a high saturation flux density, a very low remanence B_r and a small area under the B-H loop-to permit high flux density of operation with low magnetizing current and low hysteresis loss. The resistivity of the iron sheet itself is required to be high to reduce the eddy current losses. The eddy current itself is highly reduced by making the laminations very thin. If the lamination is made too thin then the production cost of steel laminations increases. The steel should not have residual mechanical stresses which reduce their magnetic properties and hence must be annealed after cutting and stacking.

                    In the case of very small transformers (from a few volt-amperes to a few kilo volt-amperes) hot rolled silicon steel laminations in the form of E & I, C & I or O as shown in Figure below as (a), (b) and (c) are used and the core cross section would be a square or a rectangle. The percentage of silicon in the steel is about 3.5. Above this value the steel becomes very brittle and also very hard to cut. The saturation flux density of the present day steel lamination is about 2 Tesla.

Broadly classifying, the core construction can be separated into core type and shell type. In a core type construction the winding surrounds the core. A few examples of single phase and three phase core type constructions are shown in Fig. 4. In a shell type on the other hand the iron surrounds the winding

The cross section of the core also would be square or rectangular. As the rating of the transformer increases the conductor size also increases. Flat conductors are preferred to round ones. To wind such conductor rectangular former is not only difficult but introduces stresses in the conductor, at the bends. From the short circuit force with stand capability point of view also this is not desirable. Also, for a given area enclosed the length of the conductor becomes more. Hence it results in more load losses. In order to avoid all these problems the coils are made cylindrical and are wound on formers on heavy duty lathes. Thus the core construction is required to be such as to fill the circular space inside the coil with steel laminations. Stepped core construction thus becomes mandatory for the core of large transformers. The figure shows a few typical stepped core constructions.

LOSSES IN THE TRANSFORMER

When input power is supplied to the primary of transformer, some portion of that power is used to compensate core losses in transformer i.e. Hysteresis loss in transformer and Eddy Current loss in transformer core and some portion of the input power is lost as I²R loss and dissipated as heat in the primary and secondary winding, as because these windings have some internal resistance in them. The first one is called core loss or iron loss in transformer and later is known as ohmic loss or copper loss in transformer. Another loss occurs in transformer, known as Stray Loss, due to Stray fluxes link with the mechanical structure and winding conductors.   Hysteresis loss and eddy current losses both depend upon magnetic properties of the materials used to construct the core of transformer and its design. So these losses in transformer are fixed and do not depend upon the load current. So core losses in transformer which is alternatively known as iron loss in transformer and can be considered as constant for all range of load.

Hysteresis loss in transformer is denoted as,

W_h = K_hfB_m^1.6     watts

Eddy Current loss in transformer is denoted as,

W_e = K_ef²K_f²B_m²     watts

                                   Where, K_h = Hysteresis Constant.

                                                   K_e = Eddy Current Constant.

                                      K_f = Form Constant.

Copper loss can simply be denoted as,

I_L²R₂′ + Stray loss

Where, I_L = I₂ = load of transformer, and R₂′ is the resistance of transformer referred to secondary.

CRGO:

Core cutting involves cutting of magnetic material (CRGO) which are available in the form of thin sheets/roles. Laminations are made using these sheets cut that when assembled together make the core of the transformer.
The magnetic material available is CRGO (COLD ROLLED GRAIN ORIENTED) Silicon Steel. CRGO electrical steel sheet with approximate silicon content of 3% is used for magnetic circuit of a transformer.

The following features influence selection of type of steel sheet.

a)      Maximum magnetic induction to obtain high induction amplitude in an alternating field.

b)      Minimum specific core loss for no load loss.

c)      Low apparent power input for low no load current.

d)      Low magneto-striction for low noise level.

e)      High-grade surface insulation.

f)       Good mechanical processing properties.

The chemical composition of CRGO:

1. Carbon
2. Manganese
3. Phosphorous
4. Sulfur
5. Silicon
6. Steel

METHODS OF REDUCING CORE LOSS:

Core loss		Core loss reducing method
Reduction in hysteresis loss		Improvement in orientation Reduction in inclusions and improvement in purity Reduction in internal strain
Reduction in Eddy current loss	Classical eddy current loss	Reduction in thickness Increase in resistivity by increase in silicon content
	Anomalous eddy current loss	Grain size Surface tension on sheet (surface film) Refinement of magnetic domains by physical means

The following types of CRGO are available:

                                                1. M-3 (thickness - 0.23 mm)

                                                2. M-4 (thickness - 0.27 mm)

                                                3. M-5  (thickness – 0.30 mm)

GRADE	THICKNESS	SP.CORE LOSS (Wt./kg at 1.7 tesla)
MOH	0.27 0.3	0.99 1.01
M4	0.27	1.22
ZDKH	0.23	0.89
ZDMH	0.23	0.78

The material is not available in India so for industries like AREVA TND India ltd. and other industries in India, it is imported from other countries.

The chief suppliers are

·         Russia

·         France

·         Germany

·         USA

·          N Korea

Machines used for cutting

There are two types of machines used for cutting the core. They are:

·         Slitting line

·         MITRE cut line

The slitting line machine is used to cut width wise and the MITRE cut line is used to cut length wise.

Specifications of core-cutting shapes and sizes

Generally following three specifications are given for cutting the sheets for any design of transformer:

1. Window height

2. Leg length

3. Yoke length

PREPARATION OF LAMINATION SHEETS:

For building the transformer cores, lamination sheets of different widths and packet heights are needed.

Flow Chart:-

Laminations are produced by the following operations.

1. Core slitting:

                            The manufacturing schedule may include cores of different diameters and different types of constructions necessitating slitting laminations in many widths and lengths. CRGOs rolls cannot be ordered in so many different widths and quantities. These rolls are available in standard widths of 790, 840, 1000 etc. For slitting operation, some widths can be combined together by suitably adjusting the cutter distances in the slitting machine.          

                   It is evident that full width of roll cannot be utilized at any time of slitting operation and the leftover material will vary from stage to stage and depending on the widths selected in combination during the process of slitting. The meticulous care in planning is imperative to minimize wastage of core steel.

2. Core cutting:

                          Different shapes and sizes of laminations are needed for different types of transformer cores. Hence, in this operation a cropping machine is for cutting laminations from slitted rolls.

             Depending upon the shape of cutting these laminations are grouped into following sets. 

1. ABCD
2. PQR
3. PQRS

PQR TYPE:

This type of shape is dominant in manufacturing and design.

Generally the corners of the laminations are cut at 45°, this jointing is known as mitred joint. Flux flow in cross grain direction for this type of jointing is minimum as the magnetic flux leaving or entering at the joint finds a smooth path flow. Hence the no load losses decreases.

3. Deburring:

                  During the process of slitting, cutting and piercing of laminations, the cut edges get some burrs. These burrs occur mainly during the cutting operation and are due to following reasons:

1.      Shearing blades blunt

2.      More clearance between blades

3.      V-notch tool blunt

Presence of burrs impairs the stacking factor, resulting in gaps, cut into the insulation coatings and bridge adjacent laminations there by increasing the eddy current losses. These burrs are removed by passing the laminations through deburring operations.

4. Varnishing:

           Caralite coating is done on the cut core legs. It is a gel that is spread over the sheets to maintain electrical insulation between two sheets. The glass film and phosphate coating (caralite coating) uniformly coated on both sides of the lamination sheets serves as surface insulation.

Lacquer – It is the antirust protection gel used to spread over the cut core sheets to make it free from oxidation and rust formation.

Term and specifications related to the shop

1. Maximum cutting width: 660 - 700 mm

2. Burrs level – It happens that due to ageing effect the cutting tool surface may not remain smooth thus the cutting edge thickness will be higher at the ends than the edge material. This difference in thickness is called as the burs level.

3. Space factor – When two sheets are placed one over other there is always some air gap due to irregularity in the surfaces it has got an ideal value. But due to some dust particles it is somewhat more than this ideal value.

            The ratio of the ideal value and the actual value is called space factor. Its value is 0.965.it is always <1.