Transformers-2

Hi folks.
Here we are going to talk about the tertiary winding of the transformer.  To start with, lets revise the facts about tertiary winding and then we will understand why it is so.

1.Tertiary winding is always connected in delta.
2. It is never used on the transformer where any of the windings (primary or secondary) are connected on delta.
3. It is never used on the transformer with 3 limbed core (implies used on transformer with 5 limbed core or 3 single phase type construction).

To understand 1st and 2nd point,

• Due to non linearity of the core, we can’t generate the sinusoidal flux in the core by applying sinusoidal voltage.
•  Without sinusoidal flux in the core, the secondary induced EMF will not be sinusoidal. This will make transformer unsuitable to supply single phase loads on secondary.
• To make flux in the core sinusoidal, we need to apply EMF that has 3rd harmonic components in it.
• Star windings don’t help the 3rd harmonic currents (co-phasal nature). But delta winding does. 
• So if any of the windings are connected in delta, 3rd harmonic voltage will be available. 
• This voltage will induce sinusoidal flux in the core and secondary EMF will be sinusoidal which is most desirable.

To understand 3rd point,

• Like 3rd harmonic flux, 3rd harmonic currents are also co-phasal in nature. They need closed path within the phases.
• If you observe, the extra 2 limbs in the 5 limbed core provide that path. So 3rd harmonic flux exists and affects the induced secondary EMF.
• If the core is 3-limbed, the 3rd harmonic flux has no closed path within the phases. So it cant flow and affect the output.

Even though we are connecting source the gives output at fundamental frequency only, from where are we getting the 3rd harmonic EMF in delta is very interesting concept. We will soon talk about it.  Also if we don’t connect the tertiary winding in the star-star 5 limbed core, problem of floating neutral will happen. You can search for above on your own.

Thank you for your time and please feel free to comment about your doubts and suggestions. You can mail me at dnachiketa1010@gmail.com about your areas of interest.