Page 165 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fig 9

Winding a small transformer

Objectives: At the end of this lesson you shall be able to:
• state the important data to be taken for rewinding the transformer
• explain the rewinding procedure for small transformers
• calculate the number of turns per volt using the formula and determine primary and secondary turns
• determine the dimensions of the transformer, size of bobbin and size of winding wire
• explain the tests to be carried out after winding the transformer.

Rewinding of small transformer
It is necessary to rewind a transformer when the winding is burnt out or badly damaged.
While dismantling transformers, care should be taken to record the necessary particulars (data) by which the
rewinding process becomes easy and the original performance of the transformer is assured.
Recording the data : The following data have to be taken from the transformer before and during disassembling.
1 Number of windings/turns/ layers.
2 Size of wires and insulation.
3 Input/output voltages & currents.

4 KVA ratings.
5 Conncetion diagrams.
6 Terminal marking / lead position
7 Types of cores / number of stampings
8 Physical condition of bobin / core.

9 Insulation schemes like size and specification of bindings, layer,interlayer,inter windings, bobin,lead wires,
sleeves etc.
If the old bobbin is reused for winding, it shall be cleaned well and shall be free from any break or crack. If a new
bobbin is used it shall be checked with the stamping (core) for proper assembly to avoid too much air gap or too
tight a fitting.
For winding, a suitable size of wire shall be selected from the data and the size of wire shall be measured as per
I.S. 4800 (Part - I) 1968.

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