Paging prc for a lighting duty transformer design question 3

[edison123]

Hi prc

After a very long time, I am designing a one-off lighting transformer 3-ph, 20 KVA, 480 V Delta / 400 V star, 60 Hz air cooled, indoor application.

With 1.5 T and a stacking factor of 0.95 , I got the following voltage/turn for different core cross section (all square CS with square coils)

A. 150 x 150 mm - 8.541 V/turn
B. 125 x 125 mm - 5.932 V/turn
C. 100 x 100 mm - 3.796 V/turn

1. Are the above volts/turn correct?

2. Which core cross section would you recommend, A, B, C or something else?

3. Is 1.5 T for modern M36 CRGO steel too low? Should I increase it to 1.7 T?

Thanks in advance.


Muthu

http://www.edison.co.in

 

[prc]

First of all, I am not familiar with design of such small transformers. But per turn voltage -I know a bit- Surprising point is this follows a formula for the past 130 years irrespective of size or type of transformer from 1 KVA to 1000 MVA .

Per turn voltage = 0.7-0.9 square root of kVA per limb. 0.9 more appropriate for 60 Hz. So per turn voltage can be 2- 2.5 V . Being in air, better to limit flux density to 1.6 T instead of 1.7 T. So you can opt for smaller core section too.

 

[edison123]

Thanks prc.

So for 20 KVA, the turn voltage would be sqrt(20/3) X 0.9? = 2.3 V

Muthu

http://www.edison.co.in

 

[prc]

You are right.

 

[edison123]

Thanks again.

Muthu

http://www.edison.co.in

 

[ZeroSeq]

prc, this formula is very useful, thanks for the insight.

Would you happen to know where it is cited (ex. J&P Transformer Book, Electric Power Transformer Engineering, etc.)?

 

[edison123]

prc - With a cross section of 60 sq cm and 1.6 T, I get 2.43 V. Is it correct?

Muthu

http://www.edison.co.in

 

[prc]

Muthu,the formula gives a band for per turn voltage, You are free to select appropriate value based on applications, constraints, supply frequency etc. Value to be increased with frequency. eg for 60 Hz -by 60/50 times increase. In fact while training young designers, I used to say that ability of designer is to judge and select correct per turn voltage. This is the most important single parameter that decides almost all other aspects and performance parameters of transformer- high or low impedance; core machine or copper machine etc.

 

[prc]

ZeroSeq- I first noticed this formula in a book by Russian author, published in India. I then checked this with designs used in 19 th century to the latest transformer designs and found this to be correct. I corrected his formula -instead of rating, KVA should be that in a core limb. Some times, single phase units may be wound in two limbs, then half the rating should be used.

1962
S.B.Vasutinsky, Principles, Operation and Design of Power Transformers, PSG College of Technology, Peelamedu, Coimbatore-4, 1962 (first transformer book to be published from India)

This Russian engineer was then working in an Engineering College in Coimbatore, India. Later I found that his book was the first transformer book Published in Chinese too. A Chinese Professor forwarded me a copy of this book in Chinese. He had also published transformer books in Russian too. Most popular transformer book from India is that of Prof Kulkarni

2004
S.V.Kulkarni, S.A.Khaparde, Transformer Engineering –Design Technology and Diagnostics, CRC Press,Ed1.0 2004, Ed2.0 Pages 729, 2013