Page 166 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The size of the wire can be measured with insulation but it shall be within the limit of tolerance. The
insulation scheme shall be followed as per the data taken. Where proper material is not available
an equivalent type and size may be selected. Turns and tapping of the winding shall be made as in
the original.
Method of stacking : Before stacking the core, stampings shall be checked for dents, bends and core insulation.
Dents on the core shall be removed, and any mangled core shall be set right. Stacking shall be done as in the
original sequence and pattern.

All the stampings available for the transformer shall be stacked without leaving out any. Fig 1 shows the different
shapes of cores used for a shell type transformer. Leads shall be properly sleeved and terminated.

Fig 1

Procedure of rewinding a transformer: As stated above, if all the necessary winding details are obtained while
disassembling the burnt out transformer, the rewinding procedure is more or less easy. However, if you have to
prepare a new transformer the following information will be of great help.
Designing a transformer : Small transformers are generally of ‘SHELL TYPE’. In shell type, both the primary
and secondary windings are mounted on the centre limb of the core. For designing of a small power transformer
proceed as stated below.
STEP NO.1

Find the total output power from the load voltage and current of the transformer.
P = E x I .........Formula 1.
2 2 2
The following example is given for your guidance.
Primary voltage - 240 V
Secondary voltage - 6V

Secondary total current - 2A
From the example the output power is calculated as 6 x 2 = 12VA.
STEP NO.2
Find the input watts.
P
....... Formula 2
2

P
1
Efficiency

%
Normally the efficiency of a transformer will be 80 to 90. As in the example
6 x 2 x 100
P 1 80 15 VA.
152 153

CITS : Power - Electrician & Wireman - Lesson 86-92