Computer Aided Electrical drawing (CAED) Subject Code: 10EE65

Subject: Computer Aided Electrical drawing (CAED) Subject Code: 10EE65

Transformer

• To know the parts of transformer

• To know the types of transformer

• To make preparatory sketch of the transformer from the given design data

• To make drawing of the sub assembly or assembly drawing using CAD

Single Phase and Three Phase Transformers (Core Type)

The elevations of single phase core type

transformer and three phase core type transformers are shown in Figure 1. The widths of cores on the two limbs are equal in single phase transformers. The widths of cores of all the three limbs are equal in three phase transformers. The height of yoke may be equal or greater than the width of core, in both cases. The depth of core will be confined to the dimension of circumcircle diameter

Single Phase and Three Phase Transformers (Shell type)

The elevations of single phase shell type transformer and three phase transformer are shown in Figure 2. The width of the extreme limbs will be about half the width of central limb. The depth of the core may equal to about two to three times the width of central limb. The height of yoke may equal to or greater than half the width of central limb

LV and HV windings

The transformer will have Low Voltage (LV) winding and High Voltage (HV) winding wound over the limbs. The LV winding is placed near the core. There will be a gap between LV and HV windings for oil circulation. The HV winding is placed away from the core. The finished windings are cylindrical in shape. The thickness and height of the cylinder will depend upon the number of turns and area of cross section of winding wire. The internal diameter will depend on the circum circle diameter, slack allowance and insulation layer. It may be noted that the sections of the cylinders so formed is confined to the window dimensions, viz, width of window and height of window. When the winding wires are thin or when the handling of finished winding is difficult because of heavy weight, the winding may be split into smaller discs. These discs may be stacked one above the other so that the required length may be formed. The views of such windings are shown in Figure 3 and Figure 4.

Rectangular coil winding

In the case of shell type transformers, the winding will be hollow- rectangular in

shape as shown in Figure 5. The inside dimension of the rectangle will depend upon- i) widthwise, the central limb width and the insulation between core and winding and ii) depth wise, depth of core and projection of fasteners like bolt, rivet head etc., protruding outside the core.

Figure 5- Rectangular coil

Core section

The core section of core type transformer may be – square type, two stepped or cruciform, three stepped, four stepped and so on. The imaginary circle passing through the corners of the core section is known as circumscribing circle or circum circle. The dimensions for obtaining the maximum area in the given circum circle, for various number of steps is given in Figure 6.

Figure 6 – Step dimensions

Problem-1

Draw the Sectional Plan of one limb of a single phase, 11kV/440 V, transformer - Core- Cruciform (Two Stepped)

Dia. of circumcircle = 33 cm

Inside and outside dia. of LV and HV windings are- 35,39,44,49 cm, respy.

Core laminations are fixed by means of two plates of 3 mm thick, by M12 bolt.

Preparatory Sketch:

As a first step a preparatory sketch is made using the given data, before taking up CAD

drawing, This greatly helps in finding the hidden dimensions, dimensions to be assumed, planning of the CAD drawing, choosing appropriate commands, etc. The preparatory sketch is shown in Figure 7. The step dimensions are calculated. The relative positions of LV and HV windings are drawn. Spacers are to be provided with assumed dimensions. Bolt is to be shown in the plan view.

Figure 7 – Preparatory sketch for Problem-1

Start the CAD drawing using the commands as shown in the following tables.

Problem-2

Figure 8- CAD Drawing for Problem-1

Draw Sectional Elevation, Sectional Plan and End-view of a 5 kVA, core type,

single phase transformer for the data given - Circumscribing Circle = 80 mm diameter

Width of Core = 55 mm, Height of Yoke = 65 mm LV –Internal diameter = 83mm

LV –External diameter = 98mm HV –Internal diameter = 114mm

HV –External diameter = 135mm

Over all width of the magnetic frame = 215mm

Over all height of the magnetic frame = 350mm Center to center distance between limbs =160mm

The preparatory sketch (Figure-9) is drawn, incorporating all the details given in the problem. The missing dimensions may be assumed. The construction of table of command and resulting figures is not necessary. In the first problem this table has been given as an illustration only. Further, there is no unique way of usage of commands for drawing a figure. However a sequence of steps for Problem – 2 is indicated as follows- With reference to center to distance marking, the core in plan is constructed.

The plan of windings is drawn.

The magnetic frame is completed.

The windings in elevation are constructed.

End view is constructed.

The CAD drawing is shown in Figure-10

Problem-3

Draw the following views of a 3 phase, core type, 250 kVA, 11 kV / 400 V transformer-

i) Front elevation in full section and ii) Plan in full section

Cross section of the core: 3 Stepped Diameter of the circum circle = 24 cm

Centre to centre distance between adjacent limbs = 42.5 cm Yoke height = 25 cm

Total height of transformer = 100 cm

Preparatory sketch – Three Phase Core Type Transformer

• Circumcircle, d = 24 cm

• Core is three stepped

• Calculate the values of three steps, viz.,

• 0.9x24 = 21.6cm

• 0.7x24 = 16.8cm

• 0.42x24 = 10.08cm

Draw center lines, 425 mm apart. Draw another center line at the bottom of the monitor. Draw the left hand side core section with three steps.

Draw the LV and HV winding profile with dia= 250, 283, 343, 415 mm respectively. Complete the magnetic frame in elevation with the given dimensions.

Draw the projection of the windings from the plan view. Hatch the core section in the plan.

Hatch the LV and HV windings in the plan. Hatch the LV & HV windings in the elevation. Copy the views to the other two limbs.

Mark the dimensions and title.

For final drawing see Figure-11.

Problem- 4

Draw the half sectional elevation and half sectional plan of a Shell Type transformer,

which is rated as 125 kVA, 2000 V/ 400 V, Single Phase, 50 Hz. The details are given below-

Dimension of core: Central leg = 14 x 35cm, Outer legs = 7 x 35 cm, Yoke = 7 x 35 cm, Window = 28 cm x 14 cm

LV winding: Sandwich, Coil dimension = 1.8 cm x 10 cm, Number of coils = 4 HV winding: Sandwich, Coil dimension = 2.0 cm x 10 cm, Number of coils = 4

Preparatory sketch

Using given data, form the magnetic frame as shown above. Study the section of the winding.

Formulate the winding dimensions.

Form the sandwich LV & HV windings alternatively

The shapes of the HV Coil and LV coil are similar, except the height. From this we can draw the plan and elevation. Final drawing is shown in figure-12Figure 12: CAD Drawing for Problem-4

 

Problem- 5

Draw the half sectional elevation, half sectional plan and end view of a 100 kVA, 2200 V

/ 480 V, single phase, core type transformer Diameter of circum circle = 245 mm,

Step dimensions are- 100,172,220 mm respectively, Window = 150mm x 300mm,

Total turns in LV winding = 66, Total turns in HV winding = 300,

Winding strip size for low voltage side = 11.8x3.5mm -2off, Winding strip size for high voltage side = 5.2x3.46 mm Delta Gap between LV and HV windings = 10 mm

Hints:

Draw the vertical and horizontal centre lines. With commands- circle, line, offset, draw

the core section in plan. Project upwards to develop elevation of the core. (Figure-13) Draw center lines. Draw the profile of the magnetic frame. Window dimension profile is in between the red lines. (Figure-14)

Note: Between core and LV winding, 5mm thick bakalized paper cylinder; Windings wound on paper discs of suitable thickness.

Show the partial drawing of placement of LV and Disc windings and the GA drawing of the 15 MVA transformer.

The details are to be worked out by trails. The final arrangement of LV and HV disc windings is shown in Figure-19. For LV winding 25 discs of 6 turns each are required.

For HV winding 21 discs of 20 turns and 2 discs of 17 turns 9(total- 23 discs) are required.

This is a huge transformer.A4 size is too small to read the details. Hence partial drawings are shown. Partial placing of LV and HV discs on one limb is shown. The spacing and insulation details are shown in Figure-20 and Figure-21. The general appearance of plan and elevation of the 15 MVA transformer is shown in Figure- 22.

Referances:

1. Performance and Design of AC Machines, MG Say, ELBS 1970

2. Electrical Engineering Drawing, KL Narang, Satya Prakashan, New Delhi,2001

3. Electrical Drafting, SF Devalapur, Eastern Book Promoters, Belgaum, 2006

4. CAD for Electrical Engineers, MS Indira, VD Sankarlal, D Beula, Fillip Learning- Elsevier, 2013

Appendix (Transformer)

Disc Coil

Insertion sequence of windings

Photo of Finished Transformer with Accessories