Mechanical Forces Acting on Transformer

         As we all know there exists forces of attraction and repulsion amongst different and same charges due to magnetic field produced by them. A current carrying conductor also generates a magnetic field which is in form of concentric circles around the conductor. 

In a transformer the two windings carry current in opposite direction so the current of one conductor interacts with the leakage field surrounding them due to conductor of other winding which causes the unbalanced forces to act on winding and core or we can say their asymmetric flux distribution is the main cause of force in transformer.


  For simplification of understanding we refer to core type transformer…..

By using magneto static analysis of PDE TOOLBOX of MATLAB software the flux distribution of our defined model can be obtained as follows.

          

Here in Fig-1 we have taken a 2400/240V transformer in which primary winding comes out of page and secondary goes into page. If we consider single conductors of both sides a schematic diagram with its leakage field can be shown as Fig-2. The leakage field can be resolved into its axial and radial components as in Fig-3 and Fig-4. The mechanical forces produced on account of interaction of two components of leakage flux with current are consequently in two perpendicular directions.
The forces in two different directions are axial and radial forces. Some times due to magnetic asymmetry in transformer some unbalanced axial forces starts acting.

  

Radial Forces:-

The radial forces produced by the axial leakage field act outwards on the outer winding tending to stretch the winding conductor, producing a tensile stress (also called as hoop stress); whereas the inner winding experiences radial forces acting inwards tending to collapse or crush it, producing a compressive stress, this is because primary and secondary winding of transformer carry current in opposite direction. Therefore the external winding is subjected to outward pull of other winding and internal winding turns are subjected to the repulsive force due to conductor of same winding.

Calculation of radial force on winding:-

           

The winding of a transformer must be designed to withstand radial forces in the worst conditions. From daily practice it is also clear that circular coil should be preferred more, because they have high tendency to with stand while other structures will be subjected to very higher forces at corners ant that may result in breaking of coil.

Axial Forces:-

          

Forces due to Asymmetry:-

In general transformer coils are symmetrical so axial forces become more effective tending to compress inner coils and through out outer coils. Asymmetry in transformer winding is due to some of the following reasons:-

·         Generally HV windings are provided with tapings.

·         When calculating no. of turns of axial length of helical winding one turn is added to no. of turns due to helical wind of turns.

·         The end turns of high voltage are reinforced to withstand the effect of voltage surge.

In case of fault condition the axial forces becomes very large due to high increase in axial component of force and asymmetrical magneto motive force distribution. Hence proper bracing of winding is necesary.

Hope this was containing some useful information for you. Thanks.